Thermodynamics and LCA analysis of biomass supercritical water gasification system using external recycle of liquid residual

Renewable Energy 141 (2019) 1117e1126

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Thermodynamics and LCA analysis of biomass supercritical water gasification system using external recycle of liquid residual Cui Wang, Hui Jin*, Pai Peng, Jia Chen State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, 28 Xianning West Road, Xi’an 710049, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 January 2019 Received in revised form 5 March 2019 Accepted 28 March 2019 Available online 4 April 2019

Biomass supercritical water gasification is clean and renewable, which can convert biomass into hydrogen rich gas. Previous studies indicated that external recycle of liquid residual could improve gas yield and gasification efficiency. However, the influence mechanism of external recycle on energy and exergy efficiency is complicated and theoretical model for system optimization is insufficient. Thermodynamic model of external recycle of liquid residual was built in this paper. Exergy efficiency of the main components and exergy loss distribution were specified and the result showed that exergy loss of reactor and preheater accounted for 26.06% and 35.88% of the total exergy loss, which were the main exergy loss sources. Effective ways to reduce exergy loss of components with large exergy loss and to improve energy and exergy efficiency of the system were proposed. Moreover, life cycle assessment of biomass gasification process was carried out. The results indicated that the increase of gasification temperature, pressure and external recycle flow rate of liquid residual and decrease of biomass concentration could improve energy and exergy efficiency of the system. Energy and exergy efficiency reached 63.67% and 48.29% respectively at the condition of gasification temperature of 560
C, pressure of 25 MPa, recycle flow ratio of 32.43%, biomass concentration of 2.78%. Besides, the increase of gasification temperature and decrease of biomass slurry concentration and pressure could decrease GWP. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Supercritical water gasification External recycle system Thermodynamics analysis LCA

1. Introduction Hydrogen, as a secondary energy, can’t be obtained from nature directly [1,2]. Supercritical water gasification (SCWG), based on the excellent physical and chemical properties of supercritical water (SCW) to reach rapid gasification, is a promising technology to produce hydrogen, which has been developed rapidly in the recent years [3e6]. SCW is water exceeding the critical point, temperature of 374
C, pressure of 22.1 MPa, having unique physical and chemical properties [7]. The solubility of gas and organic ingredients in SCW enhanced compared to the solubility in water at ambient conditions due to the hydrogen bond and dielectric constant decrease [8], which means that SCW is non-polar [9]. Higher diffusivity and lower viscosity of SCW promotes mass transfer. SCW has only one phase with no liquid-gas boundary, providing a homogeneous environment for gasification [10]. Rapid reactivity of SCW accelerates the depolymerization of aggregate structures and increases hydrogen yield [11]. Biomass is renewable, clean and

* Corresponding author. E-mail address: [email protected] (H. Jin). https://doi.org/10.1016/j.renene.2019.03.129 0960-1481/© 2019 Elsevier Ltd. All rights reserved.

abundant [12] and has been studied in many countries as a potentially important renewable resource [13,14]. Supercritical water gasification of biomass has several advantages like gas products are clean with no emission of NOx and SOx; easy separation of CO2; easy regulate of water properties due to the wide change range of water properties [15,16]. Supercritical water gasification has been studied for decades and experimental and simulation studies have been carried out [17e20]. Lu et al. [21]meant to solve the plugging problem through studying the effects of different operating parameters on biomass gasification in SCW. Parameters included pressure, temperature, residence time, reactor types, heating rate, reactor wall properties, biomass types, biomass particle size, catalysts and solution concentration. Jin et al. [22]conducted molecular dynamics researches of anthracene and furfural catalytic gasification. The influence mechanism of Cu and Ni catalyst on the supercritical gasification process, hydrogen production and carbon conversion path was figured out. Supercritical water gasification of wheat straw soda black liquor at the condition of higher temperature and longer reaction time was studied by Cao et al. [23] Jin et al. [24]proposed a novel two-step utilization of coal and studied the influence of

