Biogas upgrading technologies:Energetic analysis and environmental impact assessment☆

Yajing Xu ,Ying Huang ,Bin Wu ,Xiangping Zhang *,Suojiang Zhang *

1 Beijing Key Laboratory of Ionic Liquids Clean Process,State Key Laboratory of Multiphase Complex Systems,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2 School of Chemistry and Chemical Engineering,University of Chinese Academy of Sciences,Beijing 100049,China

Keywords:Biogas upgrading Monoethanolamine aqueous scrubbing Pressured water scrubbing Ionic liquid Green degree

ABSTRACT Biogas upgrading for removing CO2 and other trace components from raw biogas is a necessary step before the biogas to be used as a vehicle fuel or supplied to the natural gas grid.In this work,three technologies for biogas upgrading,i.e.,pressured water scrubbing(PWS),monoethanolamine aqueous scrubbing(MAS)and ionic liquid scrubbing(ILS),are studied and assessed in terms of their energy consumption and environmental impacts with the process simulation and green degree method.A non-random-two-liquid and Henry's law property method for a CO2 separation system with ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([bmim][Tf2N])is established and verified with experimental data.The assessment results indicate that the specific energy consumption of ILS and PWS is almost the same and much less than that of MAS.High purity CO2 product can be obtained by MAS and ILS methods,whereas no pure CO2 is recovered with the PWS.For the environmental aspect,ILS has the highest green degree production value,while MAS and PWS produce serious environmental impacts.

1.Introduction

Biogas produced by biomass fermentation is regarded as a supplement even substitute for natural gas[1]and green energy due to its renewability.However,besides methane(CH4),carbon dioxide(CO2)is also the main component in the raw biogas from anaerobic digestion,which accounts for 30%-47%(by volume)[2],lowering the calorific value of biogas and increasing energy demand for its compression and transportation.Therefore,raw biogas cannot be used directly as vehicle fuel or supplied to the natural gas grid before removal of CO2component.

Currently,the technologies of biogas upgrading include pressure swing adsorption,pressured water scrubbing(PWS),chemical scrubbing,membrane separation and cryogenic method[2-4].Among them,absorption is one of the most widely used technologies;for example,PWS and chemical scrubbing are employed with 40%and 25%share,respectively,among more than 200 bio-methane plants in European region[5].Generally,PWS is one of the cheapest and simplest technologies[3].Besides its high efficiency and low CH4loss,hydrogen sulfide(H2S)can be removed as well[2,3,6].However,the drawbacks of clogging from bacterial growth and low flexibility toward variation of input gas cannot be avoided[6].Additionally,water consumption is huge and waste water is discharged inevitably.Another prevailing absorption technology is amine scrubbing widely used in commercial CO2capture processes due to its relatively high absorption capacity and rate[7-9].However,the drawbacks are high energy consumption for solvent regeneration,corrosion to equipment,and significant solvent degradation and losses[8].

Ionic liquids(ILs),aspromising gas absorbents,have received increasing attentions because of their unique physical properties,including negligible vapor pressure,high thermal stability,non- flammability,high CO2solubility and designability by adjusting the combinations of anions and cations[10-15],especially for removing CO2from mixed gases[16-19].Karadas et al.[20]reviewed the use of ILs as alternative fluids for natural gas sweetening.Aparicio and Atilhan[21]used the molecular dynamics method to inferthe interaction mechanism of ILs in naturalgas sweetening.Raeissi and Peters[22]measured the solubility of CO2in[bmim][Tf2N]in pressure and temperature ranges of(0.5 to 14)MPa and(310 to 450)K.Ali et al.[23]determined the cost effective operating conditions for 1-butyl-3-methylimidazolium tetra fluoroborate([bmim][BF4])to capture CO2by simulation method.Raeissi and Peters[24]measured the solubility of CH4in[bmim][Tf2N]in pressure and temperature ranges of 1-16 MPa and 300-450 K.According to these studies,anions of ILs play a dominantrole in CO2solubility[16].For example,with cation of[bmim]+,the solubility of CO2increases in the following anion order:[NO3]＜ [DCA]＜[BF4]＜[PF6]＜ [TfO]＜[Tf2N]＜ [methide]at 25 °C[25].[bmim][Tf2N]shows higher CO2absorption capacity among these ILs,better thermodynamics stability[13],and lower viscosity[26].In this work,[bmim][Tf2N]is selected as a physical solvent to remove CO2from raw biogas.

