fluidized bed gasifier design report public)

Coordinator: Dr. Lu Aye Department of Civil and Environmental

18 MAY 2007[Energy from Biomass and Wastes] | 421‐825

DESIGN OF A LOW COST FLUIDIZED BED

GASIFIER FOR SAWDUST GASIFICATION IN

RURAL CHINA

[Vigneswaran KUMARAN] | 277492

vkurmara

Text Box

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DESIGN OF A LOW COST FLUIDIZED BED GASIFIER FOR SAWDUST GASIFICATION IN RURAL CHINA

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Abstract

The task of designing a low cost gasifier to be applied in rural China had been the motivation

of this report. The specification of 8 hour per day operation for 200 kW heat output for

heating purpose has been made as the basis of the design work. The feedstock has been

specified as pine wood sawdust from timber mills in rural China, and this feedstock is pre‐

dried before feeding to the gasifier. Due to the nature of the feedstock, a fluidized bed

gasifier had been thought appropriate for the design requirement. A simple economic

analysis was performed for the designed unit to determine the feasibility of the unit’s

application. In addition, some of the barriers related to the implementation of gasification

technology in rural China had also been addressed.

Keywords: Low cost gasifier, heating, design work, rural China, fluidized bed, feasibility,

barriers.


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Aim

The following objectives are delineated in this design report:

• A low cost gasifier design (exclude auxiliaries) is attempted for an output of 1600kWht per day

• The gasifier is to be used for gasification of sawdust in rural society of China for the purpose of heating

• A simple economic analysis was attempted to verify the applicability of the design

• An overview of technical and non‐technical barriers are presented


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Contents

Abstract 2

Aim 3

1.0 Introduction 6

2.0 Gasifier Types 7

3.0 Design Methodology for Fluidized Bed Gasifier 9

4.0 Gasifier Design Economics 16

5.0 Technical and Non‐Technical Barriers 18

6.0 Discussion and Conclusion 19

References 20

Appendix

A1 Summary of Sawdust Fluidised Bed Gasifier Design 21

A2 Thermodynamic Properties and Estimates Worksheet 25

A3 Process Parameters Calculation 27

A4 Gasifier Sizing WorkSheet 30

Figures Figure 1 Type of gasifier and elements of operation 8

Figure 2 Fluidized Bed Gasifier Diagram 15

Tables Table 1 Intermediate values for solving equation (1) to (5) 10

Table 2 Predicted product gas composition at gasifier bed temperature 11

Table 3 Higher heating value as a function of product gas temperature 12

Table 4 Process parameters and other estimates 12


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Table 5 Fluidized bed gasifier design calculation and various author’s design figures 14

Table 6 Economic data of sawdust gasification and heating system in rural China 16

Table 7 Sensitivity analysis for sawdust gasification at different operating hours 17


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1.0 INTRODUCTION

There are various technologies for the conversion of biomass to cleaner energy medium. In

this particular analysis, the gasification technology is specifically identified and discussed.

Biomass constitutes among others, three major elements which are carbon (C), hydrogen

(H) and oxygen (O). Elemental carbon and hydrogen are reactive and are combustibles that

can produce energy in the form of heat and light when reacted with oxygen. Thus, primarily

biomass is an alternative energy source that can be utilized to produce useful secondary

energy forms such as electricity and derived‐fuels. As shown on the map of China in this

page, the green fields are the massive area of biomass available in this highly populated

country.

In this large developing

country, gasification

technology has been in use in

many biomass related

industries such as agricultural

production and timber

industry. However, this

practice is not widespread [8]

contrary to what could be

expected for a country with 38

million m3 of forest residues and 0.65 billion tones of agricultural residues per annum [1].

Forest residues in the form of wood waste such as sawdust has been used in rural China [1]

and this report intends to produce a low cost gasifier unit using sawdust as the solid fuel for

gaseous fuel production which will be used for heating purposes.

Source: www.travelchinaguide.com


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2.0 GASIFIER TYPES

Gasification for this design purpose is defined as a thermochemical conversion process

whereby biomass is heated in a sub‐stoichiometric oxygen, air or steam to produce gaseous

fuel. There are various types of gasifiers and the design is specified by the feedstock type,

preparation and end‐use of product gas. The four major characteristics of the feedstock [2]

that has impact on the gasifier operation is described as follows:

2.1 Moisture content

High moisture content (above 30 %wt) in feedstock is not suitable for

gasification due to production of low heating value gaseous fuel.

2.2 Ash content

Ash in biomass tends to form solid fuses that blocks feed passage, especially

at higher combustion temperature.

2.3 Volatile compounds such as tar and higher chain hydrocarbons that needs

further removal or treatment to provide cleaner gaseous fuel.

2.4 Particle size

Depending on the type of gasifier, the particles size plays an important role in

ensuring proper flow of feedstock in the gasifier and gasification of the

feedstock.

The following figure summarizes some of the common type of gasifier and the associated

elements that could be considered for selection.


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Figure 1: Type of gasifier and elements of operation

(Source: www.gasnet.co.uk ) The fluidized type bed is selected for this design study due to the characteristic of the

feedstock and the ease of control in terms of handling of feedstock in the gasifier.

http://www.gasnet.co.uk/


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3.0 DESIGN METHODOLOGY FOR FLUIDIZED BED GASIFIER

The design methodology uses a three step approach whereby the thermodynamic and

process parameters are estimated from the predicted value of product gas composition as a

function of process temperature. These estimates are used for the sizing of the fluidized bed

gasifier for sawdust gasification.

