Fluidized Bed Gasifier Design Report Public)

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

DESIGN

OF A

GASIFIER

FOR

LOW COST FLUIDIZED BED SAWDUST GASIFICATION

IN

RURAL CHINA

[Vigneswaran KUMARAN] | 277492

---------------------------------------------Coordinator: Dr. Lu Aye

Department of Civil of Civil and Environmental

V Kumaran

Digitally signed by V Kumaran DN: cn=V Kumaran, c=MY Reason: I am the author of this document Date: 2009.06.18 12:09:44 +07'00'

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

Abstract

The task of designing 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.

2|Page


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 of heating

•

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

•

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

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Contents Abstract Aim

2 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 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 of gasifier and elements of operation of operation

8

Figure 2 Fluidized Bed Gasifier Diagram

15

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 of product gas temperature

12

Table 4 Process parameters and other estimates

12

Tables

4|Page


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

14

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

16

Table 7 Sensitivity analysis for sawdust gasification at different operating hours

17

5|Page


1.0 INTRODUCTION There are various technologies for the conversion of biomass 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 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. Source: www.travelchina uide.com

In

this

large

country,

developing gasification

technology has been in use in many

biomass

related

industries such as agricultural production industry.

and However,

timber 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 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.

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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 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, 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 of gasifier and elements of operation of operation

(Source: www.gasnet.co.uk )

The fluidized type bed is selected for this design study due to the characteristic of the of the feedstock and the ease of control of control in terms of handling of handling of feedstock of feedstock in the gasifier.

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3.0

DESIGN METHODOLOGY FOR 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 of product gas composition as a function of process of process temperature. These estimates are used for the sizing of the of the fluidized bed gasifier for sawdust gasification. 3.1

Prediction of product 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 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. 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) 0

2

3

ΔH = (J/R) + ΔA lnT + (ΔB/2)T + (ΔC/3)T ‐ ΔD/2T 0

2

2

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

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(6) (7) (8)


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 of equation are provided in Table 1. Table 1: Intermediate values for solving equation (1) to (5) Moisture content, w (mol) Gasification oxygen, m (mol) Stoichiometric combustion air, mstc.air (mol) Gasification air, mair (mol) o

0.120 0.336 3.864 1.264

Bed temperature, TB ( C)

750.00

TB (K)

1023.15

Equilibrium constant, K1

0.076


0.199 1.459

Hydrogen content in Wood, a (mol) 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 Predicted Mole Gas Components Variables Percentage (%) Values fraction 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

3.0719

1.0000

100.00

Total

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 * Q g) + (Q g * ρg * cp * ΔT)]/(Hs * Ms)

(10)

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

Volume Fraction

Molecular Weight

HHV Btu/scf

Mol. Wgt. Contribution

(= mole fraction) C1 CO2 CO H2 H2O Inerts (N2)

0.00671 0.06511 0.25372 0.16928 0.09392 0.41127

Weighted Values HHV, Btu/scf

16 44 28 2 18 28

1013.2 0 320.5 325 0 0

0.107 2.865 7.104 0.339 1.691 11.515

6.798 0.000 81.316 55.015 0.000 0.000

Total 1 23.621 143.129 T 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 Thermal efficiency, η Gasifier output, PD

321 kW 4763. 4763.80 80 kJ/N kJ/Nm m3

Gas volumetric flow, Q g

0.06 0.0641 41 Nm3/s

Gas density, ρg Gas Specific Heat, cp 1

Heating value of Solid, of Solid, Hs

Solid Fuel Consumption, Ms *Solid Fuel Consumption, Ms Solid Fuel Inlet Temp, Tis Gas Outlet Temp, Tos

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Unit 76 %

Heating value of gas, of gas, Hg

*Gas volumetric flow, Q g

1

Value

230.7 Nm3/h 3

0.28 0.2816 16 kg/m kg/m

1.39 1.391 1 kJ/k kJ/kg g oC 1512 15120 0 kJ/k kJ/kg g 0.0279

kg/s

100. 100.5 5 kg/h kg/h 27

o

C

650

o

C

2

Air Density, ρa Air Ratio, ER

1.17 1.171 1 kg/m kg/m3 0.3271

Air volumetric flow, Q a

0.03 0.0334 34 Nm3/s

Sensible Heat Recovered, H

244. 244.06 06 kJ/N kJ/Nm m3

Specific Consumption, Uc Boiler Rated Efficiency

0.33 0.33 kg/k kg/kWh Wh 82 %

Boiler Output LHV, 2 @ 27 0C, 101.325 kPa

200 kW


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 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 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 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 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‐ ρ g)*(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 B

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 Umf . The overall reaction bed height (H) is the sum of the height of sawdust of sawdust feeding point and dynamic bed height. HB = t * Umf

(14)

H = HB + Hf

(15)

B

B

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

Parameters

Sawdust

Value[11] Sugarcane bagasse

Value[9] Sugarcane bagasse

Value[8] Rice husk

Sawdust

0.0290

0.4167

0.0085

Units Type of biomass of biomass fuel

Value[12]

Feeding rate

kg/s

0.0279

0.03

Moisture (d.b.)

