Airblown gasification of woody biomass in a bubbling fluidized bed gasifier

Air-blown gasification of woody biomass in a bubbling fluidized bedgasifier Young Doo Kima, Chang Won Yanga, Beom Jong Kimb, Kwang Su Kimb, Jeung Woo Leea, Ji Hong Moonc, Won Yanga,b, Tae U

Air-blown gasification of woody biomass in a bubbling fluidized bed

gasifier

Young Doo Kima, Chang Won Yanga, Beom Jong Kimb, Kwang Su Kimb, Jeung Woo Leea, Ji Hong Moonc, Won Yanga,b, Tae U Yua,b, Uen Do Leea,b,⇑

a

Department of Green Process and System Engineering, University of Science and Technology, Republic of Korea

c

Yonsei University, Seoul, Republic of Korea

h i g h l i g h t s

Air-blown gasification of woody biomass was investigated in a pilot-scale BFB gasifier

The performance of the gasifier was investigated as a function of equivalence ratio and fluidization conditions

The average heating value of the product gas was above 4.7 MJ/Nm3

Stable operation of the integrated gasification-power generation system was achieved

a r t i c l e i n f o

Article history:

Received 25 September 2012

Received in revised form 13 March 2013

Accepted 25 March 2013

Available online 24 April 2013

Keywords:

Biomass gasification

Bubbling fluidized bed

Air-blown gasifier

Syngas

Power generation

a b s t r a c t

Air-blown gasification of woody biomass was investigated in a pilot-scale bubbling fluidized bed gasifier Air was used as the gasifying agent as well as a fluidizing gas Fuel was fed into the top of the gasifier and air was introduced from the bottom through a distributor In order to control the composition of the product gas, the amounts of feedstock and gasifying agent being fed into the gasifier were varied, and the temperature distribution in the gasifier and the composition of the syngas were monitored It was shown that the distribution of the reaction zones in the gasifier could be controlled by the air injection rate, and the composition of the syngas by the equivalence ratio of the reactants Although the obtained syngas had a low caloric value, its heating value is adequate for power generation using a syngas engine

Ó 2013 Elsevier Ltd All rights reserved

1 Introduction

Increased industrialization and energy consumption has led to

stiff international competition to secure energy resources

Accord-ing to recent research (Jacques et al., 2010), the price of oil and gas

will be twice that at present, and therefore, the widespread

exploi-tation of renewable energy is essential to avoid energy shortages,

as well for minimization of the greenhouse effect[1] Currently,

there is much interest in renewable resources in combustible

including woody biomass, sewage sludge and wastes, and various

method have been tried to convert these renewable resources to

easy-to-use fuels such as gas or liquid fuel more effectively

[2–6] Biomass gasification, in which biomass is converted to qual-ity syngas containing hydrogen, carbon monoxide, and methane without any impurities, is of particular interest[7,8] Gasification can be classified by the type of gasifier, which can be Bubbling Flu-idized Bed (BFB), Circulating FluFlu-idized Bed (CFB), moving/fixed bed, or entrained flow gasifier and also by whether the reaction oc-curs under high- or low-pressure conditions[9] It is also classified

by the oxidant, namely, air[10] steam [11], air–steam [12–14], oxygen–steam[15–17], or excess oxygen One application of syn-gas with negligible tar and dust contents and with a high heating value is as the fuel for syngas engines for the direct generation of electrical power[18] Air-blown gasification is a simple method

in comparison to steam or oxygen-blown gasification, as the com-position of the syngas can easily be controlled through the heating load, and fuel with various physical and chemical characteristics can be used by layering[19] There have been various approaches for investigating the effects of a gasifier operation on product gas 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd All rights reserved.

