experimental investigation of o2-co2 firing during pulverized coal combustion
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Institut für Verfahrenstechnik und Dampfkesselwesen Institute of Process Engineering and Power Plant Technology Direktor: Prof. Dr.-techn. G. Scheffknecht Pfaffenwaldring 23 · D-70550 Stuttgart · Tel.: 0711-685-3487 · Fax: 0711-685-3491 Universität Stuttgart
Master Thesis No. 2762
�Experimental investigation of O2/CO2 firing during pulverized coal
combustion�
Kashif Imtiaz Choudhry Matriculation No 2071972
Stuttgart, 28.02.2005
Start date: 01.06.2004 Submission date: 28.02.2005 Supervisor: Bhupesh Dhungel
This work is dedicated to my
Parents
Their prayers are always behind me
I
Acknowledgement I must firstly thank my supervisor Engr. Bhupesh Dhungel who does deserve my greatest thanks, since he provided me with incredible support, encouragement, and guidance for my thesis research work. I am thankful to Stefan Kiening & Patrick Mönckert for their generous assistance during this time. I also thank my colleagues of the MSc WASTE Process Engineering students 2003 (entry) class in universität Stuttgart Germany of for sharing experiences and knowledge during the time of study. Finally, I take this opportunity to express my profound gratitude to my beloved parent, my brothers and sisters for their moral support and patience during my study in Universität Stuttgart Germany. Engr. Kashif Imtiaz Choudhry M.Sc Engineer
II
Abstract Global warming is one of the largest environmental challenges of the present time. Increased
carbon dioxide level in the atmosphere is the dominating contributor to increased global
warming. Carbon dioxide emitted to the atmosphere through combustion of fossil fuels in
power plant, automotive engines for industrial use and for heating purposes. Three main
options for reducing CO2 emissions from fossil fuel based energy conversion are 1) increase
the fuel conversion efficiency 2) switching to a fuel with lower fossil carbon contents and 3)
capturing and storing the CO2 emitted from fossil fuel.
In order to reduce emissions of carbon dioxide from large point sources Oxy fuel combustion
technique can be used in pulverized coal fired power plants with CO2 capture. The fuel is
burnt in oxygen and recycled flue gas, yielding a high concentration of CO2 and low
emissions like NOx and SOx in the flue gas. The aims of the study are;
• Carrying out experiments for the determination of coal particle temperature profile
along the reactor axis with different O2/CO2 concentration and comparing it with base
line air condition for ignition behaviour.
• Evaluating the behaviour of two-colour pyrometer used for particle temperature
measurement at different reactor conditions and CO2 concentration
• Measuring the concentration of different gases along the reactor length during oxy fuel
combustion with different concentration of O2/CO2 and comparing it with base line air
condition for combustion behaviour.
III
Contents ACKNOWLEDGMENT .................................................................................................��.� ..I ABSTRACT�........................................................................................................��� ... .II CONTENTS ��������������������������..���� III LIST OF FIGURES ����������������������������. IV LIST OF TABLES ����������������������������... VI CHAPTER 1 �����������������������������.�...1 INTRODUCTION ������������������..���������� ..1 1.1 Coal & its classification ����������������.������ ..1 1.2 Emissions from coal combustion������������.������.. ..3 CHAPTER 2�����������������������..������ ..5 INTRODUCTION TO OXY FUEL COMBUSTION ..������������ ..5 2.1 CO2 free power generation �...�������������������. ..5
2.1.1 Pre combustion ����.������������������.. ..5 2.1.2 Post Combustion capture from conventional plants ��������. ..6 2.1.3 O2/CO2 (Oxy fuel) combustion ���������������.� ..7
2.2 Air separation �������������������������� 11 2.2.1 Cryogenic distillation �������������������� 11 2.2.2 Membrane separation �������������������� 11 2.2.3 Pressure swing adsorption ������������������ 12
2.3 CO2 sequestration ������������������������ 12 2.4 Development in Oxy fuel combustion ����������������. 13 2.5 Ignition Mechanism �����������������������. 16 CHAPTER 3 .............................................................................................................�. 20 DESCRIPTION OF EXPERIMENTS�����������������.�. 20 3.1 Test facility ��������������������������.. 20 3.2 Temperature measurement equipment ����������������. 21 3.3 Flue gas analysis ������������������������.. 23 3.4 Characterisation of the fuel ��������������������.. 24 3.5 Parameters for ignition tests ��������������������. 24 3.6 Parameters for combustion tests������������������� 25 CHAPTER 4 ...............................................................................................................� 26 IGNITION RESULTS������...������������������. 26 4.1 Influence on particle temperature for different conditions during air combustion.. 26 4.2 Ignition during Oxy fuel ���������������������... 31 CHAPTER 5 ...........................................................................................................�� 38 COMBUSTION RESULTS������...���������������.� 38 CHAPTER 6 ...........................................................................................................�.... 43 CONCLUSION AND FUTURE WORK.....������������.��.�.. 43 6.1 Conclusion ��������������������������� 43 6.2 Future work ��������������������������... 44 6.3 Future Challenges ������������������������ 44 BIBLIOGRAPHY ��������������������������� 46
IV
List of Figure Figure 1.1 Kinds of coal ............................................................................................. 2 Figure 2.1 Pre -combustion capture ������������������� 6 Figure 2.2 Post combustion capture ������������������� 6 Figure 2.3 Oxy fuel combustion capture �����������������. 7 Figure 2.4 Schematic of Oxy fuel combustion ��������������� 8 Figure 2.5 Effect of Oxygen enrichment on flame temperature ��������.. 9 Figure 2.6 Ratio of the required volume of gas in the case of combustion with
recycled flue gas over the required volume in the case of combustion in air ��������������������������� 10
Figure 2.7 Main processes in coal combustion ........................................................... 17 Figure 2.8 Stages in pulverized coal conversion ��������������. 18 Figure 2.9 Variation of concentration of oxygen and fuel with distance from the
fuel surface. Ignition occurs at some definable concentration ratio .......... 19 Figure 3.1 Schematic of pulverised coal test facility and arrangement for coal
particle temperature measurement using two-colour pyrometer with movable coal injection probe & for combustion experiments ����... 20
Figure 3.2 Optical set up of the pyrometric measurements at flow reactor ���� 21 Figure 3.3 Time dependence of primary and reference signals when fuel particle
passes through the FOV of the optical probe ����������� 22 Figure 4.1 Influence of Nitrogen/Air (Carrier gas) on coal particle temperature,
Lausitz, WT= 1100°C .............................................................................. 27 Figure 4.2 Influence of coal feeding rate, Lausitz, WT=1100°C, 90-125µm,
Carrier gas = N2����������������������. 28 Figure 4.3 Influence of particle size on coal particle temperature, Lausitz,
WT=1100°C ............................................................................................. 29 Figure 4.4 Dispersion of particles along the tip of dosing unit ��������. 30 Figure 4.5 Influence of Oxy fuel on Coal particle temperature, Lausitz
WT=1100°C, fraction = 212-315µm ........................................................ 31 Figure 4.6 Influence of Oxy fuel on Coal particle temperature, Klien Kopje,
WT = 1100 & 1300°C, fraction = 150 � 212 µm ..................................... 32 Figure 4.7 Particle temperature behaviour under air and Oxy fuel conditions
at reactor temperature 1300°C, fraction 212-315µm (Klien Kopje) ��. 33 Figure 4.8 Particle temperature behaviour under air and Oxy fuel conditions
at reactor temperature 1300°C, fraction 90� 150µm (Klien Kopje)��... 34 Figure 4.9 Particle temperature behaviour under air and Oxy fuel conditions
at reactor temperature 1300°C, fraction 90-150µm (Elcerejon)����. 35 Figure 4.10 Particle temperature behaviour under air and Oxy fuel conditions
at reactor temperature 1300°C, fraction 150 � 212µm (Elcerejon) ��... 36 Figure 4.11 Particle temperature behaviour under air and Oxy fuel conditions
at reactor temperature 1300°C, fraction 212-315µm (Elcerejon) ���. 36 Figure 4.12 Relation between detected particles over the distance from burner ��. 37
V
Figure 5.1 Emission concentration of NO, CO, SO2 and CO2, Lausitz,
WT=1100°C, O2excess= 3 % �����������������.. 38 Figure 5.2 Emission concentration of NO, CO, SO2 and CO2, Lausitz,
WT=1300°C, O2excess= 3 % ������������������ 39 Figure 5.3 Concentration of NO [ppm] in flue gas at 1100°C & 1300°C, Lausitz � 40 Figure 5.4 Concentration of SO2 [ppm] in flue gas at 1100°C & 1300°C, Lausitz � 41 Figure 5.5 Emission concentration of NO, CO, SO2 and CO2, Klien Kopje,
WT=1300°C �����������������������. 41
VI
List of tables Table 1.1 Coal elemental and proximate analysis ������������� 2 Table 1.2 Specific CO2 emissions of various fuels ������������.. 4 Table 3.1 Analyser employed for flue gas analysis with their ranges �����. 23 Table 3.2 Proximate and ultimate analysis of fuels �����������.� 24 Table 3.3 Parameters for ignition tests ����������������� 24 Table 3.4 Parameters for combustion test ���������������... 25
Introduction
1
1 Introduction
1.1 Coal & its classification
Coal is a solid, brittle, combustible, carbonaceous rock formed by the decomposition and
change of vegetation by compaction, temperature, and pressure. It varies in colour from
brown to black. Coal is a readily combustible rock containing more than 50 percent by
weight of carbonaceous material. Coal still provides a large portion of the world's energy
requirements, accounting for about 40 % [1] of the total worldwide electricity generation.
