- Email: [email protected]
Gasification and power generation characteristics of woody biomass utilizing a downdraft gasifier
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 2 1 5 e4 2 2 0
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Gasification and power generation characteristics of woody biomass utilizing a downdraft gasifier Young-Il Son, Sang Jun Yoon, Yong Ku Kim, Jae-Goo Lee* Climate Change Technology Research Division, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea
article info
abstract
Article history:
Energy utilization from biomass resources has started to attract public attention as
Received 18 December 2007
a method to reduce CO2 emissions. In this study, the characteristics of syngas production
Received in revised form
from biomass gasification were investigated in a downdraft gasifier that was combined
22 June 2011
with a small gas engine system for power generation. Syngas temperatures from the
Accepted 5 July 2011
gasifier were maintained at a level of 700e1000 C. When the air ratio for gasification was
Available online 30 July 2011
0.3e0.35, the low heating value of syngas was 1100e1200 kcal Nm3 and the cold gas efficiency 69e72%. Tar concentration in raw syngas was around 3.9e4.4 g Nm3. Syngas
Keywords:
combustion in the gas engine after purification showed that HC concentration was below
Gasification
200 ppm, and NOx concentration was below 40 ppm in the exhaust gas. ª 2011 Elsevier Ltd. All rights reserved.
Syngas Biomass Power generation
1.
Introduction
Biomass is a biological material containing energy stored in organic compounds generated by means of photosynthesis. Compared with that of fossil fuel, the energy density of biomass is too small to make it useful as an energy source. In particular, to utilize it industrially, biomass should be converted into an easy to use form. For example, if it is converted by means of liquefaction, gasification, and electrical generation, it can be used in existing equipment. To this end, diverse studies are underway, until now mainly using direct combustion methods. In terms of power generation utilizing biomass by direct combustion, there are many cases in Europe and Japan. But the disadvantage of this method is that treatment facilities must be large, because the efficiency of the steam turbines is enhanced as the scale of power generation becomes larger. For this reason, the actual circumstances are
such that the development of a gasification power generation method having comparatively high efficiency even on a small scale is actively being promoted [1e4]. Since biomass has comparatively high H/C ratio, it can be said that biomass gasification is more convenient than gasification of coal. Moreover, heavy metals, sulfur and nitrogen are present in very small amounts in biomass that the production of SOx and NOx from generated gases is small; and poisoning of the catalyst is not a big problem. It can be said that the biggest problem in commercializing the biomass gasification process is the high cost of collection and transportation of biomass. But, considering that sufficient numbers of coal and waste gasification processes are already operating in various countries, it is projected that the conversion of biomass into practical use will be very promising. Biomass gasification technologies can be provided that the supply problem of biomass is solved to some extent [5]. One of the reasons that
* Corresponding author. Tel.: þ82 42 8603353; fax: þ82 42 8603134 E-mail address: [email protected] (J.-G. Lee). 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.07.008
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gasification has not been easily commercialized, in spite of the many advantages of biomass, is that tar and soot, which are generated as impurities among the syngas generated by the gasification of biomass, stick to pipes and to the heat exchanger, interrupting continuous operation as well as reducing the efficiency of power generation equipment such as gas engines and gas turbines, etc [6,7]. To improve the quality of the syngas generated in the gasification process and to increase carbon conversion, it may be said that it would be better to convert tar into syngas by means of thermo-chemical conversion rather than by the physical elimination of components such as tar or soot, which are difficult to decompose. Therefore, in this experiment, the downdraft gasifier, having a comparatively simple structure and low tar generation volume, was desinged and subjected to experiment. This equipment is similar to the updraft type gasifier except for the point that the gasification agent and syngas flows toward the lower end of the reactor. The generated gas flows toward the lower end of the reactor, and then generates syngas through the reaction with char at about 800-1100 C. The advantages of the downdraft gasifier are approximately 90% or more of tar generated during the gasification can be eliminated, and, in the case of ash removal, the load of the dust collection equipment in the downstream can be reduced since it can be treated together with unreacted carbon in the lower end of the reactor. Moreover, it is because the process is simple like the other fixed-bed gasifier, that the initial investment cost is low, and the mothod used can be classified as a well-known technology. In this study, the characteristics of syngas generated from biomass were investigated by utilizing the downdraft gasifier, and the characteristics of application to a gas engine were investigated for the syngas actually generated.
2.2.