Indus-trial Technology, Republic of Korea Tel.: +82 41 578 8574; fax: +82 41 589 8323.

dery01@kitech.re.kr (B.J Kim), verycold@kitech.re.kr (K.S Kim), jwlee93@kitech.

re.kr (J.W Lee), mjh5635@kitech.re.kr (J.H Moon), yangwon@kitech.re.kr (W Yang),

ytu@kitech.re.kr (T.U Yu), uendol@kitech.re.kr (U.D Lee).

Contents lists available atSciVerse ScienceDirect

Applied Energy

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / a p e n e r g y

compositions and quality Low heating value (LHV) of syngas was

studied in terms of equivalence ratio (ER)[14], and the effects of

gasifier temperature, steam to biomass ratio and ER on gas

compo-sition was also investigated More specifically, the effect of ER on

the gas yield and properties of syngas were reported for different

biomass fuels such as rubber wood chip and rice husk In this

study, air-blown gasification was investigated for the production

of fuel for a syngas-engine power generator Using air as the

oxidant results in the syngas containing nitrogen and it thus has

a low caloric value Product gas compositions and temperature

distribution in the gasifier were monitored as a function of

equivalence ratio and fuel feeding rate And coupling effects of flu-idization condition and gasification reaction were investigated In addition, preliminary test of syngas power generation was con-ducted and it was found that the producer gas is adequate for power generation using a syngas engine

2 Bfb systems

Fig 1shows a schematic diagram of a BFB system and its reac-tion zones when feedstock is fed from the top and air is introduced from the bottom Influenced by the heat and mass transfer

Nomenclature

Product gas

Air+Productgas Char

Syngas Biomass

Biomass+Char

Biomass

Gasification/

Combustion

Drying/

Pyrolysis/

Gasification

Char

Combustion/

Air preheating

between the fuel particles and environment of the gasifier, the

biomass undergoes successive reaction process such as drying,

pyrolysis, gasification, and combustion and the thermo-chemical

conversion of biomass is strongly related to dimensions and shapes

of feedstock[20,21] The heat and product gases from the

combus-tion of the biomass are used for gasificacombus-tion, pyrolysis, and drying

under an appropriate atmosphere Char, tar and non-condensable

gas are representative by-products from gasification Char is a

residual solid material from devolatilization or pyrolysis of

carbo-naceous in biomass Tars are variable mixture of phenols,

polycy-clic aromatic hydrocarbons (PAHs) and heterocypolycy-clic compounds

Because it is principally the char after the gasification process that

is combusted This reaction step is particularly important in the

air-blown gasification since it supplies necessary reaction heat

An air-blown gasification system that uses air and biomass has

the advantages of enhanced fuel efficiency, the simplicity of the

system, and the ability to control the composition of the syngas

The vertical temperature profile in the system can be controlled

by the biomass feeding rate and air-flow rate and it can lead

differ-ent syngas composition as well as tar contdiffer-ent in the syngas

3 Experimental setup

Fig 2shows schematic diagrams of the experimental setup used

in this study At the bottom of the gasifier, there is an additional

fuel-combustion chamber to preheat the whole system in the beginning The screw feeder introduces biomass into the gasifier, and the cyclone located behind the gasifier separates off gas and entrained dust, sand, and unburned char

3.1 Gasifier

Fig 3a shows the BFB gasifier The inner diameter of the gasifier

is 0.4 m and its height is 3.8 m The gasifier is of the BFB type and is designed to process biomass at a rate of 10–80 kg/h Thermocou-ples of K-type were installed at eleven points to measure the tem-perature profile inside the gasifier, as shown inFig 2 They are arranged at regular intervals, except that between the distributor and TC-1

