Applied Energy xxx (2016) xxx–xxx Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy New solar-biomass power generation system integrated a two-stage gasifier Zhang Bai a,b, Qibin Liu a,b,⇑, Jing Lei c, Hui Hong a,b, Hongguang Jin a,b a Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, PR China b h i g h l i g h t s  A new solar-biomass power generation system is proposed  Endothermic reactions of the biomass gasification are driven by solar energy  The thermodynamic properties of the system are numerically investigated  The superiorities of the proposed system are validated a r t i c l e i n f o Article history: Received March 2016 Received in revised form June 2016 Accepted 17 June 2016 Available online xxxx Keywords: Solar energy Solar thermochemistry Two-stage biomass gasification Power generation Thermodynamics evaluation a b s t r a c t A new solar-biomass power generation system that integrates a two-stage gasifier is proposed in this paper In this system, two different types of solar collectors, concentrating solar thermal energy at different temperature levels, are applied to drive solar-biomass thermochemical processes of pyrolysis (at about 643 K) and gasification (at about 1150 K) for production of solar fuel The produced solar fuel, namely gasified syngas, is directly utilized by an advanced combined cycle system for power generation Numerical simulations are implemented to evaluate the on-design and off-design thermodynamic performances of the system Results indicate that the proposed system can achieve an overall energy efficiency of 27.93% and a net solar-to-electric efficiency of 19.89% under the nominal condition The proposed twostage solar-biomass gasification routine exhibits improved system thermodynamic performance compared to that in one-stage gasification technical mode, and the provided heat resource is in a good match with the requirements for the biomass gasification procedure Under given simulation conditions in this paper, the energy level upgrade ratio in the proposed two-stage solar-biomass gasification system for the introduced solar thermal energy is as high as 32.35% compared to 21.62% in one-stage gasification mode Meanwhile, the daily average net solar-to-electric efficiency on the representative days reaches to the range of 8.88–19.04%, while that of 9.97–15.71% in one-stage model The research findings provide a promising approach for efficient utilization of the abundant solar and biomass resources in western China and reduction of CO2 emission Ó 2016 Published by Elsevier Ltd Introduction Renewable energies, including solar energy and biomass, contribute to the alleviation of current energy and environment concerns due to the features of clean utilization and abundant storage [1–4] Various types of solar collectors, including flat plate collector, parabolic trough collector, solar tower and dish receiver, have been ⇑ Corresponding author at: Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, PR China E-mail address: qibinliu@mail.etp.ac.cn (Q Liu) developed to concentrate solar energy at different temperature levels [5–8] Currently concentrating solar power (CSP) technologies have been widely applied to generate power in addition to photovoltaic (PV) technology [9–12] Thermal energy concentrated by solar collectors is used to heat feed-water to superheated steam directly or through a heat transfer fluid (i.e., synthetic oil or molten salt) and then the superheated steam drives the steam turbine for power generation Due to the uneven temporal and spatial distribution of solar energy, storage of solar energy using molten salt or other phase change materials are investigated [13–17] Additionally, an emerging technology in solar thermal utilization use compressed air as heat transfer medium The first prototype of a http://dx.doi.org/10.1016/j.apenergy.2016.06.081 0306-2619/Ó 2016 Published by Elsevier Ltd Please cite this article in press as: Bai Z et al New solar-biomass power generation system integrated a two-stage gasifier Appl Energy (2016), http://dx doi.org/10.1016/j.apenergy.2016.06.081 Z Bai et al / Applied Energy xxx (2016) xxx–xxx Nomenclature A E H HHV m Q R S t T W energy level (pre-exponential factor) exergy (kJ molÀ1 or kJ kgÀ1) enthalpy (kJ molÀ1 or kJ kgÀ1) high heat value (kJ kgÀ1) mass flow rate (kg sÀ1) heat (kW) the gas constant (8.314 J/(K mol)) heliostat area (m2) time temperature (K) electric power (kW) Greek letters a reaction conversion rate b heating rate (K minÀ1) g efficiency (%) solar powered gas turbine system was tested in 2002 without major problems, and many investigations on the related issues have been conducted subsequently [18–20] The inherent properties of solar energy, such as low energy density and intermittency, provide difficulty in keeping the thermodynamic and economic performances of the solar devices at a high level Solar thermochemical utilization is a promising solution to these limitations Among current solar thermochemical utilization technologies, solar driven biomass gasification has also attracted considerable attention [21,22] Biomass is another type of renewable energy that can be utilized through chemical reactions such as combustion, pyrolysis and gasification to produce heat, tar and syngas, respectively In particular, gasification