Fig.1 shows a diagram of the gasification equipment system flow used in this study. This gasification system is constructed with a downdraft gasifier, cyclone, scrubber, dust filter, boiler, and engine. Fig. 2 shows the structure of the downdraft gasifier. In this gasification unit, syngas is produced by reaction of biomass with the supplied air as a gasification agent from the top side. The system is constructed with a hopper for supplying biomass to the combustion part. The combustion part is located in the lower end of the upper hopper. Where the biomass to be supplied from the upper hopper is combusted, the air injection part provides air for gasification, which is injected from the side of the upper combustion part. The gas discharge hole is where gas generated by the combustion and the gasification action of the biomass is discharged. The ash treatment system, which is located in the lower end of upper combustion part treats the ash component generated after the reaction of gasification. Biomass falling from the hopper is dried and partially decomposed by the flame of the combustion part, and this material is mixed again in the upper combustion part. By maintaining the flow of biomass and air together from the upper part to the lower part so that less tar is generated, generated tar is induced to decompose in the combustion part at high temperature. Char generated after combustion, and combustion gas of high temperature, are gasified while moving together to the lower part, and combustion gas becomes able to maintain temperature for gasification by means of a heat insulation layer. After the ash component remaining from gasification reaction falls to the lower part through the grate and is cooled with water, it is collected through the ash collection part.
2.3.
2.
Experimental
2.1.
Sample
Wood chip sorted into sizes ranging from 3 to 5 cm for the experiment was used as fuel. The results of proximate and ultimate analysis are shown in Table 1.
Experimental apparatus
Experimental method
In the experimental conditions implemented in this experiment, the feeding rate of wood chip is 40e45 kg h1, and the temperature was maintained at 1000 C on the basis of the combustion part. Average gasification operation conditions are shown in Table 2. the gas generated for analysis was
Table 1 e Proximate, ultimate and higher heating value analysis. Proximate analysis Item Moisture (wt%) Volatile (wt%) Fixed carbon (wt%) Ash (wt%)
Wood chip 21.7 60.9 14.3 3.9
Ultimate analysis C (wt%) H (wt%) O (wt%) N (wt%) S (wt%) HHV (kcal kg1)
46.5 5.8 43.5 0.2 0.1 4130
Fig. 1 e Flow diagram of the downdraft gasification power generation system.
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25 H2 CO CO2 CH4 O2
Gas composition (vol%)
20
15
10
5
0
500
600
700
800
900
Gasification temperature (oC)
3.
Results and discussion
3.1.
Effect of temperature on syngas composition
The inside temperature of the gasification unit was measured in the combustion zones of char and of volatile matter. In this experiment, the temperature was measured at 700e850 C in the char gasification part, and at 800e1000 C in the gasification part where volatile matters is combusted. The temperature in the zone of char combustion was a little lower than the gasification temperature in the combustion zone of volatile matters. Fig. 3 shows the relation between temperature in the middle part of the gasifier and the composition of syngas. As the temperature increases, a trend of increase in the H2 concentration and linear decrease in the CH4 concentration can be seen. Form these results, it can be seen that the reaction of CH4 þ H2O / CO þ 3H2 is dominant. The CO
Table 2 e Average gasification operation conditions. Component Name
Operational Conditions
Wood Chip Feed Rate Air ratio Gasifier Temperature (middle) Gasifier Temperature (bottom) Syngas flow
40e55 kg h1 0.2e0.85 700e850 C 800e1000 C 50e130 Nm3 h1
concentration shows a trend of linear increase as the temperature increases according to the Boudouard reaction (C þ CO2 / 2CO), and the CO2 concentration shows a trend of linear increase up to about 700 C and a trend of decrease after that temperature.
3.2.
Gasifier performance with air ration
The low heating value of syngas and the cold gas efficiency with the change of gasification air ratio are shown in Fig. 4. The gasification air ratio was calculated from the amount of oxygen fed into the gasifier divided by the amount of oxygen required for complete fuel combustion. As the gasification air ratio increased up to 0.35, the low heating value increased. However, when the gasification air ratio reached above 0.5, the syngas heating value decreased gradually. If the gasification air ratio is increased to maintain the temperature of the gasifier, it is thought that parts of the tar and combustible gas are combusted and, thereby, the heating value of syngas decreases. Moreover, the increase of nitrogen dilution amount due to the increased gasification air ratio also acts as a reason for the decreased heating value of the syngas. In this 1400
100 LHV Cold gas efficiency
1200
80
1000
60
800
40
600
20
400 0.2
0.4
0.6
0.8
Cold gas efficiency (%)
collected from the sampling port installed at the exit of the dust filter in the prior stage of the gas engine; this gas underwent pretreatment by cooling, and was then analyzed by using an on-line analyzer (GC, HP6890). Tar for analysis was collected from the sampling port installed at the exit of the gasification unit; this tar was then analyzed in accordance with “The Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases Version 3.3” proposed by J. P. A. Neeft [8].