3.2 Feeder and feedstock

Fig 3b shows the screw-type feeder, which can continuously feed a uniform amount of biomass to the gasifier The feed rate (in revolutions per minute, RPM) can be controlled, and the amount of biomass supplied by the feeder at a given RPM was determined before the gasification experiment was conducted The proximate and ultimate analysis of the wood pellet is given

in Table 1 The biomass consists of volatiles, fixed carbon, and

2

7

4

Syngas

5

Biomass

6

8

1

3

9

Fig 2 Schematic diagram of gasification system and the temperature measurement points; 1: compressed air, 2: LPG, 3: preheat chamber, 4: gasifier, 5: screw feeder, 6:

ash, and the volatile and fixed-carbon contents were 72.7% and

16.7%, respectively

3.3 Preheat chamber

Fig 3d shows the preheat chamber The preheat chamber was

located at the bottom of the gasifier in place of a wind-box to

supply air to the gasifier The chamber was equipped with an

LPG gas burner and the LPG combustion gas was supplied to the

gasifier through the distributor during the startup When the

gasification condition is achieved, the LPG supply was stopped

and the chamber acted as a simple wind-box

3.4 Gas analyzer

Fig 3e shows the device to analyze the syngas produced by the

gasifier The syngas is ignited in the stack before being released

into the atmosphere The syngas was collected by a port in the stack, and a continuous gas analyzer was used to analyze the syn-gas for H2, CO, CO2, O2, and CH4 The data from the gas analyzer was monitored in real-time and recorded on a computer For syngas analysis, the tar in the syngas must be removed, and therefore, a tar trap (Fig 3c) device was installed to absorb the tar from the high-temperature syngas

3.5 Syngas engine system

Table 3 shows the specification of syngas engine Four cycle type syngas engine was ready to use syngas from gasifier to gener-ate electric power Fuel of syngas engine is available for selection either LNG or Syngas Syngas from gasifier of atmosphere condition was supplied by ring blower and introduced into the engine while premixing with air The maximum capacity of the syngas engine is about 30 kWe Electricity from syngas engine was spent at an electronic load resistance

4 Experiment 4.1 Experimental method

In this study, silica sand was employed as the bed material At the start of the experiment, to heat up the gasifier, the LPG burner

at the bottom of the gasifier was used to preheat the air, which, in turn, heated the sand up to the temperature at which spontaneous

Table 1

Proximate and ultimate analysis of feedstock.

Proximate analysis (as received wt.%)

Ultimate analysis (d.a.f wt.%)

Fig 3 Direct photographs of the experimental setup.

Table 2 Experimental conditions.

