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2000.M.4.3.4

R&D on Energy Conservation Operation Support System for Decompressed Residual Oil Combustion Boiler

(Decompressed residual oil combustion group) �Yutaka Sakai, Tadashi Yoshimura, Shuichiro Natake,

Takashi Furuse, Takao Adachi

1. Contents of Empirical Research

There are two power generation formats that use decompressed residual oil as fuel: the IGCC format, which burns gas obtained through gasification, and the BTG format, in which fuel is burnt directly by using burners. In general, the BTG format is a commonly used method with heavy oil, but in the case of decompressed residual oil, it is believed that the oxygen concentration in the exhaust gas must be at a higher value than during combustion of regular C heavy oil, partly because there is an abundant carbon component in the fuel. The following R&D was begun in 1999 for the purpose of constructing an operational support system that reduces the concentration of oxygen in exhaust gas from the current value of 2.0% to 0.5% (0.5% energy conservation, CO2 reduction) by forecasting NOx and dust content from the combustion properties of decompressed residual oil and from boiler operating conditions, and by applying the forecasts to control of boiler combustion for power generation.

1.1 Evaluation of the basic combustibility of regular heavy oil and decompressed residual oil

An analysis was done on the fuel properties of regular heavy oil and on decompressed residual oils of different crude oil type and different distillation conditions, and an evaluation was made of basic combustibility through differential thermal analysis.

1.2 Creation of burner combustion forecast formulae

In addition to burner combustion tests in which all types of regular heavy oil were used as fuel, burner combustion tests were conducted in which operating conditions, such as furnace load and air preheating temperature, were changed; and NOx and dust content underwent regression analysis. Based on the results thereof, burner combustion forecast formulae were created which forecast NOx and dust content from fuel properties and operating conditions.

1.3 Investigation of combustion improver that can be regenerated and reused

With respect to combustion improver considered effective in reducing dust in decompressed residual oil, evaluation was made of low dust effect in combustion test furnace; combustion ash was recovered, and the potential for regeneration and reuse was investigated.

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2. Results of R&D and Analysis Thereof 2.1 Asphaltene analysis of decompressed residual oil and evaluation of basic

combustibility 2.1.1 Property analysis results

The regular heavy oils and decompressed residual oils used as fuel in tests are presented together with the results of analysis of their properties in Table 2.1-1. Included among these oils are type A and D regular heavy oils to which commercial combustion improvers A and B were added at 0.5 mass%.

2.1.2 Basic combustion test results

The basic combustibility of each regular heavy oil and of decompressed residual oils was evaluated using a differential thermal analysis balance. From the thermal analysis curve of each fuel, the amount of residual carbon and residual carbon combustion rate were determined, and an evaluation was made of the residual carbon combustion rate by relative value against regular heavy oil A.

Table 2.1-1 Properties of fuel for testing

Fuel for testing Density

(g/cm3)

Kinetic viscosity

(note)

(mm2/s)

Residual carbon content

(mass %)

Nitrogen content

(mass %)

Sulfur content

(mass %)

Asphaltene content

(mass %)

Regular heavy oil A 0.9674 205.9 11.6 0.24 2.56 6.0

Regular heavy oil A + combustion improver A

Regular heavy oil A + combustion improver B

Regular heavy oil B 0.9674 2698 16.6 0.28 3.42 10.1

Regular heavy oil C 1.0045 5924 18.1 0.31 3.52 6.9

Regular heavy oil D 0.9489 157.4 10.2 0.20 1.85 4.1

Regular heavy oil D + combustion improver A

Regular heavy oil D + combustion improver B

Decompressed residual oil A 1.0202 893 24.9 0.39 4.61 9.7

Decompressed residual oil B 1.0202 893 24.9 0.39 4.61 9.7

Decompressed residual oil C 1.0202 893 24.9 0.39 4.61 9.7

Decompressed residual oil D 1.0202 893 24.9 0.39 4.61 9.7

Decompressed residual oil E 1.0202 893 24.9 0.39 4.61 9.7

Decompressed residual oil F 1.0202 893 24.9 0.39 4.61 9.7

Decompressed residual oil G 1.0202 893 24.9 0.39 4.61 9.7

Decompressed residual oil H 1.0202 893 24.9 0.39 4.61 9.7

(Note) Kinetic viscosity values are at 50°C for regular heavy oil and at 120°C for decompressed residual oil