LHV (kcal/Nm3)
Fig. 2 e The structure of the downdraft gasifier.
Fig. 3 e Effect of temperature at the middle section in the gasifier on the composition of syngas.
0 1.0
Air ratio
Fig. 4 e Effect of gasification air ratio on low heating value of syngas, and cold gas efficiency.
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30
Table 3 e Specification of the gas engine/generator.
1400
Engine 1200
1000 3
20
LHV (kcal/Nm )
Gas composition (vol%)
25
800 15 600
H2 CO CO2 CH4 LHV
10
5
0 0
20
40
60
80
100
120
140
400
Generator
200
0 160
Model Type Number of Cylinders Displacement Bore Stroke Compression Ratio Maximum Power Maximum Torque Output Type Engine speed Voltage Frequency
e e e cc (mm mm) e PS rpm1 kg m rpm1 kW e rpm V Lh1
CD800L OHC 3 796 70 69 10 38/4900 6.3/2200 10 EK2LCT 3600 220 60
Time (min)
Fig. 5 e Syngas composition and low heating value profiles with the elapsed time.
experiment, when the gasification air ratio is around 0.3e0.35, a low heating value of 1100e1200 kcal Nm3 and the cold gas efficiency of 69e72% can be obtained. In Fig. 5, the composition of syngas and, the low heating value following the elapsed time upon operation with a gasification air ratio of 0.32e0.37, are shown. The average concentration of generated syngas is shown to be H2:16.5%, CO:15.9%, CH4:2.1%, and CO2:15.3%, and the average of the low heating value is shown to be 1100 kcal Nm3. It is generally known that the minimum calorific value required in the application of syngas to a gas engine should be above 1000 kcal Nm3 [9]. From the above experimental results, when wood chip of 40e45 kg h1 is supplied, it can be understood that syngas having a low heating value of 1100e1200 kcal Nm3 and flow of around 80-100 Nm3 h1 will be generated.
3.3.
Tar sampling and measurement
Fig.6 shows the tar collection method. This tar collection method is one in which the tar component contained in the gasification producer gas passes through the probe in the heated state of 300e350 C and then undergoes a filtering process. The first collection of tar is done to collect it in the impinger bottle below 20 C; then, the tar was again collected
in the secondly impinger bottle at 15 C. After the tar solution in the impinger bottle is filtered using a filter paper, the tar stuck to the inside wall of the impinger bottle is dissolved with isopropanol, and then the recovered solution is distilled using the evaporator for the distillation of tar. The water tank temperature of the evaporator is elevated slowly from 28 C to a maximum of 50 C. Any substance remaining after distillation is defined as tar. As a result, tar generation volume by this experiment was a level of 3.9e4.4 g Nm3 which is much lower than the level of 10e30% in the case of the other fixed-bed gasifier process. If tar can be eliminated in a reformer or purification process, it might be possible to use this syngas fully in a gas engine.
3.4. Power generation from syngas combustion in gas engine To generate power using the syngas generated in this experimental apparatus as fuel fed into a gas engine, an experiment with only LPG fuel was conducted, by means of a modification of a fuel supply system, which the same dimensions and operating conditions as those that use syngas in the CD800L reciprocating engine, which is basically designed to use LPG fuel. Also, the experiment was conducted by actually supplying syngas generated in the downdraft gasifier into the gas engine generator. The specifications of the gas engine and the generator used in this experiment are shown in Table 3. The relation 3000
Combustion pressure (kPa)
2500
2000
1500
1000 LPG Syngas 500
0 0
1
2
3
4
5
Engine load (kW)
Fig. 6 e Tar collection method.
Fig. 7 e Effect of load on the combustion pressure inside cylinder.