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different operating parameters on gas products distribution. Yanik et al. [25]conducted lignocelluloses and tannery wastes gasification experiments and researched the influence of different wastes on yield and composition of products. The conclusion that cellulose and organic materials in lignocelluloses material and the kind of lignin influenced gas composition was drawn. Jin et al. [26]conducted thermodynamics analyze of SCW coal gasification reaction. The yield and composition of gas products were studied. Moreover, the effects of different operating parameters on the gasification were investigated to solve pulping problem. Reddy et al. [27] investigated degradation routes of biomass model compounds like cellulose and lignin at the condition of near and supercritical conditions. The result showed that hydrogen yield decreased with phenol and glucose, and with lignin, char formulation decreased. Difference of gasification system influences gasification effect, composition of gas products and energy and exergy efficiency. The biomass gasification system is various and researches about system design have been conducted by scholars. Dueck et al. [28]invented a biomass gasification system to produce steam using carbonized biomass. The system is composed of main body, two cylindrical members, two cut-out members and two cylinder accommodating therein a second screw conveyors. The system could reduce thermal loss since the temperature of thermal decomposition could be quickly stabilized. Cao et al. [29]simulated a system of supercritical water gasification of black liquor and pulping. Various products were obtained like hydrogen, MP and LP steam and power. Gasification results of three different oxidants were compared. Feldmann et al. [30]created a biomass gasification system to convert biomass into gaseous fuel. At least 50e70% carbon in biomass was converted into gaseous carbon at the temperature of about 700
C. A high efficiency and low cost biomass gasification system containing a combustor and a gasifier, inside diameter of which was over 36 inches and height of which was over 40 feet was established. Dueck et al. [31] provided a new biomass gasification system used to extract heat from the biomass. The system overcome the insufficient of prior art equipment. A external recycle system was established by Jin et al. [32]. Energy efficiency and hydrogen yield increased as liquid residual was recycled due to the energy in residual was recovered. Energy and exergy analysis provides a tool for realizing system optimization and many researchers are focusing their works on relevant analysis [33,34]. Karamarkovic et al. [35]analyzed the energy and exergy efficiency of biomass air gasification with different moistures and at gasification temperatures range of 900e1373 K. The conclusion that efficiencies decreased as moisture increased and using dry biomass can promote the gasification process was drawn. SKLMF built a new experimental system for biomass gasification in SCW and chemical equilibrium of reactor, gas-liquid equilibrium of high-pressure separator and energy and exergy analysis of the whole system was conducted by Lu et al. [36]. Energy and exergy loss distribution of the system was provided. Energy, exergy and cold efficiencies of every component of the system was evaluated by Cohce et al. [37]. Thermodynamic performance of biomass gasification, steam-methane reforming and shift reactions producing hydrogen was investigated. Two methods were contained to better understand the influence of different parameters on the performance. Biomass supercritical water gasification equilibrium composition was determined by establishing a thermodynamic model [38]. Life cycle assessment (LCA) is another effective tool, which is used to investigate the environmental impacts. Researches about LCA of biomass gasification process are widely conducted [39,40]. Chen et al. [41] applied LCA analysis to assess the environmental impact of SCWG solar driven process. LCA analysis of SCWG was conducted by Edgar et al. [42] to realize the design of the system.

Though thermodynamics analysis was widely conducted by scholars, thermodynamics analysis of external recycle system is insufficient due to the complexity of the system. Thermodynamic model was established for the biomass SCW external recycle system in the text and influence of various operating parameters on the energy and exergy efficiency of the system were discussed. The components with the greatest energy and exergy loss were figured out and solutions were proposed to decrease the energy and exergy loss. Moreover, LCA of biomass gasification process was conducted to evaluate the influence of different parameters on the environment. 2. Introduction of the model of biomass SCW gasification with external recycle system Biomass SCW gasification products are mainly comprised of H2, CO2, CH4 and CO. Biomass SCW gasification process is complicated and it is often simplified as [43] steam reforming (1), water gas shift (2) and methanation reaction (3):

CHx Oy þ ð1  yÞH2 O/CO þ ðx=2 þ 1  yÞH2

(1)

CO þ H2 O4CO2 þ H2

(2)

CO þ 3H2 4CH4 þ H2 O

(3)

