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Suraj Pandey et al.: Development of Biomass Gasifier for Small Sized Internal Combustion Engines

Rentech Symposium Compendium, Volume 1, March 2012 24

Development of Biomass Gasifier for Small Sized Internal

Combustion Engines Suraj Pandey*, Bivek Baral, Sunil P. Lohani, Anant Bhusal, Aadarsh Upreti

Abstract— Biomass energy is a very important part of the

renewable energy. In face of the depletion of fossil fuel reserves

and environmental pollution caused by continuously increasing

energy demands, the utilization of biomass as an alternative of

fossil fuels has been on its boom for many years. Biomass

gasification is the major way for high efficient utilization of

biomass and it is an endothermic thermal conversion

technology where a solid fuel is converted into a combustible

gas. Conventional gasification needs sufficient air or oxygen in

the reactor to burn a part of the fuel, then releasing heat to

support gasification of the rest of the fuel. The heating value of

the product gas in typical air-blown biomass gasifier is only

47MJ/m3 because of nitrogen dilution and lower char

conversion efficiency. The current paper addresses the novel

two stage gasification with in situ pre heating of air supply. The

current on-going is expected to have higher carbon conversion

efficiency and low tar concentration in the gas output. 7

Index Terms— Biomass, Two-Stage Gasification,

Downdraft Gasifier

I. INTRODUCTION

HE current project is on process for investigation of

design and development of biomass gasifier for fuelling

small internal combustion engines of the range 5-10 kW

(electrical). The current design of the biomass gasifier is

downdraft, two-stage process type for low tar output in

syngas. The design and fabrication of the downdraft gasifier

system is now completed and has completed flare testing.

The components associated with the gasifier are cyclone, gas

expansion-buffer tank, twin-stage gas cooling unit and the

filter has been successfully developed and testing process is

going on for coupling with small single cylinder IC petrol

and self-modified diesel engine to spark ignition engines.

The gasifier rated capacity is 25 kW (thermal) for use

with hard wood chips. The fuel hopper has the capacity to

hold bulk weight of 8-10 kg of small wood chips. The

gasifier has the novel in situ design of air preheating

treatment facility for both of the stages for tar reduction.

This is the Nepal first designed and locally manufactured

gasifier for intended to run small IC engines. So far, the

testing of gasifier with different wood particle size has

been done. The study of syngas composition and tar was

analyzed for two different wood chips size. For the present

gasifier it is recommended to use wood chips size of 2.5 X

2.5 X 2.5 cm3 for low tar and highest energy value of the

syngas. The syngas generated from the gasifier has the HHV

of the range 4.5-5.5 MJ/Nm3. Moisture can vary from 5 %

for pre-dried to 50 % for freshly harvested crops. Volatile

matter in biomass is much higher than in coal, ranging from

65-85%, while fixed carbon is much lower, ranging from 7-

* Corresponding author: [email protected]

Bio-Based and Green Energy Laboratory, Department of Mechanical

Engineering, School of Engineering, Kathmandu University, Dhulikhel,

P.O.BOX: 6250, Nepal

20%. For elemental analysis, a noticeable feature is the high

oxygen content (32-45%) [1]. The study of temperature

profile along the gasifier reactor has also been measured for

better understanding of the syngas controlling factors. The

results revealed that, the higher the reduction zone

temperature, lower is the tar generation and increase in the

carbon-monoxide concentration in the syngas. The

temperature profile measurement was also conducted for the

two different wood chips size. Apparently, the tar

concentration for larger wood chips size poses higher than

for lower wood chips sizes.

II. DESCRIPTION OF TWO-STATE DOWNDRAFT

GASIFIER

A two-stage downdraft gasifier is an inverted updraft

biomass gasifier with dual (primary and secondary) air inlet

system. Primary air is supplied from the side walls just

above the combustion chamber through holes, arranged

uniformly in odd number, which is above the grate and

below the top feeding opening. And, Secondary air is

supplied from the walls of the reduction zone. This process

is also termed as in-situ tar reduction process because this

particular process helps in significant reduction of tar due to

the availability of higher temperature at reduction zone. The

higher temperature at reduction zone is possible due to the

secondary air inlet at the same zone which cracks the tar

produced after the pyrolysis of the biomass. The basic

concept of this design is to separate the pyrolysis zone from

the reduction zone. NarvPaez et al. [2] performed few

experiments in the freeboard of fluidized-bed gasifier with

secondary air; they observed a temperature rise of about

70oC and tar reduction from 28 to 16 g/m3.

III. THE DESIGNED TEST SYSTEM

The gasfier fabricated has the rated capacity of 25 kW

(thermal) for use with hard wood chips. The fuel hopper has

the capacity to hold bulk weight of 8-10 kg of small wood

chips. The gasifier has the novel in situ design of air

preheating treatment facility for both of the stages for tar

reduction. This gasifier is fuelled from the top with

gasification time of approximately 8 minutes.