3.1 Prediction of product gas composition using thermodynamic properties

The composition of the product gas is predicted using several sets of

equation as a function of gasifying temperature. The feedstock chosen for the

design is Pine Wood sawdust [3] with the physical properties and elemental

composition given in Appendix 1. The universal gasification reaction equation

[4] is given below.

CH aOb + wH2O + mO2 + 3.76mN2 → x1H2 + x2CO + x3CO2 + x4H2O + x5CH4 + 3.76mN2

The elemental mass balance provides the following sets of equation.

C: 1 = x2 + x3 + x5 (1)

H: 2w + a = 2x1 + 2x4 + 4x5 (2)

O: w +b + 2m = x2 + 2x3 + x4 (3)

The gasification equilibrium constants [4] for methane and shift gas reaction

are as below:

Methane formation: K1 = x5/(x2)2 (4)

Shift gas reaction (CO and H2): K2 = x1x3/(x2x4) (5)

The values of K1 and K2 are determined using the following temperature

dependent equilibrium equation which requires the heat of formation and

standard Gibbs function constants.

lnK = ‐(J/RT) + ΔA lnT + (ΔB/2)T + (ΔC/6)T2 + ΔD/2T2 + I) (6)

ΔH0 = (J/R) + ΔA lnT + (ΔB/2)T2 + (ΔC/3)T3 ‐ ΔD/2T (7)

ΔG0 = J ‐ RT(ΔA lnT + (ΔB/2)T + (ΔC/6)T2 + ΔD/2T2 + I) (8)


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The calculations for above constants are provided in Appendix 2. Thus,

solving the equation (1) to (5) forms a polynomial function, that can be

resolved using Excel spreadsheet as shown in the Appendix 2.

The following stoichiometric equation is used to determine the

stoichiometric combustion air required for the Pine sawdust.

CH aOb + mstcO2 + 3.76mstcN2 → CO2 + H2O +3.76mstcN2 (9) The intermediate values for the above sets of equation are provided in Table 1. Table 1: Intermediate values for solving equation (1) to (5) Moisture content, w (mol) 0.120 Gasification oxygen, m (mol) 0.336 Stoichiometric combustion air, mstc.air (mol) 3.864 Gasification air, mair (mol) 1.264 Bed temperature, TB (

oC) 750.00 TB (K) 1023.15 Equilibrium constant, K1 0.076 Equilibrium constant, K2 0.199 Hydrogen content in Wood, a (mol) 1.459 Oxygen content in Wood, b (mol) 0.676

The temperature dependent product gas composition is shown in Table 2.

Interestingly, the composition agrees with most value ranges provided in

many referenced literatures.


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Table 2: Predicted product gas composition at gasifier bed temperature

Gas Components VariablesPredicted Values

Mole fraction Percentage (%)

H2 x1 0.5200 0.1693 16.93

CO x2 0.7794 0.2537 25.37

CO2 x3 0.2000 0.0651 6.51

H2O x4 0.2885 0.0939 9.39

CH4 x5 0.0206 0.0067 0.67

N2 3.76*m 1.2634 0.4113 41.13 Total 3.0719 1.0000 100.00

3.2 Process parameter calculation and other estimates

The process parameters are calculated to determine the required air flow for

the amount of feedstock fed to the gasifier to produce 200 kW of thermal

output for heating using a gas fired boiler with an assumed thermal efficiency

of 82 %. The gasifier efficiency is assumed to be 76 %, and complete

gasification of sawdust fed is anticipated. Also, since the sawdust obtained

from the timber mill is expected to be wet, it is pre‐dried to a value below 10

% wt moisture on a wet basis, to allow proper gasification with good heating

value of product gas. The following equation [5] is used to define the

gasification efficiency and to derive the values for the feedstock required to

produce the specified output.

ηth = [(Hg * Qg) + (Qg * ρg * cp * ΔT)]/(Hs * Ms) (10)

The higher heating value (HHV) for the predicted composition of product gas

is calculated using the mole fraction and individual HHV. This value is

subsequently converted to lower heating value (LHV) of gas using the rule of

thumb that LHV is approximately 90 % of the HHV for gaseous combustibles.

This is shown in Table 3 and the process parameters are tabulated in Table 4.

The complete calculation steps are shown in Appendix 3.


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Table 3: Higher heating value as a function of product gas composition

T

Gas Component

Volume Fraction

Molecular Weight

HHV Btu/scf

Mol. Wgt. Contribution

Weighted Values

(= mole fraction)

HHV, Btu/scf

C1 0.00671 16 1013.2 0.107 6.798 CO2 0.06511 44 0 2.865 0.000 CO 0.25372 28 320.5 7.104 81.316 H2 0.16928 2 325 0.339 55.015 H2O 0.09392 18 0 1.691 0.000 Inerts (N2) 0.41127 28 0 11.515 0.000 Total 1 23.621 143.129

Using the above table and the following table, the next step is approached,

whereby the fluidized bed gasifier sizing is undertaken.