For a biogas upgrading technology,both energy consumption and environmental impact are critical screening criterion.Compared to energetic analysis[2,9],the environmental impact assessment for biogas upgrading process is relatively fewer[27,28].Starr et al.[27]evaluated three biogas upgrading technologies,i.e.,PWS,alkaline with regeneration and bottom ash upgrading,and showed that the bottom ash upgrading process has the least environmental impact even compared to the pressure swing adsorption and chemical scrubbing method.Cozma et al.[28]assessed the environmental impact of PWS process with the life cycle assessment(LCA)tool and showed that the main impact categories are the global warming,human toxicity and acidification potentials,mainly caused by the exhaust gas from desorption column and energy consumption.LCA method is comprehensive but complicated,as an alternative,green degree(GD)method proposed by Zhang et al.can quantitatively assess the environmental impacts based on nine environmental impact categories,such as substances,streams,units and systems[29,30].However,for ionic liquid scrubbing method,to the best of our knowledge,there is a little systematic study on the energetic analysis and environmental impact assessment for biogas upgrading processes.

In this work,three biogas upgrading technologies,i.e.,PWS,monoethanolamine aqueous scrubbing(MAS)and ionic liquid scrubbing(ILS)are studied in terms of their energy consumption and green degree production.Firstly,rigorous thermodynamic models are established for CO2capture system with[bmim][Tf2N]ionic liquid by fitting available experimental data[22,24].Secondly,the three biogas upgrading processes are simulated and the key parameters affecting the energy consumption are analyzed and optimized.Finally,circulating solvent,energy consumption,energy efficiency and environmental impact of each technology are evaluated and compared.

2.Methodology

Thermodynamic models are the core of simulation and design of a process.For PWS and ILS,non-random-two-liquid(NRTL)and Henry's law property methods are employed,which has been proved to be appropriate for such systems[28,31,32].For MAS,KEMEA thermodynamic package is applied based on the electrolyte non-random-twoliquid model[8].Lacking of parameters related to the ionic liquid based system,physical properties of[bmim][Tf2N]are estimated and thermodynamic models of[bmim][Tf2N]based system are established.CH4recovery ratio,specific energy consumption,energy efficiency,selectivity and GD are calculated as assessment indicators of upgrading performance.

2.1.Thermodynamics of ionic liquid systems

CO2solubility in[bmim][Tf2N]is estimated with NRTL and Henry's law method.The NRTL model is written as

where δ represents the number of components,x is the mole fraction,T is the absolute temperature,R is the gas constant,Gijis a dimensionlessinteraction parameter depending on energy interaction parameter(gij)and non-randomness factor(αij),aij,bijand αijare the binary parameters.

Table 1 NRTL binary interaction parameters of CO2/IL and CH4/IL

The heat capacity(Cp)of[bmim][Tf2N],which is a necessary parameter to estimate the energy consumption,can be calculated by DIPPR equation,which is taken from the Aspen Plus software.

The binary interaction parameters of CO2/IL and CH4/IL shown in Table 1 are regressed based on experimental solubility data in literature[22,24].The predicted values are in agreement with experimental data as shown in Figs.1 and 2.The comparison between the experimental[33]and predicted values of Cpis shown in Fig.3.Other physical properties of ILs,such as critical properties,density,surface tension and thermal conductivity are taken from literature[34].

Fig.1.CO2 solubility in[bmim][Tf2N].

Fig.2.CH4 solubility in[bmim][Tf2N].

Fig.3.Heat capacity of[bmim][Tf2N].

2.2.Assessment indicators

2.2.1.CH4 recovery ratio

The CH4recovery ratio is calculated by[3]

where ηCH4is the CH4recovery ratio,νinand νoutrepresent the flow rate(m3·h?1)of biogas and product gas,and Cinand Coutindicate the CH4concentration in biogas and product gas,respectively.

2.2.2.Specific energy consumption

The specific energy consumption SEC(kW·h·m?3)is expressed as

where TEC represents the total energy consumption(kW).

2.2.3.Energy efficiency

The energy efficiency(ηi)of a gas separation technology is calculated by

where Qin,fand Qout,prepresent the lower heat values(MWth)of feed gas and product gas,respectively,Qin,urepresents the heat required(MWth)by solvent regeneration,solvent evaporation and solvent heating for the MAS,which is usually provided by steam,Ein,erepresents the electricity required(MWe)by compressors,gas blower,solvent pump,and other driving machines in the biogas upgrading process,σ represents the conversion efficiency of steam to electricity,which is used to keep the variables in Eq.(7)in the same benchmark.The value of σ is usually between 0.2 and 0.4[35-37],so 0.3 is used in this work.The lower heating value of CH4is 50.0 MJth·kg?1[38].