The use of coal is bound to increase in the future because world reserves are large and
widespread and coal is a low-cost alternative to other fuels
Coal is formed by the physical and chemical alteration of peat by processes involving
bacterial decay, compaction, heat, and time. Coal is an agglomeration of many different
complex hydrocarbon compounds. Peat deposits are actually quite varied and contain
everything from plant parts (roots, bark etc.) to decay plants, decay products, and even to
charcoal if the peat caught fire. Peat deposits typically form in a waterlogged environment
where plant rubbish is accumulated.
In order for the peat to become coal, it must be buried by sediment. Burial cause
compaction of the peat and, water is squeezed out during the first stages of burial.
Continued burial and the addition of heat and time cause the complex hydrocarbon
compounds in the deposit to start to break down. The gaseous alteration products (methane
is one) are typically expelled from the deposit and the deposit becomes more and more
carbon-rich. The stages of this trend proceed from plant debris, peat, lignite, sub-
bituminous coal, bituminous coal, anthracite coal, to graphite (a pure carbon mineral).
The kinds of coal are lignite (brown coal), sub-bituminous, bituminous, and anthracite.
These classes are further divided into subclasses based on their degree of alteration.
Introduction
2
Table 1.1: Coal elemental and proximate analysis Coal Elemental Composition Coal Proximate Analysis
C 50 � 95 % Char 20 � 70 %
H 2 � 7 % Ash 5 � 15 %
O < 25 % Water 2 � 20 %
S < 10 % Volatiles 20 � 45 %
N 1 � 2 %
Coal consists of a complex range of materials and varies greatly in quality from deposit to
deposit, depending on the varying types of vegetation from which the coal originated, the
depth at which the deposit was formed, the temperatures and pressures exerted at that
depth, and the length of time the coal has been forming.
Figure 1.1: Kinds of coal
Coal is classified according to:
• The degree of transformation of the original plant material into carbon, ranging
from anthracite (the hardest) to bituminous, sub-bituminous and lignite
• Moisture content: coals high in carbon and low in moisture are ranked the highest
• Composition: coal is predominantly carbon but also contain hydrogen, oxygen,
nitrogen and varying amounts of sulphur.
Peat Lignite Bituminous Anthracite Graphite
Time Temperature
Introduction
3
1.2 Emissions from coal combustion
During coal combustion, carbon is converted to carbon dioxide (CO2), hydrogen is
converted to water vapours (H2O) and sulphur to sulphur dioxide (SO2) and a small amount
of trioxide (SO3). If the temperature of the furnace is high enough some of the nitrogen in
the coal and in the air is converted to nitric oxides (NOx).
Both SO2 and NOx combine with oxygen and water in the atmosphere to form acids that
return to the ground in the form of acid rain with very negative environmental impact. For
example,
SO2 + 1/2 O2 + H2O → H2SO4
When coal containing a high quantity of sulphur is burned, the sulphur dioxide produced
must be reduced to meet emission standards set by local government regulations. Another
environmental hazard in burning coal is fly ash, which consists of particles left over after
combustion. Two types of ash are produced from coal combustion. Heavier particles, called
bottom ash because they are removed from the bottom of the furnace and typically flushed
with water to a settling pond outside the plant. Fly ash, on the other hand, consists of finer
(lighter) particles that are carried away with the furnace flue gas (CO2, H2O, etc). Fly ash is
a pollutant, which like the SO2 and NOx gases must be removed from the flue gas before it
is vented to the atmosphere
Basically, the main drawback of fossil fuels is pollution. Burning any fossil fuel produces
carbon dioxide, which contributes to the greenhouse effect, and is blamed for global
warming. Burning coal produces carbon dioxide (CO2), carbon monoxide (CO), sulphur
dioxide (SO2), nitrous oxides (NOx) and particulate in exhaust gases. Their levels vary
widely, depending on fuel composition, combustion system and operating conditions. For
example, the flue gas of a coal-fired power plant may contain 300�3,000 ppm SOx, 100�
1,000 ppm NOx, and 1,000�10,000 mg/m3 particulate matter. Natural gas firing
significantly lowers the contaminant levels to less than 1 ppm SOx, 100�500 ppm NOx, and
around 10 mg/m3 particulate matter [3]. CO2 contributes to greenhouse effect associated
with global warming while SO2 and NOx contribute to acid rain. Acid rain continues to
damage sensitive lakes. NOx emissions from coal-fired power plants contribute the ground
Introduction
4
level ozone, also known as smog. Coal fired power plants are responsible for most of the
sulphate particles that cause haze and reduce visibility.
When burned in a relatively uncontrolled fashion, coal can cause a lot of damage due to
pollutants. With modern technologies and strict controls it is possible to remove most of the
SO2 and NOx before they are emitted from a power plant, however coal contains more
carbon and less hydrogen than other fossil fuels such as oil and natural gas and produces
more CO2 per unit of electricity produced than any other fuel. Burning of lignite emits 80
% [33] more carbon dioxide emissions with respect to the energy contents than burning of
natural gas. Due to this coal fired power plants have been targeted the most by
environmental agencies.
Table 1.2: Specific CO2 emissions of various fuels [33] Fuel Emissions (kgCO2/kWh)
Peat 0.38
Lignite 0.36
Hard coal 0.34
Diesel 0.27
Gasoline 0.25
Natural gas 0.20
One of the methods for capture of CO2 is oxy fuel combustion; in this method we prevent
nitrogen from being mixed with the combustion products, particularly CO2. For instance,
the concentration of CO2 in flue gas can be increased greatly by using oxygen instead of air
for combustion. Having captured the CO2 it would need to be stored securely for hundreds
or even thousands of years, in order to avoid it reaching the atmosphere.
Introduction to oxy fuel combustion
5
2 Introduction to Oxy fuel combustion
Presently coal is one of the most widely used fuels in the power generation, accounting for
around 40 % [1] of the power produced worldwide. However coal requires extensive
emission control strategies, primarily due to the NOx, SOx, Hg, Particulates and CO2
emissions. To meet the increasingly stringent emission control standards, Pulverized coal
fired plants are now required to be equipped with a variety of flue gas treatment systems.
For a power plant these devices represent a significant cost increases. Oxy-fuel combustion
is considered to be one of the effective methods to improve thermal efficiency and reduce
NOx, SOx and CO2 emission for high temperature furnaces. A major benefit of Oxy
combustion is the fact that the flue gas exhaust flow rate is significantly reduced because of
re-circulation of flue gas, can lead to significant cost savings.
Flue gases emitted from medium to large point sources are generally at or slightly above
atmospheric pressure. They typically contain 3�15% (by volume) of carbon dioxide. For
example, flue gas from a coal-fired power plant typically contains about 14% CO2, 5 % O2
and 81% N2. Flue gas from a natural gas turbine is even leaner in CO2 but higher in O2 with
a typical composition of 4% CO2, 15% O2 and 81% N2. [3].
In the past decade there has been a growing concern about greenhouse gas emission (CO2,
CH4 and N2O) and its potential impact on climate change. Since coal fired power plant
account for high percentage of CO2 emission worldwide, it has a significant impact on the
global greenhouse gas effect. To reduce CO2 emission, it is necessary to develop clean and
efficient combustion technologies for existing and new coal fired power generation plants.
2.1 CO2 free power generation
Three system approaches are possible for CO2 capture from power systems:
2.1.1 Pre-combustion capture Carbon is captured in the form of CO2 from the fuel before combustion. The fossil fuel is
first transferred (via gasification or reforming) to hydrogen and CO2 for CO2 capture.