Table 4: Process parameters and other estimates Design Parameters Value Unit Thermal efficiency, η 76 %

Gasifier output, PD 321 kW

Heating value of gas, Hg 4763.80 kJ/Nm3

Gas volumetric flow, Qg 0.0641 Nm3/s

*Gas volumetric flow, Qg 230.7 Nm3/h

Gas density, ρg 0.2816 kg/m3

Gas Specific Heat, cp 1.391 kJ/kg oC 1Heating value of Solid, Hs 15120 kJ/kg

Solid Fuel Consumption, Ms 0.0279 kg/s

*Solid Fuel Consumption, Ms 100.5 kg/h

Solid Fuel Inlet Temp, Tis 27 oC

Gas Outlet Temp, Tos 650 oC 2Air Density, ρa 1.171 kg/m3

Air Ratio, ER 0.3271

Air volumetric flow, Qa 0.0334 Nm3/s

Sensible Heat Recovered, H 244.06 kJ/Nm3

Specific Consumption, Uc 0.33 kg/kWh Boiler Rated Efficiency 82 % Boiler Output 200 kW

1 LHV, 2 @ 27 0C, 101.325 kPa


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3.3 Fluidized bed gasifier sizing

The sizing of the fluidized bed is established via several sets of assumption

and equations. These assumptions are important to enable a practical design

of the gasifier. The air flow (and the gas flow) through the bed is assumed to

be well distributed and homogeneous mixture of product gas is obtained at

averaged bed temperature of 750 0C. The minimum fluidization voidage (εmf)

of the bed [6] is assumed 0.66 (ranging between 0.5 to 0.85). The fluidized

bed is assumed to be operated with silica sand which has a particle density

(ρp) of 2600 kg/m3 and average particle diameter (dp) of 300 microns. The

bed operates at slightly above atmospheric pressure due to the air delivered

by the blower above atmospheric pressure. The first step in sizing activity is

the determination of the fluidized bed diameter [7], which uses the following

correlation:

mair = ρair * ((π * Dg^2)/4 )* Us (11)

The air flowrate (mair) through the bed is given as a function of bed diameter

(Dg) and the gas superficial velocity (Us). The following sets of equations are

required to determine the superficial velocity [7], which necessitates the

information on the bed minimum fluidization velocity (Umf).

Us = 2 * Umf (12)

Umf = [dp^2 / (150* μg)]*[g*(ρp‐ ρg)*(εmf^3)/(1‐ εmf)] (13)

Using the superficial value calculated from the above equations, the bed diameter is determined, whereby the air flow rate had already been established in previous section. The next step will require the determination of bed dynamic height (HB), which is the section of the bed where the fluidized particles (including sawdust) will rise when air is injected for reaction. This is determined using the relationship [8] between bed residence time (t) and U

B

mf. The overall reaction bed height (H) is the sum of the height of sawdust feeding point and dynamic bed height.

HB = t * UB mf (14)

H = HB + HB f (15)


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Table 5: Fluidized bed gasifier design calculation and various authors’ design figures

D ‐ Design Values; [11] ‐ van de Enden, et al.; [9] ‐ E. Olivares Gomez, et al.; [8] – Yin X.L., et al.; [12] ‐ Li X. T. et al.

Parameters ValueD Value[11] Value[9] Value[8] Value[12]

Type of biomass fuel

Units

Sawdust Sugarcane bagasse

Sugarcane bagasse

Rice husk Sawdust

Feeding rate kg/s 0.0279 0.03 0.0290 0.4167 0.0085 Moisture (d.b.) % wt 8.9 15 11.7 Feeding point M 0.26 0.3 0.26 1.8 0.89 Bed section internal diameter M 0.459 0.57 0.417 1.8 0.1 Dynamic bed height M 1.549 2 1 7.4 6.5 Freeboard internal diameter M 0.459 0.75 0.417 ‐ ‐ Freeboard height M 0.860 3 1 ‐ ‐ Disengaging Zone Diameter M 0.689 ‐ 0.835 ‐ ‐ Disengaging Zone Height M 1.362 ‐ 1.585 ‐ ‐

Air volume flow Nm3/s 0.0334 0.0320 0.0271 ‐ 0.0146

Air volume flow m3/s 0.1069 0.1366 0.0938 ‐ 0.0525

Bed temperature 0C 750 1000 760 800 800

Exiting gas flow Nm3/s 0.064 0.052 0.07 0.917 ‐

Exiting gas temperature 0C 650 800 760 527 700

Gas LHV kJ/Nm3 4764 4.7 4.3 4.4 4.7 Total thermal output kW 321 245 280 1000 105 Hot gas efficiency % 76.0 75 60 65

Superficial velocity, Us m/s 0.6447 0.535 0.75 2.8 6.681 Air Ratio 0.3271 0.28 0.22 0.25 0.29

Particle diameter, dp Micron 300 330 379 ‐ ‐ Viscosity kg/m/s 4.01E‐05 ‐ ‐ ‐ ‐

Gravitational acceleration, g m/s2 9.81

Particle density, ρd kg/m3 2600 ‐ ‐ ‐ ‐

Voidage at min. fldzn, εmf 0.66 ‐ ‐ ‐ ‐ Velocity (min. fluidization) m/s 0.322 0.268 0.375 1.4 3.340

Reynolds Number, Remf 0.6783 ‐ ‐ ‐ ‐ Velocity (max. fluidization) m/s 1.17 ‐ ‐ ‐ ‐

Terminal velocity, Ut m/s 2.15 1.78 2.50 9.33 22.27

The freeboard zone [9] is where the bed particles with terminal velocity

higher than the gas superficial velocity will leave the upper reaction bed

surface. The diameter is assumed to be equal to reaction bed diameter,

however the freeboard height (Hfb) is taken as a function of superficial

velocity ratio (between design, UsD and reference value, Us(ref)).


