The characteristics of biomass gasification in multistage heating and gradient chain gasifier

Through comparison with the normal situation, the gasification efficiency was improved significantly with the increase of the temperature of drying stage appropriately, and with the decr

The characteristics of biomass gasification in

multistage heating and gradient chain gasifier

Jianjun Caia, Shuzhong Wanga,*, Qingcheng Wangb, Cao Kuanga

aSchool of Energy and Power Engineering, Xi'an Jiao Tong University, Xi'an, 710049, China

bInstitute of Energy and Resources Comprehensive Utilization Research, Shanghai Institute of Technology, Shanghai,

201418, China

a r t i c l e i n f o

Article history:

Received 24 March 2016

Accepted 19 April 2016

Available online xxx

Keywords:

Biomass

Gasification

Optimization

Multistage heating and gradient

chain gasifier

a b s t r a c t

The characterization on gasification of biomass briquette in a sectional healing of reactor was studied in this article The results indicated that the temperature of drying stage was higher than those of oxidizing and reducing stages in multistage heating and gradient chain gasifier (MHGCG) Through comparison with the normal situation, the gasification efficiency was improved significantly with the increase of the temperature of drying stage appropriately, and with the decrease of the oxidizing and reducing temperature When the equivalence ratio (ER) was 0.28 in the oxidizing stage, the weight loss of biomass and the gasification efficiency was 43% and 56%, respectively The volume fraction of O2, CO2, H2 and CO was 4%, 10%, 14% and 24%, respectively, and the ultimate volume fraction of NO,

NOX, and SO2was about 0.024%, 0.025% and 0.032%, respectively In addition, most of the raw biomass (about 95%) was transferred to the discharge port of MHGCG Therefore, the gradient-chain has little effect on the disturbance of biomass briquetting fuel (biomass briquette) in the vertical direction

© 2016 Hydrogen Energy Publications LLC Published by Elsevier Ltd All rights reserved

Introduction

As a renewable and environmentally friendly energy source,

biomass (i.e., any organic non-fossil fuel) and its utilization

are gaining an increasingly important role in the world wide

[1] In addition, biomass has a high utilization potential and is

one of the most important energy sources in the future[2e4]

Gasification is one of the promising technologies to exploit

energy from renewable biomass, which is derived from all

living matters, and thus is located everywhere on the earth

[5e8] Specially, biomass has the potential to accelerate the

realization of hydrogen as a major fuel of the future[9] So

biomass has been considered as one of the most promising

sources of renewable energy[10] In the open literature, there

are many research works using fix bed or fluidized-bed in the area of biomass gasification This study mainly emphasizes the important aspects of the temperature, gasify agent, and biomass type[11e16] The operating conditions of gasification e.g residence time, gasification temperature, and gasifying agent is usually different in the zone of drying, pyrolysis, oxidizing, and reducing Therefore, to optimize this operating conditions needs to consider the different in the zone of dry-ing, pyrolysis, oxidizdry-ing, and reducing separately [17,18] However, the knowledge in this area is still limited in the literature We have attempted in this work to research the different demands of gasification temperature in the stage of drying, pyrolysis, oxidizing, and reducing To achieve this objective, the sectional heating furnace was made Based on the furnace, the following experimental work was carried out

* Corresponding author Tel.: þ86 2982665157; fax: þ86 29 82668708

E-mail address:szwang@aliyun.com(S Wang)

Available online at www.sciencedirect.com

ScienceDirect

journal hom epa ge: www.elsev ier.com/locate/he

http://dx.doi.org/10.1016/j.ijhydene.2016.04.128

0360-3199/© 2016 Hydrogen Energy Publications LLC Published by Elsevier Ltd All rights reserved

in order to obtain the information about the effect of

gasifi-cation temperature on different gasifigasifi-cation stages

Experimental

Biomass Briquetting Fuel (BMF) with particle size of 4

5 20 mm is used as the feedstock in the tests The BMF is

made in Shanghai Green New Energy into Technology Ser-vices Ltd The ultimate and proximate analysis results are presented inTable 1