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300
500
30000 HC CO
25000
400
350
200
20000
150
15000
100
10000
50
5000
CO (ppm)
HC (ppm)
o
Exhaust gas temperature ( C)
250
450
LPG Syngas 0
0 0
1
2
3
4
5
Engine load (kW)
300 0
1
2
3
4
5
Fig. 10 e Effect of load on the HC and CO emission in exhaust gas.
Engine load (kW)
Fig. 8 e Effect of load on exhaust gas temperature.
between the combustion pressure and the engine load at the excess air ratio of 1.0w1.1(l) is shown in Fig. 7. The experiment with only LPG fuel showed a higher combustion pressure than was the case when syngas was supplied. In all cases, as the load increased, the combustion pressure also showed a trend of increase. The relation between the temperature of the exhaust gas and the load for the excess air ratio of 1.0e1.1(l) is shown in Fig. 8. In the experiment with only LPG fuel, the exhaust gas discharged after the piston expansion was discharged within the range of 360e400 C: in the case of using syngas, this temperature range was 470e490 C. From these results, it can be seen that the temperature of exhaust gas in the case of using syngas is slightly higher. Such a temperature of the exhaust gas indicates that the utilization of recovered heat, such as that from the production of hot water and preheating of combustion air by using exhaust gas, is also possible in the case of small gas engines.
experiment with only LPG fuel, the NOx emission upon no load was about 250 ppm, but as the load increased, it showed a trend of increased NOx emission. In the case of using syngas, the NOx emission decreased considerably, and showed a level of about 30e40 ppm, although the load was increased. It can be seen that the case for using syngas is excellent also in the point of NOx discharge reduction. The relationship of pollutants (such as HO and CO) emission from syngas fuel with load change is shown in Fig. 10. As the load increased, a trend of increased HC/CO concentration was shown, and it can be seen that the CO emission in the discharged gas was significantly higher compared with the HC emission. It is thought that the CO component contained in the produced gas was not completely combusted and that some part of the CO component was discharged through the exhaust valve as a non-combusted gas.
4.
3.5. Exhaust pollutant emission from syngas combustion The effect of engine load on the NOx emission in exhaust gas at the excess air ratio of 1.0w1.1(l) is shown in Fig. 9. In the
Conclusions
In this study, by using a downdraft gasifier to induce low generation of tar, the characteristics of gasification and power generation for woody biomass were investigated and the following conclusions were obtained.
800
LPG Syngas
NOx (ppm)
600
400
200
0
0
1
2
3
4
5
Engine load (kW)
Fig. 9 e Effect of load on the NOx emission in exhaust gas.
1. The operation was possible at a gasifier temperature of around 1000 C. When the air ratio for gasification was 0.3e0.35, it is possible to obtain a low heating value of 1100e1200 kcal Nm3 and a cold gas efficiency of 69e72%. 2. Compared with the tar generation volume of around 10e30% generated by the other type of fixed-bed gasifier, the tar for the proposed downdraft gasifier applied in this study was shown to be at a level of 3.9e4.4 g Nm3. Therefore, this gasifier can enhance gasification conversion performance. 3. As a result of having conducted a gas engine power generation test using syngas at a low heating value of 1100e1200 kcal Nm3 in this study, it has been confirmed that stable power generation can be done, and that an HC emission below 200 ppm, and a NOx emission below 40 ppm, can be achieved.
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Acknowledgements This work was financially supported by the Research Cooperating Program for Agricultural Science & Technology Development, Rural Development Administration (RDA) in Korea
references
[1] VTT Technical Research Centre of Finland, Review of finish biomass gasification technologies, OPET Report 4; 2002. [2] Sugiyama S. Performance of high temperature air blown wood biomass gasification and power generation. Trans Jpn Soc Mech Engineers Ser B 2005;71:268e74.
[3] Yoshikawa K. R&D (Research and Development) on distributed power generation from solid fuels. Energy 2006;31: 1656e65. [4] Morris M, Waldheim L. Energy recovery from solid waste fuels using advanced gasification technology. Waste Manage 1998; 18:557e64. [5] Knoef HAM. Handbook biomass gasification. Gasnet; 2005. [6] Son Yi. A Study on measurement of the light tar content in the fuel gas produced from small-scale gasification and power generation systems. Proceedings of the15th Annual Meeting of the Japan Institute of Energy. Tokyo: Japan; 2006 August 3-4. [7] Thomas BR, Agua D. Handbook of biomass downdraft gasifier engine system. The Biomass Energy Foundation Press; 1998. [8] Neeft JPA. Guideline for sampling and analysis of tar and particles in biomass producer gases: version3.3; 1999-2002. [9] Quakk P, Knoef H, Stassen H. Energy from biomass: a review of combustion and gasification technology. World Bank TechPaper, No.422; .
