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Development of a Modified Biomass Updraft Fixed Bed Gasifier

with an Embedded Combustor

Lin, Jeng-Chyan Muti, Chen, Hong-Cheng & Huang, Jian-Yao Research Institute of Information and Electrical Energy

National Chinyi Institute of Technology, Taiwan

Abstract To more efficiently convert solid biomass into useful heat and power, this study develops a modified updraft fixed bed gasifier with an embedded combustor. The unique feature of the modified updraft fixed bed gasifier is the embedded combustor inside the gasifier. The inserted tube has numerous small holes on its lower wall, which allow the entrainment of the syngas generated by the gasifier. By introducing the secondary combustion air, the syngas is combusted fully inside the combustor without any assisting fuel and without any de-tar procedure. The flue gas generated by the current design is high in temperature but low in pollutions. The preliminary study shows the newly developed system possesses good flexibility in using different solid biomass and the system also demonstrated very stable and reliable performance.

Keywords: Biomass, Updraft, Fixed Bed, Gasifier, Renewable Energy

Lin, Jeng-Chyan Muti Assistant professor Research Institute of Information and Electrical Energy, National Chinyi Institute of Technology, Taiwan 35, Lane 215, Sec. 1, Chung-Shan Rd., Taiping, Taichung, 411, Taiwan 04-23924505ext6112, mutilin@mail.ncit.edu.tw

Introduction

Taiwan lacks natural energy resources. According to Taiwan Energy Bureau’s reports Taiwan

imports more than 97% of primary energy from other countries annually [1]. Taiwan is also a small, densely populated island, located in a subtropical zone with seventy three percent of land either

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mountainous or hilly [2]. Due to geological location Taiwan’s climate is full of high temperatures and heavy rainfalls. Taiwan is therefore very suitable for biomass growing.

Exacerbating the current energy situation in Taiwan is the globally dwindling of fossil energy stocks and the increasing international pressure on the global green house gas reduction. For the sake of national security and international responsibility, Taiwan government has been very aggressively pursue the renewable energy plan, which will lead to concerted efforts by all levels of the government as well as the general public to develop renewable energy and to aggressively adopt its use.

Biomass, as renewable, abundant, and domestic energy resources, represents an important but underexploited resource in Taiwan. In fact if we harvest all crops for energy on the island, the amount of energy available (at 100% conversion efficiency) is about to equal to ten times of the annual energy used in Taiwan. Furthermore, biomass when renewed can absorb carbon dioxide and is regarded as a carbon neutral energy resource. Biomass is therefore considered useful in mitigating global warming and is advocated by IPCC reports and at COP3.

In order to integrate and coordinate the tasks of promoting renewable energy, the Executive Yuan of Taiwan adopted the "Renewable Energy Development Plan" on January 17, 2002. The Council for Economic Planning and Development will be in charge of coordinating the efforts of related sectors in promoting renewables including biomass. Furthermore, a "Renewable Energy Development Bill" will be proposed to establish a legal environment for renewable energy and to facilitate the sustainable

utilization of renewable energy. [3] Biomass is converted to energy by applying biochemical processes (anaerobic digestion,

microbial digestion, acid hydrolysis) or taking thermochemical routes (combustion, pyrolysis, gasification, liquefaction.) Agricultural biomass is mainly utilized for energy purposes by thermochemical combustion or gasification processes. Depending on the air/fuel ratio any process can result in either combustion or gasification. Considering the biomass has the composition of cellulose (C6H10O5), gasification and combustion of the biomass can be represented through the following chemical reactions:

C6H10O5 + 0.5 O2 → 6CO +5 H2 + 1.85 MJ/kg heat (Gasification Process) (1) C6H10O5 + 6 O2 → 6CO2 +5 H2O + 17.5 MJ/kg heat (Combustion Process) (2)

Combustion releases much higher amounts of heat than gasification but as equation (2) indicated gasification process can generate combustible gases, which could be combusted more efficiently by using low excess air and lead to low levels of contaminants. Therefore, gasification has received

extensive attentions in the biomass energy utilization research recently [4]. Gasification technologies are increasingly being developed to provide environmentally clean and

efficient power generation from a range of fuels such as coal, biomass and oil residues. They can be

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used in IGCC, which can provide potentially high efficiencies and low environmental emissions. The main reaction pathways during gasification are:

Gasification with O2/air (partial combustion) C + 1/2O2 → CO

↓ Combustion with oxygen (air)

C + O2 → CO2 ↓

Gasification with CO2 (Boudouard reaction) C + CO2 → 2CO

↓ Gasification with steam (water gas or water-shift reaction)

C + H2O ↔ CO + H2 CO + H2O ↔ CO2 + H2

C + 1/2H2O + 1/2H2 ↔ 1/2CH4 + 1/2CO There are two major classification of biomass gasifier: fixed bed and fluidized bed. Differentiation is based on the means of supporting the biomass in the reactor vessel. Fixed bed gasifier is widely adopted for small-scale capacity (less than 1 MWth) situation. On the other hand, fluidized bed is usually developed for large-scale application. Since transporting fee of the biomass is high in biomass energy utilization, biomass power and heat utilization plant is usually localized and small in scale. With this geological limitation in mind, the current study focuses on the fixed bed biomass gasifier. According to the feeding direction of flow of both the biomass and oxidant into the reactor, the fixed bed gasifier is classified into updraft, downdraft, and cross draft types. The schematic diagrams of the three types of gasifiers are shown in Figure 1~3. Characteristics of different fixed bed gasifier types are summarized in Table 1.

Updraft fixed bed gasifier is the oldest form of gasifier and is still used for coal and biomass gasification. The Biomass is fed at the top of the reactor and move downwards as a result of the conversion of the biomass and the removal of ashes through a grate at the bottom of the reactor. The air intake is at the bottom and the gas leaves at the top. Air or oxygen and/or steam are introduced below the grate and diffuse up through the bed of biomass and char. Complete combustion of char takes place at the bottom of the bed, liberating CO2 and H2O. These hot gases (~1000℃) pass through the bed above, where they are reduced to H2 and CO and cooled to 750℃. Continuing up the

reactor, the reducing gases (H2 and CO) pyrolyze the descending dry biomass and finally dry the incoming wet biomass, leaving the reactor at low temperature (~500℃) [4, 5, 6]. Updraft gasifier is

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also called as counterflow gasifier.

Feed

Syngas

Drying zone

Pyrolysis zone

Combustion zone

Reduction zone

Ash

Air Grate

Figure 1. Updraft fixed bed gasifier

Feed

Syngas

Drying zone

Pyrolysis zone

Combustion zone

Reduction zone

Ash pit

AirGrate

Air

Figure 2. Downdraft fixed bed gasifier

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Feed

Syngas

Drying zone

Pyrolysis zone

Combustion zone

Reduction zone

Ash pit

Grate

Air

Char burning

Figure 3. Crossdraft fixed bed gasifier

Table1. Characteristics of different gasifier types

Down draft Up draft Open core Fuel (wood) - moist. Cont. (% wet basis) - ash content ( % dry basis) - size (mm)

12 (max. 25)0.5 (max. 6)

20 -100

43 (max. 60)1.4 (max. 25)

5 - 100

7 –15 (max. 15)1 – 2 (max. 20)

1 - 5 Gas exit temp (℃) 700 200 - 400 250 - 500

Tars (g/Nm3) 0.015 – 0.5 30 - 150 2 - 10 Sensitivity to load fluctuations Sensitive Not sensitive Not sensitive

Turn down ratio 3 - 4 5 - 10 5 - 10 Producer gas LHV (kJ/Nm3) 4.5 – 5.0 5.0 – 6.0 5.5 – 6.0

The advantages of the updraft fixed bed gasifier are its simplicity, high charcoal bun-out and internal heat exchange leading to low gas exit temperatures and high gasification efficiencies. Because of the internal heat exchange the fuel is dried in the top of the gasifier and therefore fuels with high moisture content (up to 60% by weight) can be used. Updraft gasifiers are also flexible in the size variation of the biomass feedstock. The major drawbacks of the updraft fixed bed gasifier are the syngas contains 10 to 20% tar by weight, which requires extensive syngas cleanup before engine, turbine or synthesis applications. However, when the tar is taken out from the syngas, at least 50% of energy in the syngas is lost.

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In order to take advantage of updraft fixed bed gasifier’s high gasification efficiency while avoiding the troublesome tar cleaning process, the current study attempts to combust the biomass generated syngas inside the reactor directly. The heat derived from the direct combustion of the syngas can be used to generate steam in a boiler or produce power via a Stirling type engine. Regardless of heat or power application, the flue gas resulted from the syngas combustion should be high in temperature and low in pollution to be useful.