Reaction (1) is endothermic and reaction (2) and (3) are exothermal. Since the gasification process is partially endothermic, some heat should be provided to complete gasification process. The heat source contains LHV of biomass, electric energy and latent heat of the recycled liquid residual [44]. 2.1. Modeling of biomass gasification in SCW Biomass SCW gasification with external recycle system was established in Aspen and the flow chart was shown as Fig. 1. DECOMP was yield balance reactor, in which biomass was decomposed into single element substance. Reactor was gasification reactor based on Gibbs minimum free energy and gasification process was completed in the component. The gasification products were cooled by water under ambient conditions and gas products were separated from liquid products. Part of the liquid residual was recovered and the remaining liquid residual was cooled by user water, providing heat for the users. In the aspen flow chart, 1 represents the biomass; 2 is the feed water; 8 is the cooling water; 12 is the recycle liquid residual and 15 is the user water. According to the previous studies [45], Soave Redlich-Kwong property method with modified Huron-Vidal mixing rule (SRKMHV2) was selected as the property method since it was suitable for chemical reactions under supercritical states. The operating parameters were as follows: gasification temperature of 500
C, pressure of 25 MPa, feed water flow rate of 850 kg/h, biomass flow rate of 70 kg/h, cooling water of 1000 kg/h, liquid recycle ratio of 32.43%. Liquid recycle ratio was defined as the ratio of recycle flow rate and the water input to the system.

Fig. 1. Flow diagram of biomass SCWG system with external recycle.

C. Wang et al. / Renewable Energy 141 (2019) 1117e1126

External recycle system was shown in Fig. 2 as red color. The recycle loop comprises with separator, splitter, high-pressure circulation pump, heat regenerator, reactor and preheater. Part of the liquid residual out of separator was recycled and mixed with the water out of heat exchanger, pressurized by high-pressure circulation pump, heated by heat regenerator and then flowed into reactor for gasification. Compared with traditional system, part of the high temperature residual was impregnated into the reactor, which saved the feed water and improved energy and exergy efficiency of the system. Moreover, some chemical ingredients like phenols and HCOOH containing in the recycle residual accelerated the gasification reaction and improved hydrogen yield. Besides, the remaining liquid residual was not discharged directly, which achieved further utilization by heating the water from users. Thus the external recycle system achieved energy and exergy recycle and realized efficient utilization of the energy contained in the liquid residual. 2.2. Biomass materials Corn stalks from Xi’an Shanxi province was selected and the elemental and proximate analyses were shown in Table 1 [46]. 2.3. Thermodynamic analysis model 2.3.1. Mass balance First, mass balance was expressed by the following equation:

X

min ¼

X mout

(4)

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Hlost was the energy loss in the process of gasification and heat exchange. Two forms of energy efficiency was defined as:

h1 ¼

Eout E1 þ Eelec

(8)

h2 ¼

Egas E1 þ Eelec

(9)

Egas ¼ mH2 qH2

(10)

In equation (9), Egas was the energy contained in the gas products; E1 represented the LHV of biomass and Eelec was on behalf of the electric energy consumed during the gasification process. In equation (10), q meant the mass flow rate of H2 and other gas products like methane, carbon dioxide and carbon monoxide were ignored since the main purpose of SCWG system was to produce hydrogen and improve hydrogen yield. 2.3.3. Exergy balance and definition of exergy efficiency Exergy represents the maximum useful work that can be acquired from a system, which is an important criterion for measuring irreversible losses based on the second law of thermodynamics. Analyzing exergy efficiency of the system and finding out components with the most exergy loss may provide a feasible solution to decrease the exergy loss. As for actual liquid fluid, exergy is composed of physical and chemical exergy.

EX ¼ EXch þ EXph

(11)

Physical exergy indicates the biggest perfect working capacity of the system due to differences in temperature [47], pressure and environment and can be calculated by:

2.3.2. Energy balance and definition of energy efficiency And energy balance of the system was described as:

Hin ¼ H1 þ Helec

(5)

Hout ¼ H14 þ H16

(6)

Hin  Hout ¼ Hlost

(7)

In these three equations, H represented the energy contained in different substances. H1 represents the energy contained in biomass, that is, LHV of biomass, which was obtained by experiment. Helec was the electric energy consumed to realize gasification.

EXph ¼ ðh  h0 Þ  T0 ðs  s0 Þ þ

v2 þ gz 2

  The specific kinetic exergy

v2 2

and potential exergy (gz) of the

actual liquid fluid are equal to zero. So expression of physical exergy is simplified to:

EXph ¼ ðh  h0 Þ  T0 ðs  s0 Þ

(13)

h and s represent enthalpy and entropy of the working medium

Water2 Gas products Fluid seperator

Heatx Biomass

Pump DECOMP 500 250bar

Reactor 500 250bar

Water

Cooler /Preheater

Residual Heatx1 Recycle system

(12)

User Water

Fig. 2. Schematic diagram of biomass SCWG system with external recycle.