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Gasification and power generation characteristics of woody biomass utilizing a downdraft gasifier Young-Il Son, Sang Jun Yoon, Yong Ku Kim, Jae-Goo Lee* Climate Change Technology Research Division, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea
article info
abstract
Article history:
Energy utilization from biomass resources has started to attract public attention as
Received 18 December 2007
a method to reduce CO2 emissions. In this study, the characteristics of syngas production
Received in revised form
from biomass gasification were investigated in a downdraft gasifier that was combined
22 June 2011
with a small gas engine system for power generation. Syngas temperatures from the
Accepted 5 July 2011
gasifier were maintained at a level of 700e1000 C. When the air ratio for gasification was
Available online 30 July 2011
0.3e0.35, the low heating value of syngas was 1100e1200 kcal Nm3 and the cold gas efficiency 69e72%. Tar concentration in raw syngas was around 3.9e4.4 g Nm3. Syngas
Keywords:
combustion in the gas engine after purification showed that HC concentration was below
Gasification
200 ppm, and NOx concentration was below 40 ppm in the exhaust gas. ª 2011 Elsevier Ltd. All rights reserved.
Syngas Biomass Power generation
1.
Introduction
Biomass is a biological material containing energy stored in organic compounds generated by means of photosynthesis. Compared with that of fossil fuel, the energy density of biomass is too small to make it useful as an energy source. In particular, to utilize it industrially, biomass should be converted into an easy to use form. For example, if it is converted by means of liquefaction, gasification, and electrical generation, it can be used in existing equipment. To this end, diverse studies are underway, until now mainly using direct combustion methods. In terms of power generation utilizing biomass by direct combustion, there are many cases in Europe and Japan. But the disadvantage of this method is that treatment facilities must be large, because the efficiency of the steam turbines is enhanced as the scale of power generation becomes larger. For this reason, the actual circumstances are
such that the development of a gasification power generation method having comparatively high efficiency even on a small scale is actively being promoted [1e4]. Since biomass has comparatively high H/C ratio, it can be said that biomass gasification is more convenient than gasification of coal. Moreover, heavy metals, sulfur and nitrogen are present in very small amounts in biomass that the production of SOx and NOx from generated gases is small; and poisoning of the catalyst is not a big problem. It can be said that the biggest problem in commercializing the biomass gasification process is the high cost of collection and transportation of biomass. But, considering that sufficient numbers of coal and waste gasification processes are already operating in various countries, it is projected that the conversion of biomass into practical use will be very promising. Biomass gasification technologies can be provided that the supply problem of biomass is solved to some extent [5]. One of the reasons that
* Corresponding author. Tel.: þ82 42 8603353; fax: þ82 42 8603134 E-mail address: [email protected] (J.-G. Lee). 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.07.008
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gasification has not been easily commercialized, in spite of the many advantages of biomass, is that tar and soot, which are generated as impurities among the syngas generated by the gasification of biomass, stick to pipes and to the heat exchanger, interrupting continuous operation as well as reducing the efficiency of power generation equipment such as gas engines and gas turbines, etc [6,7]. To improve the quality of the syngas generated in the gasification process and to increase carbon conversion, it may be said that it would be better to convert tar into syngas by means of thermo-chemical conversion rather than by the physical elimination of components such as tar or soot, which are difficult to decompose. Therefore, in this experiment, the downdraft gasifier, having a comparatively simple structure and low tar generation volume, was desinged and subjected to experiment. This equipment is similar to the updraft type gasifier except for the point that the gasification agent and syngas flows toward the lower end of the reactor. The generated gas flows toward the lower end of the reactor, and then generates syngas through the reaction with char at about 800-1100 C. The advantages of the downdraft gasifier are approximately 90% or more of tar generated during the gasification can be eliminated, and, in the case of ash removal, the load of the dust collection equipment in the downstream can be reduced since it can be treated together with unreacted carbon in the lower end of the reactor. Moreover, it is because the process is simple like the other fixed-bed gasifier, that the initial investment cost is low, and the mothod used can be classified as a well-known technology. In this study, the characteristics of syngas generated from biomass were investigated by utilizing the downdraft gasifier, and the characteristics of application to a gas engine were investigated for the syngas actually generated.