Design of a Modified Updraft Fixed Bed Gasifier with Embedded Combustor

Gasification of solid biomass can produce syngas fuel with lower carbon content. Gasification

approach is therefore considered more environmentally friendly than the direct combustion or incineration approach in converting solid biomass into energy. Nevertheless, all gasification methods produce highly viscous and very acid tars. The produced tars are hard to transport and cause serious clogging and corrosion of the piping. The syngas is therefore very difficult to be stored or utilized without removing tars. Complex de-tar system becomes necessary component for any type of gasification system. Water spraying is the most common approach to remove the tar from the syngas. Removal of tar by water spraying will inadvertently produce highly polluted water and most importantly will lose more than 50% of energy contained in the syngas.

Since wet approach of tar removal is environmentally unfriendly, recently the industry has

adopted various dry methods to clean the syngas in the modern gasification research [7, 8, 9]. Although the dry approach shows promises in reducing environmental impact, economical and reliable dry and high temperature clean up technology has not yet commercialized. This study design a modified updraft fixed bed gasifier which combusts the syngas inside the reactor and avoids the whole de-tar process completely.

The schematic diagram of the modified fixed bed gasifier proposed by the current study is shown in the Figure 4. Figure 4 shows that biomass is fed at the top of the cylindrical reactor, and a grate at the bottom of the reactor supports the reacting bed. Gasification air is introduced racially through numerous small holes on a horizontal ring that is located on top of the grate along the gasifier’s inside wall. The syngas produced is forced to squeeze in an embedded tube through many small openings on the lower half of the tube. The embedded tube inside the updraft gasifier serves two purposes: a conduit of the syngas to exit the reactor and a syngas combustor by introducing secondary combustion air to fully oxidize the syngas and release clean and hot flue gas for heat and power application.

The biomass feed moves in counter current to the gas flow, and passes through the drying zone, the pyrolysis zone, the reduction zone and the combustion zone. In the drying zone the biomass is dried. In the pyrolysis zone the biomass is decomposed in volatile gases and solid char. The heat for pyrolization and drying is mainly delivered by the upward flowing syngas and partly by radiation from the combustion zone. In the reduction zone many reactions occur involving char, carbon

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dioxide and water vapor, in which carbon is converted and carbon monoxide and hydrogen are produced as the main constituents of the syngas. In the combustion zone the remaining char is combusted providing the heat, the carbon dioxide and water vapor for the reactions involved in the reduction zone.

Feed

Drying zone

Pyrolysis zone

Combustion zone

Reduction zone

Ash

Pyrolysis zone

GasificationAir

Primary GasificationAir

Syngas Flue GasSecondaryCombustionAir

Figure 4. The schematic diagram of the modified fixed bed gasifier

The embedded combustor is a unique feature of the current modification work. The combustor

provides a space to directly combust the syngas inside the gasifier. While intensive combustion takes place in the tube, high temperature is resulted and maintained in the gasifier’s pyrolysis zones as shown in Figure 4. The creation of a high temperature pyrolysis zone certainly would increase the overall gasification intensity, and also will result in a much more stable gasification process. A 150 kWth experimental gasifier was constructed in this study. Assuming the gasification

intensity is at 100 kg/hr-m2 and biomass’s low heating value at 3500 kcal/kg [10]. The bed area could be calculated to be 0.465 m2 with a biomass to energy conversion efficiency at 80%. The gasifier volume is designed to hold enough biomass to run for at least 2 hours without refueling. Located at a distance of 47.5 cm above the grate, the embedded combustor is fixed and sealed in the gasifier with flanges. The diameter of the combustion tube is 16 cm with numerous tiny openings at the lower half of the tube. A schematic diagram of the embedded combustor is shown in Figure 5.

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CombustionAir

High TemperatureFlue Gas(T>1200℃)

Syngas Figure5. Schematic diagram of the embedded combustor

Figure6 is the snapshot of the constructed updraft fixed bed gasifier with an embedded combustor. The gasification air and secondary combustion air are provided by a 5 Hp high pressure blower manufactured by North America Inc. This blower could generate 35 m3/min airflow rates at pressure 350 mmAq. The high-pressure air is a must in this gasifier with embedded combustor combination for overcoming the high resistance existed throughout the gasifier.