% wt

8.9

15

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

C

750

1000

760

800

800

Nm3/s

0.064

0.052

0.07

0.917

‐

C

650

800

760

527

700

kJ/Nm3

4764

4.7

4.3

4.4

4.7

kW

321

245

280

1000

105

%

76.0

75

60

65

m/s

0.6447

0.535

0.75

2.8

6.681

0.3271

0.28

0.22

0.25

0.29

0

Bed temperature Exiting gas flow

0

Exiting gas temperature Gas LHV Total thermal output Hot gas efficiency Superficial velocity, Us Air Ratio

11.7

Particle diameter, dp

Micron

300

330

379

‐

‐

Viscosity

kg/m/s

4.01E‐05

‐

‐

‐

‐

Gravitational acceleration, g

m/s2

Particle density, ρd

kg/m3

2600

‐

‐

‐

‐

0.66

‐

‐

‐

‐

0.322

0.268

0.375

1.4

3.340

0.6783

‐

‐

‐

‐

m/s

1.17

‐

‐

‐

‐

m/s

2.15

2.50

9.33

22.27

Voidage at min. fldzn, εmf Velocity (min. fluidization)

m/s

Reynolds Number, Remf Velocity (max. fluidization) Terminal velocity, Ut D

‐ Design Values;

[11]

‐ van de Enden, et al.;

[9]

9.81

‐ E. Olivares Gomez, et al.;

1.78 [8]

– Yin X.L., et al.;

[12]

‐ Li X. T. et al.

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)). 14 | P a g e


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)

(Us/Ut) = 0.3

(19)

B

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 of the design calculation is shown in Table 6.

Fig 2: Fluidi Fluidi zed zed Bed Gasifier Gasifier Components

Burner for Boiler

Disengaging Zone

Cyclone

Freeboard Reactor Screw Feeder 1.8 m 3 (1.3m x 1.3m x 1m)

Ash Barrel Ash Grate

Blower

Source: Adapted and enhanced from Gomez EO EO et et al.(10)

Feeding point Bed section internal diameter Dynamic bed height Freeboard internal diameter Freeboard height Disengaging Zone Diameter Disengaging Zone Height Bed temperature

m

0.26

m

0.459 1.549

m

m

0.459 0.860

m

0.689

m

1.362 750

m

0

C

* 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 of feedstock or sand particles.

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4.0

GASIFIER DESIGN ECONOMICS

The feasibility of sawdust 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 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 of 6000 hours/year basis. Table 6: Economic data [1] of Sawdust of Sawdust gasification and heating System in rural China (2004$) 1 Capacity Purpose

200 kWt Heating

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

6.2 0.6 0.6 0.6 1.3 9.3 46.5

Operation Cost (103 US$)1 Power consumption Maintenance Labour Cost Total Operation Cost

0.5 0.3 1.2 1.9

7

Operation Cost 10‐ 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 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 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 Pr

9.26E‐07 ‐3.65E‐06

6.17E‐07 ‐1.87E‐06

4.63E‐07 ‐9.75E‐07

3.09E‐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 of the report.

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5.0

TECHNICAL AND TECHNICAL AND NON-TECHNICAL 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 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 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 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 DISCUSSION AND CONCLUSION

The design of a of a fluidized bed gasifier using sawdust as feedstock had been demonstrated for a heating output of 200 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 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. 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 th

scale gasifiers already in use in China since late 20 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 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, of Standards, Solar Energy Research Institute, US

[4]

Zainal ZA, Ali R, Lean CH, Seetharamu KN. Prediction of performance of performance of a 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 of Sawdust Fluidized 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 Thermodynamic data for for biomass biomass conversion and waste and waste incineration, National Bureau National Bureau of Standards, Solar Energy Solar Energy Research Research Institute, US

Table S2: Calculated Stoichiometric Component and Equilibrium Constant Moisture content, w (mol) Gasification oxygen, m (mol) Stoichiometric combustion air, mstc.air (mol) Gasification air, mair (mol)

0.12 0.336 3.864 1.264

o

Bed temperature, TB ( C) B

TB (K) Equilibrium constant, K1 Equilibrium constant, K2

750 1023.15 0.0762 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 of Sawdust Lower Heating Value, Hs

Value

Unit

15120

kJ/kg

Bulk density, ρb

300

kg/m

Moisture Content (d.b.)

