This document describes several options for integrating biomass drying into a biomass power plant to improve energy efficiency. The base case power plant using empty fruit bunches achieves 20.08% efficiency. Integrating a hot air dryer increases efficiency to 27.36% by drying the biomass from 70% to 4.5% moisture. A multi-stage drying system using hot air and steam dryers further increases efficiency to 29.92% by drying biomass to 4.5% moisture. Integrating a flue gas dryer achieves 24.76% efficiency and reduces biomass moisture to 57.4%. The analysis shows that biomass drying integration can significantly improve the overall energy efficiency of biomass power generation.

1. Biomass Power Plant with Integrated
Drying: Effective Utilization of Waste Heat
Tesfaldet Gebreegziabher, A. O. Oyedun, Y. Zhang , M. J. Wang , Y. Zhu , J. Liu , C. W. Hui
June 16-2014

2. Contents
1. Biomass fired power plants
2. Energy efficiency improvement options
3. Drying of biomass
4. Process description of base case
5. Dryer Integration with power plant
I. Integration study with hot air dryer(HAD)
II. Integration study with Hot air dryer (HAD) and
steam dryer(SSD)
III. Integration study with flue gas dryer (FGD)
6. Results
7. Conclusions

3. 1. Biomass fired power plants
 Use direct combustion to convert stored biomass energy to
heat and power with the help of a steam cycle.
 Have low efficiency than the conventional coal power plants.
 Low efficiency is due to relatively high moisture content, low
heating value of feedstock and flue gas heat loss (Stack losses).
Figure 1 Schematic drawing of biomass fired power plant

4. 1.1 Moisture content
 Much of the heating value is used to evaporate the water and
results in lower efficiency.
 Using dry fuel in combustion systems improves efficiency,
increases steam production, reduces fuel use, lowers
emissions and improves boiler operation.
 Although drying biomass prior to combustion process have an
impact on efficiency, it is an energy and capital intensive
process.
 Using waste heat from other sources can make drying process
more economical.

5. 1.2 Stack losses
 In power plants , air and fuel are mixed and burned to
generate heat, some of which is transferred to boiler to
produce steam.
 When the heat transfer reaches its practical limit, the
combustion gases are usually removed from the boiler
through a stack.
 At this exit condition, in reference to the ambient conditions,
the exhaust gases still hold significant heat energy and leads to
reduction in system efficiency.
 Recovering part of heat from this flue gases can lead to an
increase in steam cycle efficiency.

6. 2. Energy efficiency improvement
options
 The energy efficiency of a power plant can be improved either
by using dry feedstock in combustion process ; or
 by using waste heat recovery system to capture and use some
of the energy in the flue gas;
Figure 2 Power plant with heat recovery options

7. 3. Drying of biomass
 In our study drying of biomass was considered as efficiency
improvement option via LHV enhancement.
 Biomass considered is Empty fruit bunches(EFB).
Figure 3 LHV of EFB as function of moisture content

8. 3.1 Methodology
 A base case and three drying options for generating 12.5MW
power from empty fruit bunches containing 70% moisture
content are considered.
 Hot air dryer (HAD)
 Hot air dryer (HAD) and superheated steam dryer (SSD)
 Flue gas dryer (FGD).
 Mathematical models of the steam power plant and the drying
processes are developed.
 Water97_v13.xla is used for calculating stream properties of
the units.
 Pinch analysis is used to show the effectiveness of the different
integration options.

9. 4. Base case
Mathematical modeling
 𝑖 𝑚𝑖𝑛 = 𝑖 𝑚 𝑜𝑢𝑡 Mass Balance
 𝑖 𝐸 𝑜𝑢𝑡 + ∆ 𝐸 Energy balance
 Optimize the over all efficiency defined by:
η 𝑜𝑣𝑒𝑟𝑎𝑙𝑙 =
𝑃 𝑜𝑢𝑡
1−𝑋 𝐹,𝑑 𝑚 𝐸𝐹𝐵∗𝐿𝐻𝑉 𝐸𝐹𝐵 𝑎𝑡 0𝑤𝑡% 𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒
∗ 100%
Figure 4 Process flow diagram of base case
 By simultaneously changing the flow
rates, pressures and temperatures of
steam and boiler feed water (BFW) in the
power plant and the temperature, flow
rate and drying level of the biomass in the
drying process.
 In the base case MP and LP steam
pressures for BFW preheat are
selected arbitrarily.
Feed moisture content

10. 4.1 Base case constraints
Process constraints applied in the base case
ηturb=80%, ηboiler=90% and ηpump=100%
P12=100bar, P14=18bar, P1=3bar, P2=0.1bar, P16=3bar, P13=18bar, P5=3bar,
P7=17bar, P9=100 bar
Moisture content of EFB, X(%)=70%
LHVEFB at 0wt% moisture = 15820 kj/kg
𝑚18=𝑚10 =100kg/h
T6=330K ,T8=400K,T12=900K
Pout=12.5MW
 In the base case MP and LP steam pressures are selected
arbitrarily for preheating Boiler feed water(BFW).

11. 4.2 Base case solution
Figure 5 Base case composite curves
 Excel 2010 Standard GRG Non-linear Solver was used to solve
the optimization problem.
 While solving the optimization problem, the stream properties
of the units are determined automatically by Water97.
 The over all efficiency is calculated to be 20.08% with 45,589
kg/h EFB feed requirement.
 Stream data are extracted and composite curves are plotted as
in Figure 5.

