Analysis of a feasible trigeneration system taking solar energy and biomass as co-feeds

In addition, the primary energy saving ratio and annual total cost saving ratio compared with the separated generation system are 16.7% and 25.9%, respectively.. And the results indicate

Analysis of a feasible trigeneration system taking solar energy

and biomass as co-feeds

College of Civil Engineering, National Center for International Research Collaboration in Building Safety and Environment, Hunan University, Changsha 410082, PR China

a r t i c l e i n f o

Article history:

Received 13 January 2016

Received in revised form 20 May 2016

Accepted 22 May 2016

Keywords:

Biomass gasification

Solar energy

Internal combustion engine

Trigeneration system

System integrating

a b s t r a c t The trigeneration systems are widely used owing to high efficiency, low greenhouse gas emission and high reliability Especially, those trigeneration systems taking renewable energy as primary input are paid more and more attention This paper presents a feasible trigeneration system, which realizes bio-mass and solar energy integrating effective utilization according to energy cascade utilization and energy level upgrading of chemical reaction principle In the proposed system, the solar energy with mid-and-low temperature converted to the chemical energy of bio-gas through gasification process, then the bio-gas will be taken as the fuel for internal combustion engine (ICE) to generate electricity The jacket water as a byproduct generated from ICE is utilized in a liquid desiccant unit for providing desiccant capacity The flue gas is transported into an absorption chiller and heat exchanger consequently, supply-ing chilled water and domestic hot water The thermodynamic performance of the trigeneration system was investigated by the help of Aspen plus The results indicate that the overall energy efficiency and the electrical efficiency of the proposed system in case study are 77.4% and 17.8%, respectively The introduc-tion of solar energy decreases the consumpintroduc-tion of biomass, and the solar thermal energy input fracintroduc-tion is 8.6% In addition, the primary energy saving ratio and annual total cost saving ratio compared with the separated generation system are 16.7% and 25.9%, respectively

Ó 2016 Elsevier Ltd All rights reserved

1 Introduction

Recently, fossil fuels have been the main primary energy in the

worldwide However a series of serious problems have occurred

due to over utilization of fossil fuels, such as CO2emission, climate

change and ecological balance disruption Therefore, various

renewable energy resources are drawn increased attention for

their environmental advantages, especially solar energy and

bio-mass energy, have been widely used as a result of their unique

advantages, such as cleanliness, safety, abundant reserves and so

on[1–3]

For solar energy utilization, mid-and-low solar thermal

utiliza-tion technology obtains the widespread attenutiliza-tion for its good

ther-mal performance and economy The solar energy can not only be

used as heating driving resource, such as evaporation and

recuper-ation processes, but also can be used for chemical processes, like

decomposition and reforming Modi et al.[4]compared the

ther-modynamic performance of the Kalina cycle for a central receiver

solar thermal power with direct steam generation and a Rankine

cycle, and emphasized that Kalina cycle showed a clear advantage

when heat input was primarily from a two-tank molten-salt stor-age and Rankine cycle showed better performance than Kalina cycle when the heat input was only from the solar receiver Calise

et al.[5]designed and simulated a novel prototype of a 6 kWesolar power plant, mainly consisting of flat-plate evacuated solar collec-tors and a small Organic Rankine Cycle (ORC) to evaluate the energy and economic performance of the system At the same time, many researchers have investigated the possible of thermo-chemical utilization of solar energy Steinfeld [6] summarized and reviewed the current research on thermo-chemical production

of hydrogen by solar energy Hong et al.[7]analyzed the perfor-mance of a new solar thermal power cycle combined with middle-temperature solar thermal energy and methanol decompo-sition and concluded that the novel system was more competitive compared with conventional power system Xu et al.[8]developed

a novel combined cooling heating and power system integrated with mid-and-low temperature solar energy thermo-chemical pro-cess and the methanol decomposition, and presented an energy and exergy analysis to investigate the performance of the system Zhang et al.[9,10]proposed a solar-assisted methane chemically recuperated gas turbine system, which converted the low temper-ature solar heat into vapor latent heat and then via the reforming reactions to the syngas chemical energy Liu et al.[11]studied a

http://dx.doi.org/10.1016/j.enconman.2016.05.063

0196-8904/Ó 2016 Elsevier Ltd All rights reserved.

⇑Corresponding authors.

E-mail addresses: lhq@hnu.edu.cn (H Li), gqzhang@hnu.edu.cn (G Zhang).