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Hfb = Hfb(ref) * UsD/Us(ref) (16)

Disengaging zone [9] is defined as the area above freeboard where the gas

and entrained particles ascend to before their speed is reduced. Most of the

particles will fall back into the bed zone [10]. The height (HZ) and diameter

(DDZ) is determined using the following functions.

HDZ = UtD/Ut(ref) * HDZ(ref) (17) DDZ = 1.5 * DB (18) B

(Us/Ut) = 0.3 (19)

The last function is the relationship [6] between superficial velocity and

terminal velocity (Ut) for a fluidized bed operation. The major section of the

above calculations is shown in Appendix 4 (as part of Excel worksheet). The

output of the design calculation is shown in Table 6.

Disengaging Zone

Freeboard

Reactor

Cyclone

Burner for Boiler

Screw Feeder1.8 m3 (1.3m x 1.3m x 1m) Blower

Ash BarrelAsh Grate

Feeding point m 0.26Bed section internal diameter m 0.459Dynamic bed height m 1.549Freeboard internal diameter m 0.459Freeboard height m 0.860Disengaging Zone Diameter m 0.689Disengaging Zone Height m 1.362Bed temperature 0C 750

Fig 2: Fluidized Bed Gasifier Components

Source: Adapted and enhanced from Gomez EO et al.(10) * Refractory thickness = 160mm

The gasifier will be constructed from carbon steel of 2‐3mm thickness, and

provided with internal heat insulation along the bed and freeboard made of

cement cylinders with thickness at 160mm. The bottom distributor will be of

bubble‐cap type to prevent clogging of feedstock or sand particles.


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4.0 GASIFIER DESIGN ECONOMICS

The feasibility of sawdust gasifier application in China is very much dependent on the cost of

biomass and comparative biomass energy cost. However, for the purpose of this design

study, the biomass cost is assumed negligible, as the sawdust available in the vicinity of the

timber mills is assumed to have no other functional value other than as waste product. A

model utilised by Leung YC et al. [1], has been used here for the economic analysis. Table 7

shows an adjusted economic value to suit the capital cost and operational cost for the

gasifier specific to this design study. The operating hours had been set to 2920 hours on

annual basis against the actual analysis of 6000 hours/year basis.

Table 6: Economic data [1] of Sawdust gasification and heating System in rural China (2004$)1

Capacity 200 kWt

Purpose Heating

Capital Cost (103 US$) Gasifier and Fittings 6.2 Control Unit 0.6 Base and Buildings 0.6 Installation 0.6 Design and Regulations 1.3 Total Capital Cost 9.3 Capital Cost US$/kW 46.5

Operation Cost (103 US$)1

Power consumption 0.5 Maintenance 0.3 Labour Cost 1.2 Total Operation Cost 1.9

Operation Cost 10‐7 US$/kJ 9.3

Based on the table, the total capital cost is US$ 9300, and the operation cost is US$1300 per

year. The operations cost per unit of energy output is US$ 9.3E(‐07) for operating the

gasifier for 2920 hours annually in southern China. However these figures do not provide a

valuable perception of the exact economic outcome anticipated. Thus the following simple

equation [1] had been used to determine the profit of the biomass energy.

1 Value for 200kW is for the overall heating system


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Pr = (P1 ‐ P2/η) ‐ (C1 + C2)

Pr is defined as the profit of biomass energy, and has to be above zero to enable the project

to be feasible. The sawdust biomass energy [1] in China is taken as 1.7E(‐06) US$/kJ and

defined by P1. The cost term for sawdust is neglected in this analysis due to the reasons

stated earlier in the section of this report. A sensitivity analysis was carried out using the

above principle and the following table was established.

Table 7: Sensitivity analysis for Sawdust gasification at different operational hours in a year Operational Hours 2920 4380 5840 8760

Capital Cost (C1) US$/kJ 4.42E‐06 2.95E‐06 2.21E‐06 1.47E‐06

Operation Cost (C2) US$/kJ 9.26E‐07 6.17E‐07 4.63E‐07 3.09E‐07 Pr ‐3.65E‐06 ‐1.87E‐06 ‐9.75E‐07 ‐8.31E‐08

Based on the above analysis, the project encounters loss even for 24 hour operation on

annual basis. However, the need to consider the viability of the design application will be

further elaborated in the discussion part of the report.


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5.0 TECHNICAL AND NONTECHNICAL BARRIERS

The implementation of gasification in a small scale such as this design work would face

considerable amount of challenges, technically and non‐technically. The rural society of

China will have to benefit from the gasification project, however, the initial investment and

subsequent operations cost will have to be borne by an institution which has the capacity to

undertake the project. The timber mills in the rural areas, closer to the logging area would

effectively be the suitable owner of such undertakings. However, the technicalities involved

in defining the design efficacies, operating conditions and maintenance of the gasifier will

be outside the expertise of this mill operators. There has to be an interface between the

engineering, construction and management of the gasifier to enable the society to absorb

the technology and sustain it. As such, no frill designs need to be developed and marketed

at the least cost.

In terms of cost of operation, efficiency of fluidised bed gasifier will play a vital role since

biomass consumption would be largely affected by losses in the gasifier in the form of heat

or carbon loss. Significant improvement need to be seen in this area to make gasifier use

attractive and rewarding economically. There has to be more small‐scale higher efficiency

gasifiers developed cost effectively to suit particular need of the rural society in China,

parallel to the development and construction of medium to large scale gasifiers.