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Table 1 Elemental and proximate analysis of biomass. Proximate analysis (wt%)

Elemental analysis (wt%)

Qnet(MJ/kg)

Mad

Vad

Fad

Aad

Cad

Had

Oad

Nad

Sad

8.02

67.55

17.97

6.46

41.18

4.96

38.41

0.78

0.19

15.58

Air-dried basis.

under specified temperature and pressure conditions. h0 and s0 express the enthalpy and entropy under the condition of ambient temperature of 288.15 K and pressure of 0.1 MPa [48]. Physical exergy of biomass is zero since the influence of pressure and temperature is not obvious which can be ignored. As for gas mixture [36], EXph is the linear summation of physical exergy of each component.

EXph

X ¼ xi EiXph

(14)

i

Chemical exergy of substance refers to the maximum work that can be acquired from it by leading it to chemical equilibrium with the reference environment [49]. It is an ambient temperature of 298.15 K, an atmospheric pressure of 1 atm defined in the paper. Chemical exergy of gas mixture can be expressed as [36]:

EX;c ¼

X

xi ε0;i þ RT0

i

X

xi lnxi

(15)

i

xi represents mole fraction of the pure component i and 3 0,i is the standard chemical exergy, which is obtained from model 2 by Szargut et al. [50]. And 3 0,i represents the maximum work when a compound is taken from ambient state, based on ambient temperature T0 and pressure P0 to the dead state, based on the same ambient conditions. Chemical exergy calculation formula of gas mixture is not suitable for biomass and the calculation of biomass chemical exergy is expressed as [50]:

EXbiomass ¼ bLHVbiomass

(16)

In Eq. (16), LHVbiomass is the low calorific value of biomass and coefficient b is calculated [51]based on the ratios of carbon, oxygen and hydrogen and the equation is given by:

b¼

1:0414 þ 0:177½H=C  0:3328½O=Cf1 þ 0:0537½H=Cg 1  0:4021½O=C (17)

Exergy balance and exergy efficiency was calculated by the following equations:

Ein ¼ E1 þ E2 þ E15 þ E8 þ Eelec

(18)

Eout ¼ E6 þ E14 þ Egr

(19)

hE1

P Eout ¼ P Ein

hE2

P E ¼P 6 Ein

system and exergy input into the system. Another form of exergy efficiency, defined in this study as hE2 was the ratio of the exergy of gas products and exergy input to the system. 3. Research method of life cycle assessment (LCA) LCA is comprised by definition of purpose and scope, inventory analysis, impact assessment and result interpretation as shown in Fig. 3. The impact assessment part is a difficult and most important part of the LCA process and would be discussed in detail later [39]. 3.1. Environmental impact classification and characterization The purpose of the classification is to classify the results of the inventory analysis into environmental impact categories. The environmental impact categories include Global Warming Potential (GWP), Acidification Potential (AP), Eutrophication Potential (EP) and Ozone Layer Depletion Potential (ODP). Different contaminants cause different environmental pollution categories. The emission of SCWG system, containing CO2 and CH4, mainly influences global warming potential (GWP). Contaminants cause AP, EP and ODP like NOx, SO2 and HC are not produced. Thus only the GWP was considered in LCA analysis. Characterization meant to unite the units of inventory analysis results and merge conversion results of the same impact type and equivalent coefficient method [52] was the most common method. Equivalent coefficient of GWP was obtained from World Meteorological Organization (WMO) and the third assessment report of IPCC [53]. According to the criterion, equivalent coefficient of CO2, N2O and CH4 is 1, 23 and 296 kgCO2eq/ kg respectively. 3.2. Standardization The purpose of standardization is to better understand the relative size of various environmental impact categories parameters, which requires a selected baseline value (ie “standard person equivalent reference/capita.year”) and the standardization benchmark system of impact categories was based on environmental emissions of China in 1995 [54] (Table 2). Standardized impact potential can be obtained based on the reference value using the following equation:

Definition of purpose and scope

(20)

(21)

In these equations, E1 was chemical exergy of biomass; E2, E15 and E8 were exergy contained in feed water, cool water, and user water respectively. E6 was the exergy of gas products; E14 represented the exergy of liquid residual; Egr meant the exergy provided for the users. hE1 is the ratio of all of the exergy output from the

Inventory analysis

Impact assessment Fig. 3. Frame diagram of LCA.