Afterwards, power production is achieved.
Introduction to oxy fuel combustion
6
Figure 2.1: Pre -combustion capture
2.1.2 Post Combustion capture from conventional plants
Carbon is captured in the form of CO2 from the flue gasses after combustion. The energy
content of the fossil fuel is converted to electricity. Afterwards, CO2 is separated form the
flue gas. The power production efficiency will be lower by 7 % - 29 % and power
production cost will be rise by 1.2 � 1.5 times for separating CO2 [13]. The main reason
that causes the raising cost is difficult to separate CO2 from lower concentration of CO2
flue gas that the main component is N2.
Figure 2.2: Post combustion capture
Flue gas Gasification /
reforming
CO shift H20 + CO => H2 + CO2
CO2 Separation
Combustion
Air/O2 /H2O CO2 Air
Fuel
Energy Electricity CO2
Flue gas
Fuel
Air Power Process
CO2Separation
Introduction to oxy fuel combustion
7
2.1.3 O2/CO2 (Oxy fuel) combustion The principle of this method is to prevent nitrogen from being mixed with the combustion
products, particularly CO2. For instance, the concentration of CO2 in flue gas can be
increased greatly by using oxygen instead of air for combustion.
C + O2 => CO2
Due to oxy fuel technique CO2 concentration becomes high, resulting in easier and
economical separation.
Figure 2.3: Oxy fuel combustion capture
O2/CO2 Combustion involves burning of the fuel in an atmosphere of oxygen and recycled
flue gas instead of air. Figure below shows a schematic layout of an O2/CO2 coal power
plant. The mixed flow of oxygen and recycled flue gas is fed to the boiler together with
fuel. A part of flue gas is separated and mixed with new oxygen. The remaining part of flue
gas is treated, compressed, transported for another application or sequestered.
Recirculated flue gas Water
Electricity
CO2
Flue gas
Air Fuel
Oxygen Air
Separation
Power Process Condenser
N2
Introduction to oxy fuel combustion
8
Figure 2.4: Schematic of Oxy fuel combustion
The amount of required oxygen can be produced in an air separation unit. The oxygen is
diluted with recycled flue gas in order to attain combustion conditions. Concentration of
oxygen in the feed gas can be varied from pure oxygen to lower concentration. This mean
that it is possible retrofit an existing boiler plant to O2/CO2 combustion, even if a design of
a new power plant is more preferable, since it opens for optimisation of the oxygen
concentration in the feed gas which should yield a higher combustion efficiency. To use
pure oxygen we need material that can withstand higher temperature. With the use of pure
oxygen, combustion efficiency can be highly increased and reduce the furnace size over the
conventional furnace for the same energy output. At normal combustion of coal in the air
the concentration of carbon dioxide in the flue gas is approximately 14 %. This mean an
expansive process is necessary to increase the concentration of the carbon dioxide in order
to gain the concentrated carbon dioxide in the flue gas.
With Oxy fuel combustion system, it is possible to make CO2 concentration in the flue gas
up to 98 % [4] by separation of nitrogen from the combustion air in advance and using
oxygen and recycled flue gas, most of which is CO2. Carbon dioxide concentration in the
Air Nitrogen
Coal
Flue gas~97% CO2
Boiler
Drier
Water
Air Separation
Compressor
G
TurbineGenerator
Feed Pump
Oxygen
Flue Gas (~ 90% CO2) Recycle ~75%
Introduction to oxy fuel combustion
9
flue gas depends directly on the oxygen concentration in the feed gas, the higher the oxygen
concentration in the feed gas, the higher the carbon dioxide concentration in the flue gas.
Effect of Oxygen enrichment on flame temperature
1900200021002200230024002500260027002800
20 30 40 50 60 70 80 90 100
OXYGEN %
TE
MP
(C)
Figure 2.5: Effect of Oxygen enrichment on flame temperature [5]
Advantages
• In Oxy fuel combustion, Concentration up to 98 % carbon dioxide in the dry flue
gas may be possible, allowing direct sequestration.
• The reduced volume involved due to combustion at higher oxygen concentration,
and thus with less inert gas volume. Advantages to this reduced volume are lower
dry gas energy losses, higher plant efficiency and lower energy gas loss for gas
cleaning and separation. In figure 2.6 shows the ratio of the required volume of gas
in the case of combustion with recycled flue gas over the required volume in the
case of combustion in air
Introduction to oxy fuel combustion
10
Figure 2.6: Ratio of the required volume of gas in the case of combustion with recycled flue gas over the required volume in the case of combustion in air [4].
We can see from the figure 2.6, 5% excess oxygen situation requires lower volume
of gas than for the 1% excess oxygen case. In fact the difference between these two
cases is in the volume of pure oxygen brought into the system. For the 5% excess
oxygen case, less volume of pure oxygen is needed because more oxygen is brought
back to the system through re-circulation of the flue gas than in the case of 1%
excess oxygen. Another advantage with the higher oxygen concentration is the
ability to minimize unburned carbon.
• NOx and SO2 formed during Oxy fuel combustion is lower than from conventional
combustion system [4]
Disadvantages
• Because much oxygen is required, a large amount of parasitic power is required for
producing oxygen, resulting in high cost
• Due to recycle of exhaust gas, corrosive components in the exhaust gas become
high in concentration.
Introduction to oxy fuel combustion
11
2.2 Air separation
The difference between oxy fuel combustion plants from conventional plant from retrofit
point of view is air separation unit, where we separate Oxygen and Nitrogen. There are
many possibilities for air separation. Details of some methods are given below
2.2.1 Cryogenic distillation
Cryogenic Separation is a distillation process that occurs at temperatures close to -170° C.
At this temperature, air starts to liquefy. Before separation can occur, there are specific
operation conditions that must be achieved. Distillation requires two phases, gas and liquid.
Air must be very cold for this to happen. For this instance, at one atmosphere, nitrogen is a
liquid at -196° C. A pressure 8-10 time�s atmospheric pressure is required for this process.
These conditions are achieved via compression and heat exchange; cold air exiting the
column is used to cool air entering it. Nitrogen is more volatile than oxygen and comes off
as the distillate product. A cryogenic air separation plant is expensive and large; the
distillation column is several stories high and must be well insulated. Consequently, it only
becomes economically feasible to separate air this way when a large amount is needed.
Cryogenic separation is also capable of producing much purer nitrogen than either of the
other two processes because the number of trays in the distillation column can be increased.
2.2.2 Membrane separation
Membrane separation of air is primarily a physical process, based upon specific
characteristics of each molecule, such as size and permeation rate. The molecules in air can
be separated to form mostly pure forms of nitrogen, oxygen, or both. In a membrane
system, there is a hollow tube filled with thousands of very thin membrane fibres. Each
membrane fibre is another hollow tube in which air flows. The walls of the membrane fibre
are porous and are specially made so oxygen molecules can permeate through the wall at a
faster rate than nitrogen, allowing a nitrogen-rich stream to flow out the other end of the
fibre. Meanwhile, the air outside the fibre, in the hollow tube, is now oxygen-rich and can
be collected somewhere else.
The purity of the nitrogen generated depends primarily on two factors: the flow rate and air
pressure. If we have high air pressure, the oxygen molecules have greater incentive to
Introduction to oxy fuel combustion
12
permeate through the fiver wall. If our flow rate is slower, then oxygen has more time to
permeate through the fibre wall. We can easily adjust both of these factors to allow a
system operator to vary the amount and purity of the nitrogen generated in a very short
amount of time.
2.2.3 Pressure swing adsorption
Pressure Swing Adsorption units separate air using a special sieve that adsorbs oxygen
preferably to nitrogen. When high-pressure air flows through the sieve, oxygen molecules
are caught while nitrogen molecules pass on. The sieve continues to adsorb oxygen until a
saturation point is reached. After that, the entering air stream is cut off and the oxygen is
able to leave the tank at low pressure. In a PSA unit, two connected tanks and containing
sieves, work together to produce a near-continuous stream of nitrogen. When one tank has
become saturated and is releasing adsorbed oxygen, the entering air stream is switched to
the other tank for oxygen adsorption.
2.3 CO2 sequestration
Carbon dioxide collected from carbon dioxide removal technologies can be compressed and
transportation to the point of sequestration. Carbon dioxide sequestration can be
accomplished either by an offset (indirect sequestration) or by reduction in the emissions
from generation facility (direct sequestration) [6].