Apart from these considerations, the sustainability of fluidised bed gasifier utilisation will

depend very much on the support policy by the government of China [1], especially to

provide fund allocation for smaller units, tax reduction or relaxation for major sponsors (or

timber mill operators), and to focus on effective use of waste biomass such as sawdust.

Waste management using small scale gasifier unit (as proposed in this design work) could be

undertaken by the government to make timber industry cost effective and environmentally

sustainable.


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6.0 DISCUSSION AND CONCLUSION

The design of a fluidized bed gasifier using sawdust as feedstock had been demonstrated for

a heating output of 200 kWt to be used for rural society in China. The design output in terms

of product gas composition, heating value of gas and gasifier size, by large, agrees with

design work done by other authors’ referenced. The conversion of sawdust produces 3.0

kWh/kg of feedstock and consumes approximately 800 kg per 8h operation. The overall

system efficiency is 62.3 % when the efficiency of heating equipment is combined with

thermal efficiency of the fluidised bed gasifier.

The economic analysis was complemented with a sensitivity analysis and was found to show

that the gasifier designed may not be suitable for rural usage in terms of profitability. This is

largely due to the cost‐ineffectiveness of smaller scale gasifiers (less than 1000 kWt) in

China, whereby the total cost of implementation becomes larger and thus reduces the

profitability of biomass energy production. However, the benefits of pursuing the

application would allow cost savings in terms of biomass waste storage, transport and

pollution control which have not been included in the sensitivity analysis. Pragmatism

makes this secondary factor as a driving force for the small‐medium scale gasifier to be

installed and used in many parts of rural China, as can be proven by the hundreds of small

scale gasifiers already in use in China since late 20th century.

The technical and non‐technical aspects will be met by China as it progresses towards a

dynamic nation with large energy consumption bill in coming years. This factor will provide

the motive force for engineering, construction and operation of low cost small scale gasifiers

in rural China in future.


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REFERENCES

[1] Leung YC Dennis, Yin XL, Wu CZ. A review on the development and commercialization of biomass gasification technologies in China. Renewable and Sustainable Energy Reviews 2004; 8: 565‐580

[2] McKendry P. Energy production from biomass (part 3): gasification technologies. Bioresource Technology 2002; 83: 55‐62

[3] Thermodynamic Data for Biomass Conversion and Waste Incineration, National Bureau of Standards, Solar Energy Research Institute, US

[4] Zainal ZA, Ali R, Lean CH, Seetharamu KN. Prediction of performance of a downdraft gasifier using equilibrium modeling for different biomass materials. Energy Conversion and Management 2001; 42: 1499‐1515

[5] Wood Gas as Engine Fuel, FAO

[6] El‐Mahallawy F, El‐Din Habik S. Fundamentals and Technology of Combustion. Elsevier 2002; 677‐693

[7] Venkata Ramayya A, Eyerusalem M, Endalew M, Melaku M. Design and Simulation of Fluidized Bed Power Gasifier for a Coffee Hulling Center. Advances in Energy Research 2006; 83‐89

[8] Yin XL, Wu CZ, Zheng SP, Chen Y. Design and operation of a CFB gasification and power generation system for rice husk. Biomass and Bioenergy 2002; 23: 181‐187

[9] Gomez EO, Cortez LAB, Lora ES, Sanchez CG, Bauen A. Preliminary tests with a sugarcane bagasse fueled fluidized‐bed air gasifier. Energy Conversion and Management 1999; 40: 205‐214

[10] Gomez EO, Lora ES. Constructive features, operation and sizing of fluidized‐bed gasifiers for biomass. Energy for Sustainable Development 1995; (2) 4: 52‐57

[11] van den Enden PJ, Lora ES. Design approach for a biomass fed fluidized bed gasifier using the simulation software CSFB. Biomass and Bioenergy 2004; 26: 281‐287

[12] Li XT, Grace JR, Lim CJ, Watkinson AP, Chen HP, Kim JR. Biomass gasification in a circulating fluidized bed. Biomass and Bioenergy 2004; 26: 171‐193

[13] El‐Mahallawy F, El‐Din Habik S. Fundamentals and Technology of Combustion. Elsevier 2002; 764‐765


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Appendix 1: Summary of Sawdust Fluidized Bed Gasifier Design

Table S1: Sawdust Composition

Component wt% Normalized MW Mole Amount Mole fraction C basis

C 49.25 49.45 12 4.12 0.3190 1

H 5.99 6.01 1 6.01 0.4655 1.459

O 44.36 44.54 16 2.78 0.2155 0.676

Total 99.6 100 12.92 1 Source: Thermodynamic data for biomass conversion and waste incineration, National Bureau of Standards, Solar Energy Research Institute, US

Table S2: Calculated Stoichiometric Component and

Equilibrium Constant Moisture content, w (mol) 0.12 Gasification oxygen, m (mol)

0.336 Stoichiometric combustion air, mstc.air (mol)

3.864 Gasification air, mair (mol) 1.264 Bed temperature, TB ( C) B

o

750 TB (K)

1023.15 Equilibrium constant, K1 0.0762 Equilibrium constant, K2 0.1986 Hydrogen content in Wood, a (mol) 1.459 Oxygen content in Wood, b (mol) 0.676