Contents lists available atScienceDirect

Energy Conversion and Management

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / e n c o n m a n

hydrogen production with the integration of methanol steam

reforming and middle-temperature solar thermal energy based

on experiments The research results showed that the chemical

conversion of methanol could reach levels higher than 90% and

the maximum hydrogen yield per mole of methanol was 2.65–

2.90 mol

Biomass is the plant material derived from the photosynthesis

between CO2, water and sunlight to produce carbohydrates

[12,13], thus it is renewable and carbon–neutral resource Biomass

has some other advantages such as abundant in resources, widely

distributed, environmental friendly One of the most potential

technology of biomass utilization is gasification, by which biomass

can be transformed into bio-gas The bio-gas can be used as a

feed-stock for the production of chemicals or power[14–17] In order to

realize biomass gasification, gasifying agent like air, steam, or

oxy-gen will be required As the most availability and economy

gasify-ing agent, air is widely used in demonstration or commercial scale

biomass gasification[15,18,19] However, in this way, due to the

introduction of nitrogen, the bio-gas has a low heating value The

use of oxygen is not economical owing to the high cost of oxygen

production, although it can increase the bio-gas heating value

Gasification with steam can produce bio-gas with a heating value

of 10–14 MJ/Nm3 However, this process is an endothermic

reac-tion, which needs extra heat to sustain the gasification reaction

To summarize, air–steam gasification process may be a better

way to realize gasification The combustion reaction provides the

required heat for gasification, which is termed as auto-thermal

process

Combined cooling, heating and power (CCHP) system combines

distributed power generation with thermally activated equipments

to meet the cooling, heating and power needs for users It has been

used worldwide because of its high efficiency, low greenhouse gas

(GHG) emission and high reliability[20–22] In recent years,

com-bined heat and power (CHP) systems based on biomass and solar

energy have been widely concerned [2,23,24] Pablo et al [25]

modeled and optimized a biomass steam gasification system, which include two main parts: solar assisted steam production part and micro gas turbine power generation part The solar collec-tor generates high temperature steam (800–1200°C) as the gasifier agent The research results showed that, the overall system perfor-mance can be improved by such an integrating way Tanaka et al

[26] presented a hybrid power generation system coupling bio-mass gasification and concentrated solar collecting processes, the generated bio-gas was taken as fuel in a gas turbine in a further way Utilizing the molten-nitrate salt as heat carrier to absorb the heat from the receiver in molten salt heat storage system, the heat is used for producing steam for Rankine cycle and is con-verted to electricity Ravaghi-Ardebili et al.[27]investigated the efficiency of biomass gasification process on low temperature con-dition, which coupled with a Concentrated Solar Power (CSP) plant

As a heated working fluid molten salt produced the steam (
410 °C) to participate in the gasification reaction Angrisani

et al [28] presented a new concept solar-biomass cogeneration system using a Stirling engine for the combined production of the heat and electric power As a biomass combustion chamber, the fluidized bed simultaneously absorbed the heat concentrated from the solar collector The Stirling engine converted the heat col-lected in the fluidized bed into mechanical and then electrical power

In addition to the combined heating and power system inte-grated with biomass and solar energy, some studies have also investigated producing synthetic fuels in polygeneration systems Bai et al [29] investigated the thermodynamic analysis and the economic performances of a solar-driven biomass gasification polygeneration system for the methanol production and the power generation The solar-biomass gasifier produced raw bio-gas through absorbing the solar thermal energy reflected by heliostats The purified bio-gas was used for the methanol production as syn-gas, while the un-reacted syngas would be used for power genera-tion And the results indicated that the energy and exergy

Nomenclature

Abbreviation

CCHP combined cooling, heating and power

CGE cold gas efficiency

CHP combined heating and power

COP coefficient of performance

CSP concentrated solar power

ER equivalence ratio

FT Fischer–Tropsch

GHG greenhouse gas

HHV higher heating value

LHV lower heating value

HX heat exchanger

ICE internal combustion engine

ORC organic rankine cycle

PESR primary energy saving ratio

SBR steam/biomass ratio

VCC vapor compression cycle

Symbols

ATC annual total cost (Yuan)

ATCSR annual total cost saving ratio (%)

C cost (Yuan)

CGE cold gas efficiency (%)

COP coefficient of performance

EX exergy (kW)

F solar thermal energy input fraction (%)

HHV higher heating value (MJ/Nm3)

I interest rate (%) LHV lower heating value (MJ/Nm3)

m mass flow rate (kg/h)

N installed capacity (kW)

p service life (year)

T temperature (°C)

V volume flow rate (Nm3/h)

