observations on the combustion of torrefied biomasscj...ii abstract this thesis examines...
TRANSCRIPT
OBSERVATIONS ON THE COMBUSTION OF TORREFIED BIOMASS
A Thesis Presented
By
Mahmut Burak Tarakcioglu
To
The Department of Mechanical and Industrial Engineering
In partial fulfillment of the requirements for the degree of
Master of Science
in
Mechanical Engineering
in the field of
Thermofluids
Northeastern University Boston, Massachusetts
(December 2017)
Dedication
To my parents
ii
Abstract
This thesis examines observations on the combustion of different types of torrefied biomass. The
targeted biomass types were waste crop, herbaceous, and woody; they included Corn straw,
DDGS, Rice husk, Miscanthus, Bagasse, and Beechwood. Combustion of renewable biomass is of
technological interest because it is considered to be nearly carbon-neutral. Furthermore,
torrefaction of biomass improves the raw biomass properties and makes more “coal-like”.
Torrefied biomass has lower volatile matter content, higher fixed carbon content and higher
energy content than raw biomass. It is also having low moisture content and it is less
biodegradable. In this work, all biomass samples were torrefied at T= 275 °C for half an hour in
nitrogen. Thereafter, torrefied biomass was size classified by sieving to obtain size cuts of (75-90)
µm, (180-212) µm, (180-212) µm, (212-300) µm, (300-350) µm, (350-500) µm. The experimental
setup that was used in this investigation consisted of an electrically-heated drop-tube furnace,
operated at wall temperature 1400 K. A three-color pyrometer was interfaced with the furnace
and a high- speed high-resolution camera, to record the entire luminous particle combustion
profiles of individual particles.
As identification of the maximum biomass particle size for combustion in an existing boiler is
important, this work observed the time-temperature profiles of the aforementioned particle size
and contrasted them to those of typically-used coal particles. It was forward that torrefied
biomass particles in the size range of 212−300 μm burned in similar times as those of 75-90 μm
coal.
Keywords: Torrefied, Biomass, Combustion, Size, Pyrometry, Cinematography, Char Combustion
iii
Acknowledgement
I would like to express my sincere appreciation and gratitude to my advisor, Professor Yiannis A.
Levendis, for his excellent guidance, caring and patience.
I would like to acknowledge financial support provided by Turkish Petroleum Corporation. I would
like to thank the faculty and staff at the Department of Mechanical and Industrial Engineering at
Northeastern University. I would like to thank my lab mate Aidin Panahi for his support and
technical assistance.
I feel very fortunate to meet with great friends during my graduate studies. Special thanks to my
friends Ahmet Talha Ozcan, Orhan Buyukerzurumlu, Dr. Yasin Ozcan, Dr. Seyhmus Guler, and Dr.
Adnan Korkmaz for sharing enjoyable moments in this period of my life in Boston.
Most importantly, none of this would have been possible without the love and patience of my
family. I give all my appreciation and love to my parents, Professor Mehmet Tarakcioglu and
Serap Tarakcioglu, to whom this thesis is dedicated to, for their endless support and
encouragement. I also would like to thank my sister, Melek Sena Tarakcioglu for being such an
amazing sister. I am thankful for everything that they have done for me.
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Contents
Abstract ............................................................................................................................................ii
Acknowledgement .......................................................................................................................... iii
Contents .......................................................................................................................................... iv
Figure of Contents ............................................................................................................................ v
Table of Contents .......................................................................................................................... viii
Chapter 1 ......................................................................................................................................... 1
1. Introduction ............................................................................................................................. 1
Chapter 2 ......................................................................................................................................... 6
2. On the Particle Sizing of Torrefied Biomass for Co-firing with Pulverized Coal ...................... 6
2.1. Abstract .............................................................................................................................................. 7
2.2. Introduction ......................................................................................................................... 8
2.3. Methods ...................................................................................................................................... 10
2.3.1 Preparation of samples .............................................................................................................. 10
2.3.2. Experimental Apparatus ..................................................................................................... 13
2.4. Results and Discussion ................................................................................................................ 15
2.4.1. Observation ............................................................................................................................... 15
2.4.2. Initial particle aspect ratio .................................................................................................. 18
2.4.3. Ignition Parameters ............................................................................................................. 19
2.4.4. Combustion temperatures .................................................................................................. 20
2.4.5. Combustion Burn-out Time ................................................................................................. 22
Chapter 3 ....................................................................................................................................... 25
3. Numerical Determination of Temperatures of Burning Torrefied Biomass Char Particles .. 25
3.1. Parametric Analysis .......................................................................................................................... 37
Chapter 4 ....................................................................................................................................... 42
4. Conclusions ............................................................................................................................ 42
References .................................................................................................................................... 44
v
Figure of Contents
Figure 1 A schematic of property variation of biomass undergoing torrefaction [18]. ................. 3
Figure 2 Optical microscope photographs of various torrefied biomass fuels. (a) Corn straw, (b)
Miscanthus, (c) Sugar-cane Bagasse, (d) DDGS, (e) Rice husk and (f) Beechwood. In each case, the
left side frames show particles of 180-212 µm, whereas the right-side frames show particles of
350-500 µm: A size bar at the right bottom of this figure denotes magnification. ..................... 11
Figure 3 Scanning electron microscope images of torrefied biomass fuels in this study (a) Corn
straw, (b) Miscanthus, (c) Bagasse, (d) DDGS, (e) Rice husk and (f) Beechwood. ........................ 12
Figure 4 Schematic illustration of the drop tube furnace, particle introduction and combustion.
....................................................................................................................................................... 15
Figure 5 Images from high-speed high-resolution cinematography of single biomass particles
burning in air in a DTF operated at Twall = 1400 K. The displayed numbers in each frame are in
milliseconds, where zero does not mark the beginning of ignition, instead, it merely represents
the beginning of the depicted sequences. Nominal particle sizes were in the range of 212-300 µm.