Table S3: Sawdust Properties

Properties of Sawdust Value Unit

Lower Heating Value, Hs 15120 kJ/kg

Bulk density, ρb 300 kg/m3

Moisture Content (d.b.) 8.9 % wt

Ash Content 0.4 % wt

Average Diameter, dave 2 mm

Source: Thermodynamic data for biomass conversion

and waste incineration, National Bureau of Standards,

Solar Energy Research Institute, US


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Table S4: Predicted Product Gas Composition

Gas Components Variables Predicted Values

Mole fraction

Percentage (%)

H2 x1 0.5200 0.1693 16.93

CO x2 0.7794 0.2537 25.37

CO2 x3 0.2000 0.0651 6.511

H2O x4 0.2885 0.0939 9.392

CH4 x5 0.0206 0.0067 0.671

N2 3.76*m 1.2634 0.4113 41.13

Total 3.0719 1.0000 100

Table S5: Process Design Parameters

Design Parameters Value Unit

Thermal efficiency, η 76 %

Gasifier output, PD 321 kWt

Heating value of gas, Hg 4763.8 kJ/Nm3

AGas volumetric flow, Qg 0.0644 Nm3/s

Gas density, ρg 0.2816 kg/m3

Gas Specific Heat, cp 1.391 kJ/kg oC

1Heating value of Solid, Hs 15120 kJ/kg

BSolid Fuel Consumption, Ms 0.0279 kg/s

Solid Fuel Inlet Temp, Tis 27 oC

Gas Outlet Temp, Tos 650 oC

2Air Density, ρa 1.1714 kg/m3

Air Ratio, ER 0.327

Air volumetric flow, Qa 0.0334 Nm3/s

Sensible Heat Recovered, H 244.1 kJ/Nm3

Specific Consumption, Uc 0.3293 kg/kWh

Boiler Rated Efficiency 82 %

Boiler Output 200 kW 1 LHV A 143.8 Nm3/h 2 at 27 0C, 101.325 kPa B 62.7 kg/h


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Table S6: Economic data of Sawdust gasification

and heating system in rural China (2004 $)

Capacity 200 kWt

Purpose Heating

Capital Cost (103 US$)

Gasifier and Fittings 6.2

Control Unit 0.6

Base and Buildings 0.6

Installation 0.6

Design and Regulations 1.3

Total Capital Cost 9.3

Capital Cost US$/kW 46.5

Operation Cost (103 US$)1

Power consumption 0.5

Maintenance 0.3

Labour Cost 1.2

Total Operation Cost 1.9

Operation Cost 10‐7 US$/kJ 9.3

Adapted from D.Y.C. Leung et al. / Renewable

and Sustainable Energy Reviews 8 (2004) 571

and adjusted for 12920h operations against

6000h operations annually

Table S8: Volume of Sawdust Storage for Screw Feeder

Daily 8hrly Volume m3 2.68

Volume (with 10% Safety Factor) m3 2.95

Dimension (L x W x H) M 1.7 x 1.7 x 1


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Table S7: Gasifier Design Parameters and Comparative References

Design Element Unit ValueD Value1 Value2 Value3 Value4

Type of biomass fuel ‐ Sawdust Sugarcane Bagasse

Sugarcane Bagasse

Rice Husk Sawdust

Total thermal output kWt 200 245 280 1000 105

Feeding rate kg/s 0.0279 0.03 0.0290 0.4167 0.0085

Moisture (d.b.) % 8.9 15 11.7

Feeding point m 0.26 0.3 0.26 1.8 0.89

Bed section internal diameter m 0.459 0.57 0.417 1.8 0.1

Dynamic bed height m 1.549 2 1 7.4 6.5

Freeboard internal diameter m 0.459 0.75 0.417 ‐ ‐

Freeboard height m 0.860 3 1 ‐ ‐

Disengaging Zone Diameter m 0.689 ‐ 0.835 ‐ ‐

Disengaging Zone Height m 1.362 ‐ 1.585 ‐ ‐

Bed temperature 0C 750 1000 760 800 800

Exiting gas flow Nm3/s 0.064 0.052 0.070 0.917 ‐

Exiting gas temperature 0C 650 800 760 527 700

Air volume flow Nm3/s 0.0334 0.0320 0.0271 ‐ 0.0146

Hot gas efficiency, η % 76 75 60 65

Air Ratio m3/m3 0.327 0.28 0.22 0.25 0.29

Particle diameter, dp micron 300 330 379 ‐ ‐

Particle density, ρd kg/m3 2600 ‐ ‐ ‐ ‐

Velocity (at min fluidization) m/s 0.322 0.268 0.375 1.4 3.34 D ‐ Design Values; 1 ‐ van de Enden, et al.; 2 ‐ E. Olivares Gomez, et al.; 3 ‐ Xiu L. N., et al.; 4 ‐ Li X. T. et al.