Process Flow Description and Experimental Setup

The operation process of the current gasifier is shown in Figure 7. Starting the gasifier by igniting a small quantity of biomass outside the gasifier and then throwing in the combusting biomass and turn on the gasification air to complete the ignition step. The second step is to feed gasifier the biomass from the top inlet until the gasifier is full of the biomass and then close the top lid to start the gasification step. In less than 15 minutes or when the syngas temperature reaches about 300℃ in the

combustor, air is introduced into the inlet of the combustor and the syngas will self ignites and combusts in the combustor. The biomass flows smoothly toward the grate and the combustor can maintain its stable combustion of the syngas without needing any assisting fuel throughout the whole process.

Experimental setup to measure the gasifier’s performance is shown in Figure 8. Temperatures in the gasifier’s combustion zone, reduction zone, pyrolysis zone and drying zone are acquired by inserting K-type thermocouples into the respective zone as shown in Figure 8. In the embedded combustor’s outlet, flue gas’s temperature is measured by a R-type thermocouple and flue gas or syngas compositions are measured by Eurotron GreenLine 8000 mobile flue gas analyzer. The acquired temperature and composition data are collected, signal transformed, and rerouted to a monitoring server through near and far field ICPs. A monitoring system written by Labview is developed and used to view and analyze the measured data.

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Figure6. Snapshot of the constructed updraft fixed bed gasifier with an embedded combustor

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Ignition

Loading Biomass

Gasification

Gasification & Syngas Combustion

Biomass

Syngas

Combustion Air

GasificationAir

GasificationAir

GasificationAir

GasificationAir `

Figure7. Operation Process of the Gasifier

Feed

Drying zonePyrolysis zone

Combustion zone

Reduction zone

Ash

Pyrolysis zone

Gasification AirPrimary GasificationAir

SyngasSecondaryCombustionAir TC5

TC3TC2

TC1

Flue Gas / Syngas

Data Picking and Calculating

Monitor Output

Data Storing

History Data

LabVIEW Monitoring System

ICP Monitoring PC

TC4 Flue Gas Analyzer

Figure8. Schematic of Experimental Setup

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Results and Discussions

1. Performance test of the gasifier using three distinct solid biomass Rice husks, Acacia confusa Merr twigs and pine chips are used to test the performance of the

gasifier. The current modified updraft gasifier performs well with all three solid biomass. High heating value syngas is generated in the gasifier and the syngas is burned fully in the embedded combustor without any assisting fuel. The resulting flue gas is clean and its temperature is high. The residues in the ash pit contain very little combustibles and this demonstrates the high conversion rate of the gasifier. Figure 9, Figure 10 and Figure 11 are the three biomass used and their resulted ashes. The size of the biomass used ranges from rice husk’s 5mm to pine chip’s 25cm. Moisture content as high as 55% by weight shows no difficulty in operating the gasifier. The flexibility of the current gasifier in terms of selecting the size and moisture content of the biomass is certainly very high.

2. Detailed measurement of the gasifier by using pine chips Detailed measurement is presented here by using pine chips as the feedstock. The low heating value of the pine chip is 4100 kcal/kg and moisture content is 18% by weight. Monitoring locations are indicated in Figure 8.

Figure 9. Rice husks and ashes Figure 10. Acacia confusa Merr twigs and ashes

Figure 11. Pine Chips and ashes

2.1 Temperature Distribution Inside Gasifier Time history of the temperature distributions in gasifier’s combustion zone, reduction zone, pyrolysis zone and drying zone as indicated in Figure 8 is shown in Figure 12.

2.2 Syngas Analysis The syngas generated by the updraft fixed bed gasifier is sampled at the embedded tube exit as indicated in Figure 8. The syngas sample is filtered, cooled and drained before it is analyzed by a gas

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chromatograph for its gas composition. The gas composition of the resulted syngas is tabulated in Table 2. The syngas heating value without tars is calculated to be 1197kcal/Nm3 according to the gas compositions in Table 2.