W electricity (kW)

g efficiency (%) Subscripts

c cooling capacity

d domestic hot water

de desiccant capacity

el electricity

i/j the number of equipment

M annual maintenance sep separated generation system

th thermal heat tri trigeneration system

efficiency of the proposed system approximately reached to 56.09%

and 54.86%, respectively Hertwich et al.[30]presented a new

con-cept of producing synfuel from biomass using concentrating solar

energy, which contained 6 main parts: steam gasifier, reverse

water gas shift, hydrocarbon synthesis, heat recovery and steam

generation, and solar power system The molten-salt provided

the high temperature heat for gasification, which was obtained

from solar power system, and the H2 for reverse water gas shift

reaction was generated by electrolyzing water driven by solar

power And they modeled the production of methanol in the

pro-posed system compared with the traditional system only using

biomass or coal as a fuel Guo et al [31] studied the energetic

and environmental performance of the solar hybrid coal and

bio-mass to liquid system integrated with a solar hybrid dual fluidized

bed gasifier, the olivine was used as bed material in the gasifier to

transfer the heat from combustion reactor and/or solar receiver to

gasification reactor, and using storage units to compensate the

influence of solar radiation The purified syngas was fed into a

Fis-cher–Tropsch (FT) reactor to produce FT liquid, and the un-reacted

gas was burned to generate power in the gas turbine

At the same time, some researchers have studied the combined

cooling, heating and power (CCHP) system integrated with

bio-mass and solar energy Karellas et al.[32]investigated the

thermo-dynamic and economic analysis of a trigeneration system using

biomass and solar energy, which consisted of an Organic Rankine

Cycle (ORC) and a vapor compression cycle (VCC) Khalid et al

[33]reported that the energy and exergy analysis of an integrated

multigeneration system using biomass and solar energy It

con-tained two Rankine and gas turbine cycles, as well as an absorption

cooling cycle Biomass combustion drove Gas turbine cycles to

pro-duce electrical power and the oil heated by concentrated solar

col-lector provided Rankine cycle 2 and absorption cooling cycle with

thermal energy They concluded that system efficiency had an

obvious improvement compared with a single renewable energy

source The literature survey on biomass and solar-driven

trigener-ation system indicates that the trigenertrigener-ation system is mostly inte-grated with biomass combustion and Organic Rankine Cycle, while the research focusing on biomass gasification and Otto Cycle inte-grated trigeneration system which driven by biomass and solar energy is relatively fewer

In this paper, a small-medium trigeneration system coupled with biomass gasification and solar thermal process is suggested and discussed In the proposed system, the mid-and-low tempera-ture solar thermal energy is transformed into the chemical energy

of gas by gasification process, utilizing the sensible heat of bio-gas to produce a part of domestic hot water The internal combus-tion engine (ICE) is driven by the bio-gas to generate electricity Then, the flue gas is sent to absorption chiller and heat exchanger consequently to generate chilled water and domestic hot water The jacket water derived from ICE is utilized in a liquid desiccant unit for dehumidification So as to evaluate the system perfor-mance, the thermodynamic and economic performances of the tri-generation system are studied Several key system integrating parameters are investigated, including equivalence ratio (ER), steam/biomass ratio (SBR), air preheating temperature, solar col-lector temperature and fuel price

2 System flowsheet description The flowsheet of the suggested system is shown inFig 1 The system consists of three main parts: (1) air–steam biomass gasifi-cation and purifigasifi-cation subsystem, which contains a fluidized bed gasifier, a biomass preheater, a cyclone separator, an air splitter and heat exchangers (HX-1 and HX-2); (2) steam generation sub-system, which contains a parabolic trough solar collector and a pump; (3) internal combustion engine power generation subsys-tem, which contains an internal combustion engine, a LiBr–H2O absorption chiller, a liquid desiccant unit and a heat exchanger (HX-3)

The grinded biomass material (stream 1) is preheated by air

(stream 8, 200°C) in preheater, and reducing the biomass moisture

to about 10% Then the biomass material (stream 2) is fed into a

flu-idized bed gasifier after preheated in the biomass preheater The

preheated air (stream 8, 200°C) and steam (stream 12, 350 °C)

generated from solar collector are fed into the gasifier with

bio-mass (stream 2) The high temperature bio-gas (stream 4) after

removed the ash and char is fed into the heat exchangers (HX-1

and HX-2) Utilizing the sensible heat of bio-gas to preheat the

air (stream 6, 25°C, 1 bar) and produce domestic hot water (stream

27, 80°C) Then, as the fuel, the purified bio-gas (stream 13) is fed

into the internal combustion engine for electricity generation The

jacket water (stream 18) from the engine is used to provide low

temperature waste heat for the liquid desiccant unit, and then

the unit supplies dehumidified air (stream 20) to customers The

LiBr–H2O absorption chiller is driven by waste heat from ICE flue

gas (stream 15), in which provides cooling for users After

transfer-ring the heat to domestic hot water (80°C) in the heat exchanger

(HX-3), the exhausted gas (stream 17) is released to the

atmo-sphere at a temperature of 120°C

3 System thermal performance calculation

3.1 Assumptions

To further analyze the thermodynamic performance of the

tri-generation system, the Aspen Plus process model simulator is used

The selections of key process equations are as follows: the Peng–

Robinson thermodynamic model is selected in compression,

com-bustion, expansion and other processes of bio-gas and air The

STEAM-TA thermodynamic model is selected in water and steam

generating processes Selecting the thermodynamic equilibrium

model for the biomass gasification process And the following

assumptions are considered in modeling the fluidized bed gasifier

gasification process:

 The process is isothermal and steady state

 There is no pressure loss in the gasifier

 Biomass particles are of uniform size and temperature

 The bio-gas consists of H2, CO, CO2, CH4, H2O, and tar formation

is disregarded

 Char only contains carbon and ash, and ash is used to be inert

material

 The sulfur and nitrogen go to H2S and NH3respectively

The main chemical reactions that occurred in the biomass

gasi-fication process are presented inTable 1

In this study, rice husk is selected as the biomass material

Table 2shows biomass material characteristics used in the

simula-tion process[34] To analyze the thermodynamic performance of

the trigeneration system, a case study is investigated (the biomass

feed rate, mbiomass= 1400 kg/h; the air equivalence ratio, ER = 0.4; the steam/biomass ratio, SBR = 0.4) The main parameters are listed

inTable 3, and the initial investment costs and parameters are pre-sented inTable 4

Table 1

Gasification reactions of biomass.

Reaction name Reaction equation Heat of reaction

(kJ/mol) Carbon partial combustion C + 0.5O 2 M CO 111

Hydrogen partial combustion H 2 + 0.5O 2 M H 2 O 242

Steam-methane reforming CH 4 + H 2 O M CO + 3H 2 +206

NH 3 formation 0.5N 2 + 1.5H 2 M NH 3 –

Table 2 Characteristics of biomass material.

Proximate analysis (%, dry basis)

Ultimate analysis (%, dry basis)

Table 3 Key operating parameters of system.

Gasification temperature (°C) 890 Gasification pressure (MPa) 0.1 Solar collector temperature

(°C)

350 Solar collector efficiency (%) 60 Compression ratio of ICE 9 Flue gas pressure of ICE

(MPa)

0.12 Flue gas temperature of ICE

(°C)

450 Jacket water temperature of ICE (°C)

87 Mechanical efficiency of pump

(%)

99 Isentropic efficiency of pump (%)

75 COP of absorption chiller 1.2 COP of liquid desiccant unit 0.8 Node temperature difference

of HX-1/2 (°C)

20 Node temperature difference

of HX-3 (°C)

20

Table 4 The economic parameters of system [35–38]

Equipment investment cost (Yuan/kW) a

Gasification subsystem b

2500

Absorption chiller 1200 Electric chiller 970

Solar collector c

4525

Liquid desiccant unit 1200

Service life (year) 20 Maintenance cost ratio d

(%) 2.5 Operating hours e

Natural gas (Yuan/kW h) 0.194 Electricity (Yuan/kW h) 0.936

a

1US$ = 6.12 Yuan (RMB).

b

The gasification subsystem includes the gasifier and the gas conditioning, the former accounts for 95% of the investment, and the latter accounts for 5% of the investment.

c

The initial investment cost of the solar collector field includes the solar col-lector, the related equipment investment and the solar collector land The cost of solar collector and related equipment is 1225 Yuan/m 2

; the area of solar collector land is three times that of the solar collector, and the cost of solar collector land is

225 Yuan/m 2

.

d The maintenance cost ratio is the ratio of the maintenance cost to the invest-ment cost.

e

The annual operating hours of the trigeneration system is determined by the solar collector subsystem, according to [29] , the annual operating hours of solar collector subsystem is 2000 h.

To analyze the thermodynamic and economic performances

and the influences of the related parameters, the simulation and

analysis procedures are shown inFig 2 The inputs conditions

con-sist of system assumptions, biomass characteristics, key operating

and economic parameters By mean of the Aspen Plus simulator,

the thermodynamic performances including energy and exergy

analysis are calculated At the same time, the equipment capacity

of different components can also be obtained by Aspen Plus, which

contributes to computing the economic indicators including

annual total cost and annual total cost saving ratio Moreover,

the effects of relevant parameters on the proposed system

perfor-mances can also be analyzed through the simulation

3.2 Performance evaluation criteria

In the proposed system, the overall energy efficiency is selected

as an evaluation indicator of the thermodynamic performance of

trigeneration system, which can be defined as:

g¼Wþ Qcþ Qdþ Qde

 100%

Furthermore, the electrical efficiency has been calculated as:

where W is the electricity generation of the trigeneration system,

kW; Qcis the cooling generation of the trigeneration system, kW;

Qd is domestic hot water generation of the trigeneration system,

kW; Qdeis the desiccant capacity of the trigeneration system, kW;

Qbis the biomass energy input of the trigeneration system, kW;

mbis the mass flow rate of biomass, kg/h; Qsolis the solar energy absorbed by steam generation subsystem, kW; LHVb is the lower heating value of biomass, kJ/kg; the lower heating value is calcu-lated as[39]:

where HHVbis the higher heating value of biomass, MJ/kg; H is the percentage of hydrogen in the biomass material, %