8.9

% wt

Ash Content

0.4

% wt

Average Diameter, dave Source: Thermodynamic data Thermodynamic data for for biomass biomass conversion and waste and waste incineration, National Bureau National Bureau of Standards, of Standards, Solar Energy Solar Energy Research Research Institute, US

21 | P a g e

2

3

mm


Table S4: Predicted Product Gas Composition Predicted Values

Mole fraction

Percentage (%)

Gas Components

Variables

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

3.0719

1.0000

100

Total

Table S5: Process Design Parameters Design Parameters

Value

Thermal efficiency, η

Unit 76

Gasifier output, PD

321

% kWt 3

Heating value of gas, of gas, Hg

4763.8

kJ/Nm

A

0.0644

Nm /s

Gas density, ρg

0.2816

kg/m

Gas volumetric flow, Q g

Gas Specific Heat, cp 1

Heating value of Solid, of Solid, Hs

B

Solid Fuel Consumption, Ms

Solid Fuel Inlet Temp, Tis Gas Outlet Temp, Tos 2

Air Density, ρa

Sensible Heat Recovered, H Specific Consumption, Uc Boiler Rated Efficiency

2

at 27 at 27 C, 101.325 kPa

0

22 | P a g e

15120

kJ/kg

0.0279

kg/s

27

o

C

650

o

C 3

kg/m

3

0.0334

Nm /s

244.1

kJ/Nm

0.3293

kg/kWh

82

Boiler Output LHV

o

kJ/kg C

0.327

Air volumetric flow, Q a

1

3

1.391

1.1714

Air Ratio, ER

3

200

3

% kW

A

143.8 Nm /h

3

B

62.7 kg/h 62.7 kg/h


Table S6: Economic data of Sawdust of Sawdust gasification and heating system in rural China (2004 $) Capacity

200 kWt

Purpose

Heating 3

Capital Cost (10 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 3

46.5 1

Operation Cost (10 US$) 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. et al. / / Renewable Renewable and Sustainable and Sustainable Energy Reviews Energy Reviews 8 (2004) 8 (2004) 571 1

and adjusted for 2920h operations against 6000h operations annually

Table S8: Volume of Sawdust 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

23 | P a g e

1.7 x 1.7 x 1


Table S7: Gasifier Design Parameters and Comparative References Design Element

Unit

D

Value

1

2

3

4

Sawdust

Value Sugarcane Bagasse

Value Sugarcane Bagasse

Value Rice Husk

Value

Sawdust

Type of biomass of biomass fuel

‐

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

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

0

750

1000

760

800

800

Exiting gas flow

Nm /s

0.064

0.052

0.070

0.917

‐

Exiting gas temperature

0

650

800

760

527

700

Air volume flow

Nm /s

0.0334

0.0320

0.0271

‐

0.0146

Hot gas efficiency, η

%

76

75

60

65

Air Ratio

m /m

0.327

0.28

0.22

0.25

0.29

Particle diameter, dp

micron

300

330

379

‐

‐

Particle density, ρd

kg/m

2600

‐

‐

‐

‐

1.4

3.34

C 3

C 3

3

3

3

11.7

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

24 | P a g e


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 of Formation and Heat of Formation of Formation Estimation A1: Universal Gasification Reaction CH aOb + wH2O + mO2 + 3.76mN2 → x1H2 + x2CO + x3CO2 + x4H2O + x5CH4 +3.76mN 2 Mole Balance of Component: 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: of equations: From Eq. (1) x5 = 1 – x2 – x2 – x3

(

From Eq. (2) x4 = w + 0.5a ‐ x1 ‐ 2x5

(

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

(

From Eq. (4) x1 2 K1 = 1 – x2 –x3 x2 –x3

(

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

(

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

(

Generating the Equilibrium Constant equation from the general equations: Standard Gibbs Function 0

2

2

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

Heat of Formation of Formation 0

2

3

ΔH = (J/R) + ΔA lnT + (ΔB/2)T + (ΔC/3)T ‐ Δ D/2T

Equilibrium Constant 2

2

lnK = ‐(J/RT) + ΔA lnT + (ΔB/2)T + (ΔC/6)T + ΔD/2T + I) The values for J and I are determined by substituting the constants A,B,C and D, and the values of standard of standard Gibbs and heat of formation of formation at 298.15K. These values are tabulated as follows:

25 | P a g e


Component

Condition

ΔG298.15 (kJ/kmol)

Component

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

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

D

Source: Z.A.Zainal Source: Z.A.Zainal et et al., al., Energy Conversion Energy Conversion and Management, 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

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

‐8963.19

ΔH/R

‐4951.41

‐1880.35

Alfa (lumped constant)

‐1118.73

ΔH/R

Alfa (lumped constant) J

26 | P a g e

For K2

‐58886.80

J

‐31864.89

Condition


Gamma (lumped constant)

‐35.941

I

32.541

Gamma (lumped constant) I

‐12.730

11.420

Lambda (lumped constant)

7082.85

Lambda (lumped constant)