is one of the most important technique for processing biomass While, in gasification, reaction heat from biomass in-situ combustion is needed to drive a set of endothermic thermochemical conversion reactions for the production of syngas (a mixture composed of H2 and CO) [23–25] Therefore, it is possible to introduce solar thermal energy into the thermochemical reaction of biomass gasification in order to achieve more efficient biomass utilization In the process of solar-biomass gasification, concentrated solar energy is introduced to provide hightemperature heat resource for driving the biomass gasification reaction, in which solar thermal energy, with an amount equal to the enthalpy change of the endothermic reactions, is converted into the chemical energy of the syngas and low-carbon footprint transportation fuels [22] It is worth mentioning that biomass is composed of carbohydrates with high volatile content and exhibits favorable reactivity More importantly, the hybridized solar energy and biomass are renewable which contribute to CO2 emission reduction Currently, numerous prototype reactors, such as two-zone solar reactor, fluidized bed reactor, packed-bed reactor, have been developed for solid fuel solar gasification and a favorable solar conversion ratio can be achieved through experimental investigations [26–31] Additionally, solar gasification acts as a promising pathway for valuable liquid fuels production such as methanol and Fischer–Tropsch diesel, and in some publication, the polygeneration concept is employed to enhance system performance [32–35] In addition, gasified syngas, as a kind of solar fuel, can be directly utilized for power generation with a favorable efficiency by incorporating with the combined Brayton–Rankine cycle [36] Biomass gasification process is a set of complex reactions, in which the biomass feedstock is preheated, and then pyrolyzed to yield tar and char, then the tar is cracked and char is gasified with # reaction heat factor Subscript ASU aux bio CC day net opt parasitic ref solar sol-elec sys th air separation unit auxiliary devices biomass combined cycle daily accumulated or averaged net output power optical parasitic consumption reference system solar energy solar-to-electric system thermal the gasification agent (e.g CO2 or steam) to produce noncondensable syngas [37–39] Generally, the biomass preheat and pyrolysis steps can be implemented under a mid-temperature condition of lower than 673 K However, most previous publications only used point focus collectors to concentrate high-temperature thermal energy to drive the gasification process, which has a relatively high energy loss and capital investment compared to lowtemperature line focus collectors such as parabolic collectors, and more exergy loss due to higher temperature difference between solar energy source and biomass preheat and pyrolysis chemical reaction Therefore, the thermal heat resources should be introduced correspondingly to the individual temperature requirement of each reaction procedure A two-stage gasification concept, i.e., using high-temperature heat resource to drive the biomass gasification, mid-temperature solar thermal energy for biomass preheating and pyrolysis procedures, is an effective solution Naturally, the main objectives of this work include proposing a two-stage solarbiomass gasification concept, developing a novel solar-biomass hybrid power generation system, and assessing performances of the solar thermochemical conversion process and the developed system The main contributions are summarized as follows: (1) A novel hybrid power generation system integrated with a two-stage solar-biomass gasification process is proposed for effective utilization of solar energy and biomass The proposed system reduces fossil fuel consumption and mitigates CO2 emission (2) In the proposed system, two solar collection devices are employed to provide concentrated solar thermal energy at different temperatures In addition, the pyrolysis and gasification of the biomass feedstock are driven by concentrated solar thermal energy at appropriate temperature In this method, exergy destruction in solar collection and thermochemical conversion processes can be reduced (3) Solar thermal energy can be converted into chemical energy stored in syngas through solar-biomass gasification The energy level of the introduced solar energy is upgraded An effective integrated utilization of the renewable energies can be achieved In addition, under both design and offdesign working conditions, more favorable thermodynamics performances of the proposed system are obtained The rest of this study is organized as follows In Section 2, we propose a novel solar-biomass power generation system integrates Please cite this article in press as: Bai Z et al New solar-biomass power generation system integrated a two-stage gasifier Appl Energy (2016), http://dx doi.org/10.1016/j.apenergy.2016.06.081 Z Bai et al / Applied Energy xxx (2016) xxx–xxx a two-stage solar gasifier, and the system performance evaluation criteria are given In Section 3, the chemical composition of the biomass sample is experimentally determined, and the pyrolysis kinetic characteristics for the biomass sample are investigated The energy level upgrade for the solar-biomass