Result interpretation

C. Wang et al. / Renewable Energy 141 (2019) 1117e1126 Table 2 Standard human equivalent reference value of China in 1995. Impact categories

Base unit

Standardized reference value

GWP

kg(CO2eq)

3.59Eþ03

NIR(i)CN- 1995 ¼ IR(i)/N(i)CN/cap- 1995 NIR(i)CN- 1995 is standard person equivalent reference value of category i (kg.eq); IR(i) is impact potential of category i (kg.eq); N(i)CN/cap- 1995 is standardized impact potential of category i. 4. Results and discussions 4.1. Energy and exergy balance of the external recycle system Energy input and out from the system at the condition of gasification temperature of 500
C, pressure of 25 MPa, recycle flow ratio of 32.43%was given as (see Table 3): The energy input to the system was mainly composed of electricity energy and LHV of biomass. And energy output from the system contained energy of gas products, energy for heat supply and lost energy. It can be calculated that the energy input to the system was equal to the energy output from the system. Energy efficiency of the overall system reached 78.07% according to Eq (8). Exergy analysis supplies a more practical view of the gasification process and an effective engineering assessment. So exergy balance calculation was conducted. Exergy flow of the material input and output from system can be seen in Table 4. The number 2, 8, 1, 15 appeared in the table represented the material flow defined at the beginning of 2.1. Irreversible loss caused the exergy of the output system was less than the exergy of the input system. The reason to cause exergy loss was various. The main reason was chemical reaction and heat transfer. Two forms of exergy efficiency of the system was 78.03% and 39% respectively calculated by Eq. (20) and Eq. (21).

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water to gasification temperature; L-G is the liquid-gas separator; Heatx1 is the heat exchanger used to supply heat for users. Tables 5 and 6 indicated that exergy lost most at the REACTOR, Preheater and Heatx1. Exergy efficiency of Preheater was the lowest, reaching 64.48%. Exergy efficiency of REACTOR and Heatx1 was 84.44% and 89.87% respectively. Exergy loss of these three components accounted for 26.06%, 35.88% and 24.52% of the total exergy loss respectively. Exergy loss of REACTOR was because chemical reactions converted chemical exergy into thermal exergy. The main reason to cause low exergy efficiency of preheater was that high grade energy, electricity energy, was consumed to heat the water to gasification temperature. As for Haeatx1, low exergy efficiency was due to the irreversible loss of heat transfer process. An effective way to increase exergy efficiency of the preheater and REACTOR was that using external heat source like solar energy, high temperature waste heat to replace electricity heat. And the effective way to improve exergy efficiency of the system was external recycle of liquid residual.

4.3. Effective ways to improve the energy and exergy efficiency of the system 4.3.1. Increasing external recycle flow rate of liquid residual One of the main reasons to cause the energy and exergy efficiency of the system was not high was that part of energy and exergy contained in the liquid residual was lost. An effective method mentioned in this paper to improve the energy and exergy efficiency of the system was to establish an external recycle system of liquid residual. Besides, the influence of recycle flow rate on the energy and exergy efficiency of the system was expressed in Fig. 4. The operating conditions were gasification temperature of 500
C, pressure of 25 MPa. It can be seen that at the condition of gasification temperature of 500
C, pressure of 25 MPa, energy and exergy efficiency increased as recycle flow rate increased. Energy and exergy efficiency ranged

4.2. Exergy efficiency and exergy loss analysis Evaluating the exergy efficiency of the system and finding out the component with the largest loss provides a feasible method to improve system performance. Exergy efficiency of different components was described in Table 5 in detail. In Table 5, import and export exergy and exergy efficiency of different components were calculated. REACTOR is the gasification reactor; Heatx is the heat exchanger; Preheater is used to heat the

Table 5 Exergy efficiency of different components. Equipment

Exergy of import/kW

Exergy of export/kW

Exergy efficiency/%

REACTOR Heatx Preheater L-G Heatx1

2010.31 1843.5 1212.48 1337.31 2904.91

1697.58 1680.96 781.79 1337.30 2610.62

84.44 91.18 64.48 99.99 89.87

Table 3 Energy balance of the biomass SCWG external recycle system. Energy of inlet/kW

