Comparative Assessment of Biomass and Power-to-Gas Processes Integrated with Different Electricity-Driven Gasification Technologies

Abstract

To develop a biomass and power-to-gas (BPtG) process for renewable electricity storage and sustainable synthetic natural gas (SNG) production, this work investigated five BPtG processes integrated with different electricity-driven gasification technologies based on simulation data. These processes were evaluated for SNG composition and yield, life-cycle energy and exergy efficiencies, life-cycle carbon emissions, and the equivalent unit production cost. The results show that the energy and exergy efficiencies of SNG from those processes range between 53.1 and 58.6% and 36.4 and 41.1%, respectively. Based on the energy allocation method, the carbon emissions without and with CO₂ capture ranges from 22.0 to 34.8 and from −43.4 to −17.6, respectively, in gCO₂e/MJ_SNG. These BPtG processes can produce low-carbon SNG and even achieve negative carbon emissions with CO₂ capture. Both feedstock and electricity costs have significant influences on the profitability of the processes. The BPtG process integrated with resistance heating gasification, plasma-assisted gasification, and moderate water electrolysis are recommended for their compromise of multi-perspective performances. This paper provided the orders of the five processes based on these indicators and recommendations for different applicable scenarios.

1. Introduction

Natural gas is not a sustainable fuel for a net-zero carbon society. Thus, sustainable gaseous fuels, e.g., biomass-based synthetic natural gas (SNG), are highly expected. Biomass-based SNG can be obtained by the upgrading of biogas or equivalent syngas, which are produced by anaerobic digestion or biomass gasification, respectively. Anaerobic digestion is a mature technology but limited to biomass type and composition, reaction temperature, pH, yield, energy efficiency, etc. [1] Conventional SNG production processes commonly employed steam gasification technology to produce syngas with very low N₂ content, and indirect heating gasifiers are the feasible solution for biomass steam gasification. However, the devices are complex and expensive to design, operate, and maintain [2].

Driven by the demand for large-scale energy storage, the power-to-gas pathway is one of the current research hotspots, as methane has a very high volumetric energy density compared with hydrogen. Biomass can offer sustainable CO and CO₂ for the power-to-gas process. Then, biomass and power-to-SNG (BPtG) is named to describe the synergetic process of biomass conversion and SNG production by electricity-driven technologies. Recently, several kinds of electricity-driven gasification technologies were summarized and preliminarily analyzed [2]. When water electrolysis (WE) is applied, the oxygen gasification of biomass can be integrated to utilize the by-product oxygen [3,4]. However, the cost competitiveness of the SNG produced by this configuration is highly related to the electricity cost and capital cost of electrolyzers. When the electricity cost was very cheap or at zero, the SNG would be comparable to natural gas and the conventional SNG plant under Swedish [3], Californian [5], Italian [6], and Chinese [7] scenarios. The systematic energy efficiency of the electrolyzers varies extensively from 62% to 90% based on the higher heating value (HHV) of H₂ [8,9], and considerable energy loss occurs during the electrolysis process. Subsequently, the hydrogenation methanation of CO and CO₂ also results in considerable energy loss due to the characteristics of exothermic reactions [7]. Thus, it is of great value to analyze and improve the utilization efficiency of electrical energy in a BPtG process.

Toward that goal, the O₂-enriched biomass gasification with 80–96% O₂ purity was introduced into a BPtG process [10]. The results indicated that the BPtG process integrated with an air separation unit has obvious advantages over the BPtG integrated with water electrolysis in electricity consumption and unit production cost; however, the product has much lower CH₄ concentration. Additionally, as electrical heating technologies have very high electricity-to-heat efficiency (up to nearly 100%), a BPtG process integrated with electrical heating gasification was studied technically and economically [11]. The results showed that this process has high conversion efficiencies and a middling unit production cost. The characteristics and performances of these BPtG processes are quite different. However, the individual studies focused on determining the optimal process parameters, and these processes have different optimal parameters, especially methanation pressures. In addition, we found from the previous studies that the different combination and integration of these electricity conversion technologies can create more BPtG process configurations. Thus, it is meaningful to carry out a techno-economic assessment on these BPtG processes under the same boundaries.