gasification is analyzed in Section The nominal and off-design performances within the representative days are presented Finally, we summarize the main conclusions in Section New system and performance analysis In the power generation subsystem, the Brayton-Rankine combined cycle system uses a SGT-800 type gas turbine The steam cycle block is composed of a dual pressure heat recovery steam generator (HRSG) with a high pressure of 55 bar and a low pressure of 6.9 bar We not include an energy storage subsystem for the sake of the system simplification, and thus the power output rate will vary with solar irradiation intensity The two-stage solar-biomass gasification concept is applied in the proposed solar hybrid power generation system, and it has the following appealing advantages: (1) Biomass pyrolysis, comprising the feedstock preheating and steam generation, is driven by mid-temperature solar thermal energy Since this mid-temperature of solar energy matches better than single high-temperature with the aforementioned processes, irreversibility in this process can be reduced (2) The introduced mid-temperature solar thermal energy is concentrated by the LFC, which can achieve a relative increased collection efficiency than the PFC that operates in a higher temperature range (3) Solar energy can be readily stored in chemical form by driving biomass pyrolysis and gasification, which helps overcome the limitations of solar energy such as low-energy density and intermittency (4) The gasified syngas as a kind of solar fuel can be effectively utilized by an advanced gas turbine or used for liquid fuels production, e.g., methanol, and diesel 2.1 New system Concentrated solar energy as a heat resource is introduced to drive the biomass gasification process for chemical fuel production However, more exergy destruction will be generated during a typical solar-biomass gasification process which is only driven by high-temperature solar thermal energy To overcome this limitation, a two-stage gasification concept is employed to optimize the reaction process of solar-biomass gasification In accordance with the reaction procedure, biomass gasification process can be divided into two parts, pyrolysis and gasification, at different reaction temperatures Therefore, a two-stage solar-biomass gasification is employed in this work, and the required heat resource for each sub-process is suitably provided The produced syngas, i.e., solar fuel, is directly utilized by an advanced combined cycle for power generation The flow diagram of this two-stage solarbiomass gasification system is depicted in Fig The proposed solar-biomass power generation system consists of a solar-assisted biomass gasification subsystem and a power generation subsystem During the gasification process, the biomass pyrolysis initially produces tar and char at temperature lower than 673 K The required solar thermal energy is concentrated by linefocus solar collectors (LFC), such as parabolic trough solar collector, besides, the steam as the gasification agent is also generated by the LFC Subsequently, a point-focus collector (PFC) with the beamdown concept is applied to provide gasification reaction heat for the processes of tar cracking and char gasification at temperature above 1000 K, for syngas production Solid particles of ash and other corrosive components, such as H2S, are removed from the produced syngas via condensation and clean-up Finally, the qualified syngas as gas fuel is directly fed into combined cycle to generate power 2.2 System designated operation parameters The PFC provides high-temperature thermal energy by concentrating solar irradiation from the heliostats Heliostats, with width and length of 12 m, revolve on dual axes to track sunrays Meanwhile, solar energy used for pyrolysis is concentrated by the LPC, e.g., parabolic trough collectors The solar field is connected by numerous solar loops In this work, the ET-150 type collector is employed The fundamental parameters of the collectors are shown in Table The solar field is installed in the south–north direction and tracks sunrays automatically Solar energy resource is abundant in China, especially in western China The hybrid solar-biomass power generation system is located in Yanqi (E 86°340 , N 42°050 ), Xinjiang province in western Hyperboloid reflector GT Compressor Heliostats Combustor GT Turbine Air Biomass Syngas clean-up H 2O Solar gasifier Steam Turbine Cooling tower Condenser HRSG Fig Schematic diagram of the novel solar-biomass power generation system Please cite this article in press as: Bai Z et al New solar-biomass power generation system integrated a two-stage gasifier Appl Energy (2016), http://dx doi.org/10.1016/j.apenergy.2016.06.081 Z Bai et al / Applied Energy xxx (2016) xxx–xxx Table Parameters of ET-150 type solar collector Items Unit Aperture width Focal length Glass envelope outer/inner diameter Absorber tube outer/inner diameter Mirror reflectivity Designate optical efficiency m m m m % % 5.75 1.71 0.115/0.109 0.07/0.066 94 85.32 China The on-design point of the proposed system is the solar time of 12 h on June 21 and the nominal operation parameters for designation are listed in Table The economical collection radius of the biomass resource and the effect of solar field scale on solar