IV. PRELIMINARY EXPERIMENTS

A. Effect of Particle Size on Pyrolysis Length and

Temperature Distribution

It can be seen from Fig. 4 that, gasification with larger

wood particle size requires more time to reach the maximum

temperature than the smaller particle size of wood for the

same amount of air flow rate. The pyrolysis length for the

given constant cross section of the reactor is generally

higher in the case of large particle size of the wood. The

length of the pyrolysis zone for large particle wood size is

30 cm whereas, the length of the pyrolysis zone for small

T

Rentech Symposium Compendium, Volume 1, March 2012

particle size of the wood is just 14 cm. This is because of the

porosity increases with the increasing particle size of the

wood for the given constant cross section of the reactor. This

porosity helps heat to flow through two mode of h

transfer via radiation and convection. The smaller particle

size has lower porosity and so called packed bed. The heat

transfer through convection and radiation is less effective in

low porosity media. Interestingly, the drying zone with the

large wood

compared to the smaller particle size which is 73

Fig. 1: Overview of Chemical Process in Downdraft Gasifier

B. Effect of

Percentage of the

According to [6], it can be seen that the CO and H

components in product gas increases with reaction

temperature, while un

increasing reaction temperature. The improvement in CO

component has been observed to be more sensitive as

compared to the H

temperature encountered [6]. However, CH

Suraj Pandey










particle size has the temperature of 153


Overview of Chemical Process in Downdraft Gasifier

Fig. 2: Two

Effect of Reduction

ercentage of the Syngas



temperature, while un-reacted char and CO


ent has been observed to be more sensitive as

compared to the H2 component, for a typical range of


Suraj Pandey et al.













System

Two-Stage Downdraft Gasifier

eduction Temperature to the

yngas Composition



reacted char and CO



component, for a typical range of


et al.: Development of











compared to the smaller particle size which is 73oC.


owndraft Gasifier

emperature to the

omposition



reacted char and CO2 decrease with




temperature encountered [6]. However, CH4 percentage does

Development of Biomass Gasifier for Small Sized Internal Combustion Engines





porosity helps heat to flow through two mode of heat





particle size has the temperature of 153oC as


emperature to the Molar

According to [6], it can be seen that the CO and H2


decrease with




percentage does

not show any significant variation with reaction temperature.

The influence of bed temperature on N

was not presented by the model of author. The prediction of

kinetic modeling also shows that the char consumption

continues exponentially with increasing reaction temperature

[6].

Fig.

Fig.

compositional variation of the constituent gases measured in

the present study. The molar composition of syngas

co

sizes and temperature.

C.

dependency of the gas composition with particle size and

apparently with porosity of the bed in the reactor. From the

ex

composition is in

and the temperature in the reduction zone for constant air

supplied in both the cases. Interestingly, hydrogen does not

have much of the influence on particle size.

Biomass Gasifier for Small Sized Internal Combustion Engines






[6].

Fig. 3: Effect of Large Wood Particle Size and T

Fig. 4: Effect of Small Wood Particle Size and Temperature along

The author model in Fig.



composition is presented in Fig.


C. Effect of Particle

The gas composition shown in Fig.



experimental data, and from Fig.

composition is in










Effect of Large Wood Particle Size and T

the Gasifier

Effect of Small Wood Particle Size and Temperature along

the Gasifier R

The author model in Fig.



mposition is presented in Fig.


article Size on

gas composition shown in Fig.



perimental data, and from Fig.

composition is influenced by both the particle size of wood






The influence of bed temperature on N2




Effect of Large Wood Particle Size and T

asifier Reactor


the Gasifier Reactor

The author model in Fig. 5 clearly verifies the



mposition is presented in Fig. 6 for two different particle

ize on Syngas Composition

gas composition shown in Fig.



perimental data, and from Fig. 3 and Fig

fluenced by both the particle size of wood






component (inert)




Effect of Large Wood Particle Size and Temperature along


5 clearly verifies the



6 for two different particle

omposition

gas composition shown in Fig. 6 reflects the



3 and Fig. 4, the CO






25


component (inert)




e along


5 clearly verifies the



6 for two different particle

6 reflects the



4, the CO





D. Effect of

The tar was sampled in the filter paper and was visually

inspected. This study presents some interesting facts about

the tar production with respect the particle size.

visual inspection, it was found that, tar generation from the

small wood particle size was high at the initial stage of the

ignition, and was reduced significantly after the temperature

was reached high enough for the destruction of tar. Further,

the tar generated from the pyrolysis zone and oxidation zone

has to be passed through series of packed bed having low

porosity where it gets enough residence time to be cracked

in the high temperature region.

Fig.

On the other hand, tar generation from the large wood

particle size was low at the initial stage of the ignition and

subsequently increased with time. At the initial stage, the

temperature was too low for the gen

reason may be the, at the later stage, the heat generation in

the oxidation zone increased and thus provides high

temperature and sufficient heat for pyrolysis zone. The

significant thing that happed in the larger wood size

gasifica

yield

oxidation zone and then finally exited through reduction

Vo

lum

e co

mpo

siti

on d

ry, %


Fig. 5: Effect of R

Effect of Particle












6: Syngas Composition for









gasification is that the long length of the pyrolysis zone

yielded high amounts of tar. The tar


0

5

10

15

20

25

30

Large particle

Vo

lum

e co

mpo

siti

on d

ry, %

Large-Reduction temperature: 550

Small-Reduction temperature: 930

Gas composition with particle size

Suraj Pandey


Effect of Reduction T

Composition

article Size on T












omposition for D

Reduction Temperature









tion is that the long length of the pyrolysis zone

ed high amounts of tar. The tar


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Suraj Pandey et al. Development of Biomass Gasifier for ...ku.edu.np/renewablenepal/images/publications/vol1-7.pdf · Development of Biomass Gasifier for Small Sized Internal