Decarbonization is an increasingly important action all over the world. Several economic and environmental assessments of power-to-gas or conventional SNG were carried out regarding different configurations, water electrolysis technologies, countries, etc. [12,13,14]. However, to our best knowledge, there are few related studies on BPtG processes, especially reports on the carbon emissions of such processes under a Chinese scenario. China aims to have CO₂ emissions peak before 2030 and achieve carbon neutrality before 2060. Thus, the carbon emissions of these processes are of great significance, which must be determined and compared to clarify their differences in sustainable performances.

The objective of this work is to compare BPtG processes integrated with different electricity-driven gasification technologies from the perspectives of energy efficiency, carbon emission, and production cost. This work can guide the selection and development of suitable BPtG technologies and sustainable SNG production, and it can also promote the decarbonization progress in the fields related to natural gas, biomass and waste utilization, and even renewable electricity utilization.

2. Process Description

2.1. Requirements on SNG

The Chinese technical standards “Coal-based Synthetic Natural Gas” [15] and “Natural Gas” require that the H₂ and CO₂ concentrations should not exceed 5 mol.% and 4 mol.%, respectively [16]. There are no regulations on the CO concentration as it is very low after methanation synthesis. Further, the HHV of SNG should be equivalent to that of natural gas (≥33.7 MJ/Nm³).

2.2. Process Configurations

Processes with 25 t/h biomass as feedstock were set as a benchmark in reference to the scales of existing power plants using biomass or municipal waste as fuel in China [17]. Wheat straw, as an abundant and cheap agriculture waste, was used as feedstock. The composition, lower heating value (LHV), and specific chemical exergy (SCE) are listed in Table S1 [18]. The specific exergy (SE) of SNG consists of specific chemical and physical exergies, and the later item was calculated based on the temperature and pressure of SNG.

The BPtG process generally consisted of four units, i.e., biomass gasification, cooling and cleaning, methanation synthesis, and CO₂ separation. The key distinctions among the previous BPtG processes were the electricity-driven gasification technologies as follows [7,10,11]. Based on combinations of the unit technologies, five BPtG processes were set up and compared using the simulation data, which is introduced in a later section. Figure 1a shows the first BPtG process integrated with oxygen and steam gasification, cooling and cleaning, and methanation synthesis. The oxygen needed by the gasification process (7.4 t/h) is generated from water electrolysis. When sufficient hydrogen (4.1 t/h) is added into the methanation synthesis, the SNG with high-concentration CH₄ is obtained directly, and no post CO₂ separation is needed. To achieve this, plenty of water (36.6 t/h) is electrolyzed and plenty of surplus O₂ (25.1 t/h) is generated. Here, “sufficient water electrolysis” (SWE) is used to describe the above characteristics and this process is labeled as the “SWE process” for short.

Figure 1b indicates the second BPtG process integrated with oxygen and steam gasification, cooling and cleaning, methanation, and CO₂ separation. Its gasification unit is the same as that in the first process. However, the amount of water (8.3 t/h) is set to just satisfy the oxygen–steam gasification process and no more water is electrolyzed. Thus, the flow rate of H₂ (0.9 t/h) is significantly less than that in the SWE process. Subsequently, the crude SNG, i.e., the product of the methanation synthesis, contains high-concentration CO₂ (48.6 vol.%), and then post-CO₂ separation is necessary. Here, “moderate water electrolysis” (MWE) is used to describe the above characteristics and this process is labeled as the “MWE process” for short.

Figure 1c indicates the BPtG process integrated with vacuum pressure swing adsorption (VPSA), oxygen-enriched air gasification, cooling and cleaning, methanation synthesis, and CO₂ separation. It is labeled as “VPSA process” for short. The oxygen-enriched air with 90% O₂ purity (8.7 t/h) is produced by VPSA, and no additional H₂ is added into the methanation reactor. Therefore, the CO₂ concentration in the crude SNG from the VPSA process (60.6 vol.%) is much higher than that in MWE process (48.6 vol.%).