2.2.4.Selectivity of CH4/CO2

The solvent selectivity Siis calculated by[12]

where HCH4and HCO2represent the Henry constants of CH4and CO2at specified temperature.

2.2.5.Green degree method

Green degree(GD)is applied to quantitatively assess the environmental impact of a chemical process by evaluating the influence of all material and energy exchange with the environment[29,30,39].The GD of a process,namely green degree production of a process(ΔGDp),can be calculated by[29]

3.Process Description

3.1.Gas resource

Before upgrading,raw biogas from anaerobic fermentation unit is pretreated to remove hydrogen sulfide(H2S),particles and so on.Thus it is assumed that the feed gas consists of CH4and CO2only[40].The characteristics of the biogas are listed in Table 2,which are provided by the Nanjing University of Technology.

3.2.Process configuration

Fig.4 presents the flow diagrams of three biogas upgrading processes,i.e.,PWS,ILS and MAS.The CH4purity in the product methane gas is set at 0.98(mole fraction).The CH4recovery ratio is set as more than 95%.The product gas,byproduct and waste gas are all expanded or compressed to one atmosphere for comparison in the same benchmark.In this work,“kWth”and “kWe”represent units of thermal and electricity power,respectively.

3.2.1.Pressured water scrubbing(PWS)

Fig.4(a)is the process scheme of PWS.After compression,the biogas enters the bottom of absorber tower,while water is fed at the top of the column to achieve a gas-liquid counter flow.The operation pressure of the absorber is 0.8 MPa,which is widely used in literature[28].The column is equipped with random packings to give a large surface for gas-liquid contact.In order to minimize the losses of CH4dissolved in the pressured water,the CH4-riched water leaving the bottom of the stripper is depressed in a flash tank,and the released gas rich in CH4is recycled to the inlet of the second compressor.Water from the flash tank is sent to the top of the stripper,while air is blown from its bottom to regenerate rich solvent.The regenerated water is recycled to the absorber.The waste gas is vented to the atmosphere and the waste water is discharged directly.

Table 2 Characteristics of biogas produced in mesophilic fermentation

Fig.4.Schematic of three biogas upgrading processes.

3.2.2.Ionic liquid scrubbing(ILS)

The ionic liquid-based process shown in Fig.4(b)is a physical absorption.The difference between the ILS and PWS is the solvent regeneration section.A flash tank is used to regenerate rich IL solvent and the regenerated IL is recycled to the absorber.The gas from the top of the flash is high purity CO2,which is the byproduct of the process.

3.2.3.MEA aqueous scrubbing(MAS)

The MAS is shown in Fig.4(c).Considering that higher MEA concentration might lead to serious corrosive effects and MEA degradation,30%(by mass)MEA solution is used[8].Raw biogas is blown into the absorber from the bottom, flowing counter currently with the aqueous MEA,in which the absorbent reacts with CO2.The CH4-rich gas from the top of the absorber is dried and the product gas is acquired.The CO2rich solvent is pumped to the stripper after heated and exchanged with the lean solvent from the bottom of the stripper,while the lean solvent is cooled and recycled to the absorber.The gas from the top of the stripper is high purity CO2,which is the byproduct of the process.

4.Parametric Study and Assessment

4.1.Theoretical energy consumption for CO2/CH4 separation

According to the thermodynamic principle,the theoretical energy consumption for separating mixture ofCO2and CH4to pure components is the minimum energy demand,which is calculated in an isothermal and isobaric process and ideal gas state[41].

4.2.Parametric study

4.2.1.Pressure of the flash tank( flash1)

Because CH4is the main product of bio-methane process and also approximately 20 times stronger greenhouse gas than CO2[42],the CH4recovery ratio should be as high as possible.Flash1 unit is established to increase the CH4recovery ratio.The pressure of flash1 should be kept at reasonable value to ensure a high CH4recovery ratio,low energy consumption and less circulating solvent.A sensitivity analysis on the pressure of flash1 is necessary for PWS and ILS.According to the results in Figs.5 and 6,both circulating solvent and CH4recovery ratio decrease as the pressure of flash1 increases.Less circulating solvent leads to lower total energy consumption.As the total energy consumption decreases fasterthan CH4recovery ratio,the specific energy consumption decreases,otherwise it increases.The optimal pressure of flash1 is 0.44 MPa for the PWS and 0.35 MPa for the ILS.Tables 3 and 4 are the power requirement of the process equipment.