Besides the overall energy efficiency, the exergy efficiency of trigeneration system is defined as:

gex¼Wþ EXdþ EXcþ EXde

 100%

where EXdis the domestic hot water exergy of the system, kW; EXc

is the cooling exergy of the system, EXdeis the desiccant exergy of the system, kW; EXsolis the solar thermal exergy of the system;

EXbis the biomass exergy of the system; b is the multiplication fac-tor, which can be calculated as[40]:

b ¼1:044 þ 0:0160ðH=CÞ  0:3493ðO=CÞð1 þ 0:0531ðH=CÞÞ þ 0:0493ðN=CÞ

ð5Þ where C, H, O, N are the mass fraction of carbon, hydrogen, oxygen and nitrogen of biomass in ultimate analysis, respectively The primary energy saving ratio (PESR) is selected to compare the performance between trigeneration system and separated gen-eration system with the same products The primary energy saving ratio can be defined as:

PESR¼ 1 Qbþ Qsol

W

gsep;elþ Q c þQ de

COPsep;c gsep;elþ Qd

gsep;th

where Qsepis the fuel consumption of the separated generation

sys-tem, kW;gsep,el is the electrical efficiency of the separate power

plant, %; COPsep,cis the coefficient of performance (COP) of electrical

refrigerator and dehumidification unit;gsep,this the thermal

effi-ciency of a boiler, % In order to compare the trigeneration system

with the separated generation system on the condition of same

products, the performance parameters of the separated generation

system are as follows: the electrical efficiency of the separate power

plant is 33%, the COP of electrical refrigerator and dehumidification

unit is 3.0, the thermal efficiency of a boiler is 85%

The introduction of solar energy decreases the consumption of

biomass material, in order to determine the effect of solar energy

in the trigeneration system, the solar thermal energy input fraction

has been calculated as:

where Fsolis the solar thermal energy input fraction, %; Qsolis the

solar energy absorbed by steam generation subsystem, kW

The cold gas efficiency of the gasification process is defined as

the ratio of the energy of bio-gas to that of biomass material:

where CGE is the cold gas efficiency, %; LHVgis the lower heating

value of bio-gas, kJ/Nm3; Vgis the volume flow rate of bio-gas in

the standard state, Nm3/h; mb is the mass flow rate of biomass,

kg/h; LHVbis the lower heating value of biomass, kJ/kg

The annual total cost of the proposed system consists of three

parts: annual initial capital cost, maintenance cost and operation

cost Both the initial capital cost and maintenance cost are function

of equipment capacities The annual total cost of the trigeneration

system can be calculated as:

And the annual total cost of the separated generation system

can be calculated as:

ð10Þ where N and C are the installed capacity and the investment cost of

the equipment respectively (kW and Yuan/kW); i and j are the

number of equipments of trigeneration and separated generation system respectively; Ctri,Mand Csep,M are the annual maintenance costs of trigeneration and separated generation system respectively, Yuan Qband Cbare the annual consumption and price of the bio-mass respectively (kg and Yuan/ton); Qgasis the natural gas con-sumed by the boiler of separated generation system, kg; Ceand

Cgasare the energy charges of electricity and natural gas respec-tively, Yuan/kW h

The capital recovery factor, R, can be defined as:

p

where I is the interest rate, %; and superscript p is the service life of the equipment, year

The annual total cost saving ratio (ATCSR) is used as economic criterion to compare the performance between the trigeneration system and separated generation system It can be calculated as:

where ATCsepis the annual total cost of the separated generation system, Yuan; ATCtriis the annual total cost of the trigeneration sys-tem, Yuan

3.3 System performance calculation results

In the case system, the relevant parameters are as follows: air equivalence ratio (ER): 0.4, steam/biomass ratio (SBR): 0.4, gasifi-cation temperature: 890°C, gasification pressure: 0.1 MPa As we can see fromTable 5, the input, output and system performance are listed For the trigeneration system, in the case of input

5076 kW biomass energy, it consumes extra solar energy of

477 kW to provide the steam for biomass gasification process The input of solar energy reduces the consumption of biomass, which makes the solar thermal energy fraction reaches to 8.6% With the same products, the trigeneration system saves more pri-mary energy than separated generation system, the pripri-mary energy saving ratio reaches to 16.7% And the overall energy effi-ciency is 77.4% by utilizing biomass energy and solar energy Through Aspen Plus simulation, it can calculate the inputs and outputs exergy of the trigeneration system, which contributes to determining the exergy efficiency of the proposed system The total exergy efficiency of the proposed system is 19.2%, which is approximately 9.8% higher than the separated system (17.3%) The heating sources of absorption chiller and liquid desiccant unit

in the proposed system are from waste heat of ICE, while the

Table 5

Calculation results of trigeneration system.