Figure 1d shows the BPtG integrating with plasma-assisted gasification (PAG), cooling and cleaning, methanation synthesis, and CO₂ separation. The plasma torch directly converts electrical energy into heat energy and promotes the steam gasification process. This process is named as the “PAG process” for short. The electricity-to-heat efficiency of the plasma torch in PAG is assumed to be 90%. For the same conditions, the power of the plasma torch is 39.7 MW. Due to only steam gasification being performed, the CO₂ contents in producer gas are the lowest; after methanation and CO₂ separation, the mass flow rates of SNG and CO₂ streams are almost the same as those of the MWE process (7.7 t/h and 19.1 t/h), respectively.

Figure 1e indicates the last BPtG process integrated with resistance heating gasification (RHG), cooling and cleaning, methanation synthesis, and CO₂ separation. This process is called the “RHG process” for short. The electric resistance parts can directly convert electrical energy into heat energy and drive the gasification process. The electricity-to-heat efficiency of the electric resistance parts in RHG is assumed to be 99%. A portion of electricity is converted into sensible heat energies in the producer gas, and slag and ash. Parts of these heat energies are recovered, while the remains are wasted. The power of the electric resistance parts is 36.3 MW. Other parameters of the RHG process are almost the same as those of the PAG process. The difference in the compositions of producer gas is neglected since the chemical and phase equilibrium method is applied in gasification simulations.

2.3. Gasification Unit

In this unit, the feedstock (m_FS) is first dried by low-temperature heat recovered within this process. It is converted into producer gas (PG), tars, and residual char and ash using steam or steam and oxygen as gasifying agents. Prior to gasification, gasifying agents are preheated to 200 °C by the recovered heat energy. The same specifications for all gasification processes are as follows: (i) The gasifier is constantly operated at 800 °C and atmospheric pressure, and residence time is long enough to achieve chemical equilibrium. (ii) The steam-to-biomass mass ratio is set to 0.4. (iii) The carbon conversion efficiency is assumed to be 99%, i.e., 1% of carbon in biomass is transferred into slag and ash. (iv) The heat loss of the equipment is 3% of the input energy of biomass based on LHV. Other specifications are as follows, according to the properties of gasifying agents. The relevant parameters for the modeling are summarized in Table S2.

(1)

Pure O₂ gasification

Figure 1a,b show that the water electrolysis converts the water (m_WE) into O₂ and H₂ by renewable electricity. The exact amount of O₂ is added into the gasifier to operate it at the expected T_G. In the case of the SWE process, the surplus O₂ is sold, and the profit is considered in the economic assessment. For both processes, all the H₂ is compressed into the methanation reactor. The energy efficiency of water electrolysis (η_WE) is a key variable in the overall efficiency and unit production cost [7]. At present, it is about 85% based on HHV at the industrial scale and can be up to 90% in the experimental study [9], and the latter is applied for predicting the potential performances.

(2)

O₂-enriched air gasification

Based on the previous study [10], an O₂ purity of 90% is employed to produce eligible SNG, which is produced by a VPSA unit. The specific electricity consumption for VPSA is about 0.4 kWh/Nm³ _{pure O2}. The recovery ratio of 55.6% and the productivity of 85.6 Nm³/(t·h) are used in the following modeling [19].

(3)

Steam gasification

Both plasma-assisted gasification and resistance heating gasification involve steam gasification, and the required heat is totally converted from renewable power.

2.4. Cooling and Cleaning Unit

The producer gas must be purified and cooled prior to the compression and methanation synthesis to protect the compressor and methanation catalyst. An oil-based gas washer unit is integrated to clean producer gas [20,21]. A series of heat exchangers are arranged to recover the heat energies of producer gas at expected temperatures. Figure S1 indicates the process flow sheet of this unit. A pressure drop of 10 kPa is set to calculate the energy and exergy losses of this unit.