4.2.2.Stages of absorber and stripper for the MAS process

The effect of stages of absorber and stripper on circulating solvent and reboiler heat duty is investigated based on sensitivity analysis as shown in Fig.7.The optimal number of absorber and stripper is 4 and 8,respectively.Table 5 is the heat and power requirement of the process equipment.Electricity is mainly employed for energy assessment for PWS and ILS,so the heat required in the MAS process is converted to electricity for comparison.The conversion efficiency of heat to electricity is assumed to be 0.3 as mentioned before.

Simulation results of the three technologies are shown in Table 6.The specific energy consumption of PWS agrees with previous work[9],indicating that the simulation results are reasonable.

5.Results and Discussion

5.1.Energy consumption analysis

Fig.5.Sensitive analysis on pressure of flash1 for the PWS process.

The energy consumption and circulating solvent of three technologies are compared in Fig.8.The specific energy consumptions of ILS and PWS processes are almost the same,and about 50%less than that of MAS process.The main reason is that the former two technologies are physical absorption and absorbed CO2is released with flash or air stripping,which demands less energy consumption.The MAS is chemical absorption and heating is required to break the chemical bond and vaporize the solvent.The specific energy consumptions of the three technologies are all much higher than the theoretical value,so a large improvement space for decreasing the energy consumption of biogas upgrading is possible.

The amount of circulating solvent is in the order of PWS＞ILS＞MAS,as shown in Fig.8.The order of absorption capacity is MAS＞ILS＞PWS.Under the operation condition,1 mol MEA can absorb 0.5 mol CO2,while the absorption capacity of IL and H2O is 0.2 and 0.004 mol CO2permole solvent,respectively.The selectivity is another important indicator.According to previous work[24,43-45],the selectivity of[bmim][Tf2N]at 313.15 K is about 8.7,while that of H2O is 22.3,which means that flash1 pressure of ILS is lower than that of PWS.Moreover,MAS is a chemical process,so the circulating solvent for MAS is the least.More circulating solvent needs a larger size of absorber tower,so the capital and operating expenses will be higher.Although the quantity of circulating solvent for ILS is more than that for MAS,the solvent loss is much less because of its negligible vapor pressure and good thermal stability.The solvent loss and degradation in MAS process are significant[8],so the making up of solvent is considerable.Solvent loss for PWS is mainly due to evaporation,left with the outlet gas.The making up of water is not ignorable.

Fig.6.Sensitive analysis on pressure of flash1 for the ILS process.

Table 3 Power requirement of the PWS

Table 4 Power requirement of the ILS

Fig.7.Sensitive analysis on stages of absorber and stripper for the MAS process.

Table 5 Power requirement of the MAS

Table 6 Simulation results of the three technologies

Fig.8.Energy consumption and circulating solvent in three technologies.

Fig.9.GD production and energy efficiency of three technologies.

5.2.Green degree(GD)analysis

The GD production of the three technologies is calculated by Eq.(9)and the results are shown in Fig.9.The GD production value of ILS is the highest,followed by that of MAS and PWS,indicating that this ILS process is the most environmentally benign.The purity of CO2in the byproduct is more than 90%and no waste gas is vented to the atmosphere.The energy consumption is low and solvent loss is negligible.All these factors contribute the highest GD production.The results are based on the assumption that IL loss is zero,so the GD of IL and possible degradation of IL are not considered in this work.Since this work focuses on the global warming potential,its weight factor is assumed to be higher.The purity of CO2in byproduct of MAS is also more than 90%,but the energy consumption is high,so the GD production of MAS is lower than that of ILS.As to the PWS,CO2removed from CH4is vented to the atmosphere,which is the primary greenhouse gas,so the GD production is the lowest.Regarding the process energy efficiency,the PWS is the highest among the three routes because of the low energy consumption and high CH4recovery ratio.

6.Conclusions

In this work,three biogas upgrading technologies are studied.The CO2removal from biogas by IL[bmim][Tf2N]is evaluated through comparison with PWS and MAS.The three processes are simulated and the key parameters are optimized through sensitivity analysis.The energy consumptions of ILS and PWS are about 50%lower than that of MAS.The circulating solvent of MAS is the lowest,while the solvent making-up in ILS can be ignored due to its unique physicochemical property.The GD production value of ILS is the highest.Regarding to the energy efficiency,the PWS is the highest because of low energy consumption and high CH4recovery ratio.

H2O and MEA are cheap and IL is expensive,but the quantity of making up solvent for ILS is much less compared with the other two technologies.The prevailing PWS and MAS are mature technologies whereas ILS is still in lab stage,so more study should be conducted.In conclusion,IL[bmim][Tf2N]can be used as a physical solvent for biogas upgrading and presents some advantages compared to PWS and MAS.New ILs with higher selectivity and high absorption capacity should be developed to improve its separation performance.