2.2.
Fig.1 shows a diagram of the gasification equipment system flow used in this study. This gasification system is constructed with a downdraft gasifier, cyclone, scrubber, dust filter, boiler, and engine. Fig. 2 shows the structure of the downdraft gasifier. In this gasification unit, syngas is produced by reaction of biomass with the supplied air as a gasification agent from the top side. The system is constructed with a hopper for supplying biomass to the combustion part. The combustion part is located in the lower end of the upper hopper. Where the biomass to be supplied from the upper hopper is combusted, the air injection part provides air for gasification, which is injected from the side of the upper combustion part. The gas discharge hole is where gas generated by the combustion and the gasification action of the biomass is discharged. The ash treatment system, which is located in the lower end of upper combustion part treats the ash component generated after the reaction of gasification. Biomass falling from the hopper is dried and partially decomposed by the flame of the combustion part, and this material is mixed again in the upper combustion part. By maintaining the flow of biomass and air together from the upper part to the lower part so that less tar is generated, generated tar is induced to decompose in the combustion part at high temperature. Char generated after combustion, and combustion gas of high temperature, are gasified while moving together to the lower part, and combustion gas becomes able to maintain temperature for gasification by means of a heat insulation layer. After the ash component remaining from gasification reaction falls to the lower part through the grate and is cooled with water, it is collected through the ash collection part.
2.3.
2.
Experimental
2.1.
Sample
Wood chip sorted into sizes ranging from 3 to 5 cm for the experiment was used as fuel. The results of proximate and ultimate analysis are shown in Table 1.
Experimental apparatus
Experimental method
In the experimental conditions implemented in this experiment, the feeding rate of wood chip is 40e45 kg h1, and the temperature was maintained at 1000 C on the basis of the combustion part. Average gasification operation conditions are shown in Table 2. the gas generated for analysis was
Table 1 e Proximate, ultimate and higher heating value analysis. Proximate analysis Item Moisture (wt%) Volatile (wt%) Fixed carbon (wt%) Ash (wt%)
Wood chip 21.7 60.9 14.3 3.9
Ultimate analysis C (wt%) H (wt%) O (wt%) N (wt%) S (wt%) HHV (kcal kg1)
46.5 5.8 43.5 0.2 0.1 4130
Fig. 1 e Flow diagram of the downdraft gasification power generation system.
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25 H2 CO CO2 CH4 O2
Gas composition (vol%)
20
15
10
5
0
500
600
700
800
900
Gasification temperature (oC)
3.
Results and discussion
3.1.
Effect of temperature on syngas composition
The inside temperature of the gasification unit was measured in the combustion zones of char and of volatile matter. In this experiment, the temperature was measured at 700e850 C in the char gasification part, and at 800e1000 C in the gasification part where volatile matters is combusted. The temperature in the zone of char combustion was a little lower than the gasification temperature in the combustion zone of volatile matters. Fig. 3 shows the relation between temperature in the middle part of the gasifier and the composition of syngas. As the temperature increases, a trend of increase in the H2 concentration and linear decrease in the CH4 concentration can be seen. Form these results, it can be seen that the reaction of CH4 þ H2O / CO þ 3H2 is dominant. The CO
Table 2 e Average gasification operation conditions. Component Name
Operational Conditions
Wood Chip Feed Rate Air ratio Gasifier Temperature (middle) Gasifier Temperature (bottom) Syngas flow
40e55 kg h1 0.2e0.85 700e850 C 800e1000 C 50e130 Nm3 h1
concentration shows a trend of linear increase as the temperature increases according to the Boudouard reaction (C þ CO2 / 2CO), and the CO2 concentration shows a trend of linear increase up to about 700 C and a trend of decrease after that temperature.
3.2.
Gasifier performance with air ration
The low heating value of syngas and the cold gas efficiency with the change of gasification air ratio are shown in Fig. 4. The gasification air ratio was calculated from the amount of oxygen fed into the gasifier divided by the amount of oxygen required for complete fuel combustion. As the gasification air ratio increased up to 0.35, the low heating value increased. However, when the gasification air ratio reached above 0.5, the syngas heating value decreased gradually. If the gasification air ratio is increased to maintain the temperature of the gasifier, it is thought that parts of the tar and combustible gas are combusted and, thereby, the heating value of syngas decreases. Moreover, the increase of nitrogen dilution amount due to the increased gasification air ratio also acts as a reason for the decreased heating value of the syngas. In this 1400
100 LHV Cold gas efficiency
1200
80
1000
60
800
40
600
20
400 0.2
0.4
0.6
0.8
Cold gas efficiency (%)
collected from the sampling port installed at the exit of the dust filter in the prior stage of the gas engine; this gas underwent pretreatment by cooling, and was then analyzed by using an on-line analyzer (GC, HP6890). Tar for analysis was collected from the sampling port installed at the exit of the gasification unit; this tar was then analyzed in accordance with “The Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases Version 3.3” proposed by J. P. A. Neeft [8].