Energy of outlet/kW

Inlet enthalpy Electricity energy

323.75 1396.82

Sum

1720.57

Enthalpy of gas Heat supply Heat loss

774 568 377.34 1720.57

Table 4 Exergy balance of the biomass SCWG external recycle system. Exergy of inlet/kW 2 8 1 15 Electric exergy Sum

Exergy of outlet/kW 124.03 150.65 371.02 1087 1396.82 3129.53

Gas Residual Heat supply

1220.52 168.9 1052.48

2441.89

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C. Wang et al. / Renewable Energy 141 (2019) 1117e1126

Table 6 Exergy loss rates distribution of biomass SCWG external recycle system. Import/kW Exergy destruction

REACTOR Heatx Preheater L-G Heatx1

Export exergy Export and destruction exergy

Energy and exergy efficiency /%

Energy efficiency

Exergy 4330.61

Exergy loss rate/%

Exergy loss proportion/%

312.73 162.54 430.68 0.0054 294.29 3130.37 4330.61

26.06 13.54 35.88 0.001 24.52 100

7.22 3.75 9.95 0.0003 6.79 72.28 100

recovered with external recycle, which increased energy and exergy efficiency of the system. Besides, certain substances contained in the residual like phenol and acetic acid which could promote hydrogen production were recycled. Moreover, the recycle of liquid residual caused the concentration of the raw material decreased, according to chemical equilibrium principle, low concentrate of raw material can accelerate gasification reaction.

Exergy efficiency

40

20

0

0

200

400

-1

600

Recycle flow rate / kg·h

Fig. 4. Influence of recycle flow rate on the energy and exergy efficiency of the biomass SCWG system with external recycle.

from 31.11% to 45.23% and from 22.88% to 39.14% separately. The lowest energy and exergy efficiency at the condition of 25 MPa, 500
C, appeared at the recycle flow rate of zero and the highest energy and exergy efficiency of the system appeared at the recycle flow ratio of 32.43%. Energy and exergy of liquid residual were

4.3.2. Increasing gasification temperature The effect of gasification temperature on energy and exergy efficiency at the operating parameters of pressure of 25 MPa, external recycle flow ratio of 32.43%, variation of reaction temperature of 500e560
C was discussed. The highest reaction temperature of biomass gasification was 560
C since the purpose of this paper was to discuss the influence of various temperatures on the gasification characteristics and the temperature range in this paper was only part of the gasification temperature in practical applications. It indicated that the influence of gasification temperature on energy and exergy efficiency was obvious (Fig. 5). Energy and exergy efficiency increased sharply as gasification temperature increased, providing a viable way to improve energy and exergy efficiency of the system. The lowest and highest energy efficiency of the system was 45.23% and 63.67% separately at the gasification temperature of 500
C and 560
C. Exergy efficiency changed in the range of 39.14%e48.29%. One of the main reasons was that reactive activity of biomass increased as gasification temperature increased, promoting the gasification process. Besides,

Fig. 5. Influence of gasification temperature on the energy and exergy efficiency of the biomass SCWG system with external recycle.

C. Wang et al. / Renewable Energy 141 (2019) 1117e1126

excess water resulted in a preference for the production of hydrogen and carbon dioxide instead of carbon monoxide. The increase of gasification temperature accelerated water-gas shift reaction, that is, the reaction of carbon monoxide and water, since this process was an endothermic reaction, accompanied by the increase of hydrogen. Increase of hydrogen yield improved the energy and exergy efficiency of the system. Moreover, increase of gasification temperature increased hydrogen production since gas production reactions were frequently classified as free radical reactions and it could be reinforced at higher temperature.

4.3.3. Increasing gasification pressure Fig. 6 expressed the influence of gasification pressure on the energy and exergy efficiency of the system at the condition of gasification temperature of 500
C, liquid residual recycle flow ratio of 32.43%. The lowest gasification pressure conducted in this paper was 23 MPa in order to exert the excellent chemical and physical properties of supercritical water. However, excessive pressure puts higher demands on material properties, so the maximum operating pressure was 25 MPa. Energy and exergy efficiency increased as gasification pressure increased and change value of them were 2.34% and 3.44% separately, indicating that the influence of pressure was not obvious. The phenomenon was proved by many scholars [55,56]. The influence mechanisms were various. One of the influence factor can’t be ignored was that properties of SCW can be regulated by operating parameters like temperature and pressure [50]. Buhler [57] considered that the gasification process of biomass in supercritical water may be the result of the mechanism of ionic reaction and of free radical reaction. Kruse [58] also figures out that the dominant position of the ion reaction and the free radical reaction changes with the variation of gasification temperature and pressure. Freeradical reactions are inhibited at high pressure in SCW since the density and ion product increase as pressure increases, promoting ionic reactions. Gases like hydrogen are mainly formed through free-radical reactions, so the increase of pressure prevented gas production and caused energy and exergy efficiency decreasing. For another, it can be seen that the main equations in gasification including water-gas shift reaction, methanation reaction and steam reforming reaction were all expansion reactions. These three reactions were all inhibited at a higher pressure according to chemical equilibrium movement theorem, which reduced yield of gas products like hydrogen and carbon dioxide. All of the mechanisms supported the results in this paper that high pressure favors