12. 4.3 Optimum BFW Preheat temperature
and pressure of base case
 In this study LP and MP steam are allowed to vary in the following
ranges: (2 bar ≤ LP ≤ 5 bar and 15 bar ≤ MP ≤ 40 bar)
 The cycle efficiency was improved to 20.80% with 42,934 kg/h EFB
feed requirement.
 LP = 2.4 bar and MP = 19.8 bar (3 bar and 18 bar-base case).
 T of Stream 6 and 8 are 411K and 491K respectively. (330 and
400K-base case)
Figure 5 Base case composite curves

13. 5. Dryer Integration with power plant
 During thermal drying, EFB is dehydrated through heat transfer
with air, steam or Flue gas .
 The optimum temperature and pressure values of LP and MP
,and the optimum temperature of BFW are used in all the
integration study.
 Models of both HAD and SSD are based on gross Pout
prediction with out considering combustion reaction of EFB.
(Only LHV value is used for mass and energy balance).
 Mathematical model of flue gas based dryer is based on
combustion analysis of EFB.

14. 5.1 HAD Integrated with power plant
Parameters Value
Moisture content of the EFB feed, X1 70%
Temperature of the EFB feed, (K) 298
Pressure of the air feed, P 101325Pa
Temperature of the air feed, (K) 298
Relative Humidity of the air feed, RH1 (%) 50
Specific Heat of Air, Cpda( kJ/kg-C) 1.006
Specific Heat of EFB, 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
Maximum relative humidity of air (%) 95
Minimum LHV of the dried EFB (kj/kg) 15,000
Modeling
 Air heater- Psychometric relations.
 HAD- Mass and energy balance.
Figure 6 HAD integrated plant

15. 5.1.1 HAD results
LP
MP
Air heater
LP
MP
 The cycle efficiency is 27.36% with 34,322 kg/h EFB requirement.
Fig7: HAD mass balance
Fig 8: HAD composite curves

16. 5.2 Multi stage dryer
Figure 9 Multi stage dryer
LP/
 MP and LP steam can be extracted for Air preheat and SSD.
 As a huge temperature difference is observed between MP
and SSD ,LP extraction is considered in this study.

17. 5.2.1 Detailed Multi stage dryer
Figure 10 Multi stage dryer integrated plant
Modeling
 Air heater- Psychometric relations.
 HAD- Mass and energy balance.
 SSD-Mass and energy balance.

18. 5.2.3 HAD-SSD results
Figure 11 mass and Energy balance Multi stage dryer

19. 5.2.4 HAD -SSD composite curve
LP(SSD)
LP(PP)
MP
SSD
Air heater
Figure 12 Composite curve of HAD-SSD
 The cycle efficiency is 29.92% with 31,576 kg/h EFB requirement.

20. 5.3 FGD integrated power plant
 Unlike the HAD and SSD integration cases EFB combustion process
is assumed to predict the possible amount of heat recovery from
flue gases for drying biomass.
Q recoverable
calculation
FGD
Flue gas
Exhaust gas
EFB
Moist
Flame
Temperature
FG Temperature
Exhaust Temperature
mev calculation
Figure 13 Calculations involved in FGD
TFGDE ≥ Tdw
TFGDE ≥ Tda

21. 5.3.1 EFB properties for combustion
Ultimate Analysis wt% Molecular weight
(Kg/kmol)
Wt/kg fuel
Carbon 45.53 12 0.4553
Hydrogen 5.46 1 0.0546
Nitrogen 0.45 14 0.0045
Sulphur 0.04 32 0.0004
Oxygen 43.4 16 0.434
Ash 5.12 0.0512
Total 100 1
 The modeling of the combustion process is based on the ultimate
analysis of EFB.

22. 5.3.2 FGD integrated power plant
Figure 14 FGD integrated plant
Process constraints
ηturb=80%, ηpump=100%
P12=100bar, P2=0.1bar, P9=100
bar,P5=P6=3,P7=P8=15
Moisture content of EFB, X(%)=70%
LHVEFB at 0wt% moisture = 15820 kJ/kg
m18=m10 =100kg/hr
T12=900K
Pout=12.5MW
TFGOB= 10K+ T11
T3= Tsat2-5K
T18=298K
5 bar < P13<40 bar
2 bar <P16<15 bar
hev =2260kj/kg

23. 5.3.3 FGD composite curves
LP steam
MP steam
LP steam
MP steam
Figure 15 FGD integrated plant composite curves
 The calculated cycle efficiency is 24.76% with 38297
kg/h EFB containing 57.4 %moisture.
 About 13% moisture removal.

24. 6. Summary of results
System EFB F Requirement
kg/hr
Air for drying
kg/hr
Final moisture
(%)
Efficiency
%
Base Case 45,589 - 70 20.08
Base Case preheat and optimum P 42,934 - 70 22.80
Integration with FGD 38297 - 57.4 24.76
Integration with HAD 34322 958544 4.5 27.36
Integration with HAD and SSD using LP 31576 355110 4.5 29.92
 The analysis shows that integration of drying to power
generation from EFB increases the overall energy efficiency.
 3 multistage dryers is not considered yet.

25. 7. Conclusion
 HAD and SSD dryers in multistage manner operating by
extraction of LP steam have better efficiency.
 In multistage drying the steam generated at SSD is a LP
steam and can be used for air preheating and this can save
LP steam of power plant.
 Composite curves of the overall plant are plotted for each
case to indicate the effectiveness of heat integration and to
provide insights for further improvement.
 Water 97 is important tool and can be used for modeling and
simulation of chemical process involving steam cycles in
EXCEL successfully.
 Future work will be analysis of multistage dryers combining
air, steam and flue gas as drying medium.

26. Thank you