LHV (kcal/Nm3)
Fig. 2 e The structure of the downdraft gasifier.
Fig. 3 e Effect of temperature at the middle section in the gasifier on the composition of syngas.
0 1.0
Air ratio
Fig. 4 e Effect of gasification air ratio on low heating value of syngas, and cold gas efficiency.
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30
Table 3 e Specification of the gas engine/generator.
1400
Engine 1200
1000 3
20
LHV (kcal/Nm )
Gas composition (vol%)
25
800 15 600
H2 CO CO2 CH4 LHV
10
5
0 0
20
40
60
80
100
120
140
400
Generator
200
0 160
Model Type Number of Cylinders Displacement Bore Stroke Compression Ratio Maximum Power Maximum Torque Output Type Engine speed Voltage Frequency
e e e cc (mm mm) e PS rpm1 kg m rpm1 kW e rpm V Lh1
CD800L OHC 3 796 70 69 10 38/4900 6.3/2200 10 EK2LCT 3600 220 60
Time (min)
Fig. 5 e Syngas composition and low heating value profiles with the elapsed time.
experiment, when the gasification air ratio is around 0.3e0.35, a low heating value of 1100e1200 kcal Nm3 and the cold gas efficiency of 69e72% can be obtained. In Fig. 5, the composition of syngas and, the low heating value following the elapsed time upon operation with a gasification air ratio of 0.32e0.37, are shown. The average concentration of generated syngas is shown to be H2:16.5%, CO:15.9%, CH4:2.1%, and CO2:15.3%, and the average of the low heating value is shown to be 1100 kcal Nm3. It is generally known that the minimum calorific value required in the application of syngas to a gas engine should be above 1000 kcal Nm3 [9]. From the above experimental results, when wood chip of 40e45 kg h1 is supplied, it can be understood that syngas having a low heating value of 1100e1200 kcal Nm3 and flow of around 80-100 Nm3 h1 will be generated.
3.3.
Tar sampling and measurement
Fig.6 shows the tar collection method. This tar collection method is one in which the tar component contained in the gasification producer gas passes through the probe in the heated state of 300e350 C and then undergoes a filtering process. The first collection of tar is done to collect it in the impinger bottle below 20 C; then, the tar was again collected
in the secondly impinger bottle at 15 C. After the tar solution in the impinger bottle is filtered using a filter paper, the tar stuck to the inside wall of the impinger bottle is dissolved with isopropanol, and then the recovered solution is distilled using the evaporator for the distillation of tar. The water tank temperature of the evaporator is elevated slowly from 28 C to a maximum of 50 C. Any substance remaining after distillation is defined as tar. As a result, tar generation volume by this experiment was a level of 3.9e4.4 g Nm3 which is much lower than the level of 10e30% in the case of the other fixed-bed gasifier process. If tar can be eliminated in a reformer or purification process, it might be possible to use this syngas fully in a gas engine.
3.4. Power generation from syngas combustion in gas engine To generate power using the syngas generated in this experimental apparatus as fuel fed into a gas engine, an experiment with only LPG fuel was conducted, by means of a modification of a fuel supply system, which the same dimensions and operating conditions as those that use syngas in the CD800L reciprocating engine, which is basically designed to use LPG fuel. Also, the experiment was conducted by actually supplying syngas generated in the downdraft gasifier into the gas engine generator. The specifications of the gas engine and the generator used in this experiment are shown in Table 3. The relation 3000
Combustion pressure (kPa)
2500
2000
1500
1000 LPG Syngas 500
0 0
1
2
3
4
5
Engine load (kW)
Fig. 6 e Tar collection method.
Fig. 7 e Effect of load on the combustion pressure inside cylinder.