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biomass gasification. 4.3.4. Adjusting concentration of the biomass material The effect of slurry concentration on energy and exergy efficiency of the system with gasification temperature of 500
C, pressure of 25 MPa, liquid residual recycle flow ratio of 32.43% was described in Fig. 7. It can be seen from Fig. 7 that the energy and exergy efficiency decreased as biomass concentration increased. The energy efficiency varieties in the range of 34.42%e45.23% and exergy efficiency changed from 30.49% to 39.14% as biomass concentration decreased from 2.95% to 2.78%. Previous researches [59] indicated that hydrogen yield decreased as slurry concentration increased and the hydrogen gasification efficiency and carbon gasification efficiency also decreased [60]. Chemical equilibrium principle could explain the phenomenon. It can be seen from Eqs (1)e(3) that H2O and H2, CO were always the reactants and products and the decrease of slurry concentration indicated that H2O contained in feedstock increased, which then promoted three reactions, Eqs (1)e(3). Besides, high concentration of the biomass concentration may cause reactor plugging, further reducing the efficiency of the system. 4.4. Life cycle assessment (LCA) 4.4.1. Definition of purpose and scope The research object of this paper was biomass gasification system, aimed to calculate the environmental emissions and impacts. Life cycle was defined from the biomass raw materials entering the factory to generation of the target products, which was shown in Fig. 8. Main raw material of hydrogen production process were biomass and water and the energy input to provide heat for reactor and preheater was electricity. Gasification products were mainly comprised by H2, CH4 CO2 and CO. Basic function unit was the production of 1 kg H2. Consumed energy and raw material of different parts of the life cycle was calculated referred to the production of 1 kg H2. 4.4.2. Impact assessment The variation of characterized environmental impact potential value and standardized impact potential value of GWP with variant operating parameters were discussed to obtain the influence of

Energy efficiency Exergy efficiency

Energy and exergy efficiency /%

Energy and exergy efficiency /%

Energy efficiency 40

30

20

10

0

40

30

20

10

0 23

24

Gasification pressure / MPa

25

Fig. 6. Influence of gasification pressure on the energy and exergy efficiency of the biomass SCWG system with external recycle.

Exergy efficiency

2.76

2.79

2.82

2.85

2.88

2.91

2.94

Biomass slurry concentration / %

2.97

Fig. 7. Influence of biomass concentration on the energy and exergy efficiency of the biomass SCWG system with external recycle.

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C. Wang et al. / Renewable Energy 141 (2019) 1117e1126

DECOMP

REACTOR

Heatx

L-G

Pump

FSPL

Heatx1

Preheater Fig. 8. LCA of biomass gasification in SCW.

more energy to heat the feedwater and required better ultimate tensile strength materials, so gasification temperature cannot be too high. It can be seen from Fig. 9 (b) that GWP and standardized GWP increased 7.26% and 7.59% respectively as biomass slurry concentration increased, indicating that the increase of concentration had minor impact on the GWP. As mentioned before, the increase of concentration could decrease energy and exergy efficiency of the system, The increase of CH4 and decrease of H2 was a nonnegligible reason to cause thermodynamic efficiency decrease and GWP increase. Fig. 9 (c) showed that GWP increased sharply as gasification pressure increased, indicating the increase of pressure could aggravate environmental pollution. The main reason to cause serious GWP increase was the increase of methane and decrease of

operating parameters on the environmental index. It can be seen from Fig. 9 (a) that characterized and standardized GWP all decreased as gasification temperature increased from 500
C to 560
C. It can be explained from the reaction mechanism and gas products composition. Hydrogen and carbon dioxide yield increased as gasification temperature increased, while the methane yield decreased since methane decomposed at higher temperature, proved by Abuadala et al. [61]. CH4 influences GWP obviously due to high equivalent elasticity coefficient. The decrease of CH4 and increase of hydrogen caused decrease of GWP. Thermo dynamical analysis conducted in 3.3.3 showed that energy and exergy efficiency increased as temperature increased. Improving gasification temperature is a feasible way to improve efficiency and to reduce environmental pollutions. However, higher temperature required