combustion occurs (400 °C) The temperature of the gasifier was

monitored by a computer After biomass was input into gasifier,

its spontaneous combustion caused the temperature to rapidly

in-crease, and the LPG burner was turned off[22,23] Then, a uniform

amount of biomass was fed by the screw feeder such that the

equivalence ratio (ER) (see Section4.2) was maintained at about

1.3

The combustion conditions were maintained until the bed

tem-perature of the gasifier had reached 800 °C at TC-3 The air inflow

rate was reduced to change ER when the bed temperature was

suf-ficiently high The heat from biomass combustion was used to heat

the sand and for the endothermic pyrolysis and gasification

reactions

4.2 Experimental conditions

The experimental conditions were controlled by the feed rates

of biomass and air, as shown inTable 2 The biomass feed rate

ranged from 25 to 55 kg/h, and air-flow rate from 33 to 54 Nm3/

h The ratio of the biomass and air feed rates is defined as the

equivalence ratio (ER), and the four representative experimental

conditions of this study were ER = 0.32, 0.27, 0.24, and 0.19 CASE

2 and CASE 3 are the two cases in which the biomass feeding rate

is the same (around 55 kg/h) but the air feeding rate is different,

and CASE 3 has the lowest ER (0.19) In addition, long-term

opera-tion performance was tested over two days (CASE 4) For all cases,

the heating value of the syngas satisfies the minimum requirement

of the power generation engine while showing the effect of

fluid-ization number and equivalence ratio on the gasification process

in the bubbling fluidized bed gasifier

5 Results

5.1 Temperature of the gasifier

The temperature profiles in the gasifier are shown inFig 4 The

temperature of the gasifier is mainly controlled by the ER and fuel

feed rates of the biomass For fixed feed rates of biomass, the

average temperature increases as ER decreases and for fixed ER,

it is proportional to the fuel feed rates It is notable that the

tem-perature distribution is significantly affected by ER or fuel feed

rates As shown in Fig 4, the difference between the maximum

and minimum temperature in the gasifier becomes large when

ER decreases or fuel feed rates decreases In a bubbling fluidized

bed with top feeding, the gasifier temperature is also highly

cou-pled with fluidization number (FN) which represents fluidization

condition of the fluidized bed With higher FN, fluidization occurs

more vigorously and it enhances fuel mixing and heat transfer in

the reaction zone

In order to investigate the details of the thermo-chemical

con-version process of the gasifier, vertical temperature distribution

of each case was plotted inFig 5 It is notable that varying the feed rates of biomass or air induced changes the temperature profile which leads to the reaction conditions inside the gasifier signifi-cantly As discussed inFig 1, a bubbling fluidized bed with top fuel feeding system has sequential reaction pathway from the top In the view point of fuel, it experiences drying, pyrolysis, gasification and combustion In the view point of oxidizer, it experiences preheating and combustion and it turns into combustion gases and then take part in the gasification process Again, the product gas from gasification, acts as a heat transfer medium during the pyrolysis and drying process

It is interesting that changes of reaction zones can be estimated from the vertical temperature profile of the gasifier As depicted in

Fig 5, each reaction zone is overlapped to each other but we can esti-mate a boundary or layer of a specific reaction zone between the overlaps For convenience, we can tell that main gasification zone

is coincided with the constant temperature region though some par-tial oxidation or pyrolysis occurs simultaneously in the same region

Fig 5a shows the temperature profile of Case 1 Comparing to Cases 2 and 3, main gasification zone is relatively narrow and difference between the maximum and minimum temperature is

Table 3

Syngas engine specification.

ER : 0.27 LHV:4.7MJ/Nm 3

ER : 0.24 LHV:5.3MJ/Nm 3

ER : 0.19 LHV:5.7MJ/Nm 3

500 600 700 800

900

TC-1 TC-2 TC-3 TC-4 TC-5 TC-6 TC-7 TC-8 TC-9 TC-10 TC-11

o C)

Time (m) Fig 4 Temperature of the gasifier for each case.

Combustion/

Air preheating

Drying/

Pyrolysis/

Gasification

Gasification/

Combustion

Case 2

0 2 4

FN=3.2 ER=0.27

(c)

Temperature (oC)

FN=3.7 ER=0.19

FN=4.6 ER=0.24

Fig 5 Vertical temperature distribution of the gasifier as a function of FN.

larger than other cases This is because the feed rates of biomass is

less than the other cases and this means that combustion heat and

hot product gas yield are less than the other cases while the heat

loss to the wall or sensible heat loss of the product gas to the fresh

feedstock is relatively similar to the other cases It is also notable

that the temperature of the very first measuring point (TC-1) of

Case 1 is less than the second point (TC-2) which implies that

com-bustion zone is also narrow for Case 1 The reason why is less fuel

feed rates and lower FN Since the amount of fuel is not enough to

reach the very bottom of the gasifier, the main combustion

reac-tion occurs within narrower regions comparing to other cases In

addition, leveling-off of temperature in the fluidized bed is less

since FN is less than the other cases

For Cases 2 and 3 increase of fuel feed rates as well as air flow

rate increases gasification zone The area of gasification zone (i.e

constant temperature region) increases as FN increases This is

be-cause of increase of bed expansion, increase of solid entrainment,

and total mass flow of the product gas Note that the temperature

maximum occurs in the very first measuring point which implies

that the main combustion zone is lower than Case 1 and fuel can

reach to the very bottom of the gasifier The enhanced mixing by

increased FN can also contribute to the fuel transportation The

decrease of drying and pyrolysis zone is also another interesting

result For the same fuel feed rates (Cases 2 and 3), the drying

and pyrolysis zone is highly affected by ER which results in the

heat and mass flow of the product gas which becomes heating

medium for the drying and pyrolysis process

5.2 Syngas composition

Table 4shows the syngas composition as a function of ER, and

Fig 6shows how its composition varies with time as well as ER

The concentration of syngas tended to increase as ER went from 0.27 to 0.19 The hydrogen concentration, which is important for combustion of CO in syngas engines, increased from 14.5% to 16.5%, carbon monoxide increased from 13.8% to 16.1%, and CH4

increased from 4.0% to 5.3% The total volume of the product gas decreased as ER was reduced The caloric value for each ER is