energy source of electrical refrigerator is from high grade

electric-ity From the perspective of the waste heat utilization, in spite of

the lower energy grade of flue gas and jacket water, the absorption

refrigeration and liquid desiccant technology make full use of the

waste heat And these measures also improve the exergy efficiency

of the proposed system

Moreover, the equipment capacity of the components could be

determined in the case study The initial capital cost of system can

be calculated by the equipment investment cost inTable 4, thus

the operation cost and maintenance cost can be obtained by the

economic formula subsequently Finally, the annual total cost

and annual total cost saving ratio are determined based on the

above results In the case study, it shows that the annual initial

capital cost of the proposed system is larger than the separated

generation system, while the operation cost is obviously lower

than the separated generation system The annual total cost saving

ratio (ATCSR) is approximately 25.9% compared with the separated

generation system

These results indicate that, the novel trigeneration system with

the combination of renewable energy can improve the overall

energy efficiency of system and provide various products for

customers

4 Discussion

In order to know better about the novel system, air equivalence

ratio (ER), steam/biomass ratio (SBR), air preheating temperature,

solar collector temperature and fuel price are selected as key

oper-ating parameters to analyze the performance of the proposed

system

4.1 Effect of SBR on the gasification temperature with various ERs

Gasification temperature is critical for air–steam gasification

process Both air flow rate and steam flow rate have an effect on

gasification temperature in the adiabatic condition In this study,

the gasification temperature is varied from 700°C to 1000 °C

And the performance analysis is performed in the range of

0.356 ER 6 0.5 and 0 6 SBR 6 4.0

Fig 3illustrates the effects of steam/biomass ratio (SBR) and

equivalence ratio (ER) on the gasification temperature It can be

seen that the high equivalence ratio and low steam/biomass ratio

favor the increase in gasification temperature With the increase

in SBR, the gasification temperature decreases In addition, the

equivalence ratio has a significant effect as the gasification

temper-ature increases with the increase in the equivalence ratio As we know, steam gasification requires sufficient heat for endothermic gasification reaction The higher air flow rate contributes to gener-ating more combustion heat, which is favorable to steam gasifica-tion reacgasifica-tion When keeping equivalence ratio constant, the endothermic reaction of water–gas and steam-methane reforming are strengthened with the increase in the steam flow rate, then leading to the decrease in gasification temperature

4.2 Effect of SBR on the bio-gas composition

Fig 4shows the variation of bio-gas composition as a function

of the SBR over the range of 0–4.0 With the increase in steam/bio-mass ratio, the content of N2and CO decrease gradually, and H2 and CO2 content increase gradually However, the variation of

CH4content is not obvious, though the trend is decreasing With the increase in steam flow rate, the reaction of water–gas and CO shift is enhanced, which consumes more steam and CO and pro-duces more H2and CO2 Although keeping the equivalence ratio constant, the mole of combustible gas increases Therefore the N2 content introduced by the air is diluted in the bio-gas And the reaction of steam-methane reforming is strengthened with the increase in steam flow rate, which decreases the CH4content 4.3 Effect of SBR on the bio-gas yield with various ERs

Fig 5depicts the effect of steam/biomass ratio on bio-gas yield

at different ER With the increase in steam flow rate, the reaction of water gas and CO shift is enhanced, which promotes the yield of bio-gas As shown inFig 5, the bio-gas yield increases significantly with the increase in steam/biomass ratio For example, when keep-ing equivalence ratio at 0.35, the bio-gas yield increases from 2.22

to 3.45 While increases from 3.88 to 7.52 at ER of 0.5 Moreover, due to the introduction of N2in the air, the gas yield enhances However, the increase in bio-gas yield is not obvious with the increase in ER For example, the bio-gas yield increases from 3.45

to 3.88 at SBR of 1.0

4.4 Effect of SBR on the cold gas efficiency with various ERs Cold gas efficiency is an important indicator to evaluate the per-formance of the gasifier Fig 6presents the cold gas efficiency (CGE) at different steam/biomass ratios and equivalence ratios of the gasification process Either the increase in the steam/biomass ratio or the equivalence ratio leads to the decrease in cold gas