The real figure of the MHGCG system is shown inFig 1 The unique tailor-made configuration of the MHGCG system mainly consists of a instrument control box (Fig 1-A), MHGCG (Fig 1-B), the supply system of gasifying agent (Fig 1-C), and a blower (Fig 1-D) The temperatures, transmission speeds,

Table 1e Proximate and ultimate analysis of BMF

15.36% 74.92% 1.81% 7.91% 46.88% 5.27% 37.94% 0.05% 0.14%

Fig 1e The real figure of the MHGCG system

Fig 2e The distribution of TCs in the MHGCG system

residence time, and the opening degree of blower at different

zones (e.g drying, pyrolysis, oxidizing, and reducing) are

controlled by the instrument control box

Compared with the traditional fixed bed gasification

[9,19e21], the MHGCG has the following advantages: The

operating conditions of gasification (e.g residence time,

gasification temperature, and gasifying agent in the zone of

drying, pyrolysis, oxidizing, and reducing) are easily

controlled by MHGCG Therefore, this operating conditions is

considered separately in the zone of drying, pyrolysis, oxidizing, and reducing It is suitable for large batch of the raw biomass gasification The requirement of raw biomass size is not strict in MHGCG The MHGCG has lots of other preferences e.g simple structure, good sealing performance, flexible pro-cess layout and lower power consumption

The distribution of thermocouples (TCs) is shown inFig 2 The MHGCG system totally has 8 thermocouples They are located on different positions These positions are listed in the Table 2

The flue gas analyzer (E4400-S) is made in America E-In-struments ltd It is used to measure the composition of flue gas e.g O2, CO, CO2, NO, NO2, and SO2 The temperature of flue gas (T flue), the temperature of air (T air), the efficiency of biomass gasification (Efficiency), and the rate of biomass weight loss (Losses) also can be measured by E4400-S Based on theoretical calculation and practical consider-ations for suitable MHGCG gasification regime, the experi-mental parameters of MHGCG are shown inTable 3

Table 2e The description of the MHGCG system

TC02 Gas outlet 2 TC06 Below the pyrolysis zone TC03 Above the pyrolysis zone TC07 Below the oxidizing zone TC04 Above the oxidizing zone TC08 The reducing zone

Table 3e The experimental parameter of MHGCG system

Terms Drying Pyrolysis Oxidation Reduction

Thickness of BMF (m) 0.03 0.03 0.03 0.03

Weight of BMF (kg) 0.945 0.945 0.945 0.945

Volume of air (m3) 0 0 3.9 0

Residence time (min) 3.57 3.57 3.57 3.57

Flow rate of air (m3/h) 0 0 4.57 0

Temperature (C) 100 100 600 400

Fig 3e The temperature of TC01-04 collected vs the residence time

1) To assure the flow rate of gasifying agent at different

zones

According to the ultimate analysis of BMF, the theoretical

air of BMF (L0) is 4.31 m3/kg In all experiments, air is injected

in the oxidizing zone Initial height of BMF in the oxidizing

zone is 0.03 m, and the weight of BMF is 1.89 kg The porosity

of biomass one the chain is 0.5 Therefore, the actual weight of

biomass is 0.945 kg According to the rotating speed of

trans-mission shaft, the residence time is 3.57 min at different

zones When ER is 0.28, the actual volume of air (L) is 1.09 m3

Therefore, the flow rate of air is 4.57 m3/h

2) To assure the heating temperature at different zones

In the oxidizing zone, the fix carbon is oxidized by the

re-action of oxidation, and large heat is released to the furnace

Therefore, the heat is used to dry BMF, as well as, to increase

the temperature of the pyrolysis and reducing zones

More-over, the temperature are measured by K-type thermocouples

As shown in Table 3, the drying, pyrolysis, oxidizing, and

reducing zone is preset as 100 C, 100C, 600 C, 400 C,

respectively

Results and discussion

collected In 0e24 min, the pre-heating temperature was

collected by TCs, as show in the region of AFigs 3 and 4 As the actual temperature of MHGCG’ furnace was heating up to the preset temperature, the motors of transmission were opened