Direct CO2 sequestration involves capturing CO2 at its point of generation before it is
released to the atmosphere. The CO2 is then put in long-term (hundreds to thousands of
years) environmentally sound storage, usually in a deep geological formation. Removing
CO2 from the exhaust streams of factories and electric plants and storing it deep
underground would be an example of direct sequestration Direct Sequestration technologies
includes:
• Injection into oil and gas reservoirs
• Injection into deep, unmineable coal seams.
• Injection into saline aquifers.
• Injection of liquid carbon dioxide into deep Ocean.
Introduction to oxy fuel combustion
13
Indirect CO2 sequestration involves capturing CO2 that has already been released to the
atmosphere. CO2 is removed from the atmosphere through intake by plants or by fixing
carbon in the soil. The CO2 captured by indirect sequestration occurs through a variety of
means, both natural and anthropogenic.
2.4 Development in Oxy fuel combustion
In the power generation there are tremendous efforts due to economical and ecological
reason to reduce the emission without reducing the efficiency or increasing the capital costs
significantly. There are lot of research works going on to get the desired goals. Some of the
information regarding those research works towards betters efficiency, economic and
emission reduction is given below
A retrofit oxygen fired plats with different capacities of 30, 100, 200, and 500 MW power
output [1], consisting of Air separation unit, boiler, recalculated flue gas, DeNOx system,
Hg removal system, Electrostatic precipitator, flue gas desulphurisation, carbon dioxide
conditioning and sequestration, and stack. It was found that full oxy combustion will lower
the required heat transfer areas by ~ 50 %, making the boiler more compact and less capital
intensive NOx removal efficiency 80 � 95 % and particulate removal efficiencies of 99 �
99.9 % is achieved. The analysis shows that, under the assumed conditions, the total
annualised cost of the oxy-fired plants is comparable to those of the air-fired cases. For new
plants, which would also include advanced, compact, full oxy fired boilers; the total costs
of the oxy-fired plants are lower than those of the air fired plants. Due to economic scaling
factors, the oxy-fired plants are more economically viable at smaller sizes. Finally it is
noted that oxy-fired operation generates a flue gas rich in carbon dioxide, which may be
easily captured in order to sequester.
In the enriched oxygen coal fired combustion scheme [4] the carbon dioxide concentration
in the flue gas depends directly on the oxygen concentration in the feed gas, the higher the
oxygen concentration in the feed gas, the higher the carbon dioxide concentration in the
flue gas and up to 98 % dry volume, carbon dioxide concentration can be achieved in the
flue gas. Research shows that O2/CO2 recycle system reduced volume involved due to
combustion at higher oxygen concentration, and thus with less inert gas volume, like 5 %
excess oxygen situation requires lower volume of gas than 1 % excess oxygen case. All the
Introduction to oxy fuel combustion
14
experiments performed at a firing rate of 0.2 MW for Eastern bituminous coal, 5 % vol dry
excess oxygen in flue gas and oxygen in the feed varies between 28 and 42 % vol on a dry
basis and with oxygen purity of 100 % and 90 %. The flame temperature increases with the
concentration of oxygen in the feed gas, the temperature rises from about 1300°C at 28 %
oxygen to about 1500°C at 42 % Oxygen. Results shows that SO2 chemistry is not affected
by the presence of high concentration of carbon dioxide and oxygen, at least in the range of
28 to 35 % for the oxygen. At 28 % and 35 % with flue gas re-circulation shows much
lower SO2 emission rates than non-recycle runs. CANMET energy technology centre
mentioned in an research paper � Oxy fuel combustion research at CANMET energy
technology centre� [24] that 30 % to 35 % oxygen in recycled flue gas (dry or wet) is the
most feasible retrofit option since temperature profile and heat transfer matches to
conventional air combustion.
Combustion test facility [10] with vertical cylinder type furnace and combustion capacity of
1.2 MWt (equivalent to coal firing rate of 150 kg/h). The fuel ratio of coal used in this
study was 0.8, 1.3 and 1.6. Pulverized coal used with the grain size from 53 to 63 µm. With
O2 / RFG pulverized coal combustion system, it is possible to make the CO2 concentration
in the exhaust gas 95 % or highest by separation nitrogen from combustion air in advance
and using oxygen and recycled flue gas, most of which is CO2.
A pilot scale demonstration of oxy combustion with flue gas re-circulation in a pulverized
coal fired boiler [16] had an experimental set-up for 1.5 MW pulverized coal boiler with re-
circulated flue gas (without flue treatment). The operating condition were primary Oxidant
was 15 to 20 % of total, secondary oxidant was 50 to 85 of total, Tertiary oxidant was 0 to
30 % of the total and outlet oxygen concentration was 3 %. Only 80 % of the air stream was
replaced by recycled flue gas. It was found NOx reduction up to 76 % during staged oxy
combustion when compare to un-stage air firing and 50 % reduction of Hg during oxy
firing. During oxy firing unburned carbon were 22 % lesser without staging and 48 % lesser
with staging when compared to non-staged air firing.
A joint research was carried out from Air Liquide, Illinois Clean Coal Institute, Illinois
Department of Commerce & Economic Opportunity and Babcock & Wilcox Company [17]
on 1.5 MW pulverized coal boiler. They used FLUENT as a simulation tool to optimise the
amount, location and injection of oxygen in the boiler. All the experiments were carried out
Introduction to oxy fuel combustion
15
in different conditions without staging (Base air firing, Oxygen enrichment case) and with
air staging and oxy enriched staging. With the simulation, it was found that NOx emission
was 15 to 21 % lower than base air firing during oxygen enrichment.
A 1.2 MW Boiler [20] used to analysis of the flame formed during oxidation of pulverized
coal by O2 � CO2 mixture. Oxygen was supplied in two streams, one was mixed with re-
circulated flue gas and the second was injected directly into the furnace. It was found that
the temperature of gases during O2 � CO2 firing near the burner zone was 200°C lower than
air firing and the concentration of oxygen in the combustion gas was never more than 27 %.
Drying of re-circulated flue gas transportation increased the temperature near the burner
zone by about 150°C indicating better ignition stability.
A 44 MW wall fired boiler [21] test facility with bituminous coal was investigated for NOx
reduction. It was found that loss on ignition was very small when stoichiometric ratio was
reduced from 0.9 to 0.85, when small amount of oxygen (< 5 %) was added from staged
stream. Oxygen addition (< 5 %) during staging resulted almost 40 % NOx reduction
compared to base line air staged combustion without any restriction on loss on ignition and
burner stability.
The research paper �Development of the CO2 recovery type pulverized coal fired power
plant applied oxygen and recycled flue gas combustion� [22] consists of micro gravity test,
combustion test and design & simulation. In the micro gravity test, spherical combustion
camber (200 mm inner diameter) with micro gravity field to form uniform coal particle
cloud was used. The used fuel rations were 0.8, 1.3 and 1.6 with the particle size of 53 to 63
µm for the investigation. It was found that flame propagation velocity was higher for more
volatile coals, the velocity and brightness of flame were much lower in CO2/O2
environment. The maximum velocity was found at the concentration when the coal particles
were at the distance of 10 times the particle size.
In a combustion test, 1.2 MW (inner diameter 1.3 m & height of 7 m) boiler with the
Bituminous coal as fuel. It was found SO2 and SO3 emissions were higher in O2/RFG
combustion. Dust concentration was 2 times higher than air combustion and higher
desulphurisation rate was observed for O2/RFG combustion with increasing Ca/S molar
ratio.
Introduction to oxy fuel combustion
16
In the design and simulation, experimental results were used for the simulation of 1000
MW pulverized coal fired power plant and after simulation it was found optimum oxygen
purity of 97.5 % at which total power consumption becomes least for both CO2 liquefy
fraction and gas recovery. Due to high cost of oxygen production and CO2 treatment, net
thermal efficiency decreased to 30 % from 39 % with conventional plant.
A promising technology for CO2 Capture �Oxy combustion process in pulverized coal fired
boilers� [26] investigation set up consists of boiler with capacity of 1.5 WM, video camera
for flame picture/ temperature mapping with the flue gas recycle (FGR) from 80 to 95 %
and mass flow rate was constant. It was noted that furnace exit temperature was lower for
O2/RFG firing than for baseline air firing. NOx reduction of nearly 70 % achieved with
O2/RFG. CO2 concentration in the flue gas was 80 % due to air infiltration.