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0.00 0.50 1.00 1.50 2.00

Decompressed residual oil H

Relative combustion rate constant

Decompressed residual oil G

Decompressed residual oil F

Decompressed residual oil E

Decompressed residual oil D

Decompressed residual oil C

Decompressed residual oil B

Decompressed residual oil A

Combustion improver B + regular heavy oil D

Combustion improver A + regular heavy oil D

Regular heavy oil D

Regular heavy oil C

Regular heavy oil B

Combustion improver B + regular heavy oil A

Combustion improver A + regular heavy oil A

Regular heavy oil A

0.00 1.00 2.00 3.00 4.00

Decompressed residual oil H

Differential thermal analysis residual carbon content (mg/10 mg)

Decompressed residual oil G

Decompressed residual oil F

Decompressed residual oil E

Decompressed residual oil D

Decompressed residual oil C

Decompressed residual oil B

Decompressed residual oil A

Combustion improver B + regular heavy oil D

Combustion improver A + regular heavy oil D

Regular heavy oil D

Regular heavy oil C

Regular heavy oil B

Combustion improver A + regular heavy oil B

Combustion improver A + regular heavy oil A

Regular heavy oil A

Figure 2.1-1 Comparison of each fuel by differential thermal analysis

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2.2 Creation of burner combustion forecast formulae 2.2.1 Investigation of relationship between fuel properties and exhaust gas properties

With respect to regular heavy oils A, C and D, comparisons were made of combustibility index by differential thermal analysis and of NOx and dust content by burner combustion test in combustion test furnace. The specifications of the combustion test furnace used in burner combustion tests, compared to those of large-scale boiler for power generation by burning decompressed residual oil, are presented in Table 2.2-1. The combustion test furnace contains combustion air preheater and atomizing steam super- heater, and combustion conditions such as air preheating temperature and atomizing steam super-heating temperature can be set equivalent to those in practical boiler for power generation. Combustion test conditions and results are presented in Table 2.2-2 and Figures 2.2-1 to 2. NOx concentration is represented by actual NOx concentration in combustion exhaust gas without conversion of exhaust gas oxygen concentration. Respecting NOx, a linear relationship with nitrogen content in fuel could be recognized. As for dust content, a linear relationship could be recognized with residual carbon combustion rate and residual carbon content under conditions in which the exhaust gas oxygen concentration exceeds 0.5%.

Table 2.2-1 Specifications of combustion test furnace

Combustion test furnace

Boiler for power generation by decompressed residual oil burning

Structure Horizontal water-cooling cylindrical type, Fireproof inner lining

Water tube boiler

Scale (evaporation) (-) From 300 t/h

Number of burners 1 burner 8 to 12 burners

Combustion volume per burner

Up to 300 L/h 2000 to 3000 L/h

Burner arrangement Front firing format Corner firing format (front firing format)

Burner type Intermediate mixture type, Internal mixture type

Intermediate mixture type

Furnace load capacity 0.2 to 0.5 MW/m3 0.2 to 0.5 MW/m3

Air preheating temperature Room temperature to 623K

473 to 573K

Residual oxygen concentration

0.3 to 4.0% 1.5 to 2.0%

Atomizing steam heating temperature

453 to 673K 523 to 673K

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Table 2.2-2 Burner combustion test conditions (comparison tests of regular heavy oil A, C and D)