in the zone of pyrolysis, oxidation, and reduction In addition, when the residue time had been set up, the motors of trans-mission in the zone of dry was opened later Meanwhile, the biomass feeding was started from the hopper, the tempera-ture at this moment in MHGCG system was shown in the re-gion of B inFigs 3 and 4 When BMF had exhausted in the hopper, the temperature was collected by TCs at this moment,

as shown in the region of C inFigs 3 and 4

The average temperature at different positions is shown in Fig 5 The temperature of the region below the oxidizing zone (TC07 collected) was highest, and followed by the temperature

of the region above the oxidizing zone (TC04 collected), but the temperature of the region flue gas outlet 1 (TC01 collected) was lowest Theoretically, the rang of temperature in the drying, pyrolysis, oxidizing, and reducing zone is 200e300C,

500e600 C, 1000e1200 C, and 700e900 C, respectively However, in the MHGCG system, the highest temperature was found in the region of oxidation (as show inFigs 3e5the TC 04 and TC 07 collected, about 607.5C), followed by the region of pyrolysis (as show inFigs 3e5the TC 03 and TC 06 collected, about 549.5C), and the lowest temperature was found in the region of reducing (as show inFigs 3e5the TC 08 collected, about 466C)

The gap between the experimental results and the theo-retical results is shownTable 4 The average experimental temperature for pyrolysis was lightly lower than the

Fig 4e The temperature of TC05-08 collected vs the residence time

theoretical temperature, and the average experimental

tem-perature for reduction was obviously lower than the

theoret-ical temperature, but the average experimental temperature

for drying, oxidizing, were obvious higher than the theoretical

temperature Specially, the average experimental

tempera-ture for drying was obvious higher than the theoretical

tem-perature Therefore, the storage security of BMF decreased

Compared with the theoretical temperature, the average

experimental temperature for oxidizing and reducing were

lower 44% and 41%, respectively The plausible reason of this phenomenon could be explained that the flue gas with high temperature largely flow form the zone of oxidation into the zone of drying and pyrolysis However, the zone of reduction located in the end of the chain In addition, there was virtually nonexistent the flue gas with high temperature Therefore, the temperature of reducing zone decreased sharply Through comparison with the normal situation, the gasification effi-ciency was improved significantly with increase the temper-ature of drying zone appropriately, and decrease the temperature of oxidizing and reducing zone The experi-mental results indicated that the MHGCG system still played positive roles in BMF gasification The content of the results of this research should be detail analyzed in the further research

The concentration of gas flue components in the tested is shown inFig 6 With the residue time increasing, the volume fraction of O2decreased initially and then kept stable gradu-ally But the volume fraction of CO2increased initially and then kept stable gradually In addition, increasing the residue time in the tested range obvious increased the volume frac-tion of H2and CO When the experiment was beginning, the BMF feeding was started from the hopper Because of the residues time was 3.57 min at the zone of dry and pyrolysis respectively, BMF was transferred to the zone of oxidation at the time of 7.14 min At the same time, the volume fraction of

CO2was increased sharply, but the volume fraction of O2was decreased, but a large amount of energy was liberated Therefore, the volume fraction of H2and CO was increased In addition, ER was 0.28 at the zone of oxidation, and the fixed carbon was combusted incompletely Therefore, the volume fraction of CO was increased slowly With BMF was trans-ferred to the zone of oxidation continuously, the volume fraction of O2, CO2, H2and CO kept stable gradually At this point, the volume fraction of O2, CO2, H2and CO was 4%, 10%, 14% and 24%, respectively In the MHGCG system, the volume fraction of CO was lower than other systems The plausible reason of this phenomenon can be explained by the reaction

of Boudouard, water-gas, and steam reforming reduced in the zone of reduction

Fig 6e The concentration of gas flue components vs the residence time at outlet

Fig 5e The average temperature of different TCs collected

Table 4e The contrast analysis of experimental and

theoretical temperature

Terms drying Pyrolysis Oxidation Reduction

Experimental

temperature (C)

572.4 549.5 607.5 466.0 Theoretical

temperature (C)

250 550 1100 800 Temperature

difference (C)

322.4 0.5 492.5 334.0 Absolute percentage

(%)