....................................................................................................................................................... 17
Figure 6 Average initial aspect ratio of torrefied biomass fuels in this study for five different sieve
sizes. .............................................................................................................................................. 18
Figure 7 Average torrefied biomass fuels ignition delay times in air for various particle size group
in the DTF at Tg=1350 K. ............................................................................................................... 19
Figure 8 Average deduced flame temperatures for all biomass fuels of this study in air, at
Tgas=1350 K. In each case the first column is (75-90) µm, the second column is (180-212) µm, the
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third column is (212-300) µm, the fourth column is (300-350) µm and the last column is (350-500)
µm. ................................................................................................................................................ 20
Figure 9 Average deduced char temperatures for all biomass fuels of this study in air, at
Tgas=1350 K. ................................................................................................................................. 22
Figure 10 Experimental data on combustion times of (a) volatile flames (b) chars of torrefied
biomass particles in three ranges within 75-350 μm in a DTF at Tg=1350 K in air. ..................... 23
Figure 11 Comparison of cumulative combustion times of single particles of coal in the nominal
size range of 75-90 μm and torrefied biomass burning in a DTF at 1350 K in air. ....................... 23
Figure 12 Miscanthus torrefied char particle of 175 µm, (which was derived for torrefied biomass
particle 180-212 µm) in the air in the electrically-heated laminar-flow drop-tube furnace ....... 26
Figure 13 Calculated Diffusion Coefficients for O2-N2 Gas Pairs at Pressure of 1 atm. .............. 30
Figure 14 Kinetic parameters for torrefied Miscanthus and Beachwood biomass as well as for a
lignite coal. Pre-exponential Factor Values[126]. ......................................................................... 32
Figure 15 Variation of heat loses from a torrefied biomass char particle of 175 µm, (which was
derived for torrefied biomass particles 180-212 µm in diameter) burning in air versus
temperature. ................................................................................................................................. 33
Figure 16 Superposition of rates of heat generation and heat loss from a torrefied biomass char
particle of 175 µm, (which was derived for torrefied biomass particle 180-212 µm in diameter)
burning in air versus its temperature. .......................................................................................... 33
Figure 17 Experimentally measured and theoretically predicted char particle temperatures versus
initial torrefied biomass particle size.. .......................................................................................... 36
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Figure 18 The effect of char particle emissivity on char particle temperatures versus initial
torrefied biomass particle size.. .................................................................................................... 38
Figure 19 The effect of the assumed char particle size on char particle temperatures versus initial
torrefied biomass particle size.. .................................................................................................... 39
Figure 20 The effect of the assumed chemical reaction activation energy on char particle
temperatures versus initial torrefied biomass particle size. ........................................................ 40
Figure 21 The effect of the assumed chemical reaction pre-exponential factor on char particle
temperatures versus initial torrefied biomass particle size.. ....................................................... 41
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Table of Contents
Table 1 Mass loss during torrefaction of biomass .......................................................................... 4
Table 2 Chemical compositions of torrefied biomass .................................................................. 13
Table 3 Torrefied biomass initial particle sizes and char particle sizes ........................................ 31
Table 4 Averages of experimentally measured peak char temperatures .................................... 35
Table 5 Averages of experimentally measured average char temperatures ............................... 35
1
Chapter 1
1. Introduction
Around 10% of the total energy consumption in the USA in 2016 was from renewable energy,
and nearly half of that renewable energy was directly attributed to biomass according to Energy
Information Administration[1]. The interest to harvest renewable energy sources has been
growing significantly in response to (a) the increasing amount of greenhouse gases in the
atmosphere causing global warming and (b) to the decrease of conventional fossil fuel sources[2].
These problems have motived researchers to find, and improve alternative sources of energy. In
addition to coal, petroleum and natural gas, biomass is the next abundant carbon-based energy
resource. Around 40% of global electricity comes from coal-fired power plants[3]. Co-firing is the
combustion of the biomass and coal for power production[4]. Co-firing biomass with coal
represents a cost effective and efficient renewable option that promises reduction of CO2
emissions effect, NOx emissions, and it has several benefits for society[5, 6]. Many different types
of biomass can be co-fired with coal. Co-firing with different coal types has been studied earlier
to reduce harmful emissions from coal combustion [3, 7-9].
Biomass is an organic matter such as wood, crops, seaweed, animal wastes etc., that can be used
in transportation, electricity and heating. Biomass can be considered a renewable source of
energy, because it is plentiful, naturally grown, and it can be grown again at same location after
harvesting[10]. Biomass is considered by scientists and economic planners as the fuel of the
future, and its steadily increasing role in the world supports this view. Renewable biomass has
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advantages of near-zero net CO2 emission, as it absorbs carbon from atmospheric carbon dioxide
while it grows and then it returns carbon dioxide to the atmosphere when it is burned; and this
creates a closed-loop carbon cycle. However, biomass has some disadvantages as fuel (low
calorific value, high moisture content, low grindability, high biodegradability, high emissions of
acid gases, and smoking during combustion)[11-13].
Biomass can be classified in several ways. The origin of biomass and the biomass properties are
the main approaches for biomass classification: primary residues (by products of food and forest
products), secondary residues (by-products of biomass processing for production of food
products or biomass materials), tertiary residues (by-products of used biomass derived
commodities, and energy crops[14]. The classification can also be based on properties: wood and
woody fuel (hard and soft wood, wood residues, demolition wood, etc.), herbaceous fuels (straw,
grasses, etc.), wastes (sewage sludge, municipal waste water, etc.), derivatives (wastes from food
industries, etc.), aquatic (algae, etc.), energy crops (particularly cultivated for energy
purposes)[15].
Biomass can be converted into energy using several different ways, each with their pros and cons.
Producing energy from biomass has three main categories: thermo-chemical, bio-chemical, and
mechanical[16]. Thermo-chemical conversion has four sub-divisions: combustion, pyrolysis,
gasification, liquefaction, and torrefaction [11, 16, 17].
Torrefaction is a mild pyrolysis pre-treatment process for improving the properties of raw
biomass as a fuel. Torrefied biomass is more stable, has higher energy density, lower atomic O/C
and H/C ratios and moisture content, higher friability and grindability, less biodegradability, less
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smoking during combustion, and higher hydrophobicity [11, 12, 18-21]. Figure 1 shows changes
in biomass properties before and after torrefaction, taken from Chen at al. (2015)[20].
