Energetic analysis of gasification of biomass by partial oxidation in supercritical water☆

Qingqing Guan ,Chaohai Wei*,Xinsheng Chai,Ping Ning ,Senlin Tian ,Junjie Gu ,Qiuling Chen ,Rongrong Miao

1 Faculty of Environmental Science and Engineering,Kunming University of Science and Technology,Kunming 650500,China

2 The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters,College of Environment and Energy,South China University of Technology,Guangzhou 510006,China

3 State Key Laboratory of Pulp and Paper Engineering,South China University of Technology,Guangzhou 510006,China

Keywords:Autothermal Gasification Supercritical water Biomass Energetic model

ABSTRACT

1.Introduction

Supercritical water(SCW)is water at temperatures and pressures that exceed its thermodynamic critical point(TC=647 K,PC=22.1 MPa).SCW exhibits properties that are very different from ambient liquid water,having a lower dielectric constant,fewer and weaker hydrogen bonds,and a higher isothermal compressibility than liquid water[1].When temperatures and pressures are over the critical point,small organic compounds,even gases become completely miscible in SCW[2].Therefore,a single homogeneous phase can exist at SCW conditions,leading to dramatically increase the reaction rates.

Since SCWoffers unique advantages,there are increasing interests in using SCW for fuels production[3],biomass processing[4]and waste treatment[5,6].Supercritical water oxidation(SCWO)is the process of oxidation treatment of wastewater or waste in supercritical water.As SCWO can rapidly destruct organic wastes,commercial SCWO facility for treating industrial wastewater became operational early in 1994[7].

With the current shortage of fossil fuel,it has become of increased strategic importance to use biomass for energy[8-10].Supercritical water gasification(SCWG)is the process that makes gaseous fuels in supercritical water.Some researches indicate that SCWG can be described as a steam reforming Reaction(1),water-gas shift Reaction(2)and methanation Reaction(3)[11].

Since SCW serves as both reactant and reaction medium,supercritical water gasification(SCWG)can lead to low tar and char formation and high hydrogen yields.Gasification of biomass in supercritical water provides a potential way to convert biomass to fuel-rich gas containing H2and/or CH4[12,13].Commercial SCWG facility can be found for treatment of surplus active sludge for gaseous fuels in Japan,but Matsumura's evaluation indicates that SCWG requires a significant amount of energy to pre-heat and maintain a desired high temperature for the reactions[14].To reduce the energy-cost is still the key issue for its industrialized application.

Partial oxidation in SCW is the process that gasification of biomass with less amount of oxygen than the required amount for complete oxidation by stoichiometry.As the amount of oxygen is less than that required by stoichiometry,the reactions lead to significant amount of CO and intermediates to further produce gaseous fuels such as H2and CH4by water gas shift reaction and SCWG[15,16].Therefore,partial oxidation in SCW is a combined process between SCWO and SCWG.It was found that the partial oxidation gasification could significantly reduce the energy consumption[17,18]since heat released from the exothermic reactions by oxidation can provide a supplemental energy source.Recently,some results even showed that when gasification of vinasse in supercritical water(SCW)in the presence of air,it might occur to keep the process in an auto-thermal status according to energetic analysis[19,20].Some researches have already shown that the reforming of ethanol to produce hydrogen from a mixture of ethanol with oxygen and water[21-23]can be conducted under autothermal conditions,suggesting a commercial way to reform ethanol.

There have been some previous studies of SCWG by partial oxidation,but relatively few that deal with thermodynamics.The results of earlier gasification of glycerol under autothermal mode were based on the use of Gibbs free energy minimization with equation of state of Peng-Robinson(PR)to predict both the product compounds and the mass enthalpy without considering the energy loss in the exchanger and the reactor and with high gasification efficiency[24].Partial oxidative gasification of biomass indicates that gasification efficiency such as lignin is far less than 100%[25].Gasification process is controlled by kinetics of reactions in experimental cases,and it is difficult to reach equilibrium as theory.Also,reforming of ethanol to produce hydrogen from a mixture of ethanol with oxygen and water[21-23]is different from partial oxidation in supercritical water since the amount of water is predominant in SCW system.

As the effect of lower gasification efficiency and energy loss,autothermal process in SCW was not reported by Jin et al.even with very high oxidant equivalent ratio(ER,0.6)and 24%(by mass)biomass loading[26].Understanding the energetic process can provide basic knowledge for optimal thermodynamic conditions to make the process self-sustaining.Therefore,it is necessary to conduct a thermodynamic analysis of the biomass gasification in SCW process by partial oxidation.The present paper focuses on the key factors such as oxidant equivalent ratio,energy recovery efficiency,biomass loading and temperature upon which autothermal gasification of biomass by partial oxidation might depend,which tries to provide detailed information to optimize the process to achieve an autothermal status for commercial operation to get fuel gases.