Item Condition

Fuel

Combustion volume

Air preheating temperature

Atomizing steam temperature

Atomizing steam volume

Exhaust gas oxygen concentration

Furnace load

Regular heavy oil A, C and D

200 L/h

473K

533K

42 kg/h

0.5 to 4.0%

0.218 MW/m3

0.0

0.1

0.2

0.3

0.4

0.5

0 100 200 300

O2 4% 2% 1% 0.5%

Nitr

ogen

con

tent

(mas

s %

)

Nox ppm

Figure 2.2-1 Nitrogen content versus NOx

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0.00

1.00

2.00

3.00

0 100 200 300 400

Rel

ativ

e co

mbu

stio

n ra

te c

onst

ant O2 4%

2% 1% 0.5% 0.3%

Dust (mg/Nm3)

0.00

1.00

2.00

3.00

4.00

0 100 200 300 400

O2 4% 2% 1% 0.5% 0.3%

Diff

eren

tial t

herm

al a

naly

sis

resi

dual

car

bon

cont

ent m

g/10

mg

Dust (mg/Nm3)

Figure 2.2-2 Differential thermal analysis combustibility index versus dust content

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2.2.2 Investigation of relationship between operating conditions and exhaust gas properties

In order to forecast NOx and dust content in boiler for power generation when operating conditions have been changed, combustion tests were performed while changing furnace load and air preheating temperature in a combustion test furnace. A partition was placed in the combustion chamber and by moving it so as to alter the combustion chamber capacity, the furnace load was changed. Together with a change in air preheating temperature, the combustion air linear velocity in the burner changes, and because a change is produced in the status of the initial mixture of fuel spray and combustion air, and in order to evaluate the impact of this change on NOx and dust content, the nozzle position was varied for each air preheating temperature and NOx and dust content were determined. The positional relationship between burner and nozzle is shown in Figure 2.2-3. The burner tile reduction component is presented as the reference position for nozzle and the furnace interior is represented by positive values. Under the test conditions shown in Table 2.2-3, furnace load, air preheating temperature and nozzle position were changed while other conditions were held constant, and measurements of NOx and dust content were taken. The results are given in Figures 2.2-4 to 6. NOx concentration is represented by actual NOx concentration in combustion exhaust gas without conversion of exhaust gas oxygen concentration.

Burner component detail

Nozzle position

Figure 2.2-3 Positional relationship between burner and nozzle

Table 2.2-3 Combustion test conditions under variable furnace load Item Condition

Fuel Regular heavy oil D

Combustion volume 200 L/h

Air preheating temperature 473 to 573K

Atomizing steam temperature 533K

Atomizing steam volume 42 kg/h

Exhaust gas oxygen concentration 0.3 to 2.0%

Furnace load 0.218 to 0.436 MW/m3

Nozzle position -25 mm to + 30 mm

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Exhaust gas oxygen concentration (%) Exhaust gas oxygen concentration (%)

Nox

ppm

Dus

t (m

g/N

m3 )

Figure 2.2-4 Results of combustion test under variable furnace load (air preheating temperature 573K/nozzle position -25 mm)

Exhaust gas oxygen concentration (%)

Nox

ppm

Dus

t (m

g/N

m3 )

Exhaust gas oxygen concentration (%)

Figure 2.2-5 Results of combustion test under variable air preheating temperature (nozzle position -25 mm/furnace load 0.218 MW/m3)

Exhaust gas oxygen concentration (%) Exhaust gas oxygen concentration (%)

Nox

ppm

Dus

t (m

g/N

m3 )

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2.2.3 Creation of Forecast Formulae by Regression Analysis

With respect to NOx and dust content in test furnace combustion, a quadratic multiple regression analysis was made, taking fuel properties and operating conditions as the variables; and burner combustion forecast formulae were created by determining each coefficient of quadratic polynomials based on the results of the aforesaid analysis. The variables and range of operating conditions considered in the forecast formulae are presented in Table 2.2-4. Figure 2.2-7 gives a comparison of values calculated by the same formulae and actual measurement values. We see that within the range presented in Table 2.2-4, the NOx and dust content when each condition has been changed can be forecast at good accuracy by the formulae.