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Appendix 2: Thermodynamic Properties and Estimates Worksheet

A. Prediction of Product Gas Composition based on Biomass Composition using Universal Gasification Reaction Equation,

Reaction Equilibrium Constants' Equation, Gibbs function of Formation and Heat of Formation Estimation

A1: Universal Gasification Reaction

CH aOb + wH2O + mO2 + 3.76mN2 → x1H2 + x2CO + x3CO2 + x4H2O + x5CH4 +3.76mN2

Mole Balance of Component:

C: 1 = x2 + x3 + x5 (1)

H: 2w + a = 2x1 + 2x4 + 4x5 (2)

O: w +b + 2m = x2 + 2x3 + x4 (3)

A2: Equilibrium Constants in Gasification reaction:

A2.1 Methane formation

K1 = x5/(x2)2 (4)

A2.2 Shift reaction (CO and H2 formation)

K2 = x1x3/(x2x4) (5)

Solving the simultaneous sets of equations:

From Eq. (1) x5 = 1 – x2 – x3 (6

From Eq. (2) x4 = w + 0.5a ‐ x1 ‐ 2x5 (7

Using x5 from Eq.(1) to Eq. (2) x4 = – x1 + 2x2 + 2x3 + w‐2+a/2 (8

From Eq. (4) x1 2 K1 = 1 – x2 –x3 (9

Substituting value of x4 from the Eq. (7) to Eq. (3) – x1 + 3x2 + 4x3 = 2m + 2+b‐a/2 (1

Substituting x4 in Eq.(7)to Eq.(5) x1x3 = K2 x2 [ – x1 + 2x2 + 2x3 + w –2+a/2] (1

Generating the Equilibrium Constant equation from the general equations:

Standard Gibbs Function

ΔG0 = J ‐ RT(ΔA lnT + (ΔB/2)T + (ΔC/6)T2 + ΔD/2T2 + I)

Heat of Formation

ΔH0 = (J/R) + ΔA lnT + (ΔB/2)T2 + (ΔC/3)T3 ‐ ΔD/2T

Equilibrium Constant

lnK = ‐(J/RT) + ΔA lnT + (ΔB/2)T + (ΔC/6)T2 + ΔD/2T2 + I)

The values for J and I are determined by substituting the constants A,B,C and D, and

the values of standard Gibbs and heat of formation at 298.15K. These values are tabulated

as follows:


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Component Condition ΔG298.15 (kJ/kmol) Component Condition

H2O l ‐237129 H2O l

H2O g ‐228572 H2O g

CO2 g ‐394359 CO2 g

CO g ‐137169 CO g

CH4 g ‐50460 CH4 g

H2 g 0 H2 g

N2 g 0 N2 g

Component A B C D

CH4 1.702 9.08E‐03 ‐2.164E‐06

H2 3.249 4.22E‐04 8300

CO2 3.376 5.57E‐04 ‐3100

CO 5.457 1.05E‐03 ‐115700

N2 3.28 5.93E‐04 4000

H2O 3.47 1.45E‐03 12100

C 1.771 7.71E‐04 ‐86700

Source: Z.A.Zainal et al., Energy Conversion and Management, 42 (2001) 1499‐1515

The following intermediate values have been generated to derive the final equilibrium

constant equation:

R (kJ/kmol.K) 8.314

T (K) 298.15

For K1 For K2

ΔA ‐6.567 Del A ‐2.302

ΔB 7.47E‐03 Del B ‐1.52E‐03

ΔC ‐2.16E‐06 Del C 0

ΔD 70100 Del D 108800

ΔH ‐74520 Del H ‐41166

ΔG ‐50460 Del G ‐28618

ΔH/R ‐8963.19 ΔH/R ‐4951.41

Alfa (lumped constant) ‐1880.35 Alfa (lumped constant) ‐1118.73

J ‐58886.80 J ‐31864.89


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Gamma (lumped constant) ‐35.941 Gamma (lumped constant) ‐12.730

I 32.541 I 11.420

Lambda (lumped constant) 7082.85 Lambda (lumped constant) 3832.68

ΔA ‐6.5670 ΔA ‐2.302

ΔB/2 3.73E‐03 ΔB/2 ‐7.59E‐04

ΔC/6 ‐3.61E‐07 ΔC/6 0

ΔD/2 35050.00 ΔD/2 54400

Substituting the above calculated values, the following sets of equilibrium constant

equations are formed:

ln K1 =[ 7082.848/T – 6.567 ln T + 7.466E‐3 T/2 – 2.164E‐6 T2/6 + 0.701E‐5/(2 T2)+32.541] (12) ln K2 = [ 3832.679/T ‐ 2.302 ln T – 7.6E‐4 T + 54400/(T2) + 11.42] (13)

The values of a and b in the Gasification Reaction is determined using the Sawdust

composition from literature:

Component wt% Normalized MW Mole

Amount Mole

fraction C

basis

C 49.25 49.45 12 4.121 0.319 1.000

H 5.99 6.01 1 6.014 0.466 1.459

O 44.36 44.54 16 2.784 0.215 0.676

Total 99.6 100.00 12.918 1.000 Source: Thermodynamic data for biomass conversion and waste incineration, National Bureau of Standards, Solar Energy Research Institute, US

The set of equations (7) to (11) can be solved as follows. Firstly, the value of m and w is specified. Then for a known

temperature T(isothermal), K1 & K2 is determined using Eq. (12) and Eq. (13). Next x1, x2, & x3 are found using Eq. (9), (10), & Eq. (11) respectively. Subsequently x4 & x5 are determined using Eq. (6) & Eq. (7) respectively. Solving the above sets of equations will produce polynomial equations. In order to determine the values of x1, x2 and x3,

a trial‐and‐error method was used via this Excel spreadsheet.