0

200

400

600

800

1000

1200

0 50 100 150 200

Operation Time (Minutes)

Temperature (℃)

Combustion Zone

Reduction Zone

Pyrolysis Zone

Drying Zone

Figure 12. Temperature distribution inside gasifier

Table 2. Syngas Compositions

Syngas Components

H2 O2 N2 CO CH4 CO2

Contents (Vol%) 14.1 0.055 52.9 18.5 2.94 11.5

2.3 Flue Gas Analysis of the Embedded Combustor When secondary combustion air is introduced to the embedded combustor to mix with the entrained syngas with high temperature (more than 500℃) from the gasifier. An intensive flame will

be established and maintained in the tube. Figure 13 is a snapshot of the combustion taking place in the embedded combustor. Time history of the represented flame temperature or flue gas temperature of the tube is recorded in Figure 14. As Figure 14 indicated that this gasifier and embedded combustor combination is capable of producing intensive combustion with flue gas temperature as high as 1320℃. The flue gas compositions are measured by the Eurotron GreenLine 8000 mobile

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flue gas analyzer and are shown in Figure 15. The flue gas composition measurement shows that the flue gas contains about 0.05% of CO, 5% of CO2 and 0.2% of O2 by volume. From the measurement results it is clear that the current design is capable of producing clean and very hot flue gas. The stable flame temperature and flue gas compositions as indicated in Figure 14 & 15 also show the stability and reliability of the current design.

2.4 Overall performance of the modified updraft fixed bed gasifier Gasification intensity (kg/hr-m2), which is defined as the biomass consumption rate per unit

gasifier bed area, is used to characterize the overall performance of a fixed bed gasifier. The average gasification intensity of the current modified updraft fixed bed gasifier is derived from a batch experiment. Full load of biomass weight of the gasifier per unit bed area is divided by the duration of the gasification required to dispose of all the biomass in the system. The Gasification intensity of the current design is found to be 120 kg/hr-m2 and the corresponding thermal output would be 212 kwth for the pine chips used at the current study.

2.5 Future improvements The combustion flame in the embedded combustor belongs to the category of diffusion flame. The flame length is therefore comparatively long in the current study. In some applications where space constraint is an issue the long flame will pose problems. To shorten the length of the flame and also increase the stability of the flame a swirling generator in the inlet of the combustion air is being studied to tackle this problem. Preheating of the gasification air is also under consideration to improve the gasification intensity of the current design. In the current design a simple recuperation from the combustor outlet could easily achieve this purpose. Replacing partial gasification air with high temperature steam to enhance the syngas quality is also being investigated. Steam as a gasification agent has been proved to be able to increase the hydrogen content in the syngas by the underlying water shift reaction. The heating value of steam gasified syngas is also much higher than air gasified syngas due to the decrease of Nitrogen, which contributes little to the heating value of the resulting syngas.

Conclusions

Biomass, as renewable, abundant, and domestic energy resources, represents an important but underexploited resource in Taiwan. Biomass when renewed can absorb carbon dioxide and is regarded as a carbon neutral energy resource. Biomass is therefore considered useful in mitigating global warming. Low energy density and wide spread in space characterize biomass as an energy source. Small-scale biomass gasification approach is adopted in this current study to try to develop

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an economically feasible biomass gasifier. A modified updraft fixed bed gasifier with embedded combustor is successfully developed in the current study. The preliminary study of the system demonstrates good functionality and operativity. The unique feature of the embedded combustor inside the biomass gasifier is shown to produce very clean and very hot flue gas for heat and power application. The current design also possesses the coveted stability and flexibility of a biomass gasifier. Heat and power application of this biomass gasifier is being undertaking in three areas. The first is to integrate this biomass gasifier with an external power generator, Stirling engine. The second is to use the biomass generated flue gas, which could be adjusted to reducing flame, to carbonize bamboo for the production of popular bamboo charcoals. The third application is to use the clean and hot flue gas for drying purposes.

Figure13. Photo of the combustion flame in the embedded combustor

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0

200

400

600

800

1000

1200

1400

0 50 100 150 200

Operation Time (Minutes)

Temperature (℃)

Figure14. Represented flame temperature history at the exit of the embedded combustor

0

1

2

3

4

5

6

0 50 100 150 200

Operation Time (Minutes)

Contents (% by volume

)

CO2COO2

Figure 15. Flue gas compositions history from the embedded combustor

Acknowledgements

The authors would like to gratefully acknowledge the supports from the National Science Council of Taiwan under the contract numbers: NSC92-2212-E-243-001, NSC 93-2212-E-243-002 and NSC 93-2622-E-167-006-CC3.

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