2.5 Ignition Mechanism
Coal particles ignite homogeneously, heterogeneously, or through a combination of both
mechanisms. Ignition of coal particles can be either homogeneously, i.e. prior pyrolysis and
subsequent ignition of the volatiles, or heterogeneously, i.e. direct oxygen attack on the
whole coal particle. The ignition mechanism depends on volatile matter content of coal,
flammability of volatiles and their transport from the particle. Generally, larger coal
particles ignite homogeneously and smaller particles ignite heterogeneously with a
separating boundary at a particle diameter, depending on the ambient conditions. Despite of
the fact, that pulverized coal particles are known to be irregular shaped
Introduction to oxy fuel combustion
17
Figure 2.7: Main Processes in Coal Combustion [37]
Homogenous ignition is a 2 step process; primary step is the initial ignition of the volatiles.
Following this, the combustion of the volatile; secondary ignition, of the char, then occurs
as pyrolysis terminates.
Heterogeneous ignition can involve 3-stages. The primary ignition is by direct attack of the
reactant gas on the solid. This solid is the whole coal, not just a char, and the heterogeneous
reaction removes material that would otherwise be expelled as volatiles. As a parallel to
homogeneous case, this reaction can sometimes be quenches as pyrolysis becomes
appreciable, even if the volatiles do not burn. Secondary ignition, when it occurs, is that
volatiles, and this may be followed in due course by a re-ignition of the char at the end of
pyrolysis. One step ignition, of the course, is when only the first ignition process occurs in
either case.
Coal particle D= 30 �70µm
Devolatilization
Volatiles
Char
Homogeneous Combustion
Heterogeneous Combustion
tchar=1-2sectvolatiles=50-100mstdevolatile=1-5ms
t
Introduction to oxy fuel combustion
18
Figure 2.8: Stages in pulverized coal conversion [36]
Ignition temperature is an invariant of the fuel, two major factors that can influence the
ignition source temperature, in addition to concentration and particle size, are the speed of
the cloud past the ignition source and the size of the ignition source. The homogenous
ignition temperature is inversely proportional to particle size and oxygen concentration.
Oxygen and volatiles are diffuse into each other, ignition will occur in a narrow region
where the gas temperature, oxygen, and volatiles concentration reaches the flammability
limits.
1 Coal dust
+ Air
2 Radiative Preheat 3
Conductive heating
4Pyrolysis
onset
5Radiation
+ Pyrolysis
10 Radiation
9 Gaseous reaction
8Gaseous diffusion
7Volatiles oxidation
6Volatilesdiffusion
CO, H2
CO
H
CO + O ! CO2H + OH ! H2O
PRE HEAT ZONE FLAME ZONE POST FLAME ZONE
Distance
Particle Temperatur
Introduction to oxy fuel combustion
19
Homogeneous ignition occurs first at low oxygen concentration since sufficient oxygen is
not available for heterogeneous reactions. At higher oxygen concentration however
heterogeneous ignition occurs first and heterogeneous ignition temperature decreases with
the increase of the particle diameter.
CO
NC
ENTR
ATI
ON
DISTANCE
Figure 2.9: Variation of concentration of oxygen and fuel with distance from the fuel surface. Ignition occurs at some definable concentration ratio.
Ignition Zone
O2
Fuel
Description of experiments
20
3 Description of experiments 3.1 Test facility
The pulverised coal combustion test facility at Institute of Process Engineering and Power
Plant technology (IVD) Universität Stuttgart Germany was used for oxy fuel combustion
investigation. The test facility [figure 3.1] consists of electrically heated ceramic tube with
reaction zone of 2500 mm length and 200 mm diameter. Electrical heating is in order to
adjust a constant wall temperature. Wall temperature can be varied from 1100°C to 1400°C
during the experiments.
For ignition experiments, the pulverised coal is fed into the reactor by means of a water-
cooled feeding probe, which can be moved in vertical direction enabling particle
temperature measurement at different positions from the burner. The combustion gases are
injected through an annular clearance between burner and top section of reactor and are
divided into primary and secondary streams.
Figure 3.1: Schematic of pulverised coal test facility and arrangement for coal
particle temperature measurement using two-colour pyrometer with movable coal injection probe & for combustion experiments.
Description of experiments
21
For the combustion experiments, the pulverised coal is fed by carrier gas to the top
mounted burner through which it is injected into the combustion chamber. The feeding
system consists of a gravimetric conveyor and screw feeder. The combustion gases are
injected through an annular clearance between burner and top section of reactor and are
divided into primary and secondary streams. Flue gas sampling probe are mounted at the
bottom of reactor from where flue gas is extracted to obtain a gas sample for the emission
analysis.
For oxy fuel tests independent mixing device were installed with facility to adjust gas
composition of O2 and CO2. For temperature measurement of single particles, two-colour
pyrometer was used. A process control system was used to monitor all the relevant process
data including the gas analysis data and to control the whole facility. The flue gas is
extracted at the final section of the heated reaction tube for the emission analysis of O2,
CO2, CO, NO, Nox, HCN, SO2 and char particles.
3.2 Temperature measurement equipment
For the detection of the surface temperature of single particles a two-colour pyrometer was
used. Pyrometer measures the temperature of objects without touching them. Every object
whose temperature is above absolute zero (-273.15 °C) emits radiation. This emission is
heat radiation and is dependent upon temperature. The term infrared radiation is also in use
because the wavelengths of the majority of this radiation lie in the electro-magnetic
spectrum above the visible red light, in the infrared domain. Temperature is the determining
factor of radiation and energy. Infrared radiation transports energy. This radiated energy is
used to help determine the temperature of a body being measured.
Types of Pyrometers
• Spectral Band Pyrometers (Narrow and Broad band)
• Total band Pyrometers
• Two-colour Pyrometers
Description of experiments
22
Working principal of two-colour Pyrometer
The optical set up of the pyrometric measurements in the reactor is show in the [figure 3.2].
The radiation emitted by a coal particle is collected through the lens system and focused to
the end of an optic fibre bundle. In the middle of the bundle is one large fibre, called the
primary fibre. This fibre is used for pyrometry. The primary fibre is surrounded by smaller
fibres, called reference fibres. They are used for particle discrimination. The radiation
entering the primary fibre is measured in a radiometric units over two separate wavelength
bands, and the radiation entering the reference fibres is measured collectively in a separate
unit over a single wavelength band.
Figure 3.2: Optical set up of the pyrometric measurements at flow
reactor [31].
Figure 3.3: Time dependence of primary and reference signals when fuel particle passes through the FOV of the optical probe [31].
When a coal particle hotter than the background passes through the field of view (FOV) of
the primary fibre, a corresponding increase is detected in the pyrometric single levels.
Figure 3.3 shows the trajectory of the particle image in the plane of the end face of the fibre
bundle and the corresponding signals as function of time. The particle temperature is solved
using the ratio of the pulse height measured at the two wavelength bands. Temperature of
Particle Tp can be solved from the equation below[34].
Description of experiments
23
(R1 � R01) / (R2 � R02) = [ F1(Tp) � R01] / [ F2(Tp) � R02]
Where Ri � R0i is the pulse height at wavelength band λ i (i = 1,2), R0i is the system response
when no particles are in the FOV (i.e., a signal value between the peaks), and Fi(T) is the
system response calibrated against a blackbody radiator at temperature T.
Advantages The main advantages of the two colour Pyrometers are firstly, it is a non-contact
temperature measurement instrument so that it does not affect the target and the material of
the equipment. Secondly it has high response speed for temperature fluctuations which in
not possible with contact measuring equipments. Because of the Pyrometer�s quick
response time; temperature of the moving object can be measured accurately. Finally, the
temperature reading is independent of the particle area and emissivity fluctuation. Five
factors contributing to uncertainty of measured particles are noise, accuracy of calibration,
the emissivity of the particle, the accuracy of the temperature determination and solid angle
between particle and collecting pyrometer optics
3.3 Flue gas analysis
For the measurements of flue gas components, flue gas sampling probe is mounted at the
bottom of reactor from where flue gas is extracted to obtain a gas sample. The measuring
ranges of analysers and techniques are listening below [Table 3.1]. Nox is calibrated in the
CO2 environment during oxy fuel test to avoid quenching effect.
Table 3.1: Analyser employed for flue gas analysis with their ranges Gas Component Measuring technique Measurement range
O2 Paramagnetism 0 � 50 % vol
CO2 Non dispersive infra red 0 � 100 % vol
CO Non dispersive infra red 0 � 5,000 ppmv
NO, NOx Chemiluminence 0 � 10,000 ppmv
Description of experiments
24
3.4 Characterisation of the fuel
The coals used for this investigation were low volatile Bituminous coal (Klien Kopje),
medium volatile Bituminous coal (Elecrejon) and high volatile brown coal (Lausitz). The
fuel was analysed for it proximate and ultimate analysis. The proximate analysis divided
the fuel into the components water, volatile matters, fixed C and ash, the ultimate analysis
into carbon, hydrogen, nitrogen, sulphur and oxygen [Table 3.2].