tower collection performances should be taken into account For PFC, the solar field layout is designed and optimized with the assistance of the System Advisor Model (SAM) software, which is developed by the National Renewable Energy Laboratory in USA [40] The solar field is chosen based on the required thermal solar energy, the design point condition, and the power capacity The solar multiplier is set to 1.0 since the energy storage subsystem is not considered The aperture of the LFC is 20,772 m2, while the heliostat area of the PFC is about 78,830 m2 and the maximum distance of the heliostat from the tower is about 475 m Since the solar irradiation and ambient condition are variable, the operating parameters of the hybrid system should be adjusted because energy storage systems are not included The performance of the gas and steam turbines is influenced by operating condi- Table Nominal parameters for the proposed solar thermal power system Items Location Altitude Direct nominal irradiation (DNI) Ambient temperature Pyrolysis temperature Gasification pressure Gasification temperature Solar collection temperature for LFC/PFC Mass ratio of steam to biomass Pressure ratio of GT compressor Gas turbine inert temperature Gas turbine exhaust temperature Parameters of the high-pressure steam Parameters of the low-pressure steam Pinch-point temperature difference of HRSG Yanqi, Xinjiang 1055 m 751 W/m2 298.35 K 643 K 18 bar 1150 K 643–1150 K 0.5 20 1561.15 K 819.15 K 805.15 K/55 bar 533.15 K/6.9 bar 20 K tions, including power load ratio and ambient temperature As a result, the off-design behavior of the proposed systems should be evaluated Four representative days (19 March, 21 June, 22 September, and 21 December) were selected for analysis The direct nominal irradiation profiles vary during the four selected days, as depicted in Fig 2.3 Performance evaluation method In this section, system thermodynamic performance and solar energy conversion efficiency are evaluated [33,41] A typical biomass integrated combined cycle (BIGCC) system is selected and used as the reference system in this work, so that the electric power generated from the input biomass energy with typical gasification routines and the contribution of the introduced solar energy for the power generation can be calculated Biomass is directly gasified with purified O2, and the gasified syngas is used in a combined cycle for generating power Both on-design and off-design thermodynamic performance evaluations of the proposed solar hybrid power system are implemented The reference system is also simulated under the same operating conditions For the proposed hybrid power system with two-stage solarbiomass gasification, the net generated electricity Wnet and the incremental solar power production Wsol,elec are defined as: W net ¼ W À W CC;aux À W sol;parasitic ð1Þ W ref;net ¼ mref;syngas Á HHVref;syngas Á gCC;net À W ASU ð2Þ W sol;elec ¼ W net À W ref;net ð3Þ where W and Wnet represent the total generated power of the proposed system and the net output power, respectively; WCC,aux, Wsol,parasitic and WASU indicate the power consumption of the combined cycle auxiliary devices, parasitic consumption of solar field operation and the power consumption of air separation unit, respectively; Wref, net is the net power output of the reference system, and gCC,net is the net efficiency of the combined cycle in the proposed system The overall energy efficiency gth,sys and the net solar-to-electric efficiency gsol-elec are used as basic criteria for performance evaluation of the proposed solar-biomass hybrid systems, which are defined as: gth; sys ¼ W net DNI Á Ssolar þ mbio HHVbio gsolÀelec ¼ W net À W ref;net DNI Á Ssolar ð4Þ ð5Þ where Ssolar is the area of the solar field; HHV represents the higher heat value; and m represents the mass rate For the off-design performances of the system on representative days, the accumulated amount of the net generated electricity Wnet,day is considered, and can be calculated by: W net; Fig DNI profiles of the representative days day ¼ X W net Á Dt ð6Þ If the power load ratio of the gas turbine is lower than 10%, the power generation system will be shut down, because the system efficiency declines sharply and the generated electricity cannot even compensate the basic power consumption of the auxiliary devices at such low power load Meanwhile, the one-stage solar-biomass gasification system with equivalent biomass gasification feed rate is evaluated, and used to reveal the potential thermodynamic performance improvement of the two-stage solar-biomass gasification power generation system The only difference between the two systems is that the Please cite this article in press as: Bai Z et al New solar-biomass power generation system integrated a two-stage gasifier Appl Energy (2016), http://dx doi.org/10.1016/j.apenergy.2016.06.081 Z Bai et al / Applied Energy xxx (2016) xxx–xxx one-stage solar-biomass gasification system is driven only by hightemperature solar thermal energy collected by the PFC Biomass sample determination 3.1 Chemical composition of the biomass sample Corn straw is the most abundant herbaceous biomass resource in China, and is thus selected as the gasification feedstock The biomass sample of corn straw was selected follows The