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300
500
30000 HC CO
25000
400
350
200
20000
150
15000
100
10000
50
5000
CO (ppm)
HC (ppm)
o
Exhaust gas temperature ( C)
250
450
LPG Syngas 0
0 0
1
2
3
4
5
Engine load (kW)
300 0
1
2
3
4
5
Fig. 10 e Effect of load on the HC and CO emission in exhaust gas.
Engine load (kW)
Fig. 8 e Effect of load on exhaust gas temperature.
between the combustion pressure and the engine load at the excess air ratio of 1.0w1.1(l) is shown in Fig. 7. The experiment with only LPG fuel showed a higher combustion pressure than was the case when syngas was supplied. In all cases, as the load increased, the combustion pressure also showed a trend of increase. The relation between the temperature of the exhaust gas and the load for the excess air ratio of 1.0e1.1(l) is shown in Fig. 8. In the experiment with only LPG fuel, the exhaust gas discharged after the piston expansion was discharged within the range of 360e400 C: in the case of using syngas, this temperature range was 470e490 C. From these results, it can be seen that the temperature of exhaust gas in the case of using syngas is slightly higher. Such a temperature of the exhaust gas indicates that the utilization of recovered heat, such as that from the production of hot water and preheating of combustion air by using exhaust gas, is also possible in the case of small gas engines.
experiment with only LPG fuel, the NOx emission upon no load was about 250 ppm, but as the load increased, it showed a trend of increased NOx emission. In the case of using syngas, the NOx emission decreased considerably, and showed a level of about 30e40 ppm, although the load was increased. It can be seen that the case for using syngas is excellent also in the point of NOx discharge reduction. The relationship of pollutants (such as HO and CO) emission from syngas fuel with load change is shown in Fig. 10. As the load increased, a trend of increased HC/CO concentration was shown, and it can be seen that the CO emission in the discharged gas was significantly higher compared with the HC emission. It is thought that the CO component contained in the produced gas was not completely combusted and that some part of the CO component was discharged through the exhaust valve as a non-combusted gas.
4.
3.5. Exhaust pollutant emission from syngas combustion The effect of engine load on the NOx emission in exhaust gas at the excess air ratio of 1.0w1.1(l) is shown in Fig. 9. In the
Conclusions
In this study, by using a downdraft gasifier to induce low generation of tar, the characteristics of gasification and power generation for woody biomass were investigated and the following conclusions were obtained.
800
LPG Syngas
NOx (ppm)
600
400
200
0
0
1
2
3
4
5
Engine load (kW)
Fig. 9 e Effect of load on the NOx emission in exhaust gas.
1. The operation was possible at a gasifier temperature of around 1000 C. When the air ratio for gasification was 0.3e0.35, it is possible to obtain a low heating value of 1100e1200 kcal Nm3 and a cold gas efficiency of 69e72%. 2. Compared with the tar generation volume of around 10e30% generated by the other type of fixed-bed gasifier, the tar for the proposed downdraft gasifier applied in this study was shown to be at a level of 3.9e4.4 g Nm3. Therefore, this gasifier can enhance gasification conversion performance. 3. As a result of having conducted a gas engine power generation test using syngas at a low heating value of 1100e1200 kcal Nm3 in this study, it has been confirmed that stable power generation can be done, and that an HC emission below 200 ppm, and a NOx emission below 40 ppm, can be achieved.
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Acknowledgements This work was financially supported by the Research Cooperating Program for Agricultural Science & Technology Development, Rural Development Administration (RDA) in Korea
references
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[3] Yoshikawa K. R&D (Research and Development) on distributed power generation from solid fuels. Energy 2006;31: 1656e65. [4] Morris M, Waldheim L. Energy recovery from solid waste fuels using advanced gasification technology. Waste Manage 1998; 18:557e64. [5] Knoef HAM. Handbook biomass gasification. Gasnet; 2005. [6] Son Yi. A Study on measurement of the light tar content in the fuel gas produced from small-scale gasification and power generation systems. Proceedings of the15th Annual Meeting of the Japan Institute of Energy. Tokyo: Japan; 2006 August 3-4. [7] Thomas BR, Agua D. Handbook of biomass downdraft gasifier engine system. The Biomass Energy Foundation Press; 1998. [8] Neeft JPA. Guideline for sampling and analysis of tar and particles in biomass producer gases: version3.3; 1999-2002. [9] Quakk P, Knoef H, Stassen H. Energy from biomass: a review of combustion and gasification technology. World Bank TechPaper, No.422; .