Table 4

Concentration of syngas composition.

0

5

10

15

20

25

30

0 2 4 6

3 )

H2

CO2

O2 CO CH 4

Time (m)

LHV

Fig 6 Syngas composition and lower heating value as a function of ER.

Table 5 Comparison of gasifier performance with previous studies.

SEI – Southern Electric International Inc / ISU – Iowa State University / EPI – Energy Product of Idaho.

0 5 10 15 20 25

500 600 700 800

CO

CO2

H2

CH4

O2

o C)

Time (day)

TC-1 TC-2 TC-3 TC-6 TC-8 TC-10

Fig 7 Continuous operation results of BFB systems.

0 4 8 12 16 20

0 2 4 6 8 10 12 14 16 18 20

H

2

CO

2

CO

CH4 Electronic Power

Time (min) Fig 8 Preliminary test result of syngas power generation (electric power output and simultaneous syngas composition).

shown inTable 4, and it can be seen that in all cases it exceeded

4.7 MJ/Nm3, which means the syngas produced here is suitable to

be used as fuel for syngas-engines.Table 5shows a comparison

of our result and the results of previous research, and

demon-strates that the performance of our gasifier is good in comparison

to other systems It is notable that hydrogen content of our study is

higher than the previous results, which is good for syngas engine

operation It can be explained by the long residence time of the

reactant and product gas due to our gasifier configuration and

top fuel feeding system Water–gas shift reaction (i.e CO + H

2-O  >H2+ CO2) also contributes to the increase of the hydrogen

content because CO of this study is less than the other results

and CO2 is more than the other results (Table 5) Fig 7 shows

long-term operation results of BFB system The experimental

con-dition and average syngas compositions are presented inTable 2

(Case 4) andTable 4respectively As shown in theFig 7, stable

and continuous operation of BFB gasification system has been

proved and syngas quality is appropriate for power generation

with a syngas engine

5.3 Syngas engine test

Fig 8 shows the preliminary test result of integration of

biomass gasification-gas cleaning-syngas engine A gas cleaning

system composed of a gas cooler, bag filter, scrubber and I.D fan

was adopted for removing tar and dust in the product gas The

original product gas composition of the gasifier was similar to Case

2, but the final product gas composition can be altered by some air

leakage from the bag filter caused by the suction operation of I.D

fan However, this can be compensated in the premixing stage of

the syngas engine operation As a result, stable and continuous

operation was conducted with the integrated system and

electric-ity of 12–14 kWe was generated in the test run[24]

6 Conclusions

Air-blown gasification was investigated for the production of

syngas in a (BFB) gasifier The feed rates of biomass and air were

controlled to change the ER and vary the internal conditions

Changes in the biomass and air feed rates affected the product

gas composition and temperature profiles in the gasifier Based

on the temperature profiles, the dynamics of reaction zones were

investigated in terms of ER, fuel feed rates, and FN The

composi-tion of the syngas was significantly affected by ER The

concentra-tion of hydrogen is relatively higher than previous researches and

it results from the configuration of the gasifier: longer free board

and top fuel feeding The caloric value of the product gas was above

4.7 MJ/Nm3, and thus satisfactory for use in syngas engines

Preli-minary test of integrated gasification-power generation was

conducted

Acknowledgement

The authors would like to gratefully acknowledge to institution

support program by the Ministry of strategy and Finance of Korea

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