Torrefaction process reduces the particle aspect ratios[19]. Table 1 was constructed, the mass
loss of the raw biomass, upon torrefaction taken by Ren et al. (2017)[19]. Torrefaction of biomass
occurs at the 200-300°C temperature range, can be categorized into light, mild, and severe
torrefaction processes, the temperatures are approximately 200-235, 235-275, and 275-300°C,
in our experiments furnace was heated to 275°C[11, 20-22]. Nitrogen is the widely used carrier
gas to provide a non-oxidizing atmosphere in most of laboratory tests[19]. Torrefied biomass has
been successfully used, such as a fuel in the heating sector, power generation (co-firing),
gasification and steel production as well as an upgraded fuel in electric power plants and
gasification plants[11, 22, 23].
Figure 1 A schematic of property variation of biomass undergoing torrefaction [18].
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Table 1 Mass loss during torrefaction of biomass
Biomass Mass loss during torrefaction (%)
Corn Straw 25 Miscanthus 32 Bagasse 34 Corn DDGS 38 Rice Husk 34 Beech Wood 31
The size and shape of biomass are both significant factors of that can affect combustion
properties, these properties of biomass also effect of fuel preparation[4]. In power plants,
biomass size is an important fluidization parameter[24]. The finer particle size and the greater
the range of particle sizes, the greater the cohesive strength, and the lower the flow rate[25].
Increase the contact area by minimizing the size, increases the cohesive forces. The size of
biomass particles in pulverized combustion is expected to be larger than that of coal particles
because of their typically lower bulk density, faster devolatilization rates and higher volatiles to
fixed carbon ratios. Also, due to the different physical properties of biomass and coal, biomass
particles will not be pulverized to the same size as coal particles.
Chapter 2 investigated combustion characteristics in air of different torrefied biomass particles
size ranges. The targeted biomass types were waste crop, herbaceous and woody. Five different
sizes (75-90 µm, 180-212 µm, 212-300 µm, 300-350 µm, 350-500 µm) for each of torrefied
biomass were used in this work. The experimental setup that was, employed in this investigation
consisted of a drop-tube furnace, operated at a wall temperature of 1400 K and, a three color of
pyrometer, interfaced with this furnace and a high speed high-resolution camera. Individual
torrefied biomass particle combustion behaviors were recorded. We reported volatile and char
combustion phases, temperature, and time. Results are compared with relevant past data on the
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combustion characteristics of single coal particles. The goal of this chapter is to identify the
appropriate size of torrefied biomass particles whose combustion durations match those of 75-
90 µm pulverized coal particles, which is a size typically used in pulverized fuel boilers. Such data
will be useful in deciding the fuel sizing for co-firing coal with biomass.
Chapter 3 reports on a theoretical determination of char temperatures of burning torrefied
burning particles of various sizes using a model proposed by Field[26]. Torrefied char biomass
particles were assumed to burn with oxygen in air and produce carbon monoxide in their
immediate vicinity. In this chapter, the experimentally obtained torrefied char biomass particles
temperatures were compared with those predicted by the model.
Chapter 4 includes the conclusions of this work.
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Chapter 2
2. On the Particle Sizing of Torrefied Biomass for Co-firing with
Pulverized Coal
Aidin Panahi, Mahmut Tarakcioglu, Michael Delichatsios
and Yiannis A. Levendis*
Mechanical and Industrial Engineering Department, Northeastern University, Boston, MA, USA
*Corresponding Author Email: [email protected]
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2.1. Abstract
As grinding may have the largest cost impact on biomass feedstock harvesting and preparation,
a fundamental investigation was conducted to shed light on a targeted maximum biomass
particle size cut that can be co-fired with coal in existing pulverized fuel boilers. The method used
was to grind and sieve biomass particles to various size cuts, and then determine the largest
biomass size cut that burns in similar time frames as those of typically-sized pulverized coal
particles (e.g., 75-90 microns). This work focused on torrefied biomass derived from different
origins, herbaceous, woody, or crop related. The torrefaction of the biomass was conducted at T
= 275 °C for half an hour in nitrogen. The experimental setup that was used in this investigation
consisted of a drop-tube furnace, operated at a wall temperature of 1400 K, a three-color
pyrometer, interfaced with the furnace and a high-speed high-resolution camera. Entire
luminous particle combustion profiles of individual particles were recorded. Results are
compared with relevant past data on the combustion characteristics of single coal particles of
different ranks, burned in the same furnace under identical operating conditions. Such data is
useful in selecting the fuel size for co-firing coal with biomass. Another goal was to contrast the
combustion characteristics of the biomass samples of the selected size with those of coal.
Keywords: Combustion, Torrefied, Biomass, Size, Burn-out Time, Temperature, Pyrometry,
Cinematography, Co-firing
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2.2. Introduction
Around 30.4% of the total power produced in the USA in 2016 was from coal according to Energy
Information Administration and American coalition for clean cola electricity [27-29]. Co-firing
different coal types has been studied earlier [3, 7, 9, 30-36] to reduce harmful emissions from
coal combustion. Co-firing renewable fuels such as biomass along with coal is also a promising
technique for reducing pollutants from coal combustion as well [37-46].
Since biomass is plentiful and naturally grown, it is categorized as a renewable energy source [15,
47-50]. However, its low calorific value, high moisture content, hygroscopic nature and smoking
during combustion reduce the appeal of biomass as a fuel. Torrefaction is a practical method for
improving the properties of biomass as a fuel [5, 11, 20, 51-60]. Torrefaction is a thermochemical
pretreatment process, which can ameliorate the biomass utilization characteristics, including
heating value, grindability and resistance to decay [17, 18, 59-68]. Torrefied biomass can be a
suitable alternative fuel in existing large-scale pulverized coal boilers because torrefaction
renders the properties of biomass to be closer to those of coal [5, 54, 69-74]. In power generation
industry biomass and coal, grinding is a necessary and key step [47, 75-78].
Biomass particle shape and size are both important physical properties which could influence the
combustion properties of solid particles and fuel preparation [4, 79, 80]. Biomass particle size
plays an important role in fluidization in power plant boilers [24, 81-86]. Regarding this matter,
Marinelli et al. [25] reported that the finer the particle size, the greater the cohesive strength of
the powder, which impedes fluidization.