2.Model Development

Jin et al.have investigated gasification of glucose and lignin by partial oxidation under wide conditions in supercritical water[26].In this work,we referred their results such as gaseous yields and gasification efficiency to analyze reactions.Base on those reactions and the typical gasification process,we develop the energy balance of reactions and the energetic model.

2.1.System under consideration

Fig.1.System under study.S—separator;R—reactor.

The typical system under study is shown in Fig.1.This system is composed of two high pressure pumps(one for feedstock and one for oxygen),a heat exchanger(HE),a reactor,a cooler and three separators for high rich hydrogen gas.For a simplified evaluation of thermodynamic process and the energy exchange took place in the system,thermostatic and isobaric reactor model was considered.The biomass mixed with water at room temperature(298 K)was pressurized into HE and then entered the reactor.Meanwhile,the oxygen was pressurized into the reactor directly for partial oxidative gasification of biomass.The feedstock was heated up quickly at the reaction temperature at the inlet of reactor.After leaving the reactor,the effluent was cooled down to about 373 K in the HE for recycling energy and then cooled at room temperature in the cooler.In the separator,gaseous products were separated.

2.2.Development of the thermodynamic model

In an auto-thermal system,the energy and enthalpy output by partial oxidative reactions and recovered in HE should supply enough energy for system heating-up,energy loss and so on.As the reactor and HE were the key parts for this system,in which the largest enthalpy changes took place,the energy change of influent and effluent in HE and reactor was calculated.The flows of the mass and heat are shown schematically in Fig.2.

In supercritical water state,water is like a dense gas and has salvation properties like non-polar fluids.Therefore,hydrocarbon,gases and supercritical water may be assumed to be single homogeneous fluid.PR equation of state is used to conduct analysis of supercritical water reactions.PR equation of state for purefluids[27]is as

with

Fig.2.Mass balance and enthalpy changed.

for purefluids;

where

and

To extend to a mixture,van der Waals mixing rule is applied,i.e.,

and

where kijis estimated according to Abrams et al.[28],i.e.,

The thermodynamic data and critical properties of all substances are listed in Table 1[29].

To calculate the enthalpy of real gas under supercritical state,we used

and H*is the enthalpy of ideal gas

where Z is compressibility factor,

and the values of Z0and Z′can be obtained from chemical engineering.

2.2.1.Thermodynamic process in the HE

In the HE,the influent was heated quickly to temperature T1without chemical reaction.Thus,the enthalpy changed during the adiabatic temperature risen from room temperature of 25°C can be expressed as

where j is two for H2O and the biomass feedstock at the inlet.Considering the system under constant pressure P,can be calculated by expression,

The adiabatic temperature rise is related to the decrease in temperature of the products resulting from the change in enthalpy of the products which emerge from the heat exchanger at 373 K

where k=6 for H2,H2O,CO,CH4,CO2and other products at the outlet.Andis

A heat transfer efficiency(η=75%)was assumed within the HE[14,30],then,

2.2.2.Thermodynamic process in the reactor

According to Fig.2,theΔH2associated with the temperature rise to T can be expressed as

Table 1 Heat capacity of components at the reference state and physical properties used in calculation

l is 1 for O2and m is 2 for H2O and feedstock at the inlet.H2Ris

The autothermal system depends on energy ΔH3=niΔH0i（ T2）.The theoretical alternative enthalpy of the reactions was contributed by nimoles of the i th feedstock compound,which is calculated by

n is 3 for O2,H2O and feedstock.

Enthalpy loss is considered.The enthalpy loss was calculated by a simplified way,

where K is heat transfer coefficient and S is the surface of the reactor.In this system,reactor with volume about0.24 m3and surface 2.26 m2was assumed.As walls of reactor are commonly made by nickel alloy,heat transfer coefficient K=25 kJ·h?1was considered firstly[31],whose effect will be discussed in the next section.

Therefore,the supplied enthalpy is

and the enthalpy required is

The process is calculated and the equation is solved by Matlab software.The results are presented in the next section.