Table 3.2: Proximate and ultimate analysis of fuels Brown coal
(Lausitz) High volatile
Bituminous Coal (Elcerejon) Medium volatile
Bituminous Coal (Klien Kopje) Low volatile
Volatiles [%, waf] 57.36 40.30 27.76
Ash [%, wf] 5.46 8.11 19.29
Moisture [%, ar] 10.20 06.30 03.60
Carbon [%, waf] 66.78 81.07 83.93
Hydrogen [%, waf] 6.60 6.16 5.01
Nitrogen [%, waf] 0.65 1.51 1.67
Sulphur [%, waf] 0.72 0.44 0.36
Oxygen [%, waf] 25.25 10.82 9.03
3.5 Parameters for ignition tests
Table 3.3: Parameters for ignition tests Wall temperature 1100°C, 1300°C
Pressure Atmospheric
Coal Particle 90-125µm*, 90-150µm, 150-212µm & 212-315µm
Inlet oxygen concentration 21 %, 27 %, 35 % and Air as base line
Distance from Burner (cm) 2.5, 5, 10,15,20,25,30 and 40
Primary stream Air or O2/CO2 mixture, (3.5 m3/h)
Secondary stream Air or O2/CO2 mixture, (5.3 m3/h)
Carrier stream Same as primary and secondary stream, (100 l/h) * Old fraction 90-125µm used for some experiments for comparison with new fraction results
Note: Whenever the volumetric fraction of O2 is mentioned, remaining portion in the mixture is always CO2.
Description of experiments
25
3.6 Parameters for combustion tests
Table 3.4: Parameters for combustion test Wall temperature 1300°C
Pressure Atmospheric
Coal Particle D50 ≈ 60µm
Inlet oxygen concentration 21 %, 27 %, 35 % and Air as base line
Primary stream Air or O2/CO2 mixture, (3.5 m3/h)
Secondary stream Air or O2/CO2 mixture, (5.3 m3/h)
Carrier stream Same as primary and secondary stream, (1.5 m3/h) Note: Whenever the volumetric fraction of O2 is mentioned, remaining portion in the mixture is always CO2.
Ignition results
26
4 Ignition results
Experiments were conducted to investigate the influence of furnace temperature, coal
particle size, carrier gas (e.g. Air, N2), coal feeding rate and oxy fuel on coal ignition. The
experimental combustion facility has electric furnace with water-cooled movable single
particle dosing probe. Coal particles are transported in the inner part of the probe with the
carrier gas and primary gas introduced from the outer part of probe. Due to movable probe
it gives the possibility to measure the coal particle temperature at different positions in the
combustion chamber. All experiments were performed without flue gas re-circulation and
CO2 is directly supplied for combustion. Coal particle temperature was measured by 2-
colour pyrometer installed at a collinear optical port at 1.55 m as shown in figure 3.1.
Three different types of coals named as Brown coal, high volatile bituminous coal and
medium volatile bituminous coal with a fraction of 90-150 µm, 150-212 µm and 212-315
µm were investigated. Furnace temperature was maintained at 1100 °C and 1300 °C during
experiments with the variation of Oxygen supply from 21 % to 35 % for Oxy fuel
behaviour and air was used as a base line for comparison.
4.1 Influence on particle temperature for different conditions during air
combustion
The figure 4.1 shows the influence of nitrogen/air as carrier gas on coal particle
temperature. The result shows that nitrogen, as carrier gas does not give a significance
change in the coal particle temperature as compared to air. The nitrogen concentration
(carrier gas) is very low as compared to total gas in reactor to show any significant changes.
Ignition results
27
1600
1650
1700
1750
1800
1850
1900
0 5 10 15 20 25 30 35 40 45
Distance from Burner [cm]
Part
icle
Tem
pera
ture
[°C
]
150-212µm CG=air150-212µm CG=N290-125µm CG=air90-125µm CG=N2
Figure 4.1: Influence of Nitrogen / Air (Carrier gas) on coal particle temperature, Lausitz, WT= 1100°C
Figure 4.2 shows the effect of coal feeding rate on the coal particle temperature. During the
experiments the behaviour of coal particle temperature under the feeding rate from 5 Hz
(~7g/h) to 50 Hz (~160 g/h) with air and nitrogen as carrier gas was observed.
Coal feeding rate shows strong influence on particle temperature. Results have indicated
that the higher the coal-feeding rate the lower is the particle temperature. During the
experiments, oxygen concentration around 21% at exit was always maintained, so for high
feeding rate, one cannot claim that oxygen was not enough for ignition. The reason could
be the working principle of two-colour pyrometer. In two-colour pyrometer the coal particle
temperature is the function of radiative energy of particle (E1) and radiative energy of
background, which in this case is the reactor wall (E2).
Temperature =f (E2E1 )
Ignition results
28
1550
1600
1650
1700
1750
1800
1850
0 5 10 15 20 25 30 35 40 45
Distannce from Burner [cm]
Part
icle
Tem
pera
ture
[°C
]
5Hz10Hz20Hz50Hz
Figure 4.2: Influence of coal feeding rate, Lausitz, WT=1100°C, 90-125µm, Carrier
gas = N2
During higher particle feeding rate complete flame was formed, which could be the reason
to increase the value of background radiative energy (Reactor wall) E2 and decreasing the
ratio of E1 / E2, which shows the low temperature of the coal particles. Another reason for
this lower temperature with higher feeding rate could be intra particle energy loss as
particles are much closer to each other in this case.
The rate of reaction between coal and oxygen is affected by particle size, for the smaller
particle size the coal becomes much more reactive. For smaller particles, the rate of heat
generation is higher for two reasons. First, the effectiveness factor is closer to 1, leading to
greater oxidation rate per unit volume. Second, the mass transfer coefficient increases with
decrease in the particle size, from figure 4.3 behaviour of particle temperature over the
particle size in the presence of air as carrier gas can be seen.
Ignition results
29
1600
1650
1700
1750
1800
1850
0 5 10 15 20 25 30 35 40 45
Distance from Burner [cm]
Part
icle
Tem
pera
ture
[°C
]
air 212-315µm
air 150-212µm
air 90-125µm
Figure 4.3: Influence of particle size on coal particle temperature, Lausitz, WT=1100°C
Ignition/combustion of coal particle can be generally regarded as a two-step process. First
the surrounding/surface ignites due to release of volatiles, and then inner surface ignites.
First step is strongly influenced by availability of oxygen and release of ignitable volatiles.
Second step is influenced by the rate of diffusion of oxygen inside the coal particle for
combustion of char. Therefore larger particle theoretically should have higher temperature
in the beginning due to larger surface area that comes in contact with oxygen and lower
temperature afterwards since diffusion of oxygen inside the particles takes longer time. This
is exactly what the trend of the curves for particle fraction of 90-125µm and 150-212µm
shows. However the largest fraction do not show such tendency. This is most probably by
too much release of volatiles resulting in oxygen deficiency or longer time required for
release of volatiles due to longer time required to heat up larger particles.
The most distinguishable and understandable trend among three particle fractions is the
temperature peaks. The peak of particle temperature for smallest fraction is nearest from the
burner (around 5cm if the curve follow the same trend) confirming that smaller the particle,
the faster it ignites. Peaks for other two larger fractions are further away from the tip of
burner, indicating longer ignition time. For the clear understanding of the relation between
Ignition results
30
particle diameter and particle temperature, it was observed that temperature of the particle
decreases with the increase in the particle diameter.
As we move nearer to the burner the density of particle cloud increases as shown in figure
4.4, since the particles dispersal has approximately a form of conical shape. So when we
move nearer to burner, for pyrometer it�s difficult to measure a single particle and take
many particles as a particle cloud and this can influence the accuracy of coal particle
temperature measurements.
Figure 4.4: Dispersion of particles along the tip of dosing unit
Ignition results
31
4.2 Ignition during Oxy fuel
Ignition behaviour of the particles in the presence of different oxygen concentration for
different types of coals were investigated. The reaction between coal and oxygen is affected
mainly by two factors: first one involves the chemistry such as carbon type, active sites etc.,
and the second one covers physical characteristics, such as specific surface area, pore or
surface diffusion, etc. For different coals, the physical properties differ, so that activation
energy and reactivity change with the type of coal. Activation energy depends on the
chemical structure of the coal and individual particle temperature. The reactivity, which
represents the effective number of collision between carbon and oxygen molecules,
depends heavily on coal properties.