2.5. Methanation Synthesis Unit

CO and/or CO₂ with H₂ are converted into CH₄ and H₂O depending on the methanation reactions and the water–gas shift reaction [22]. The stoichiometric coefficients of the methanation reactions of CO and CO₂ allow the determining of the amount of H₂ that is needed to completely reform CO and CO₂ into CH₄. The stoichiometric number (SN) of the incoming gas stream to characterize the achievable methane yield is defined as below:

$$SN={\displaystyle \frac{{ml}_{\mathrm{H}2,\mathrm{P}\mathrm{G}}+{ml}_{\mathrm{H}2,\mathrm{W}\mathrm{E}}}{{3·ml}_{\mathrm{C}\mathrm{O},\mathrm{P}\mathrm{G}}+{4·ml}_{\mathrm{C}\mathrm{O}2,\mathrm{P}\mathrm{G}}}}$$

(1)

where ml_H2,PG and ml_H2,WE are the molar flow rates of H₂ in producer gas and that are generated by water electrolysis, respectively. ml_CO,PG and ml_CO2,PG are the molar flow rates of CO and CO₂ in producer gas, respectively. To obtain a highly pure methane stream, SN should be close to unity. For the SWE process, sufficient H₂ is generated by water electrolysis. For MWE, VPSA, PAG, and RHG processes, the methanation synthesis is carried out without the addition of external H₂, and SN is obviously less than 1. In these processes, the water–gas shift reaction converts partial CO into CO₂ and generates H₂.

The single isothermal once-through fluidized bed methanation synthesis reactor (MSR) is employed in this study [23]. A certain amount of water (m_WM) is heated to 200 °C and then added into the methanation reactor to suppress carbon formation. The reactor is assumed to operate constantly at 300 °C. The released heat (Q₄) is recovered and reused (Figure S1). Lower methanation pressure (p_M) favors the conversion efficiencies, because the compression of producer gas consumes less power. However, to meet the requirements of H₂ and CO₂ contents in SNG, the minimum p_M is quite different from one process to another [7,10,11], due to the differences in SN and N₂ content. According to the simulation data in this study, the p_M of the SWE process, MWE process, and VPSA process is set to 65 bar, 40 bar, and 45 bar, respectively. Meanwhile, the p_M in the PAG process and RHG process is set to 30 bar. The product of methanation synthesis is cooled to 80 °C and the released heat (Q₅) is also recovered and reused (Figure S1). Then, it is further cooled and condensed to recover water and adjust moisture content for the following CO₂ separation. The pressure drop from the methanation reactor to the coolers is assumed to be 0.5 bar.

2.6. CO₂ Separation and Compression

The SWE process can directly produce eligible SNG without CO₂ separation. However, the crude SNG produced by other processes contains high concentrations of CO₂. To meet the requirement of SNG composition and HHV, CO₂ separation must be performed. This work employs the Selexol absorption process [24]. The CO₂ separation efficiency is set to 98%, the CH₄ loss is estimated to 1%. A process of catalytic oxidation is employed to eliminate the global warming potential of the CH₄ loss in the separated CO₂ stream. Finally, the SNG is compressed to 64 bar.

2.7. Process Simulation and Heat Integration

The simulation of the five processes was based on our latest simulation works including the conventional SNG process [25] and SNG production processes integrated with water electrolysis [7], O₂-enriched air gasification [10], and electrical heating gasification [11,26]. The minor modifications were upgraded according to the above-mentioned specifications, such as methanation pressure and final pressure. The model blocks and operation parameters of key devices are briefly summarized in Table S2. As indicated in Figure S1, the heat streams (Q₁~Q₅) are recovered and used to generate electricity by an organic Rankine cycle unit (ORC) to compensate for the power consumption of the whole process. The heat recovered by CL_PG,2 is used preferably for steam generation and O₂ preheating, while that by CL_PG,3 is used preferably for biomass drying. The heat integrations of all processes are checked, and the results conform to the principles of heat transfer. The energy and exergy flow rates of feedstock, renewable power, and SNG are presented in Tables S3 and S4, respectively.

3. Assessment Methodology

3.1. Life-Cycle Model

The life-cycle method was applied to evaluate the efficiencies and carbon emissions. The economic assessment was also based on this concept with some simplifications. The life-cycle model of SNG includes five stages as follows: biomass cultivation and collection (C&C), feedstock transportation (FT), plant constructing and dismantling (C&D), SNG production (SP) and end use (EU).

