HKUST EVNG Seminar series

optimum Biomass Drying for Combustion – A Modeling Approach

Tesfaldet Gtgaa@ust.hk

March-11-2013

Contents

• Biomass Types • Biomass to Energy Routes • Biomass drying for heating value Enhancement-Modeling

and optimization• Results• Discussions• Future work

Biomass Types• Not limited to woody biomass and crops .• Includes some organic wastes with high moisture contents

like sludge ,microalgae and etc .

Biomass to Energy Routes

http://www.toyo-eng.co.jp/en/product_line/environment/baiomass/index.html

•While biomass with moisture levels of 55 - 65wt% can sustain combustion, the optimum moisture content is 10wt% - 15wt%.

Why Drying?• Burning biomass with high moisture content :

• reduces the combustion temperature • incomplete combustion • undesirable reaction products• requires a large amount of auxiliary fuel to make it combustible

• Drying of biomass:– Can increase the combustion efficiency, reduce pollution and improve

operation.

• BUT– Drying of biomass is an expensive process that requires huge capital

investment and energy input.

• Therefore ,the level of drying should be optimized.

Biomass Drying for combustion-Modeling

GOAL

• To develop a validated model of the drying kinetics of solids like wood, solid wastes and sludge for heating value enhancement before combustion process.

• To use the model to analyze the drying time , moisture removal rate ,Energy requirement and design parameters for practical drying of solids.

How?

• Material and energy balances, heat transfer and drying kinetics.

Drying process

• Regardless of drying particle shape, there exists simultaneous heat and mass transport.

Dryer HeaterAir in Hot air Exit air

Solid in

Solid out

Heat source?

Air•Temperature ……☑•Humidity ………….☑•Velocity ……………☒

•TGA………………..…☑•TG-DSC……………. ☒•Dryer Type …....…☒•Drying models……☒

Drying…..

Things to do

Air Heater• Mathematical model- Psychometric analysis

Dryer• Drying kinetics models-• Experiment-Thermogravimetric analysis (TGA)• Sample (Type, shape, moisture content)

Product value• Heating value enhancement.

Air heater • The corresponding property change of air for moisture removal

from the solid is calculated by making use of psychometric relations (equations).

• The air heater cost was determined for the required air property change.

Dryer• For sizing the dryer as well as determining the level of the

drying, it is important to obtain the drying kinetics of which the drying rate inside the dryer is calculated.

Drying Kinetics models

• General forms:1. Diffusion based model….Fick’s Diffusion Equation2. Thin layer drying curve based models (log, modified log)

Fick’s Diffusion EquationL

rc

rs

Experimental Drying kinetics determination

• The moisture ratio is determined using TGA.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600

Moi

stur

e Ra

tio

Drying Time (sec)

60°C

70°C

80°C

90°C

0.0027 0.0028 0.0029 0.003 0.0031-21

-20.9

-20.8

-20.7

-20.6

-20.5

-20.4

-20.3

-20.2

-20.1

-20

f(x) = − 2045.36715143528 x − 14.7163148605645R² = 0.936509534819916

lnDeff

1/T

ln(D

eff)

The product value

• Monetary value gained upon drying.

Over all process

Objective Function• Maximizing the net annual profit (AP) of the process

represented by :AP= Product value- (Heater cost +Dryer cost+ Operating Cost)

• Operating cost=Steam costConstraints

1. Air preheated temperature, 60oC Ta,2 110oC

2. Relative Humidity of the exhaust air, RH3 60%

3. The LHV of the final product, LHV 15MJ/Kg4. Residence time > 100sec

• Solver• Standard GRG Non-linear Solver-Excel.

Design parameters

Parameters Value

Feed rate of wood chips, Ws,1 (Kg/hr) 5000

Moisture ratio of the wood feed, X1 1

Temperature of the wood feed, Ts,2 (C) 25

Pressure of the air feed, Pa,1 1atm or 101325Pa

Temperature of the air feed, Ta,1 (C) 25

Relative Humidity of the air feed, RH1 (%) 50

Temperature difference of air and solid at the dryer outlet, DT (C) 5

Air heater Cost, Chtr ($/yr) 500 + 100 A 0.8

Dryer Cost , Cdyr ($/yr) 5000 + 5000 V 0.6

Steam Cost, CLP ($/kW) 0.1

Heat value of wood ($/kW) 0.05

Specific Heat of Air, Cpda( kJ/kg-C) 1.006

Specific Heat of Wood, Cpds, (kJ/kg) 1.2566

Specific Heat of Water Vapor, Cpv , (kJ/kg-C 1.89

Specific Heat of Water, Cpw, (kJ/kg-C) 4.186

Latent Heat of Water, LHw, (kJ/kg-C) 2270

Latent heat of LP steam, LHLP (KJ/Kg) 1999

Size of the wood , L, (m) 0.0005

Annual operation time, top (hrs) 2000

ResultsBase Case/1 (L=0.0005m)

Case 2 (L=0.005m )

Case 3 (L=0.05m)

Case 4 (L=0.05m)

Moisture content of the dried wood, X3 (wt %) 17 17 17 43

LHV of the dried wood, LHVs (KJ/kg) 15000 15000 15000 9698

Solid residence time, t (hr) 0.11 8 247 0.03

Air feed rate, Wa,1 (kg/hr) 179,496 83,358 158,187 59,842

Air preheated temperature, Ta,2 (°C) 62 97 107 60

Air Exhausted Temperature Ta,3 (°C) 35 43 78 35

Heater Cost ($/yr) 50,250 71,138 160,818 21,161

Dryer Cost ($/yr) 21,495 219,008 1,700,822 12,237

Energy Cost ($/yr) 357,913 353,818 766,027 119,324

Product Value ($/yr) 1,260,453 1,260,453 1,260,453 1,172,979

Annual Profit ($/Yr) 830,795 616,489 -1,367,214 1,020,258

Discussion• The optimum solutions indicated that the size of the wood chips strongly

affects the economy.

• When the size of the wood chips become too large, the drying time

becomes too long thus significantly increases the dryer size and energy

cost.

• Another observation is that if a lower heating value of the dried wood

chips is acceptable, the profitability is resumed.

• in order to satisfy the constraint of minimum heating value without

upsetting the environment and/or operation of the combustion process

when drying too large or too wet biomass, auxiliary fuel is one of the

options to be considered.

Research on pipe line,,,,,,

Differential Scanning Calorimetry (DSC)-Exactly we can predict heat flow!

Thank you

Go green!