As we know from the literatures, higher concentrations of oxygen will result in higher
temperature and early ignition. In Lausitz 1100°C Curve (figure 4.5) for 35% oxygen case
indicates the most rapid ignition rate, which is clearly visible from the steep slope of the
curve after 15cm and in case of 21 % oxygen the ignition delay is maximum and giving
maximum temperature at the distance of 30 cm from burner. It was observed that curve of
27 % oxygen and air for the fraction size of 212-315 µm have a similar trend.
1650
1700
1750
1800
1850
1900
1950
2000
0 5 10 15 20 25 30 35 40 45
Distance from Burner [cm]
Part
icle
Tem
pera
ture
[°C
]
21% O227% O2,35% O2Air
Figure 4.5: Influence of Oxy fuel on Coal particle temperature, Lausitz
WT = 1100°C, fraction = 212-315µm
Ignition results
32
In the figure 4.6, effect of the reactor wall temperature of 1100°C and 1300°C for the
fraction of 150 to 212 µm (Klien Kopje) at the 21 % oxygen concentration and air can be
observed. In case of Air, ignition delay is greater for 1100°C as compare 1300°C.
In case of 21 % oxygen with the wall temperature of 1100°C, no particles were detected
from 10 cm to 40 cm for the burner distance. Which means the temperature of particles
were too low for detection by two-colour pyrometer or ignition delay was so high, on other
hand particle detection can be seen for 10 cm to 30 cm distance from the burner at 1300°C.
On the basis of the results that no particles were detected for 21% O2 at 1100°C, and the
realistic furnace temperature for hard coals are ~ 1300°C, It was decided to carry out
experiments only at 1300°C.
1400
1450
1500
1550
1600
1650
1700
0 5 10 15 20 25 30 35 40 45
Distance from Burner [cm]
Part
icle
Tem
pera
ture
[°C
]
Air (1100°C)Air (1300°C)21% O2 (1300°C)
Figure 4.6: Influence of Oxy fuel on Coal particle temperature, Klien Kopje, WT = 1100 & 1300°C, fraction = 150 � 212 µm
Figure 4.7 show the particle temperature behaviour under air and oxy fuel conditions at
1300°C reactor temperature for the fraction of 212-315µm for Klien Kopje. The trends of
the graph indicate that at a distance of more than 10 cm, burnout phase of ignition is
already reached and to see the heat-up and pyrolysis of ignition, measurements at distances
less than 10cm is necessary which was not possible during the time when the experiment
Ignition results
33
was carried out due to the length of the fuel feeding probe. To see these phases of ignition,
a new fuel probe was constructed enabling measurements at nearer distances.
1500
1550
1600
1650
1700
1750
1800
1850
1900
1950
2000
0 5 10 15 20 25 30 35 40 45
Distance from Burner [cm]
Part
icle
Tem
pera
ture
[°C
]
Air35% O227% O221% O2
Figure 4.7: Particle temperature behaviour under air and Oxy fuel conditions at
reactor temperature 1300°C, fraction 212-315µm (Klien Kopje)
With the new fuel-feeding probe, experiments were carried out for the coal particle
temperature measurements up to 2.5 cm from the burner distance. Figure 4.8 shows the
particle temperature behaviour under air and oxy fuel conditions at 1300°C reactor
temperature for the fraction of 90-150µm (Low volatile Bituminous coal = Klien Kopje).
The temperature measurements were carried out for different distances starting from 40 cm
away from the burner. The maximum temperature of coal particle was observed at 35 % O2.
In case of 27 % O2 the coal particle peak temperature can be seen at the distance of 7.5 cm
from burner as compare to 5cm for air, but the trend of coal particles temperature profile
were similar for both air and 27 %. Particle fraction 90-150µm were detected only up to 30
cm from the burner as compare to particle fraction of 212-315µm indicating rapid burnout
due to small particle size.
Ignition results
34
1400
1500
1600
1700
1800
1900
2000
0 5 10 15 20 25 30 35 40 45
Distance from Burner [cm]
Part
icle
Tem
pera
ture
[°C
]
Air
21% O2
27% O2
35% O2
Figure 4.8: Particle temperature behaviour under air and Oxy fuel conditions at
reactor temperature 1300°C, fraction 90� 150µm (Klien Kopje)
Figure 4.9 shows Particle temperature behaviour under air and oxy fuel conditions at
reactor temperature 1300°C for fraction 90-150µm (medium volatile bituminous coal
=Elcerejon). Pyrometer detected particle temperature up to 25 cm for oxy fuel conditions
and 30 cm in case of air, as the fraction size was smallest and the fuel was more volatile.
Coal particle temperature was maximum for the 35 % O2 and minimum for 21 % O2.
Although sampling was carried out up to the distance of 2.5 cm near the burner but peak of
coal particle temperature was not appeared except 27 % O2 in feed gas at 5 cm away from
burner.
Ignition results
35
1400
1500
1600
1700
1800
1900
2000
0 5 10 15 20 25 30 35 40 45
Distance from Burner [cm]
Part
icle
Tem
pera
ture
[°C
]Air21% O227% O235% O2
Figure 4.9: Particle temperature behaviour under air and Oxy fuel conditions at
reactor temperature 1300°C, fraction 90-150µm (Elcerejon)
Figure 4.10 shows Particle temperature behaviour under air and oxy fuel conditions at
reactor temperature 1300°C, fraction 150 � 212µm (Elcerejon). It was observed that
particle detection was up to 30 cm away from the burner in case of oxy fuel and 40 cm in
case of air. Maximum Coal particles temperature was observed at 35 % O2 concentration at
2.5 cm away from burner.
Particle temperature behaviour under air and oxy fuel conditions at reactor temperature
1300°C, fraction 212-315µm (Elcerejon) is shown in figure 4.11. It was found that
pyrometer detected coal particles up to 40 cm away from burner for both air and oxy fuel
test, because of bigger fraction of coal as compared to 25 cm for the smallest fraction of 90-
150µm. No clear peak of coal particles temperature was detected for air or oxy fuel tests
but highest temperature was observed for 35 % and lowest for 21 % with air and 27 % O2
laying in between them and showing similar trend.
Ignition results
36
1400
1500
1600
1700
1800
1900
2000
0 5 10 15 20 25 30 35 40 45
Distance from Burner [cm]
Part
icle
Tem
pera
ture
[°C
]Air21% O227% O235% O2
Figure 4.10: Particle temperature behaviour under air and Oxy fuel conditions at reactor temperature 1300°C, fraction 150 � 212µm (Elcerejon)
1400
1500
1600
1700
1800
1900
2000
0 5 10 15 20 25 30 35 40 45
Distance from Burner [cm]
Part
icle
Tem
pera
ture
[°C
]
Air21% O227% O235% O2
Figure 4.11: Particle temperature behaviour under air and Oxy fuel conditions at
reactor temperature 1300°C, fraction 212-315µm (Elcerejon)
Ignition results
37
Figure 4.12 shows the relation between number of detected particles for certain amount of
time over the distance from burner during tests. It was found that as feeding probe distance
decrease from 10 cm to 2.5 cm to the burner, the detection of coal particles by pyrometer
decreased considerably.
0
100
200
300
400
500
600
700
0 5 10 15 20 25 30 35 40 45
Distance from Burner [cm]
Part
icle
s D
etct
ed
21% O2
27% O2
35% O2
Figure 4.12: Relation between detected particles over the distance from burner
It was observed that at such close distance some particles were fully ignited, some were
about to ignite and some were not ignited at all. Temperature was calculated on the basis of
average of total detected particles and since the two-colour pyrometer detected only those
particles which were already ignited, the average temperature of particles kept on
increasing. In reality the average particle temperature should have decreased at distance less
than 10 cm for this case indicating the heat up phase of ignition but due to the detection
limit of pyrometer, the average particle temperature kept on rising.
Combustion results
38
5 Combustion results
Combustion experiments too were carried out with air as well as oxy fuel. For experiments
in O2/CO2 mixture, the nitrogen is replaced by CO2. Every experiment is done at ~ 3 %
excess oxygen in the flue gas. Oxygen gas concentration varies in feed gas between 21 %,
27 % and 35 % vol on a dry basis for oxy fuel combustion tests. Concentration of the
oxygen in flue gas was controlled by variation of supplied coal in reactor.