effi-0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

650

700

750

800

850

900

950

1000

1050

Steam/Biomass Ratio

ER=0.35 ER=0.4 ER=0.45 ER=0.5

0 10 20 30 40 50

CH4

CO2

CO

H2

N2

Steam/Biomass Ratio

ciency The cold gas efficiency experiences a obvious reduction

when the equivalence ratio is increased For example, the cold

gas efficiency decreases from 63.7% to 45.8% with the increase in

ER from 0.35 to 0.5, when keeping the steam/biomass ratio at

1.0 Similarly, with the increase in steam flow rate, the cold gas

efficiency decreases rapidly at low value of ER, then decreases

slowly at high value of ER As can be seen fromFig 6, the cold

gas efficiency varies from 67.0% to 64.1% at ER of 0.35, and varies

from 45.8% to 43.8% at ER of 0.5

4.5 Effect of SBR on the overall energy efficiency and solar thermal

energy input fraction

The curves presented in Fig 7show the variation of overall

energy efficiency at different SBR and ER The increase in steam/

biomass ratio causes the increase in overall energy efficiency on

account of solar thermal energy input Obviously, the high steam

flow rate requires more solar thermal energy input Because the

solar collector provides the heat to raise the temperature of steam,

therefore the consumption of biomass material could be reduced

Solar thermal energy input fraction is selected to evaluate the

con-tribution of solar thermal energy As shown inFig 8the solar

ther-mal energy input fraction increases with the increase in steam/

biomass ratio at different solar collector temperature The solar thermal energy input fraction can be reached up to 48.1% when steam/biomass varies from 0 to 4.0 at solar collector temperature

of 350°C In addition, it can be seen from Fig 7 that with the increase in ER, the overall energy efficiency increases For example, the overall energy efficiency increases from 77.2% to 82.3% with the increase in ER from 0.35 to 0.5, when keeping the steam/bio-mass ratio at 1.0

4.6 Effect of SBR on the primary energy saving ratio with various ERs The effect of SBR on the primary energy saving ratio (PESR) at different ER is shown inFig 9 Primary energy saving ratio (PESR) has been calculated to assess the performance between trigenera-tion system and conventrigenera-tional separated generatrigenera-tion system.Fig 9

presents that PESR decreases with the increase in SBR and ER When the gasification process operates at a lower steam and air flow rate, the PESR drops obviously with the increase in mass ratio, but decreases slowly with the increase in steam/bio-mass ratio As can be seen from Fig 9 that the PESR decreases from 19.5% to 15.5% with increase in SBR from 0 to 1.0 at ER of 0.35, however decreases from 13.7% to 12.9% with increase in SBR from 1.0 to 4.0 at ER of 0 5 And PESR decreases to a constant

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

2

3

4

5

6

7

8

3 /kg)

Steam/Biomass Ratio

ER=0.35 ER=0.4 ER=0.45 ER=0.5

Fig 5 Effect of SBR on the bio-gas yield with various ERs.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

40

45

50

55

60

65

70

Steam/Biomass Ratio

ER=0.35 ER=0.4 ER=0.45 ER=0.5

Fig 6 Effect of SBR on the cold gas efficiency of bio-gas with various ERs.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 70

72 74 76 78 80 82 84 86 88

90

ER=0.35 ER=0.4 ER=0.45 ER=0.5

Steam/Biomass Ratio

Fig 7 Effect of SBR on the overall energy efficiency with various ERs.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0

10 20 30 40 50

Steam/Biomass Ratio

Fig 8 Effect of SBR on the solar thermal energy input fraction for various solar collector temperatures.

value of 12.9% when the SBR increases from 2.5 to 4.0 at ER of 0.5.

The results show that the proposed system has an apparent

advan-tage to the separated generation system, especially for saving the

fossil fuels

4.7 Effect of SBR on the system products and efficiency

Figs 10 and 11describe the distribution of system products and

efficiency at different SBR respectively The results inFig 10

indi-cate that SBR has a significant influence on domestic hot water

generation As shown inFig 5, the bio-gas yield increases with

the increase in SBR when the solar collector temperature is

main-tained at 350°C, therefore increasing the sensible heat of bio-gas

and the domestic hot water obtained by heat exchanger (HX-2)

Besides that, the electricity, cooling generation and desiccant

capacity decrease with the increase in SBR, but not obviously

Due to the decrease in lower heating value of bio-gas, the input

energy of ICE goes down while the bio-gas yield increases with

the increase in steam flow rate.Fig 11shows the system

perfor-mance for various SBRs at ER of 0.4 The electrical efficiency

decreases with the increase in SBR, as it can be seen fromFig 10,

domestic hot water increases significantly compared with other

products, consequently increasing the thermal efficiency with the

increase in SBR However the bio-gas yield increases with the

increase in steam flow rate, the LHV of bio-gas is reduced, which

decreases the electrical efficiency

4.8 Effect of air preheating temperature on the overall energy efficiency with various ERs

The trigeneration system uses air and steam as gasification agent, and the air preheating temperature has a significant impact

on the overall energy efficiency.Fig 12represents the variation of overall energy efficiency with the air preheating temperature at different ER As shown inFig 12, the higher temperature of air improves the gasification performance more The overall energy efficiency increases from 77.8% to 82.8% with increase in air pre-heating temperature from 100°C to 500 °C at ER of 0.4 Moreover, the overall energy efficiency increases with the increase in ER For example, the overall energy efficiency increases from 77.2% to 82.3% with increase in ER from 0.35 to 0.5 at air preheating tem-perature of 200°C