Table 2.2-4 Variables considered in creating burner combustion forecast formulae Forecast variable

Operating condition

Exhaust gas property

Fuel property

Variable Range

NOx Nitrogen content Exhaust gas oxygen concentration

Furnace load

Air preheating temperature

Nozzle position

0.3 to 4.0% 0.218 to 0.436 MW/m3

473 to 573K

-25 mm to + 30 mm

Dust content

Differential thermal analysis

Residual carbon content

Differential thermal analysis

Residual carbon combustion rate

Exhaust gas oxygen concentration

Furnace load

Air preheating temperature

Nozzle position

Retention time (furnace inner length)

1.0 to 4.0% 0.218 to 0.436 MW/m3

473 to 573K

-25 mm to + 30 mm

1.75 to 3.5 m

10

NO

x pp

m (a

ctua

l mea

sure

men

ts)

Regression statistics

Multiple correlation R 0.98Major decision R2 0.95Correction R2 0.94Standard error 4.89Number of observations 67

Regression statistics

Multiple correlation R 0.95 Major decision R2 0.90 Correction R2 0.85 Standard error 20.1 Number of observations 45

NOx ppm (forecast value) Dust mg/Nm3 (forecast value)

Dus

t mg/

Nm

3 (act

ual m

easu

rem

ents

)

Figure 2.2-7 Comparison of forecast values and actual measurements

2.3 Investigation of combustion improver that can be regenerated and reused

In order to investigate combustion improver that can be regenerated and reused, the dust content produced by burner combustion test, using regular heavy oils A and D as fuel, was measured for commercial combustion improvers A and B. Combustion test conditions are given in Table 2.3-1 and results are shown in Figure 2.3-1. With regular heavy oil A, a dust reduction effect by combustion improver can be noted in the domain of low oxygen concentration; with regular heavy oil D, a reduction effect cannot be noted in the domain where exhaust gas oxygen concentration is 0.5% or below. This indicates that in regular heavy oils A and D, the properties of the residual carbon produced are different because of the differences in fuel properties so that the conditions of exhaust gas oxygen concentration under which the effects of combustion improver can be obtained become varied.

In order to evaluate the potential of each combustion improver for regeneration and reuse, the combustion improver combustion ash obtained from burner combustion tests was recovered and added to regular heavy oil D, and a differential thermal analysis was conducted. Shown in Figure 2.3-2 are the combustion rates of residual carbon, taking regular heavy oil D as the standard, when combustion improver has been added and when a combustion ash containing an equivalent volume of combustion improver ingredient has been added. The combustion rate of residual carbon is lower when combustion ash has been added than when combustion improver has been added, and this is ascribed to the fact that the chemical configurations of effective ingredients in combustion improver are modified by burner combustion. In comparison to cases of no additions, a significant increase in combustion rate can be noted, and we find that it is possible to reduce dust content through regeneration and reuse of combustion improver.

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Table 2.3-1 Burner combustion test conditions (Commercial combustion improver A and B)

Item Condition

Fuel Regular heavy oil A and D

Combustion volume 200 L/h

Air preheating temperature 473K

Atomizing steam temperature 533K

Atomizing steam volume 42 kg/h

Exhaust gas oxygen concentration 0.3 to 4.0%

Furnace load 0.218 MW/m3

Dus

t (m

g/N

m3 )

Exhaust gas oxygen concentration (%) Exhaust gas oxygen concentration (%)

Regular heavy oil D Regular heavy oil D + combustion improver ARegular heavy oil D + combustion improver B

Regular heavy oil A Regular heavy oil A + combustion improver A Regular heavy oil A + combustion improver B