pyrolysis experiment of corn straw was first conducted, by a program-controlled electrical furnace, at temperature below 673 K, the tar yield ratio reached 19.5% as reported in Table The chemical composition as air-dry basis of the biomass sample and the char (solid product from pyrolysis) were determined and summarized in Table Fig TG and DTG curves for pyrolysis of corn straw 3.2 Kinetic analysis of biomass pyrolysis -8.5 a¼ mo À mt mo À mf ð7Þ where mo and mf are the initial and the final masses of the sample, respectively; mt stands for the mass of the sample at time (t) It is assumed that in the nonisothermal and heterogeneous reaction of biomass pyrolysis, the general reaction rate (da/dT) is calculated as a function of conversion rate a and rate constant k (T), as follows: da ¼ k Á f ðaÞ dT b ð8Þ where b is the heating rate; and f (a) refers to the reasonable model of the reaction mechanism Table Product yield of pyrolysis (wt.%) Corn straw 0.15 0.1 0.05 -9.0 ln(β/Τ 2) To investigate the two-stage solar-biomass gasification concept, the thermogravimetric analysis (TGA) of the biomass feedstock pyrolysis has been applied The reaction temperature of the start and end can be measured, and the biomass pyrolysis kinetic characteristics can also be obtained by analyzing the TGA data The TGA measurements were performed using a Mettler Toledo TGA/DSC-1 with a gas flow of 50 mL/min of nitrogen In the TGA experiments, the sample was pulverized to a particle size of about 0.2 mm, and heated from room temperature to 1000 K at a heating rate of 10– 50 K/min The thermogravimetric curve (TG) and differential thermogravimetric (DTG) curves of the samples on TGA are shown in Fig The main weight loss procedure occurs within the temperature range of 500–650 K, and the solar collection temperature of the parabolic trough collector can satisfy such technical requirement In this study, the reaction conversion rate of the biomass sample denoted by a can be expressed as follows: 0.95 -9.5 -10.0 -10.5 1.50 10K/min 15K/min 20K/min 50K/min 1.55 1.60 1.65 1.70 1.75 1.80 1000/T K-1 Fig Arrhenius plot of b/T2 versus 1/T at selected conversion ratios The reaction rate constant k is dependent on the temperature and can be expressed by:   E k ¼ A exp À RT ð9Þ where A is the pre-exponential factor; E represents the activation energy and R indicates the universal gas constant [8.314 J/(K mol)] In kinetics investigation, the distributed activation energy model (DAEM) is widely used because it fits the DTG curve DTG by a series of parallel, first-order reactions The DAEM is based on the assumption that the system consists of a series of irreversible parallel first order reactions The details were elucidated by Miura [42–44] Finally, the reaction conversion rate can be simplified as: a¼1À Z UðE; TÞf ðEÞdE ð10Þ Tar Water Char Gas 19.50 22.13 38.26 20.11 where f(E) is the distribution function of the activation energy, and U(E,T) can be approximated as follows: Table Chemical composition of the biomass sample a HHV (MJ kgÀ1) Sample Proximate analysis (wt.%) Ultimate analysis (wt.%) Mad Aad Vad FCad Cad Had Nad Sad Oad Corn straw Chara 3.94 0.36 7.1 18.65 69.56 22.81 19.39 58.18 41.49 59.28 6.05 3.90 2.35 4.60 0.19 0.25 38.88 12.96 16.51 25.67 Produced by pyrolysis Please cite this article in press as: Bai Z et al New solar-biomass power generation system integrated a two-stage gasifier Appl Energy (2016), http://dx doi.org/10.1016/j.apenergy.2016.06.081 Z Bai et al / Applied Energy xxx (2016) xxx–xxx UðE; TÞ ffi exp À ART ÀE=RT e bE ! ð11Þ In the TGA experiments, pyrolysis characteristics were measured at heating rates of 10, 15, 20 and 50 K/min The Arrhenius plot of b/T2 versus 1/T was determined through mathematical analysis as illustrated in Fig The activation energy E at selected conversion ratio can be determined, and the overall activation energy E is 223.95 kJ/mol Results and discussion 4.1 Energy level upgrade from solar thermal energy to chemical energy The energy level A and energy-utilization diagram methodology (EUD) proposed by Ishida and Kawamura [45] are applied to analyze energy level upgrading Energy level A is a dimensionless criterion and defined as the ratio of exergy change DE to energy change DH, namely A = DE/DH = À T0DS/DH For transferred heat, the energy level AT can be simplified to AT = À T0/T For an energy-conversion system, energy is released by the energy donor (Aed) and accepted by the energy acceptor (Aea) Correspondingly, the EUD can be illustrated and determined by the energy level (A) versus the energy-conversion quantity (DH), which graphically shows the variations of energy quality and energy quantity of the process With the assistance of the EUD, exergy destruction can be obtained easily from the shaded area between the curves for Aed and Aea Meanwhile, the energy level degradation of each process and the driving force as the energy level difference can be graphically shown Concentrated solar energy is used to drive the biomass gasification, and the solar energy is converted into chemical energy From the viewpoint of the energy level, the energy level of the solar energy can be upgraded to that of produced solar fuel, which can then be used in numerous energy applications with increased efficiency For the given solar-biomass