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The size of biomass particles in pulverized combustion is typically larger than that of coal particles
because of their typically lower bulk density, faster devolatilization rates and higher volatiles to
fixed carbon ratios [38, 87-91]. Also, due to the fibrous nature of raw biomass particles cannot
not be pulverized to the same size small particle sizes as coal [77, 92-97]. On the other hand, the
larger particle sizes of biomass can induce lengthier ignition delays and can generate larger
quantities of unburned residues than coal [12, 77, 98-101]. Hence finding the right biomass
particle size to co-fire with coal is important to achieve good combustion efficiency. This has been
addressed theoretically by Sastamoinen et al. [99] and experimentally by Mock et al. [102]. The
latter authors burned pulverized torrefied wood, raw sewage sludge and raw coffee waste and
found that particles as large as those in the range of (355-425) µm when exposed to a hot
upwards moving gas stream (1340 K) moved in the direction of the stream and burned
completely. Larger particles fell to the bottom of the furnace and did not burn completely.
Given that biomass total grinding costs can be the largest cost impact factor, (reportedly at $23-
$26 per oven-dry tonne impact range [103, 104]) as well as the aforesaid importance of biomass
particle shape and size [105] on particle transportation, blending with coal and fluidization in
power generation sector, this research examined the impact of the nominal particle size (based
on sieving) on the burnout times of biomass particles relative to coal. This work focused on
torrefied biomass, which has been identified as a better choice for co-firing with coal in existing
boilers [106, 107]. Besides the overall burnout times, this work examined the contributions of
the volatile and char combustion phases to the cumulative burnout times. It is also reporting on
pyrometrically-deduced particle combustion temperatures and measured ignition delays.
Different types of pulverized torrefied biomass fuels in size cuts of (75-90) µm, (180-212) µm,
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(212-300) µm, (300-350) µm and (350-500) µm were burned at a gas temperature comparable to
that used in the coal combustion studies of Refs [98, 99, 102, 108]. Based on all these combined
observations, the appropriate size of torrefied biomass particles that can be ignited and burned
in time-frames that are comparable to those of a typical coal particle size (75-90 µm) used in
utility boilers was determined.
2.3. Methods
2.3.1 Preparation of samples
Pulverized corn straw and rice husk were obtained from the Harbin Institute of Technology,
China. Pulverized miscanthus and beechwood were supplied by Ruhr-University Bochum,
Germany. Sugarcane bagasse was obtained from a bio-ethanol production plant in Brazil. DDGS
(Distiller’s Dried Grains with Soluble) was provided by a North American ethanol-producing
company. Torrefaction of all samples was carried out in a laboratory-scale muffle furnace in
nitrogen. The furnaces were charged with small amounts (a few grams) of millimeter-size
particles of biomass and, subsequently, they were heated to 275°C with heating rates in the order
of 104 °C/min. Upon reaching the final temperature, each sample was treated at constant
conditions for 30 min. All torrefied biomass fuels were dried, chopped in a household blender,
and size classified by sieving to obtain size cuts of (75-90) µm, (180-212) µm, (212-300) µm, (300-
350) µm and (350-500) µm. Optical microscope photographs and scanning electron microscope
photographs of each torrefied biomass are shown in Fig. 2 and Fig.3 respectively. Therein, it can
be observed that most types of biomass, except DDGS and rice husk, are needle-like and, thus,
have high length-to-diameter aspect ratios. In the previous work in this laboratory it was
observed that the torrefaction process reduced the particle aspect ratios [19]. All coal samples
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were procured from the Penn-State Coal Bank. The proximate analysis and the ultimate analysis
of the fuels, both on a dry basis, are given in Table 2. Proximate and ultimate analysis of the coal
and biomass samples was carried out at Harbin institute of technology in China [109].
Figure 2 Optical microscope photographs of various torrefied biomass fuels. (a) Corn straw, (b)
Miscanthus, (c) Sugar-cane Bagasse, (d) DDGS, (e) Rice husk and (f) Beechwood. In each case, the left side frames show particles of 180-212 µm, whereas the right-side frames show particles of 350-500 µm:
A size bar at the right bottom of this figure denotes magnification.
12
Figure 3 Scanning electron microscope images of torrefied biomass fuels in this study (a) Corn straw, (b)
Miscanthus, (c) Bagasse, (d) DDGS, (e) Rice husk and (f) Beechwood.
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Table 2 Chemical compositions of torrefied biomass
Rank / Fuel
Source
Biomass(Torrefied) Coal Herbaceous Crop-Derived Woody
Corn Straw Miscanthus Sugarcane
bagasse Corn DDGS Rice Husk Beechwood Anthracite Semi-Anthracite Bituminous Sub-bituminous Lignite
Proximate Analysis (dry basis)
Volatile matter (%) 67.55 75.00 73.74 71.46 55.62 72.20 3.6 9.2 33.6 33.1 44.2
Fixed Carbon (%) 24.56 22.30 23.49 21.09 21.58 27.30 82.2 80.1 50.6 35.1 12.0
Ash (%) 7.89 2.70 2.71 7.45 22.80 0.50 14.2 10.7 13.3 5.6 15.3
Ultimate Analysis (dry basis)
Carbon (%) 52.77 52.80 55.82 58.22 44.16 55.40 94.7 91.7 71.9 69.8 56.8
Hydrogen (%) 5.32 5.70 5.45 6.32 4.41 5.30 1.6 3.5 4.7 5.7 4.1
Oxygen (%) (by
difference) 32.46 38.6 34.94 22.98 26.73 38.88 2.0 1.3 6.9 15.6 15.8
Nitrogen (%) 1.50 0.18 1.00 4.00 1.22 0.18 1.0 1.9 1.4 0.9 1.1
Sulfur (%) 0.07 0.46 0.03 1.01 0.03 0.31 0.7 1.6 1.4 0.4 0.7
Calcium (%) 0.56 0.55 0.44 0.15 0.35 0.19 NA NA NA NA NA
Sodium (%) 0.02 0.04 0.03 0.80 0.02 0.01 NA NA NA NA NA
Potassium (%) 1.19 0.97 0.06 1.50 0.73 0.19 NA NA NA NA NA
Magnesium (%) 0.29 0.14 0.06 0.32 0.08 0.07 NA NA NA NA NA
Chlorine (%) 0.18 0.03 0.01 0.12 0.04 0.02 NA NA NA NA NA
Heating Value
(MJ/kg) 19.4 20.1 20.3 23.7 16.1 20.9 29.2 31.8 31.5 28.2 23.0
2.3.2. Experimental Apparatus
The combustion of free-falling fuel particles took place in an electrically heated, laminar flow,
vertical drop tube furnace at a gas temperature of ~1350K. The radiation cavity of this furnace
(an ATS unit) was 25 cm long and was heated by hanging molybdenum disilicide elements. A
vertical 7 cm i.d. transparent quartz tube was fitted in this furnace. Air was introduced into this
14
tube through a water-cooled stainless steel injector and, also, through a flow straightener placed
coaxially to the furnace injector, see Fig. 4. To enable single particle combustion, fuel particles
were introduced through a port at the top of the injector by first placing them on the tip of a
beveled needle syringe. Gentle taps on the needle allowed single particles to enter the injector
and, subsequently, the furnace. Pyrometric observations of single particles were conducted from
the top of the furnace injector, viewing downwards alongside the central axis of the furnace, Fig.