120 Standardized GWP

GWP

0.04

Standardized GWP 0.06

Standardized GWP/AP

40

GWP

0.03

60

0.05

50

0.04

0.02

20 0.03

0.01 500

510

520

530

540

550

Gasification temperature/°C

2.8

560

2.9

Biomass slurry concentration /%

(b)

(a) GWP

120

Standardized GWP 0.032

Standardized GWP

0

0

110

0.030

GWP

GWP

100

0.03

80

0.028

100

0.026 90

21

22

23

24

Gasification pressure/MPa

25

26

(c) Fig. 9. Influence of operating parameters on GWP biomass SCWG system with external recycle (a) Temperature; (b) Concentration; (c) Pressure.

3.0

Standardized GWP/AP

GWP 100

C. Wang et al. / Renewable Energy 141 (2019) 1117e1126

hydrogen. Thus though energy and exergy efficiency increased a little as gasification pressure increased, considering environmental pollution problem, gasification pressure was not the higher, the better. 5. Conclusions Biomass SCWG with external recycle system of liquid residual was established to produce hydrogen and to provide heat for users. Compared with traditional system, part of the liquid residual was recovered and flowed into the reactor to realize re-gasification after it was preheated by the preheater. The advantage of the system lies in the external recycle system, which can not only recover energy contained in the residual to reduce energy loss but also decrease final concentration of feedstock to realize high gas yield and then increase efficiency. (1) Exergy efficiency of different components were analyzed and the components with high exergy loss were preheater and reactor, which accounted for 35.88% and 26.06% of the total exergy loss. (2) Feasible method, using external heat source like solar energy, high temperature waste heat to replace electricity was proposed to reduce exergy loss of the highest exergy loss component. And the effective way to improve exergy efficiency of the system was external recycle of liquid residual. (3) Effective approaches to improve energy and exergy efficiency of the system were put forward in this paper. Results indicated that with external recycle, energy and exergy efficiency of the system reached 45.23% and 39.14% respectively at 500
C 25 MPa, recycle flow ratio of 32.43%. (4) Increase of gasification temperature and decrease of slurry concentration and pressure could decrease GWP, reducing environmental pollution.

Conflicts of interest The authors declared that they have no conflicts of interest to this work. Acknowledgment This work was supported by the Basic Science Center Program for Ordered Energy Conversion of the National Natural Science Foundation of China (No. 51888103); National Key R&D Program of China [grant number 2016YFB0600100]; and the National Natural Science Foundation of China [grant numbers 51776169]. References [1] M. Momirlan, T.N. Veziroglu, Current status of hydrogen energy, Renew. Sustain. Energy Rev. 6 (1) (2002) 141e179. [2] A. Midilli, M. Ay, I. Dincer, M.A. Rosen, On hydrogen and hydrogen energy strategies : I: current status and needs, Renew. Sustain. Energy Rev. 9 (3) (2005) 255e271. [3] Z. Yan, X. Tan, Hydrogen generation from oily wastewater via supercritical water gasification (SCWG), J. Ind. Eng. Chem. 23 (2015) 44e49. [4] I.-G. Lee, S.-K. Ihm, Hydrogen production by SCWG treatment of wastewater from amino acid production process, Ind. Eng. Chem. Res. 49 (21) (2010) 10974e10980. [5] T. Richard, J. Poirier, Selection of ceramics and composites as materials for a supercritical water gasification (SCWG) reactor, Adv. Sci. Technol. 72 (2010) 129e134. [6] R. Li, Q. Chen, H. Zhang, Detailed investigation on sodium (Na) species release and transformation mechanism during pyrolysis and char gasification of highNa zhundong coal, Energy Fuels 31 (6) (2017) 5902e5912. [7] E.S. Fois, M. Sprik, M. Parrinello, Properties of supercritical water: an ab initio simulation, Chem. Phys. Lett. 223 (5e6) (1994) 411e415.

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