gasification process (see Fig 5), the energy and exergy balances can be expressed as follows: m3 h3 ¼ m1 h1 þ DH2 ð12Þ m3 e3 ¼ m1 e1 þ DE2 À DEw ð13Þ where m, h and e represent the mass, specific enthalpy and specific exergy, respectively; DEw is the reaction exergy destruction during the gasification process In accordance with the definition of the energy level, we obtain the following: A1 ¼ m1 e1 =m1 h1 ¼ e1 =h1 ð14Þ A2 ¼ DE2 =DH2 ¼ À T =T ð15Þ A3 ¼ m1 e1 þ DE2 À DEw m1 h1 þ DH2 ð16Þ The reaction heat factor # denotes the ratio of the absorbed solar thermal energy to the feedstock chemical energy, namely # = DH2/m1h1, thus Eq (13) is changed to: m1 e1 DE À DE w þ ¼ A3 m1 h1 þ #m1 h1 DH2 =# þ DH2 A1 # þ ðA2 À DAw Þ ¼ 1þ# 1þ# A3 ¼ where DAw = DEw/DH2 is the energy level reduction of the reaction heat, and it is caused by the mismatch of energy levels between the solar thermal energy and biomass gasification Through the solar thermochemical process of driving biomass gasification, a part of solar thermal energy is converted to chemical fuel of syngas with an upgraded energy level The relative upgrade ratio in the energy level of solar thermal energy may be formulated as:   ðA3 À A2 Þ A1 # A2 ¼ ðA2 À DAw Þ À A2 þ A2 1þ# 1þ# ¼ ðA1 À A2 Þ # DA w À ð1 þ #ÞA2 þ # A2 ð18Þ The energy level difference between biomass feedstock A1 and syngas A3 serves as a ‘‘driving force” to improve the solar thermal energy to the higher one in chemical energy Meanwhile, the energy level upgrade ratio is dependent on DAw produced by the mismatch of energy levels between the reaction heat resource and biomass gasification The reduction of DAw is one way to enhance the performance of solar-biomass gasification Consequently, the two-stage solar-biomass gasification concept is proposed in this work, with a main motivation to decrease exergy destruction during the midtemperature reaction procedure The EUD for the solar-biomass gasification process is illustrated in Fig For the typical solarbiomass gasification process with high-temperature solar energy introduced (1150 K for the case study), the energy level of solar energy can be improved from 0.74 to 0.9 by conversion into the produced syngas While, if the gasification process employs the proposed two-stage solar-biomass gasification technical mode, the energy level of the required solar energy is reduced to 0.68, and the increased energy level upgrade ratio of 32.35% for the solar energy conversion can be achieved In addition, compared to the one-stage gasification mode, the proposed system can convert more heat resource of solar energy into the chemical form, accounting for 9.25% of the required net exergy of the solar thermal energy 1.25 Abiomass 1.0 Asyngas ATIT A Substituting Eqs (14) and (15) into Eq (13), the energy level of the syngas as produced solar fuel can be expressed as: A'solar ΔEextra 0.5 biomass Gasification steam H1, E1, A1 ð17Þ Asolar syngas H3, E3, A3 H2, E2, A2 Solar energy (thermal resource) Fig The process of solar assistant biomass-steam gasification 0.0 50 100 200 ΔH / MW Fig EUD of solar-biomass gasification process Please cite this article in press as: Bai Z et al New solar-biomass power generation system integrated a two-stage gasifier Appl Energy (2016), http://dx doi.org/10.1016/j.apenergy.2016.06.081 Z Bai et al / Applied Energy xxx (2016) xxx–xxx 4.2 System evaluation under the on-design condition System performance evaluation on two-stage solar-biomass gasification concept under the nominal condition was first conducted using the aforementioned evaluation criteria, the net generated power is 42.11 MW, with a the biomass feed rate of 4.6 kg/s and introduced solar energy of 74.8 MW In the solar-biomass gasification process, the net solar share is about 49.61%, whereas the overall system energy efficiency and exergy efficiency reach 27.93% and 31.62%, respectively By comparison with the reference system, gsol-elec can be calculated and is about 19.89% In the system with one-stage biomass gasification, if the biomass processing rate is equal to the proposed system, then the solar energy requirement is increased to 85.39 MW, resulting in gth,sys and gsol-elec reduction by 6.56% and 12.4%, respectively Furthermore, detailed investigations, including energy and exergy balance analysis, were implemented, and the results are summarized in Table Under the given on-design operation conditions, the largest energy and exergy loss in the proposed system are produced in the solar collection processes, which accounts for the total energy input of 27.36% and total exergy input of 22.13%, respectively compared to the one-stage gasification mode, the exergy loss in solar collection for the proposed system and solar thermochemical process are reduced by 23.25% and 20.22%, respectively Additionally, the heat loss of the stack gas and the steam condensation contributes to the second largest energy loss of 29.83% While, for the exergy analysis, the second largest energy loss item is generated in the syngas combustion processes and accounts for 16.37% of the total input In particular, for the power generation subsystem in the combined cycle, the illustrated EUD is shown in Fig 7, five energy conversion sub-processes, namely gas combustion, air compression, gas expansion, heat exchange in HRSG