4, i.e., along with the particle’s path line. Details of the pyrometer optics, electronics, calibration,
and performance were given by Levendis et al. [110]. The voltage signals generated by the three
detectors were amplified and then were processed by a microcomputer using the LabView
software.
A high-speed high-resolution camera was located at one side of the furnace and viewed through
the slotted side quartz windows to record the particle combustion histories, against a backlight
frosted glass collated at the diametrically-opposite side of the furnace. An Edgertronic Self-
Contained Digital High-Speed Broadband video camera was used, at speeds of 1000 frames per
second. The camera was fitted with an Olympus-Infinity Model K2 long-distance microscope lens
to provide high resolution images of the combustion events.
15
Figure 4 Schematic illustration of the drop tube furnace, particle introduction and combustion.
2.4. Results and Discussion
2.4.1. Observation
High-speed high-resolution cinematography sequences of single particles burning in air are
shown in Fig.5.The combustion behaviors of biomass particles are amazingly the same according
16
to their different physical and chemical compositions. All torrefied biomass particles of this size
range ignited homogeneously and burned in two phases (volatiles flame and char combustion).
Volatile flames were sooty and luminous forming strikingly spherical envelope flames. Upon
extinction of the volatile flame char combustion took place. This has been shown in previous
work in this laboratory [10].
17
Figure 5 Images from high-speed high-resolution cinematography of single biomass particles burning in air in a DTF operated at Twall = 1400 K. The displayed numbers in each frame are in milliseconds, where zero does not mark the beginning of ignition, instead, it merely represents the beginning of the depicted
sequences. Nominal particle sizes were in the range of 212-300 µm.
18
2.4.2. Initial particle aspect ratio
By measuring 500 particle images photographed by the optical microscope, the average aspect
ratios of biomass particles in five different size ranges were obtained, as shown in Fig. 5, where
the average aspect ratio is the ratio of average length to average width.
As also shown in Fig.6, the average widths of biomass particles are all in the ranges of sieve sizes,
but the average lengths of all biomass particles exceeded the sieve sizes. It presents that most of
the biomass samples are needle shape. It is noticeable that for all the biomass samples except
DDGS and rice husk with increasing the sieve size the aspect ratios increase. It is in agreement
with the optical microscopy and SEM observations displayed in Fig. 2 and Fig.3.
Figure 6 Average initial aspect ratio of torrefied biomass fuels in this study for five different sieve sizes.
In comparison with coal particles which they are mostly sphere [100], biomass particles both raw
and torrefied have larger aspect ratio conferring to their needle shape.
19
2.4.3. Ignition Parameters
Single torrefied biomass particles burning in air appeared to ignite homogeneously forming
spherical flames. Other biomass samples both raw and torrefied were also burned at the author’s
laboratory [10], and similar homogeneous ignition behaviors were observed.
The ignition delay defines as the time passed from a particle’s exit from the cold DTF injector to
the moment where it experienced visible ignition in cinematographic images. There have been
some efforts to study the ignition behavior of single particle biomass fuels [12, 111-115], and
studies focusing on the ignition delay of the different type of the torrefied biomass are rather scarce.
Fig.7 displays the ignition delay times for torrefied biomass fuels of this study. It reveals that finer
biomass particles present lower ignition delay times than the bigger size torrefied biomass fuels,
and it is in agreement with Austin et al. [116] which they concluded that the raw biomass ignition
delay times increased with the increase of the initial particle diameter.
Figure 7 Average torrefied biomass fuels ignition delay times in air for various particle size group in the
DTF at Tg=1350 K.
20
2.4.4. Combustion temperatures
2.4.4.1. Flame temperature
Experimental biomass single particle flame temperature is presented in Fig.8. Such temperatures
were deduced from pyrometric measurements assuming graybody radiation from the soot
mantle in the envelope flame [117].
The presented values of the volatile temperatures herein are peak temperatures recorded in
single particle temperature-time histories, averaged over the 40 individual biomass particles that
were successfully detected by the pyrometer in each case. Standard deviation bars are shown on
each point. The radiation intensity signals captured by the pyrometer during the combustion of
biomass particles were strong and it was comparable to than those captured during combustion
of coal particles under identical experimental conditions [118], especially during the phase of
volatiles combustion.
Figure 8 Average deduced flame temperatures for all biomass fuels of this study in air, at Tgas=1350 K.
In each case the first column is (75-90) µm, the second column is (180-212) µm, the third column is (212-300) µm, the fourth column is (300-350) µm and the last column is (350-500) µm.
21
Results commonly show particle volatile temperatures in the range of 2100-2300 K in air. The
volatile flame (soot mantle) temperature can significantly exceed the surrounding gas
temperature, partly because of heat release during exothermic surface reaction. Particle size
appears to have an impact on volatile flame temperature, as bigger particles appear to be in
hotter than smaller ones.
Different volatile temperature between the three biomass types might be because of the gas and
tar content of each biomass. The cellulose and hemicellulose components of biomass decompose
to small molecules in the form of volatile gases, tars and pyrolytic water [119-121]. The volatile
flames of torrefied biomass fuels have been observed less sootier than those of high rank coals
[122]. Oxygen content in biomass fuels is much higher than that of coals. Hence, the volatiles
contain high quantities of CO2 ,hydrogen and light hydrocarbons [123].