ΔA

‐6.5670

ΔA

3832.68 ‐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 of equilibrium constant equations are formed: 2

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

(12) (13)

The values of a of a and b in the Gasification Reaction is determined using the Sawdust composition from literature:

wt%

Normalized

MW

Mole Amount

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

Component

Mole fraction

C basis

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

The set of equations of equations (7) to (11) can be solved as follows. Firstly, the value of m 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 of equations will produce polynomial equations. In order to determine the values of x1, 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

27 | P a g e


Stoichiometric combustion air, mstc.air (mol)

3.864

Gasification air, mair (mol)

1.264

o

Bed temperature, TB ( C)

750.00

B

TB (K)

1023.15


0.076


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

3.0719

1.0000

100.00

Total

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

28 | P a g e


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

cp,I(T) = ai,0 + ai,1*T and Technology where ai,0 and ai,1 are constants obtained from Fundamentals and Technology of Combustion, of Combustion, El ‐Mahallawy F., Mahallawy F., El ‐Din Habik S.(p764 Habik S.(p764‐765)

Specific heat calculation o

Gas mixture at Bed Temperature

T= C

750.00

Kelvin

1023.15

Gas Composition

% vol

Cp

Weighted cp

MW

Weighted MW

CO H2 CH4

25.37 16.93 0.67

33.252 30.261 35.062

8.436 5.122 0.235

28.01 2.02 16.04

7.107 0.342 0.108

CO2

6.51

43.168

2.810

44.01

2.865

N2 H2O Total

41.13 9.39 100.00

32.769 29.888

13.477 2.807 32.888

28.02 18.016

11.524 1.692 23.64

Gas cp

32.888

kJ/kmole.degC

Gas cp

1.391

kJ/kg.degC

B1.2: The Heating Value of Product of Product Gas is calculated using the composition generated from the Thermodynamic estimation and the Heating Values of individual of individual component

Volume Fraction Gas Component

Molecular Weight

(= mole fraction)

HHV

Mol. Wgt. Contribution

Btu/scf

Weighted Values 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

23.621

143.129

Total

1

HHV

143.13

Btu/scf

LHV

128.8

Btu/scf

4763.8

kJ/Nm3

Heating Value of Gas of Gas (LHV)

Appendix 4: Gasifier Sizing Worksheet 29 | P a g e


The following sets of equation of equation had been used to formulate the sizing of the of the gasifier A.3 The air flow rate through the gasifier as a function of superficial of superficial velocity (Us) and gasifier internal diameter (Dg) mair = ρair * ((π * Dg^2)/4 )* Us Ref: Venkata Ramayya A., Ramayya A., et. al; Design and Simulation and Simulation of Fluidised of Fluidised Bed Bed Power Power Gasifier Gasifier for a for a Coffee Hulling Center

A.4 Relation between superficial velocity (Us) and minimum fluidisation velocity (Umf ) Us = 2 * Umf Ref: Venkata Ramayya A., Ramayya A., et al.; et al.; Design and Simulation and Simulation of Fluidised of Fluidised Bed Bed Power Power Gasifier Gasifier for a 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 + Hf B

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

A.6 The determination of minimum of minimum fluidisation velocity, Umf Umf = [dp^2 / (150* μg)]*[g*(ρp‐ ρ g)*(εmf ^3)/(1‐ ε mf )] where, μg is gas viscosity ρp is bed fluidising particle density ρg is gas density εmf is bed voidage at minimum fluidization

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

A.7 Maximum fluidisation velocity, Umax Umax = (8/6)*([dp*(ρp‐ ρ g)*g/(Cd*ρg)]^0.5) where, Cd is drag coefficient Ref: El ‐Mahallawy et Mahallawy et al.; al.; Fundamentals and Technology and Technology of of Combustion, 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 Ref: El ‐Mahallawy et Mahallawy et al.; al.; Fundamentals and Technology and Technology of of Combustion, Combustion, 677 ‐ 693

A.9 Bed pressure drop, DelP ΔP = (ρp ‐ ρ g)*g*H*[1 ‐ ε mf ]

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

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

30 | P a g e


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

HDZ = UtD/Ut(ref) * HDZ(ref) (2)

where, Ut(ref) is Ut reference from literature

(2)

HDZ(ref) is HDZ reference from design

A.12 The disengaging zone diameter and bed internal diameter Dz = 1.5 * DB Ref: E. Olivares Gomez et al.; et al.; Energy Conversion Energy Conversion & Management 40 Management 40 (1999) (1999) 205 ‐ 214

A.13 The assumed relationship between freeboard height and superficial velocity Hfb = Hfb(ref) * UsD/Us(ref) (2)

where, Us(ref) is Us reference from literature

Hfb(ref) is Hfb reference from literature

31 | P a g e

(2)

Fluidized Bed Gasifier Design Report Public)

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