and steam expansion, are included For the gas combustion process in the GT gas combustor, the combustion of fuel plays the role of energy donor in heating fuel and air which are energy acceptors, and Aea1 denotes the heating of the fuel gas and air, and the exergy destruction in the combustor reaches 21.8 MW The gas turbine (Aed,GT) and compressor (Aea,comp) serve as the energy donors and acceptor, respectively, Fig EUD of the power generation subsystem with a total exergy destruction of 7.08 MW While, the width of Aed,GT and Aea,comp indicates the gas turbine’s work output and the compressor’s power consumption, the GT net power output reaches 32.27 MW During the heat recovery sub-process within the HRSG, the energy level of flue gas exiting the gas turbine (Aed2) acts as the energy donor, and its energy level decreased from 0.64 to 0.19, while the feed water of the steam Rankine cycle represents an energy acceptor with energy level improved from 0.06 to 0.63 The area between the curves Aed2 and Aea2 indicates the exergy destruction in the HRSG, which is 4.04 MW or 3.03% of the total exergy input 4.3 Off-design evaluation in representative days Owing to the fact that the performance of the solar collection and combined cycle are affected by operation conditions, we evaluated the off-design behavior in four representative days Solar collection efficiency, including heliostat optical efficiency and receiver thermal efficiency, is computed based on the optimized solar field layout and the local meteorological data In addition, the integrated Brayton-Rankine cycle is referred to SGT-800 type gas tur- Table Energy and exergy balances in the proposed systems Two-stage solar-biomass gasification One-stage solar-biomass gasification Energy balance Energy balance Exergy balance Input Value (kW) Ratio (%) Value (kW) Biomass Solar energy 643 K Solar energy 1150 K 75973.00 15599.77 59200.28 50.39 10.35 39.26 80948.82 8366.38 43851.96 Total 150773.05 100.00 133167.16 Output Generated power 42107.84 27.93 42107.84 Ratio (%) 60.79 6.28 32.93 100.00 31.62 Energy loss/exergy loss Exergy balance Value (kW) Ratio (%) Value (kW) Ratio (%) 75973.00 – 85389.33 47.08 – 52.92 80948.82 – 63251.22 56.14 – 43.86 161362.33 100.00 144200.03 100.00 42107.84 26.10 42107.84 29.20 Energy loss/exergy loss Solar collection-643 K Solar collection-1150 K Solar-steam generation Gasification unit Syngas condensation Gas combustor Gas turbine Steam turbine HRSG Stack loss Condenser Aux power Others 5311.91 35944.60 – – 14292.45 – 2155.40 1123.86 – 14131.72 30851.00 2632.50 2221.78 3.52 23.84 – – 9.48 – 1.43 0.75 – 9.37 20.46 1.75 1.47 2848.85 26625.57 1435.40 8299.88 7507.91 21801.00 7083.70 2444.60 4041.00 90.48 1941.50 2632.50 4306.92 Total 150773.05 100.00 133167.16 2.14 19.99 1.08 6.23 5.64 16.37 5.32 1.84 3.03 0.07 1.46 1.98 3.23 100.00 – 51845.79 – – 14292.45 – 2155.40 1123.86 – 14131.72 30851.00 2632.50 2221.78 – 32.13 – – 8.86 – 1.34 0.70 – 8.76 19.12 1.63 1.38 – 38404.20 1435.40 10402.97 7507.91 21801.00 7083.70 2444.60 4041.00 90.48 1941.50 2632.50 4306.92 – 26.63 1.00 7.21 5.21 15.12 4.91 1.70 2.80 0.06 1.35 1.83 2.99 161362.33 100.00 144200.03 100.00 Please cite this article in press as: Bai Z et al New solar-biomass power generation system integrated a two-stage gasifier Appl Energy (2016), http://dx doi.org/10.1016/j.apenergy.2016.06.081 Z Bai et al / Applied Energy xxx (2016) xxx–xxx bine, the power cycle off-design performance is simulated considering the influences of the ambient temperature, power load, gas fuel composition The results are calculated based on a one-hour basis, and Fig presents the system performance represented by system energy efficiencies of gth,sys and the net power output of Wnet Electricity generated by the combined cycle varies instantaneously with solar irradiation because neither thermal energy storage system nor chemical energy storage system is adopted in the proposed solar hybrid power system On June 21st, the daily maximum power of Fig Hourly net generated electricity and the system energy efficiency in the representative days Fig Hourly solar-to-electric efficiency of the systems in the representative days Please cite this article in press as: Bai Z et al New solar-biomass power generation system integrated a two-stage gasifier Appl Energy (2016), http://dx doi.org/10.1016/j.apenergy.2016.06.081 Z Bai et al / Applied Energy xxx (2016) xxx–xxx Wnet / MW⋅h 450 30 300 20 150 10 Wnet ηsys,th ηsol-elec 600 (b) one-stage solar-biomass gasification 3.19 6.21 9.22 12.21 450 40 30 ηsys,th /ηsol-elec / % 3.19 6.21 9.22 12.21 40 Wnet / MW⋅h (a) two-stage solar-biomass gasification ηsys,th /ηsol-elec / % 600 300 20 150 10 Wnet ηsys,th ηsol-elec Fig 10 Daily average system performances in the representative days 44.31 MW h with the highest gth,sys of 28.47% is achieved at 12 h, and the operational time last for 10 h For the typical days in March and September, the maximum power is 26.63% and 26.89%, respectively, and gth,sys for both days is above 25% Furthermore, in the sunny hours of the December day, compared to other days, the system performances experience a sharp performance reduction, with the highest hourly net power of only 19.01 MW h and the gth,sys of 16.29% The main reasons are that the operation time is significantly