2.4.4.2. Char temperature
The results shown in Fig. 9, illustrates char temperatures with increasing particle size for three
different types of biomass.
Particle size in the range of (75-90) µm, (212-300) µm, (300-350) µm and (350-500) µm
experienced lower char temperatures than size cut of (180-212) µm. A single film model in
appendix A has been used to investigate the char temperatures for different particle size and It
is noticeable that the calculated results support the notion that the char temperatures increase
mildly with the char particle size and upon reaching a maximum value decrease mildly.
22
In general, biomass chars burned at a somewhat higher average temperature than bituminous
char particles by 50-100 K, as can be confirmed by comparing the results shown in previous
studies in this laboratory [108].
Figure 9 Average deduced char temperatures for all biomass fuels of this study in air, at Tgas=1350 K.
2.4.5. Combustion Burn-out Time
Fig. 10a and 10b illustrate average combustion durations of volatiles and char particles of the five
torrefied biomass types with nominal initial sizes in the ranges of (75-90) µm, (180-212) µm and
(212-300) µm, (300-350) µm and (350-500) µm. In each range, the combustion histories of over
40 particles were recorded and average times were calculated. Shorter burn-out times were
observed for the smaller size cut in volatile and char phases. DDGS experienced the longest
combined burn-out times.
23
Figure 10 Experimental data on combustion times of (a) volatile flames (b) chars of torrefied biomass
particles in three ranges within 75-350 μm in a DTF at Tg=1350 K in air.
Based on experimental observations, particles of (75-90) μm, (180−212) μm and (212-300) μm
size cuts experienced complete burn-outs in the DTF, used in this study, without leaving any
visible residues at its exit, where a white filter was placed. Cumulative (total) volatile and char
burnout times for these particles are plotted in Fig. 11; they span the time-frame of 60-160 s.
Based on these results, appropriate torrefied biomass particle size would be suggested for co-
firing with coal.
Figure 11 Comparison of cumulative combustion times of single particles of coal in the nominal size
range of 75-90 μm and torrefied biomass burning in a DTF at 1350 K in air.
24
Previous studies in this laboratory burned single particles from five different ranks of coals [100,
117, 118] in the same DTF, in air, also at Tg=1350 K. Their size was 75-90 μm, which is a size
commonly burned in pulverized coal boilers. Burn-out times for lignite, sub-bituminous and
bituminous coal particles were in the same range as those of the torrefied biomass particles
mentioned above. Burnout times of anthracite and semi-anthracite particles were longer, but
those coals are not commonly burned.
25
Chapter 3
3. Numerical Determination of Temperatures of Burning Torrefied
Biomass Char Particles
26
Temperatures of single torrefied biomass particles burning in air inside the electrically-heated
laminar-flow drop-tube furnace (DTF) were calculated based on Field’s model for combustion of
a carbon sphere [26]. A carbonaceous char particle was assumed to burn with oxygen and
produce mostly carbon monoxide in its immediate surroundings, see Fig 12, according to the
reaction C+O2 → CO. While CO2 is also a possible combustion product, it was assumed that at the
CO was by far the most prevalent at the high temperature conditions in the DTF, based on past
research findings [124, 125].
Figure 12 Miscanthus torrefied char particle of 175 µm, (which was derived for torrefied biomass particle
180-212 µm) in the air in the electrically-heated laminar-flow drop-tube furnace
27
Thermal equilibrium is the state of a system in which its heat flow is balanced with its
surroundings, meaning the temperatures of the system and surroundings are the same. A system
at a higher temperature will transfer heat to a system at a lower temperature when they are in
contact, until their temperatures are equal. Thermal equilibrium is an important concept for
thermodynamics and blackbody radiation. The main idea is in our model the carbon surface
temperature tends towards the equilibrium temperature that the rate of heat generation and
the rate of heat loss are equal.
Energy Balance Equation,
𝐻𝑔 − 𝐻𝑐 − 𝐻𝑟 = 0 (1)
Where 𝐻𝑔 = rate of heat generation per unit area,
𝐻𝑐 = rate of heat loss by conduction
𝐻𝑟 = rate of heat loss by radiation
The rate of heat generation at particle surface to the rate of heat loss due to conduction and
radiation. The resulting expression is:
𝐻𝑔 = 𝐻𝑐 + 𝐻𝑟 (2)
It is need to express each of three terms in Equation (1) for find the equilibrium particle
temperature.
The rate of heat loss by conduction is given by:
𝐻𝑐 =2𝜆(𝑇𝑠−𝑇𝑔)
𝑥 (3)
28
Where λ = mean thermal conductivity of the gas around the particle, cal cm s °K⁄ .
Thermal conductivity is a measure of the ability of a material to transfer heat. The thermal
conductivity of the gas increases with temperature and as a convenient approximation is
assumed to be proportional to the absolute temperature to the power 0.75 [26]. The thermal
conductivity at the mean of the surface and gas temperature is:
λ = λ0 (𝑇𝑠+𝑇𝑔
2𝑇0)
0.75
(4)
Where λ0 is the thermal conductivity at a reference temperatureT0. The thermal conductivity is
independent of pressure[10]. λ0 = 0.0002 cal cm s K⁄ .
The rate of heat loss by radiation is given by:
𝐻𝑟 = ϵσ(𝑇𝑠4 − 𝑇𝑤
4 ) (5)
Where σ = Stefean − Boltzman constant = 1.36 ∗ 10−12 cal cm2 s K4⁄ ,
ϵ = emissivity of particle,
𝑇𝑤 = effective wall temperature of the surfaces which surround the particle, °K
We took emissivity of particle as 0.9 but values of 0.8 and 1.0 were also explored [10, 126, 127].
The rate of heat generation is given by:
𝐻𝑔 = 𝑞𝑄 (6)
𝑄 Is the heat release at surface per unit mass of carbon burnt, cal/g. The value of Q depends on
the mechanism and also slightly on the temperature. φ Is a mechanism factor which takes the
value 1 when carbon dioxide is transported away and 2 when carbon monoxide is transported
29
away. When the carbon is transported away as carbon monoxide, the heat released at the surface
is 2340 cal/g, but when carbon dioxide is formed, the heat released is 7900 cal/g. Both heat
quantities increase slightly with temperature increase. The values quoted are those given by
Lavrov, Korobov & Filippova (1963).