(1)

Cultivation and collection

Based on the survey on the energy consumptions of cultivation in China [27], the average consumptions of diesel and electricity were about 18.7 kg/t-wheat and 139.4 kWh/t-wheat, respectively. The emissions from the cultivation stage can be classified into direct and indirect emissions. In terms of direct emissions, biomass is regarded as a carbon-neutral resource, and the CO₂ emission and absorption of biomass are not considered. The greenhouse gasses CO₂ and N₂O discharged from the soil were estimated to be 549.9 kg/t-wheat and 0.42 kg/t-wheat. The indirect emissions are related to the productions of the input energies and materials, including fertilizers, electricity, and diesel. It is assumed that all power consumed in this stage is wind and photovoltaic power. Table S5 lists the carbon emission data from China’s products’ carbon footprint factors database (CPCD) [28].

As the wheat cultivation produces both wheat grain and wheat straw, the distribution of the environmental impacts between grain and straw should be considered. The weighting coefficient for wheat straw can be determined by the energy allocation method or the economic allocation method. The former is the ratio of wheat straw’s energy content to the total energy content based on LHV; and the latter is the ratio of wheat straw’s economic value to the total economic value [27]. The weighting coefficients with energy and economic allocation methods were calculated to be 0.4377 and 0.1245, respectively. Alternatively, the environmental impact of the cultivation stage can be excluded. Carbon emission factors relevant to the three methods are listed in Table S5.

Power and diesel are the major consumed energies during the stage of wheat straw collection, which are roughly 14.5 kWh/t and 1.45 kg/t [29]. The carbon emissions associated with the manufacture of the tools used in this stage are not considered.

(2)

Feedstock transportation

Wheat straw is assumed to be transported by medium-duty diesel trucks with a load of 8 tons in this stage. According to the fuel consumption limit for heavy-duty commercial vehicles in China, the diesel consumption of each truck is on average 0.02 kg/(t·km), considering both fully loaded and unloaded trucks. The transportation radius is 32.26 km based on an estimation model in a previous report [29]. In addition, the road tortuosity is assumed to be 1.4, and the return trip with an empty load is also considered, multiplied by coefficient 2. As the packing density of straw is only about 500 kg/m³, the adjustment coefficient of freight volume is set to 0.9.

(3)

Constructing and dismantling

The consumption of materials related to plant construction refers to an SNG plant integrated with dual fluidized beds gasifier [30]. The differences in the plant construction stage among these processes are ignored. Based on the modification with plant scale, the amount of steel, iron, aluminum, and cement consumed in plant construction are estimated to be 594 t, 4 t, 7 t, and 1730 t, respectively. The emission data of these materials are also from CPCD [28]. The emissions during the project’s abandoning and dismantling are estimated to be about 10% of those in the project construction stage, and 30% steel is assumed to be recycled.

(4)

SNG production

In this stage, the amounts of the raw materials, power consumption, and emissions are derived from the simulation data. Regarding this stage, only the carbon emissions related to renewable power are considered.

(5)

End use

One of the aims in the end-use stage is to efficiently use SNG to improve the life-cycle energy and exergy efficiency. The cogeneration of heat and power by using a gas turbine or gas engine can achieve an energy efficiency of 90% or even higher, thus the cogeneration is considered for this stage. Table S6 also presents the emission factors of stationary combustion from the literature [31].

3.2. Technical Assessment

To evaluate the technical competitiveness of these processes, the following parameters are compared in this study: composition, yield, LHV, and SCE of the product [7]. The energy and exergy efficiencies of SNG production stage (η_En,SP and η_Ex,SP) are defined as follows:

$${\eta }_{En,\mathrm{S}\mathrm{P}}={\displaystyle \frac{{En}_{\mathrm{S}\mathrm{N}\mathrm{G}}}{{En}_{\mathrm{F}\mathrm{S}}+E_{\mathrm{R}\mathrm{E},\mathrm{S}\mathrm{P}}}}\times 100\%$$

(2)

$${\eta }_{Ex,\mathrm{S}\mathrm{P}}={\displaystyle \frac{{Ex}_{\mathrm{S}\mathrm{N}\mathrm{G}}}{{Ex}_{\mathrm{F}\mathrm{S}}+E_{\mathrm{R}\mathrm{E},\mathrm{S}\mathrm{P}}}}\times 100\%$$

(3)

where En_SNG is the energy flow rate of SNG based on LHV, in MJ/h; Ex_SNG is the exergy flow rate of SNG including chemical and physical exergies, in MJ/h; En_FS and Ex_FS are the energy and exergy rates of biomass, respectively, in MJ/h. E_RE,SP is the total power consumed in the SNG production stage, in MJ/h (Tables S3 and S4).