129 0.09 44.8 41.8

With the residence time increasing, the volume fraction of

NO and NOX decreased initially and then increased lightly,

and the changing trend of volume fraction of NO and NOXwas

accordant The results indicated that most of the flue gas of

NOXoriginated in NO In addition, the plausible reason of this

phenomenon can be explained that the the volume fraction of

NO and NOXdecreases with increasing temperature However,

the volume fraction of SO2increased initially and then kept

stable gradually with the residence time increasing The

plausible reason of this phenomenon could be explained that

BMF was transferred to the zone of oxidation at the time of

7.14 min Therefore, most of sulfur in the BMF was then

con-verted to the SO2 The ultimate volume fraction of NO, NOX,

and SO2was about 0.024%, 0.025% and 0.032%, respectively

The average temperature of flue gas is shown in Fig 7

Increasing the residue time in the tested range increased the

average temperature of flue gas The plausible reason of this

phenomenon can be explained that the heat transfer in

MHGCG system was not installed leading to poor heat

dissi-pation The content of the optimization of the heat dissipation

in the MHGCG system will be detail analyzed in the further

research

The weight loss rate of BMF in the MHGCG system is shown

inFig 8 With the residue time increasing, the rate of weight

loss decreased gradually, and finally kept stable The weight

loss of BMF was increased initially and then decreased

dramatically in range of time 0e6 min Those may be due to

the reaction of BMF oxidation happened The weight loss of

BMF decreased gradually to 43% at the time 12 min

The gasification efficiency of BMF is shownFig 9 In the

time 0e7 min, the gasification efficiency of BMF increased

gradually At the time 8 min, the gasification efficiency of BMF

was beginning to stabilize, about 56%

The distribution of ash at different zones is shown in

Fig 10 In addition, the distribution of ash at the zone of

dry-ing, pyrolysis, oxidation, and reduction is shown inFig 10-A,

B, C, and D, respectively The distribution of ash at the end of

the discharge port is shown inFig 10-E Specially, the partial

enlargement of ash at different zones e.g drying, pyrolysis,

oxidation, reduction, and the discharge port is shown in Fig 10-a, b, c, d, and e, respectively As show inFig 10-A, BMF still retained the relatively complete appearance But was lightly blackening as show inFig 10-a This proofs that BMF is little changed to char under the high temperature atmo-sphere As show in Fig 10- B/b, the appearance of BMF is mostly blackening This proofed that BMF was mostly turned into char As show inFig 10-C/c, the appearance of BMF was mostly turned into gray white This proofed that BMF was mostly oxidized to ash As show inFig 10-D/d, the appearance

of BMF was mostly turned into gray white This proofed that BMF was mostly turned into white ash But As show inFig 10-E/e, the appearance of BMF still retained lightly block char Overall, BMF was mostly dropped into the discharge port, about 95%, and the shape of ash was strip-like Therefore, the gradient-chain has little effect on the disturbance of BMF in the vertical direction

Fig 9e The combustion efficiency of BMF vs the residence time

Fig 8e The weight loss of BMF vs the residence time

Fig 7e The average temperature of flue gas vs the

residence time at the outlet

This work investigated the characteristics of BMF gasification

in the MHGCG system Important conclusions drawn from the

present study as follows:

1) In the MHGCG system, the temperature of drying zone was

higher than the temperature of oxidizing and reducing

zones Through comparison with the normal situation, the

gasification efficiency was improved significantly with

increase the temperature of drying zone appropriately,

and decrease the temperature of oxidizing and reducing

zone

2) When ER was 0.28 in the oxidizing zone, the weight loss of

biomass and the gasification efficiency was 43% and 56%,

respectively In addition, the volume fraction of O2, CO2, H2

and CO was 4%, 10%, 14% and 24%, respectively And the

ultimate volume fraction of NO, NOX, and SO2was about

0.024%, 0.025% and 0.032%, respectively

3) The mostly raw biomass (about 95%) was transferred to the

discharge port of MHGCG, and the shape of ash was

strip-like Therefore, the gradient-chain has little effect on the

disturbance of BMF in the vertical direction

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