3.Results and Discussion

3.1.Effect of oxidant equivalent ratio(ER)

As glucose and lignin are the typical compounds found in biomass feeds,they were used as the model compounds of feedstock for the discussion of the energetic process using partial oxidation.In this case,water is supposed to be fed with a 1 mol·s?1flow rate into HE then enters the reactor.Firstly,the thermodynamic process of partial gasification of glucose and lignin is discussed under different oxidant equivalent ratios at 600°C and 25 MPa with 10%(by mass)loading in supercritical water.As biomass loading is 10%(by mass),the feedstock flow rates are 7.2 kg·h?1for both glucose and lignin.Related to experimental data[26],partial oxidation gasification reactions of glucose and lignin are summarized,as shown in Tables 2 and 3.To evaluate the enthalpy of the reactions and energetic process,LHV is considered in this study.It is noted that lignin has cross-linked racemic macromolecule with molecular masses in excess of 10,000 u and the typical molecular formula is(C6H10O5)mbasing on elemental analysis.In this study,the molecular formula C6H10O5is used to represent the unit lignin,which is consistent with the elemental analysis of the feedstock.

From Table 2,it is clear that the alternative enthalpy of the reactions changes from?755 to?977 kJ with the increase of ER from 0.2 to 0.8.For partial gasification of glucose,the increase of ER improves the gasification efficiency but decreases the hydrogen content dramatically.Previous study of partial oxidation of phenol showed that a higher concentration oxygen resulted in the oxidation of acids and CO predominating since a higher concentration oxygen increased the oxidant reaction rates for both acids and CO.Therefore,the increase of ER might enhance combustion of both glucose and hydrogen gas,leading to the decrease of fuel gases.

The effect of ER on the energetics of the process for glucose is also shown in Fig.3(a).As more oxygen is added into the system,the energy required increases from 26.5 to 28.3 kJ·s?1.Although the energy supplied increases dramatically,the results still indicate that even at the highest ER,the energy derived is insufficient to maintain an autothermal process.That is,some additional energy has to be introduced to the system.

The similar results were observed for the partial gasification of lignin in supercritical water.Table 3 shows that when ER increases from 0.1 to 0.4,the alternative enthalpies of partial gasification of lignin change dramatically from?150 to?866 kJ and the energy supplied increases from 3.8 to 12.1 kJ·s?1,as shown in Fig.3(b).The energy required to keep the process autothermal is about 27 kJ·s?1at this case.Therefore,even the highest ER is used for the partial gasification of lignin,it is still insufficient to maintain the system in an autothermal status.

These results are some different from those reported previously[19,20],in which the autothermal reforming of glycerol in supercritical water can be achieved at 900°C and 24 MPa with only 5%by mass glycerol loading and O2/glycerol of 1 by theoretical assessment.In low gasification rates,there are thermodynamic limits of the reactions to be completed,leading to low energetic efficiency.Additionally,the preheating efficiency(75%)in HE is lower than that of ideal HE.The results also show that enthalpy loss takes up 35%of the total energy required,which is not taken into account in other studies.Those also might be the main reasons that even at the highest ER the energy derived is insufficient to maintain an autothermal gasification of biomass at base case.Since the preheating efficiency and enthalpy loss play important roles on the achievement of autothermal status,the effect of preheating efficiency and energy loss will be discussed in the next section.

The increase of ER certainly can improve the energy supply but decrease the fuel gases.Lee et al.reported that direct gasification of glucose can produce the yields of about 6.2 mol·mol?1H2and 0.5 mol·mol?1CH4[32]at similar conditions without oxygen.But in this case,the highest H2yield is about 1.35 mol·mol?1and CH4yield is 0.15 mol·mol?1with ER 0.2.Therefore,partial oxidation of gasification of glucose in supercritical water might be a way that combustion of glucose and fuel gases for energy due to efficient gasification of glucose in supercritical water.Meanwhile,direct gasification of lignin at 600 °C can produce yields of about 1.5 mmol·g?1H2,1.6 mmol·g?1CO and 4.5 mmol·g?1CH4[33].It is noted that partial oxidative gasification of lignin can get gaseous yields of about 8 mmol·g?1H2and a few CH4and CO at ER 0.2.Lignin is relatively aromatic in nature which is difficult to be gasified in SCW,and partial oxidation of lignin might be an efficient way for both energy and fuel gases.

Table 2 Reaction of partial oxidative gasification for glucose

Table 3 Reaction of partial oxidative gasification for lignin

Fig.3.Effect of ER on energy of(a)glucose,(b)lignin(□ the energy supplied,○ the energy required)at T=600 °C,P=25 MPa with η =75%and K=25 kJ·h?1.