Moisture content, w (mol) 0.120 Gasification oxygen, m (mol) 0.336


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Stoichiometric combustion air, mstc.air (mol) 3.864

Gasification air, mair (mol) 1.264 Bed temperature, TB ( C) B

o

750.00 TB (K) 1023.15 Equilibrium constant, K1 0.076 Equilibrium constant, K2 0.199 Hydrogen content in Wood, a (mol) 1.459 Oxygen content in Wood, b (mol) 0.676

Gas Components Variables Predicted Values Mole fraction Percentage (%)

H2 x1 0.5200 0.1693 16.93

CO x2 0.7794 0.2537 25.37

CO2 x3 0.2000 0.0651 6.51

H2O x4 0.2885 0.0939 9.39

CH4 x5 0.0206 0.0067 0.67

N2 3.76*m 1.2634 0.4113 41.13

Total 3.0719 1.0000 100.00

Equation No. Error LHS RHS

Eqn (9) 0.000 0.021 0.021

Eqn (10) 0.000 2.618 2.618

Eqn (11) 0.059 0.104 0.045

Eqn (7) 0.000 0.289 0.289

Eqn (6) 0.000 0.021 0.021

Appendix 3: Process Parameter Calculation


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B1.1: Specific heat of product gas is calculated using the following equation and constants:

cp,I(T) = ai,0 + ai,1*T

where ai,0 and ai,1 are constants obtained from Fundamentals and Technology

of Combustion, El‐Mahallawy F., El‐Din Habik S.(p764‐765)

Specific heat calculation

Gas mixture at Bed Temperature T= oC 750.00

Kelvin 1023.15

Gas Composition % vol Cp Weighted cp MW Weighted MW

CO 25.37 33.252 8.436 28.01 7.107 H2 16.93 30.261 5.122 2.02 0.342 CH4 0.67 35.062 0.235 16.04 0.108 CO2 6.51 43.168 2.810 44.01 2.865

N2 41.13 32.769 13.477 28.02 11.524 H2O 9.39 29.888 2.807 18.016 1.692 Total 100.00 32.888 23.64

Gas cp 32.888 kJ/kmole.degC

Gas cp 1.391 kJ/kg.degC

B1.2: The Heating Value of Product Gas is calculated using the composition generated from the Thermodynamic

estimation and the Heating Values of individual component

Volume Fraction

Molecular Weight

HHV Mol. Wgt. Contribution Weighted

Values

Gas Component (= mole fraction) Btu/scf HHV, Btu/scf

C1 0.00671 16 1013.2 0.107 6.798

CO2 0.06511 44 0 2.865 0.000

CO 0.25372 28 320.5 7.104 81.316

H2 0.16928 2 325 0.339 55.015

H2O 0.09392 18 0 1.691 0.000

Inerts (N2) 0.41127 28 0 11.515 0.000

Total 1 23.621 143.129

HHV 143.13 Btu/scf

LHV 128.8 Btu/scf

Heating Value of Gas (LHV) 4763.8 kJ/Nm3

Appendix 4: Gasifier Sizing Worksheet


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The following sets of equation had been used to formulate the sizing of the gasifier

A.3 The air flow rate through the gasifier as a function of superficial velocity (Us) and gasifier internal diameter (Dg)

mair = ρair * ((π * Dg^2)/4 )* Us

Ref: Venkata Ramayya A., et. al; Design and Simulation of Fluidised Bed Power Gasifier for a Coffee Hulling Center

A.4 Relation between superficial velocity (Us) and minimum fluidisation velocity (Umf)

Us = 2 * Umf

Ref: Venkata Ramayya A., et al.; Design and Simulation of Fluidised Bed Power Gasifier for a Coffee Hulling Center

A.5 The relationship between minimum fluidisation velocity, minimum dynamic bed height (HB) and residence time (t)

HB = t * Umf

H = HB + HB f

Ref: Yin X. L., et al.; Biomass and Bioenergy 23 (2002) 181 – 187

A.6 The determination of minimum fluidisation velocity, Umf

Umf = [dp^2 / (150* μg)]*[g*(ρp‐ ρg)*(εmf^3)/(1‐ εmf)]

where, μg is gas viscosity

ρp is bed fluidising particle density

ρg is gas density

εmf is bed voidage at minimum fluidization


A.7 Maximum fluidisation velocity, Umax

Umax = (8/6)*([dp*(ρp‐ ρg)*g/(Cd*ρg)]^0.5)

where, Cd is drag coefficient

Ref: El‐Mahallawy et al.; Fundamentals and Technology of Combustion, 677 ‐ 693

A.8 Drag coefficient determination

Remf = (ρg * Umf * dp)/μg Remf is Reynolds number for minimum fluidization

Cd = 24/Re for low Re

Cd = 0.44 for Re>= 10^3


A.9 Bed pressure drop, DelP

ΔP = (ρp ‐ ρg)*g*H*[1 ‐ εmf]


A.10 Gas viscosity (μg) for the predicted composition is estimated using on‐line software available at www.firecad.net

μg = 4.014E‐05 kg/m.s

A.11 The assumed relationship between disengaging zone height and terminal velocity

(Us/Ut) = 0.3

http://www.firecad.net/


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HDZ = UtD/Ut(ref) * HDZ(ref)

where, Ut(ref) is Ut reference from literature(2)

HDZ(ref) is HDZ reference from design(2)

A.12 The disengaging zone diameter and bed internal diameter

Dz = 1.5 * DB

Ref: E. Olivares Gomez et al.; Energy Conversion & Management 40 (1999) 205 ‐ 214

A.13 The assumed relationship between freeboard height and superficial velocity

Hfb = Hfb(ref) * UsD/Us(ref)

where, Us(ref) is Us reference from literature(2)

Hfb(ref) is Hfb reference from literature(2)

fluidized bed gasifier design report public)

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