𝑄 = 7900(2 ⁄ φ − 1) + 2340(2 − 2 ⁄ φ)
As mentioned earlier, the combustion product was assumed to be CO, thus the value of φ was
set equal to 2[124]. In this case:
Q= 2340 cal/gr
In the mass and energy equations, q is the overall particle burning rate per unit external surface
area.
Rate of consumption of carbon, q, can be denoted as
𝑞 =𝑝𝑔
(1
𝐾𝑑𝑖𝑓𝑓+
1
𝐾𝑠)
(8)
The diffusional reaction rate coefficient, 𝐾𝑑𝑖𝑓𝑓, depends on the particle size, the mechanism
factor and the mean temperature. Reaction rate coefficient, 𝐾𝑑𝑖𝑓𝑓 defined as:
𝐾𝑑𝑖𝑓𝑓 = 24𝜑𝐷 𝑥𝑅′𝑇𝑚⁄ (9)
Where D = diffusion coefficient of oxygen in the gas, cm2 s⁄ ,
𝑅′ = gas constant (= 82.06 atm cm3 mole °K⁄ )
𝑇𝑚 = the mean temperature between the gas and temperature, °K
30
𝑥 = 2𝑎 = char particle diameter, cm
𝑇𝑚 =𝑇𝑔+𝑇𝑠
2
Fig. 13 was constructed for the Diffusion coefficient of oxygen in the gas (D) based on Data on
Fuel and Combustion Properties [Fossil Fuel combustion, Wiley Interscience].
Figure 13 Calculated Diffusion Coefficients for O2-N2 Gas Pairs at Pressure of 1 atm.
The torrefied biomass char particle diameters x, were calculated based on data retrieved from
Panahi et al. (2017) [10], for chars collected upon devolatilization of Miscanthus biomass particles
with initial diameters of 180-212µm. Char particle sizes for torrefied biomass particles of larger
or smaller initial size cuts were estimated by extrapolation, and results are shown Table 3.
31
Table 3 Torrefied biomass initial particle sizes and char particle sizes
Initial Particle Diameter (µm) Average Char Diameter (µm) 75-90 52 (± 10 )
180-212 175 (± 10 )
212-300 195 (± 10 )
300-350 252 (± 10)
350-500 392 (± 10)
Where Ks is the surface reaction rate coefficient.
𝐾s = 𝐴exp(−𝐸 𝑅𝑇𝑠)⁄ (10)
Where 𝐸 = activation energy, cal mole,⁄
𝑅 = gas constant (1.986 cal mole K), and ⁄
𝐴 = pre − exponential factor, g cm2s atm⁄
Activation energy and pre-exponential factor values were taken from Vorobiev et al. [128]. The
activation energy is assumed to be 80 kJ/mol and the pre-exponential factor was taken as
18 g cm2s atm⁄ , see Fig. 14.
32
Figure 14 Kinetic parameters for torrefied Miscanthus and Beachwood biomass as well as for a lignite
coal. Pre-exponential Factor Values[126].
The equilibrium particle temperature can be found from Equation (1) using the above expressions
for heat generation and heat loss. Heat generation is defined as Equation (6). Heat loses is defined
as Equation (2). We can plot the rate of heat generation and the rate of heat loss against particle
temperature is shown Fig 15. The intersection point of the two curves represents the predicted
particle temperature for model, as shown Fig.16. The intersection point temperature is different
for different char particle sizes.
33
Figure 15 Variation of heat loses from a torrefied biomass char particle of 175 µm, (which was derived
for torrefied biomass particles 180-212 µm in diameter) burning in air versus temperature.
Figure 16 Superposition of rates of heat generation and heat loss from a torrefied biomass char particle
of 175 µm, (which was derived for torrefied biomass particle 180-212 µm in diameter) burning in air versus its temperature.
34
Here, we describe in detail experimental results obtained for six different types of torrefied
biomass. Pulverized corn straw and rice husk were obtained from the Harbin Institute of
Technology, China. Pulverized miscanthus and beechwood were supplied by Ruhr-University
Bochum, Germany. Sugarcane bagasse was obtained from a bio-ethanol production plant in
Brazil. DDGS (Distiller’s Dried Grains with Soluble) was provided by a North American ethanol-
producing company. Torrefaction of all samples was carried out in a laboratory-scale muffle
furnace in nitrogen. Thereafter, all fuels were dried, chopped in a household blender, and size
classified by sieving to obtain size cuts of (75-90) µm, (180-212) µm, (180-212) µm, (212-300) µm,
(300-350) µm, (350-500) µm.
The combustion of free-falling fuel particles took place in an electrically heated, laminar flow,
vertical drop tube furnace at a gas temperature of 1400°K. To enable single particle combustion,
fuel particles were introduced through a port of at the top of the injector by first placing them
on the tip of a beveled needle syringe. Pyrometric observations of single particles were
conducted from the top of the furnace injector, viewing downwards alongside the central axis of
the furnace. The voltage signals generated by the three photodetectors of the pyrometer were
amplified and then they were processed by a microcomputer using the LabVIEW software. In
each range, the combustion histories of 20-30 particles were recorded. Thereafter, averages of
their peak char temperatures in each pyrometric profile and averages of their average char
temperatures through the entire Temperature-time profiles were calculated. Table 4 illustrates
the former char temperatures for all types of biomass, whereas Table 5 illustrates the latter char
temperatures. The typical temperature difference between the two measurements was 11°K.
35
Table 4 Averages of experimentally measured peak char temperatures
Miscanthus Bagasse Beechwood Corn Straw DDGS Rice Husk
75-90 1771 1762 1831 1743 1708 1704
180-212 1834 1848 1875 1769 1768 1752
212-300 1806 1805 1862 1755 1752 1674
300-350 1804 1800 1853 1750 1750 1676
350-500 1800 1795 1849 1746 1719 1672
Table 5 Averages of experimentally measured average char temperatures
Miscanthus Bagasse Beechwood Corn Straw DDGS Rice Husk
75-90 1765 1752 1825 1731 1700 1694
180-212 1828 1839 1869 1764 1761 1748
212-300 1800 1796 1855 1749 1744 1670
300-350 1794 1789 1848 1745 1739 1671
350-500 1785 1785 1840 1740 1711 1668
36
Figure 17 Experimentally measured and theoretically predicted char particle temperatures versus initial
torrefied biomass particle size.