The energy and exergy efficiencies (η_En,CP and η_Ex,CP) from the cultivation stage to the production stage are calculated as follows:

$${\eta }_{En,\mathrm{C}\mathrm{P}}={\displaystyle \frac{{En}_{\mathrm{S}\mathrm{N}\mathrm{G}}}{E_{\mathrm{R}\mathrm{E},\mathrm{L}\mathrm{C}}+{En}_{\mathrm{d}\mathrm{i}\mathrm{e}\mathrm{s}\mathrm{e}\mathrm{l}}+{En}_{\mathrm{F}\mathrm{S}}}}\times 100\%$$

(4)

$${\eta }_{Ex,\mathrm{C}\mathrm{P}}={\displaystyle \frac{{Ex}_{\mathrm{S}\mathrm{N}\mathrm{G}}}{E_{\mathrm{R}\mathrm{E},\mathrm{L}\mathrm{C}}+{Ex}_{\mathrm{d}\mathrm{i}\mathrm{e}\mathrm{s}\mathrm{e}\mathrm{l}}+{Ex}_{\mathrm{F}\mathrm{S}}}}\times 100\%$$

(5)

where E_RE,LC is the total power consumed in stages from C&H to SP, in MJ/h. En_diesel and Ex_diesel are the energy and exergy flow rates of diesel consumed in stages from C&H to C&T, respectively, in MJ/h.

The life-cycle energy and exergy efficiencies are calculated by taking the conversion efficiencies of the end-use stage, which are expressed as follows:

$${\eta }_{En,\mathrm{L}\mathrm{C}}={\eta }_{En,\mathrm{C}\mathrm{P}}\times {\eta }_{En,\mathrm{E}\mathrm{U}}$$

(6)

$${\eta }_{Ex,\mathrm{L}\mathrm{C}}={\eta }_{Ex,\mathrm{C}\mathrm{P}}\times {\eta }_{Ex,\mathrm{E}\mathrm{U}}$$

(7)

where η_En,EU and η_Ex,EU are the energy and exergy efficiencies of the end-use stage. For the cogeneration of heat and power, η_En,EU and η_Ex,EU are set to 90% and 63%, respectively, based on the advanced performance of cogeneration technologies [32].

3.3. Carbon Emission Assessment

The functional unit is 1 MJ_SNG based on HHV. The life-cycle carbon emission of SNG is evaluated by 100-year global warming potential (GWP), and CO₂, CH_4, and N₂O were considered. The GWP of biogenic CO₂ generated by the combustion of biofuels is usually excluded in the assessment [30,33]. The biogenic CO₂ emissions of other materials or services are also neglected in this study. The life-cycle GWP of SNG (GWP_LC) is then calculated as follows:

$${GWP}_{LC}={\displaystyle \frac{\sum {GWP}_i}{{{HHV}_{\mathrm{S}\mathrm{N}\mathrm{G}}\times V}_{\mathrm{S}\mathrm{N}\mathrm{G}}·\times 6000\times 20}}({\mathrm{gCO}}_2\mathrm{e}/{\mathrm{MJ}}_{\mathrm{SNG}})$$

(8)

where GWP_i denotes the GWP contributed by the stage i, including C&C, FT, C&D, SP, EU. When CO₂ capture (CP) of the separated CO₂ stream is applied, the life-cycle GWP of SNG (GWP_LC+CP) is calculated as below:

$${GWP}_{LC+CP}={\displaystyle \frac{\sum {GWP}_i-{GWP}_{\mathrm{C}\mathrm{P}}}{{{HHV}_{\mathrm{S}\mathrm{N}\mathrm{G}}\times V}_{\mathrm{S}\mathrm{N}\mathrm{G}}·\times EOH\times lifespan}}({\mathrm{gCO}}_2\mathrm{e}/{\mathrm{MJ}}_{\mathrm{SNG}})$$

(9)

where AOH and lifespan refer to the annual operation hours and years of lifetime, respectively. V_SNG is the volume flow rate of SNG, in Nm³/h.