reduced to h, and the solar collection efficiencies are weakened, which resulting in a lower efficiency for power cycle and decreased power ratio The comparison on performance represented by gsol-elec between the proposed system with two-stage and one-stage solar-biomass gasification mode is conducted, and daily results are summarized in Fig Generally, gsol-elec for the two-stage gasification mode is higher than that for the one-stage gasification mode, although the overall variation trends are similar In summer time such as in June day, the gsol-elec for the novel two-stage system is higher than the three other selected days with a peak value of 20.7% The daily efficiency is also higher than the one-stage gasification mode which in the range of 12.28–30.61% However, in the winter time such as in December, the performance of novel twostage solar-biomass gasification mode is worse than the onestage solar-biomass gasification mode since the reduction ratio of gsol-elec reaches from 0.64% to 16.11% The reasons are that the solar collection efficiency of the LFC deteriorates substantially in winter as the cosine effect is increased, while the performances of the PFC still remains at a reasonably high level, especially when dual-axis tracked heliostats are applied The daily average performances of the developed system in the representative days are summarized in Fig 10 The daily maximum net generated electricity output produced in June day is 409.65 MW h with the highest averaged gth,sys of 29.14% and gsolelec of 19.04% Although the daily operation times in March and September days are relatively shorter than that in June day, gas turbines can operate with a favorable performance, with an average gsol-elec up to 17.15% and 16.07%, respectively This is mainly because the contributions of higher intercepted solar heat and the lower ambient temperature, while the system with one-stage gasification technical mode can only achieve 15.71% and 14.81% The above studies indicate the proposed novel solar-biomass power generation system integrated with a two-stage gasifier exhibits remarkable performance, which achieves the daily average efficiency of the gsol-elec above 15% except in December day Although the solar-biomass gasification can only be realized at the laboratory scale currently, the conventional biomass gasification and the concentrated solar power technologies have become mature, they will contribute to make a breakthrough and facilitate the real applications of solar-biomass gasification technology Therefore, the solar-biomass thermal gasification technology will accelerate the commercial operation in the near future, which will bridge current fossil-fuel-based technologies and future solar thermochemical technologies Thus, the proposed system provides a promising approach for efficient utilization of the abundant solar and biomass resources in the western China Conclusions In this work, we propose a new solar hybrid power generation system that integrated with a two-stage gasifier The on-design and off-design thermodynamic performances are simulated and analyzed, and the main research findings can be outlined as follows: (1) A new system with two-stage solar-biomass gasification route is proposed for efficient utilization of the solar energy The proposed system applies mid-temperature solar energy collected by the LPC for the biomass pyrolysis, which matches well with the reaction temperature, and the exergy destruction during the gasification process and the solar energy collection process can be reduced by 23.25% and 20.22% compared to the one-stage solar-biomass gasification mode, respectively (2) By using concentrated solar energy to drive the biomass gasification, solar energy is converted into the chemical fuel in the form of gasified syngas The energy level of the introduced solar thermal energy in the proposed two-stage solarbiomass gasification system was improved from 0.68 to 0.9, which results in an energy level upgrading ratio of 32.35% compared to 21.62% in one-stage gasification mode (3) Under the nominal condition, the overall energy efficiency and the net solar-to-electric efficiency for the proposed novel system reached 27.93% and 19.89%, respectively Additionally, the proposed system exhibits satisfactory thermodynamics performances except in December days during system off-design evaluation In addition, and the daily average net solar-to-electric efficiency achieved the improvement in the range of 8.6–21.33% compared to the onestage gasification thermochemical system The proposed hybrid solar power generation integrating twostage gasification routine provides a promising approach for the efficient utilization of the abundant renewable solar and biomass energy resources in western China Acknowledgements The authors appreciate financial support provided by the National Natural Science Foundation of China (No 51276214, No 51236008) Please cite this article in press as: Bai Z et al New solar-biomass power generation system integrated a two-stage gasifier Appl Energy (2016), http://dx doi.org/10.1016/j.apenergy.2016.06.081 10 Z Bai et al / Applied Energy xxx (2016) xxx–xxx References [1] Moriarty P, Honnery D What is the global potential for renewable energy? 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