4.9 Effect of solar collector temperature on the overall energy efficiency with various ERs

As mentioned above, the solar collector temperature has an important effect on the overall energy efficiency similarly.Fig 13

illustrates the overall energy efficiency with solar collector tem-perature at different ER The solar collector provides heat with

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

12

13

14

15

16

17

18

19

20

Steam/Biomass Ratio

ER=0.35 ER=0.4 ER=0.45 ER=0.5

Fig 9 Effect of SBR on the primary energy saving ratio with various ERs.

0

1000

2000

3000

4000

Steam/Biomass Ratio

Electricity Domestic hot water Cooling generation Desiccant capacity

0 10 20 30 40 50 60 70 80 90 100

Steam/Biomass Ratio

Thermal efficiency Electrical efficiency Overall energy efficiency

Fig 11 Effect of SBR on the system efficiency (ER = 0.4).

70 75 80 85 90

Air Preheating Temperature ( )

ER=0.35 ER=0.4 ER=0.45 ER=0.5

Fig 12 Effect of air preheating temperature on the overall energy efficiency at

steam for gasification reaction, so the temperature of steam

changes with the solar collector temperature From Fig 13, the

higher solar collector temperature enhances the overall energy

efficiency As described above, it is also showed that the increase

in ER increases the overall energy efficiency of trigeneration

system

4.10 The comparison of annual total cost compositions between

trigeneration and separated generation system

The annual total cost compositions of the proposed and

sepa-rated generation system are shown inFig 14 It can be found that

the annual initial capital cost of the trigeneration system is

obvi-ously higher than that of the separated generation system The

annual initial capital cost of proposed system is approximately

seven times than that of the separated generation system

How-ever, the annual operation costs between the separated generation

system and trigeneration system are 3,626,740 Yuan (RMB) and

980,000 Yuan (RMB) respectively, which the separated generation

system is about 3.7 times than the trigeneration system In

addi-tion, fromFig 14it can be seen that the equipment initial capital

cost of the trigeneration system mainly accounts for 51.5% of the

annual total cost, and the operation cost is about 33.9% of the

annual total cost in the trigeneration system For the separated generation system, the operation cost is much higher than the equipment initial capital cost, which accounts for 92.9% of the annual total cost

4.11 Effect of fuel price on ATCSR Considering the fluctuation of the market fuel price, it is imper-ative to analyze the variation of annual total cost saving ratio (ATCSR) at different fuel prices, such as biomass, natural gas and electricity The ATCSR sensitive analysis between the trigeneration system and separated generation system is shown inFig 15 It can

be seen from Fig 15 that the annual total cost saving ratio decreases lineally with the increase of the biomass price Due to the increase in biomass price, the operation cost of trigeneration system increases, which leads to the increase of annual total cost (ATC) subsequently Moreover, the annual total cost saving ratio increases nonlinearly with the increase of prices of natural gas and electricity The effect of electricity on the annual total cost sav-ing ratio is greater than that of the natural gas under the same change multiple of the prices

5 Conclusion

In this study, a feasible trigeneration system coupled with bio-mass gasification and solar thermal process is proposed Trans-forming mid-and-low solar thermal energy into chemical energy

of bio-gas by heating the steam indirectly, and providing fuel for internal combustion engine and exhaust heat recovery subsystem Simulation and performance analysis of the trigeneration system are performed to investigate the effects of key operating parame-ters on its performance The main research and conclusion are as follows:

(1) Air equivalence ratio, steam/biomass ratio and air preheat-ing temperature have a significant effect on biomass gasifi-cation reaction, and consequently affect the overall energy efficiency of the trigeneration system

(2) The introduction of mid-and-low solar thermal energy in tri-generation system decreases the extra consumption of bio-mass, and the solar thermal energy input fraction can be reached up to 48.1% when steam/biomass varies from 0 to 4.0 at solar collector temperature of 350°C Similarly, the higher solar collector temperature improves the overall energy efficiency of the trigeneration system

100 150 200 250 300 350 400 450

75

80

85

Solar Collector Temperature( )

ER=0.35 ER=0.4 ER=0.45 ER=0.5

Fig 13 Effect of solar collector temperature on the overall energy efficiency for

various ERs (SBR = 1.0).

0

1x106

2x106

3x106

4x106

Operation Maintenance Equipment

Annual Total Cost Composition

Trigeneration system Separated generation system

Fig 14 The comparison between trigeneration system and separated generation

0 10 20 30 40 50 60

The Change Multiple of Price Biomass Natural gas Electricity

Fig 15 The effect of fuel price on the annual total cost saving ratio.