Figure 5.1 shows the emission concentration of NO, CO, SO2 and CO2 in flue gas at
1100°C for Lausitz coal. The maximum achieved concentration of carbon dioxide was ~ 95
% in the flue gas in case of oxy fuel combustion, reactor was operated at positive pressure.
Trend of emissions concentration in flue gas shows, higher the oxygen concentration in the
feed gas leads to increase the NOx and SOx. As we have seen from ignition results, increase
the oxygen concentration in feed gas leads to higher temperature, leads to increase in NOx
emission in flue gas.
0
400
800
1200
1600
2000
Air 21 27 35Oxygen [vol %]
NO
, CO
, SO
2 [pp
m]
0
20
40
60
80
100
O2,
CO
2 [vo
l %]
Coal [g/h] SO2 CO NO O2 CO2
Figure 5.1: Emission concentration of NO, CO, SO2 and CO2, Lausitz, WT=1100°C
O2excess= 3 %
Combustion results
39
It was found that NOx concentration in the flue gas was higher in case of oxy fuel (27 %
and 35%) than air because of higher conversion rate of fuel N to NO due to higher fuel
feeding rate and oxygen concentration. Similarly higher concentration of SO2 in flue gas
because of higher fuel sulphur (S) contents for combustion with higher feeding rate of coal.
In case of combustion in air and 21 % O2, when all the input parameters were same during
the experiments, we found reduction in the NOx emissions in flue gas. An increase in CO
concentration can be seen in the flue gas due to dissociation of CO2, normally it can be
observed at temperature greater than 1267°C and at atmospheric reaction pressure [35].
CO2 → (1 � x) CO2 + x CO + (x/2) O2
CO concentration was observed on the online gas analyser although coal feeding was not
yet started for combustion, which confirms that temperature was not uniform in the reactor.
Figure 5.2 shows the emission concentration of NO, CO, SO2 and CO2 in flue gas at
1300°C for Lausitz coal for 3 % excess Oxygen in flue gas. Approximately 94 % CO2 was
found in the flue gas for oxy fuel combustion at 1300°C.
Figure 5.2: Emission concentration of NO, CO, SO2 and CO2, Lausitz, WT=1300°C O2excess= 3 %
0
400
800
1200
1600
2000
Air 21 27 35Oxygen [vol %]
NO
, CO
, SO
2 [pp
m]
0
20
40
60
80
100
O2,
CO
2 [vo
l %]
Coal [g/h] SO2 CO NO O2 CO2
Combustion results
40
Results showing continuous increase in the emission of NO and SO2 with the increase of
oxygen concentration in feed gas. Maximum concentration of NO and SO2 in the flue gas
was found at 35 % O2 case. With the comparison of results of Lausitz for 1100°C &
1300°C, higher concentration of NO was found at higher temperature (825.9 ppm at
1300°C & 703 ppm at 1100°C) for the 35 % O2 in feed gas and 3 % excess O2 in flue gas
condition. Trend of the graph for air and 27 % O2 shows, similarity in case of concentration
of emissions in the flue gas. Behaviour of the NO, SO2 and CO emission were same like
previous result.
Figure 5.3 and 5.4 shows the relationship between temperature and emissions concentration
(NO, SO2). It was clearly found, increase of emission concentration (NO, SO2) in the flue
gas with the increase of temperature from 1100°C to 1300°C for the 3 % excess oxygen
condition in the flue gas. Very small difference in the concentration of NO in flue gas was
found between combustion in air and 27 % O2 at 1300°C
200
400
600
800
1050 1100 1150 1200 1250 1300 1350
Temperature [°C]
NO
[ppm
]
Air 21% O2 27% O2 35% O2
Figure 5.3: Concentration of NO [ppm] in flue gas at 1100°C & 1300°C, Lausitz
Combustion results
41
400
600
800
1000
1200
1050 1100 1150 1200 1250 1300 1350
Temperature [°C]
SO2 [
ppm
]Air 21% O2 27% O2 35% O2
Figure 5.4: Concentration of SO2 [ppm] in flue gas at 1100°C & 1300°C, Lausitz
0
400
800
1200
1600
2000
Air 21 27 35Oxygen [vol %]
NO
, CO
, SO
2 [pp
m]
0
20
40
60
80
100
O2,
CO
2 [vo
l %]
Coal [g/h] SO2 CO NO O2 CO2
Figure 5.5: Emission concentration of NO, CO, SO2 and CO2, Klien Kopje, WT=1300°C
Combustion results
42
Figure 5.5 shows the emissions concentration in the flue gas for the combustion at
temperature of 1300°C, low volatile Bituminous coal (Klien Kopje) and for 3 % excess O2
in flue gas. It was found that the concentration of NO and SO2 were maximum at 35 % O2
and lower in 21 % O2 in feed gas as compare to Air as base line. Same trend of emissions
concentration can be noticed from this graph like previous results, where the concentration
of emission were lower in case of 21 % O2 and higher for 35 % O2 but have similarity in
case of combustion in 27 % O2 and air.
Conclusions and future work
43
6 Conclusions and future challenges
6.1 Conclusions
This experimental investigation of O2/CO2 firing during pulverised combustion was done to
observe ignition behaviour of coal particle, behaviour of two-colour pyrometer for particle
temperature and emission concentration in the flue gas during combustion.
It was found from ignitions experiments that coal particle temperature decrease with the
increase in coal feeding rate. The results show that coal particle temperature is inversely
proportional to the particle size and smaller particles are faster to ignite. Increasing the
amount of oxygen in the feed gas leads to increase the coal particle temperature and early
ignition. It was found that coal particle temperature profile for 27 % O2 concentration in the
feed gas showing similar trend as air.
During the investigation of pyrometer behaviour for coal particle temperature measurement,
it was found that for higher coal feeding rate / near to the burner, its difficult for pyrometer
to measure single and take many particles as a particle cloud and this can influence the
accuracy of coal particle measurements.
It was observed that emissions concentration CO2, NO, SO2 and CO leads to increase with
the increase of oxygen concentration in the feed gas. The maximum achieved concentration
of carbon dioxide was ~ 95 % in the flue gas in oxy fuel combustion conditions. NO
emission trend shows the similarity between 27 % O2 in feed gas to combustion in air.
The ignition and combustion analysis shows that, under the assumed conditions, oxy fuel
fired plants operated at 27 % O2 in feed gas are comparable to those of the air-fired cases.
Finally, it is noted that the oxy fuel fired operation generates a flue gas rich in carbon
dioxide ~ 95 %, which may be easily captured in order to be sequestered. Thus, the boiler
operation can be efficiently converted to a zero emission plant.
Conclusions and future work
44
6.2 Future work
For the future work, following points are suggested to get better results and better
understanding on oxy fuel firing experiments.
Improvement is needed in measurement method near the burner to see better coal particle
temperature profile. Because it was found that as feeding probe distance decrease from 10
cm to 2.5 cm to the burner, the detection of coal particles by pyrometer decreased
considerably.
Instead of air, pure oxygen and CO2 is fed to the reactor and the effect of water is
neglected, so in future water stream in the feed gas is to be added to see the effect of water
in O2/CO2 firing.
We have to consider the effect of HNO3 to get better results.NO2 dissolves into the water
and forms HNO2 and HNO3 (Nitrous and Nitric acid )
2NO2 + H2O → HNO2 + HNO3
But HNO2 is unstable and will be changed into HNO3 finally
2HNO2 + O2 → 2HNO3
Experiments were done at constant 3 % excess oxygen in the flue gas which means the air /
fuel ratio was different in case of 21 %, 27 % and 35 % oxygen in the feed gas. For future
investigation, it is suggested to observe the emission concentration in the flue gas at
constant air / fuel ratio. 6.3 Future challenges
Combustion of coal in an O2/CO2 atmosphere has been investigated to increase the
knowledge of combustion characteristic.
Unlike the N2 molecules, the CO2 and H2O molecules are emitters of thermal radiation,
meaning that when N2 is substituted with CO2 in the plant, the heat transfer characteristics
will change. There will be a need for verification and validation of reliable heat transfer
models that include the changed thermal radiation characteristics.
Conclusions and future work
45
Combustion of coal in pure oxygen gives a high temperature, which leads to increase Nox.
The solution can be re-circulation of flue gas.
Higher the CO2 concentration in the flue gas means heat flux to the wall will be higher and
higher temperature corrosion is therefore likely occur more rapidly in an O2/CO2
combustion boiler than in an air-fired boiler, corrosion testing is there necessary.
Future challenges concerning oxygen production technology for the O2/CO2 combustion of
coal.
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46
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