Experimentally measured and theoretically predicted particle char temperatures are shown in
Figure 17, as a function of the initial torrefied particle size cut. The experimental results are taken
from Table 4, model char temperatures were calculated using the model described above. Both
the experimental and calculated results support the notion that the char temperatures increase
mildly with the char particle size and upon reaching a maximum value decrease mildly. When the
initial torrefied particle size increased, heat loses and heat generation values were decreased.
The maximum temperature corresponds to a biomass char size of 175 µm, which, in turn,
corresponds to an initial torrefied particle size of 180-212 µm. The minimum temperature to a
biomass char size of 52 µm, which, in turn, corresponds to an initial torrefeid particle size of 75-
90 µm.
37
3.1. Parametric Analysis
In this section the values of the following parameters: emissivity of particle, particle diameter,
activation energy, and pre-exponential factor were varied according to prior reporting in the
literature. Results on their predicted effects on the calculated char particle temperatures are
shown below:
(a) The effect of the emissivity of a particle, ε, is shown Figure 18. Only heat radiation
equation was affected by the emissivity of particle. The emissivity of particle used to
achieve the modeling results is shown in Figure 17, was 0.9. However, as emissivity values
for carbonaceous particles have been reported in the literature to span the range of 0.8-
1 [127], the model was perturbed to reflect the results of the char particle temperatures
when ε took values in this range, as shown in Fig. 18. When ε increased to 1, the particle
temperature decreased by 32 K, whereas when ε decreased to 0.8, the particle
temperature increased by also 35 K. This is because higher emissivity increased the heat
loss form a particle.
38
Figure 18. The effect of char particle emissivity on char particle temperatures versus initial torrefied
biomass particle size.
(b) The effect of a change in particle diameter, x, is shown in Figure 19. Both the diffusion
term in the heat generation equation and the conduction term in the heat loss equation
are affected by a change in the particle diameter, whereas the chemical term and the
radiation term in these equations remain unaffected. For example, torrefied Miscanthus
in the size cut of 180-212 µm, upon pyrolysis generated char particles with estimated
diameters of 175(± 10) µm [10]. Char sizes for this and other torrefied biomass particle
size cuts are shown in Table 3. The model was perturbed to reflect the results of the char
particle temperatures when x took values in this range (175(± 10)), and results are shown
in Fig. 17. The effect of reducing the particle diameter is to reduce the particle
temperature for 75-90 µm size but increase the particle temperature for other particle
sizes. The effect of increasing the particle diameter is to increase the calculated particle
39
temperature for 75-90 µm size but decrease the particle temperatures for other particle
sizes.
Figure 19 The effect of the assumed char particle size on char particle temperatures versus initial
torrefied biomass particle size.
(c) The effect of perturbing the assumed activation energy, E, in the kinetic expression of Eq.
10 is shown in Figure 20. The calculated heat generation equation is effected by the value
of the activation energy. The activation energy used to obtain the modeling results shown
in Figure 17 was 19000 cal mole⁄ [128]. This value was mildly perturbed to 18000
cal mole⁄ and 20000 cal mole⁄ . When E increased to 20000 cal mole⁄ , the particle
temperature decreased by 88K, whereas when E decreased to 18000 cal mole⁄ , the
40
particle temperature increased by 83 K. Lowering the activation energy enhances the
combustion reactions and, thereby, increases the heat generation.
Figure 20 The effect of the assumed chemical reaction activation energy on char particle temperatures
versus initial torrefied biomass particle size.
(d) The effect of perturbing the assumed pre-exponential factor, A, in the kinetic expression
of Eq. 10 is shown in Figure 21. Only the heat generation equation term is affected by the
pre-exponential factor. The pre-exponential factor used to achieve the modeling results
shown in Figure 17 was 18 g cm2s atm⁄ . However, as pre-exponential factor values for
biomass have been reported to be in the range of 13-20g cm2s atm⁄ [128], the model was
41
perturbed to reflect the results of the char particle temperatures when A took values in
this range, as shown in Fig. 21. When the factor A increased to 20 g cm2s atm⁄ , the
particle temperature increased by 29K, while when A decreased to 13g cm2s atm⁄ , the
particle temperature decreased by 84 K.
Figure 21 The effect of the assumed chemical reaction pre-exponential factor on char particle
temperatures versus initial torrefied biomass particle size.
42
Chapter 4
4. Conclusions
This master thesis puts forward a detailed study of combustion of different types of pulverized
torrefied biomass for energy harvesting. The biomass types were waste crop, herbaceous, and
woody; they included corn straw, DDGS, rice husk, Miscanthus, sugarcane bagasse, and
Beechwood. Renewable biomass is an important energy source. Combustion studies were
conducted in a drop tube furnace (DTF) and temperature –time measurements were conducted
pyrometrically and cinematographically.
The determination of appropriate size of biomass particles that can be fired or co-fired with coal
in existing pulverized fuel utility is very important. From the experimental results, particles with
nominal sizes in the size cut of (212−300) μm or less burned completely in air in a DTF at Tg=1350
K. Larger particles in the size cut of (300-350) μm and (350-500) μm occasionally exited the
bottom of the furnace, partially burned. Accordingly, the maximum size of torrefied particles that
could be reliably burned completely in this particular DTF at a gas temperature of 1350 K was in
size range of (212−300) μm. After that, a comparison was made between burnout times of such
torrefied biomass particles and pulverized coal. It was observed that combustion durations of all
types of torrefied biomass particles of 212-300 µm match those of 75-90 µm pulverized coal
particles, which is a typical size-cut currently used in pulverized fuel boilers.
In addition to the determination of biomass particle burnout times, particle char temperatures
were deduced pyrometrically, and were compared with predicted temperatures from a simplified
particle combustion model. Such comparisons allowed for evaluation of particle temperature
43
trends with biomass char particle size and evaluation of various combustion parameters, such as
emissivity of particle, char particle diameter, combustion reaction activation energy and pre-
exponential factor, and thermal conductivity of torrefied biomass particles burning in air inside
the electrically-heated laminar-flow drop-tube furnace.
44
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