Therefore,there are some economic differences among diver feed stocks by partial oxidative gasification in supercritical water.For those materials which can be efficiently gasified,partial oxidation might be a way that combustion of fuel gases for energy.But for aromatic materials such as lignin or coal,it is more potential since partial oxidation can produce similar amount of fuel gases as direct gasification and offer additional energy.It also should be partial oxidation gasification with different ERs for different materials.

Although Ortiz et al.[20]reported that the autothermal gasification of glycerol can be achieved with the ER less than 0.2 at the given conditions(i.e.,glycerol concentration=30%by mass based on Gibbs free energy minimization calculation).The experimental results indicate that the ER should be over 0.3 for the autothermal process but less than 0.6 for fuel gas production when the energetic efficiency is lower than that of idealcase.In future studies,it might be also important to the catalytic gasification of biomass by partial oxidation for improving energetic efficiency.

3.2.Effect of energy recovery efficiency

In this section,the effect of preheating efficiency and energy loss in a reactor on the process was examined.The efficiency of heat exchanger was changed from75%to 95%and heat transfer coefficient was changed from 5 to 25 kJ·h?1.From Fig.4,it is clear that when the efficiency of heat exchanger improves from 75%to 95%,the energy required decreases by around 13 kJ·s?1for both glucose and lignin under the conditions that at600°C and 25 MPa with 10%by mass loading.The results also indicate that the largest heat requirement is at the stage of feedstock heating up,which is consistent with those reported by Matsumura[14].When the efficiency of heat exchanger changes from 75%to 95%,the temperature of feedstock at outlet of HE dramatically improves from 243.5 to 513.2°C for partial oxidation gasification of glucose and from 225.9 to 488°C for partial oxidation gasification of lignin.Therefore,the increase of efficiency of heat exchanger is an effective way to improve the energy recovered.

The effects of heat transfer coefficient are also shown in Fig.4.Clearly,the energy required dramatically decreases as the decrease of heat transfer coefficient.With the decrease of heat transfer coefficient from 25 to 5 kJ·h?1,the energy required decreases by around 6 kJ·s?1for both glucose and lignin.Therefore,the decrease of heat transfer coefficient is also an effective way to cut down the energy required.

It is noted that when HE efficiency is higher than 85%,the value of K is less than 15 kJ·h?1for gasification of glucose.The curve of the energy required is below the curve of the energy supplied,indicating that the energy derived is sufficient to supply the gasification process,i.e.,the energy derived can maintain the system in autothermal status,as shown in Fig.4.Similar results can be observed when HE efficiency is higher than 90%and the value of K is less than 10 kJ·h?1for gasification of lignin.

Fig.4.Effect of energy recovery efficiency on energy at T=600°C,P=25 MPa and ER=0.4 for(a)glucose and 0.3 for(b)lignin(□the energy supplied).

All these results indicate that the effect of energy recovered pays a key role to achieve an autothermal process.To keep the process autothermal,it is necessary to improve HE efficiency and decrease the energy loss in the reactor by means of such as heat insulation to decrease heat transfer coefficient.Since HE efficiency normally is around 75%,there is a great challenge for future research to optimize heat exchanger for supercritical water gasification system.But comparing with the increase of combustion of fuel gases for energy,cutting down energy loss should be still advantageous.Here,we also note that HE efficiency above 80%and heat transfer coefficient below 15 are necessary factors to keep the system autothermal.

3.3.Effect of biomass loading

The biomass loading has significant effect on the process of partial oxidation in supercritical water.The research by Smith et al.shows that the concentration of feedstock is the key factor for maintaining the process in an autothermal mode[17].Among these enthalpies,ΔH3is the only enthalpy upon which the autothermal process is dependent.To investigate the biomass loading on the reactions,the energetics of the process as a function of concentration at 600°C and 25 MPa with K=25 kJ·h?1are shown in Fig.5.

Fig.5.Effect of concentration on energetic process at T=600°C,P=25 MPa,K=25 kJ·h?1,with η =75%and ER=0.4 for glucose.

From Fig.5,the results show that the energy supplied increases with the increase of biomass loading but the energy required changes slightly.For partial oxidation gasification of glucose with 24%biomass loading,the energy supplied increases to 26.8 kJ·s?1while the energy required is around 28.8 kJ·s?1.In fact,there is 9.02 kJ·s?1energy loss in the reactor but only about 3 kJ·s?1energy is needed to keep the process energy balance.Therefore,autothermal process will be promising when the concentration of organic mass in water is over 17.5%by mass.From this view,to keep a high concentration of feedstock will also be a key factor for developing a self-sustaining gasification of biomass process.