Dus

t (m

g/N

m3 )

Figure 2.3-1 Dust content measurement results (Combustion improver A and B)

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0.00 0.50 1.00 1.50 2.00

般重油

助然剤 般重油

助燃剤 般重油

燃焼 般重油

燃焼 般重油

Relative combustion rate constant

Combustion ash B+ regular heavy oil D

Combustion ash A+ regular heavy oil D

Combustion improver B+ regular heavy oil D

Combustion improver A+ regular heavy oil D

Regular heavy oil D

Figure 2.3-2 Results of differential thermal analysis of combustion

improver and of fuel with combustion ash added (Combustion improver A and B)

3. Results of Empirical Research (1) A regression analysis was made of combustion exhaust gas properties in burner

combustion tests in which regular heavy oil was used as fuel and fuel properties and operating conditions were varied. As a result, it was possible to create burner combustion forecast formulae in which fuel properties and operating conditions are taken as variables and combustion exhaust gas properties can be forecast quantitatively.

(2) Combustion improver that can be regenerated and reused was investigated, and the possibility of promoting combustion by effective ingredients in combustion improver combustion ash was confirmed through differential thermal analysis.

4. Synopsis

For each type of regular heavy oil, differential thermal analysis values were taken as indexes of the combustibility of fuels, and the properties of combustion exhaust gas when fuel properties and operating conditions are varied could be forecast at good accuracy by means of polynomials in which each coefficient was determined by quadratic multiple regression analysis. The same method was applied to combustion of decompressed residual oil in test furnace and in practical boiler, and in light of feasibility studies, construction of an operating support system has been scheduled for the future.

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It was revealed that the dust content from burner combustion test of combustion improver, under low concentrations of exhaust gas oxygen, tends to vary from the results of differential thermal analysis, and it became clear that the dust reduction effect when combustion improver is used varies depending on the target fuel and exhaust gas oxygen concentration. For combustion of decompressed residual oil with low oxygen concentration in the exhaust gas, the action mechanisms of combustion improver under conditions of low exhaust gas oxygen concentration must be clarified, and a system effective in reducing dust under the same conditions must be obtained. Moreover, in the combustion of decompressed residual oil in large-scale boiler for power generation, having an electric dust precipitator, combustion improver that can be regenerated and reused is believed to be essential from an economic standpoint. Respecting the two types of combustion improver tested in the current fiscal year, the possibilities for regeneration and reuse could be confirmed. In future, systems more effective in regeneration and reuse will be sought out, and studies of regeneration/reuse systems will be scheduled.

In the regression analysis conducted on test furnace combustion data, nozzle position and combustion chamber length were used as parameters in order to analyze the impact of the initial mixture of fuel and spray and the impact of dust retention time in the furnace; these are characteristic variables of the test furnace. In order to forecast NOx and dust at good accuracy in practical boiler, parameters other than the aforementioned must be newly examined so that the initial mixture of fuel and spray in actual combustion and dust burn up in the furnace can be evaluated properly. In addition, the impact on NOx and dust content of burner type, of differences in furnace format and of differences in combustion scale must be investigated.

If the two indexes of combustibility, namely, residual carbon content and residual carbon combustion rate, can be determined from differential thermal analysis, it will be possible to forecast the dust content in burner combustion by using the burner combustion forecast formula established this fiscal year. However, it is believed that the relationships between fuel properties and these combustibility indexes must be investigated. In the fuel tested this fiscal year, a favorable correlation could be recognized between the residual carbon content from differential thermal analysis and the JIS residual carbon content, which traditionally has been used as an index of fuel combustibility. With respect to combustion rate of residual carbon in differential thermal analysis, however, correlations with asphaltene content or with any other known fuel property could not be recognized. In order to identify the associations with fuel properties, a detailed investigation covering asphaltene structure, etc., is considered mandatory.

These issues will be investigated in R&D from the year 2000.

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