But the biomass loading has a composite effect on both reactions and energy process.As the increase of biomass loading,the efficiency of gasification and gaseous yields decrease dramatically due to the lower steam reforming reactions[12,13].It is found that as the concentration of glucose increases from 10 to 24%,the hydrogen yield decreases from 8.9 to 2 mol·kg?1and the CO yield decreases from 7.9 to 5.9 mol·kg?1.The dramatic decrease of gasification efficiency and gaseous yields takes place when biomass loading is over 20%.Therefore,from the view of energy recovery,it should gasify biomass under 20%biomass loading.Meanwhile the increase of biomass loading can improve the energy supplied dramatically.To keep the concentration of feedstock above 15%is necessary for an autothermal operation.

These results are also different from those based on the calculation of partial oxidation of vinasse in supercritical water[34].In the theoretical process,the concentration of vinasse over 40%by mass can be used as feedstock in calculation.But as we note,the gasification efficiency and gaseous yields decrease with the increase of biomass loading.High biomass loading also has a risk for the valve blockage during the gasification of common biomass materials.Therefore,biomass loading between 15 and 20%is suitable for an autothermal gasification process.

It also should be pointed that there are some difficulties to autothermally gasify the wet biomass samples(such as sewage sludge and algae which contains nearly 85%water[12,14])in SCW,due to the low concentration of biomass (about 15%).

3.4.Effect of temperature

The effect of temperature on the process was investigated at 550,600 and 650°C.The energetic analysis is shown in Fig.6.The results indicate that as the gasification efficiency improves from 82%to 92%,with the increase of temperature the energy supplied increase from 9.9 to 12.3 kJ·s?1correspondingly.However,the increase of temperature also increases the energy of heating up and energy loss in the reactor,which results in the increase of the energy required from 25.8 to 27.9 kJ·s?1.

Fig.6.Effect of temperature on energy of glucose at T=600°C,P=25 MPa,K=25 kJ·h?1,with η =75%and ER=0.4.

Temperature has complicated effect on energetic process.It accelerates the reaction rates and improves the gasification efficiency meanwhile increases the energy required.As methanation reaction is backward with the increase of temperature,hydrogen yield increases from 4.4 to 8.9 mol·kg?1and methane yield decreases from 1.5 to 0.8 mol·kg?1as the increase of temperature from 550 to 650 °C.

To determine a suitable temperature for gasification of biomass under energetic consideration,it still will be necessary to perform an optimal analysis under the certain conditions.But in general,partial oxidation gasification of biomass in SCW between the temperature 400 and 650°C is favorable since a higher temperature not only leads to a higher energy consumption,but also increases the cost of equipment.

4.Conclusions

There are many factors such as oxidant equivalent ratio,energy recovery efficiency,biomass loading and temperature on which autothermal process by partial oxidation is depended.In this work,an energetic model to investigate the partial oxidative gasification of biomass in SCW is developed based on experimental results.The results indicate that ER should be over 0.3 but less than 0.6 to keep an efficient autothermal gasification process.There are some economic differences among diver feed stocks by partial oxidative gasification in supercritical water.For materials which can be efficiently gasified,partial oxidation might be a way that combustion of fuel gases for energy.While for aromatic materials such as lignin or coal,it is more potential since partial oxidation can produce similar amount of fuel gases as direct gasification and offer additional energy.Energy recovered plays a key role to achieve an autothermal process.Comparing with the increase of combustion of fuel gases for energy,cutting down energy loss should be still advantageous.The results also show that the biomass loading should be above 15%but under 20%for an autothermal gasification since the increase of biomass loading could improve the energy supplied but decrease the efficiency of gasification and gaseous yields.In general,some specific conditions exist among different materials.

Nomenclature

a Peng-Robinson temperature-dependent attraction parameter,m4·kmol?2

ammixture a,m4·kmol?2

b Peng-Robinson temperature-independent repulsion parameter,m3·kmol?1

bmmixture b,m3·kmol?1

Cpheat capacity,J·mol?1·K?1

?ωfunction of the acentric factor

HRresidual part enthalpy,kJ

ΔHialternative enthalpy,kJ

kijbinary interaction coefficient between species i and j

n molar flow of water,mol·s?1

nimolar flow of component i,mol·s?1

P pressure,Pa

PCicritical pressure of species i,Pa

R universal gas constant,kJ·kmol?1·K?1

T temperature,K

TCicritical temperature of species i,K

V volume of the fluid mixture,m3

VCicritical volume of species i,m3·kmol?1

Z compressibility factor

η energy efficiency

ωiacentric factor of species i