3.4. Economic Assessment

The unit production cost (UPC) is a key index of economic analysis. The UPC was calculated based on the total capital investment (TCI) and the total production cost (TPC). Table S7 presents the TCI model and the percentage of each component, which can be estimated by the equipment costs of the devices (Table S8) [7,10,11]. Table S9 presents the TPC model that primarily consists of raw material, utilities, maintenance and operating, depreciation, plant overhead cost, administrative cost, and distribution and selling cost [17]. The depreciation is calculated according to the straight-line method with a salvage value of 4% and a lifespan of 20 years. UPC is then calculated based on the TPC value and the annual yield of the product. However, as the SNGs produced by those processes are different in HHV, the equivalent UPC (eUPC) is defined to normalize and compare the costs at the same baseline, which is expressed below [10]:

$$eUPC=UPC\times {\displaystyle \frac{{HHV}_0}{{HHV}_{\mathrm{S}\mathrm{N}\mathrm{G}}}}={\displaystyle \frac{TPC}{annual \mathrm{S}\mathrm{N}\mathrm{G} yield}}\times {\displaystyle \frac{{HHV}_0}{{HHV}_{\mathrm{S}\mathrm{N}\mathrm{G}}}}$$

(10)

where HHV₀ is the benchmark value of natural gas (36.49 MJ/Nm³) according to the typical value in the Chinese technical standard [16].

The eUPC only denotes the basic production cost. When CO₂ capture is applied, the separated CO₂ stream can be sold on the carbon trading market, and the relevant profit should be subtracted from TPC, which is expressed below:

$${eUPC}_{+\mathrm{C}\mathrm{P}}={\displaystyle \frac{TPC-P_{\mathrm{C}\mathrm{O}2,\mathrm{S}\mathrm{P}}}{annual \mathrm{S}\mathrm{N}\mathrm{G} yield}}\times {\displaystyle \frac{{HHV}_0}{{HHV}_{\mathrm{S}\mathrm{N}\mathrm{G}}}}$$

(11)

where P_CO2,SP is the annual profit of the separated CO₂ stream during the SP stage, and the carbon price is about 40 CNY/ton based on the data in the carbon trading market [34].

4. Results and Discussion

4.1. Product Properties

Table 1. Properties of SNG produced by different processes.

BPtG Process	SWE	MWE	VPSA	PAG	RHG
Feedstock (t/h)	25	25	25	25	25
O2 purity (%)	100	100	90	—	—
SN	0.933	0.151	0.164	0.300	0.300
pM (bar)	65	40	45	30	30
CH4 (vol.%)	92.25	93.68	85.00	93.62	93.62
H2 (vol.%)	3.38	2.86	2.65	2.92	2.92
CO (vol.%)	0.001	0.02	0.02	0.02	0.02
CO2 (vol.%)	3.57	1.87	3.01	1.87	1.87
N2 (vol.%)	0.61	1.19	8.85	1.19	1.19
HHV (MJ/Nm3)	36.39	37.59	34.12	37.58	37.58
SCE (MJ/Nm3)	33.94	35.06	31.82	35.05	35.05
SE (MJ/Nm3)	34.39	35.51	32.27	35.49	35.49
Yield (Nm3/kg)	0.829	0.422	0.336	0.422	0.422
PIN (MW)	192.2	43.8	7.0	39.7	36.3

shows the key variables and properties of SNG produced by these BPtG processes. Due to the characteristics of hydrogen methanation in the SWE process, suitable p_M (65 bar) can directly generate eligible SNG when the SN is 0.933. The SWE process has the highest yield of SNG (0.829 Nm³/kg) at the price of the highest input power (P_IN, 192.2 MW). Conversely, SNG from the VPSA process has the highest N₂ content (8.85 vol.%) and lowest CH₄ content (only 85 vol.%) due to the characteristics of O₂-enriched air. It also has the lowest yield (0.336 Nm³/kg) and input power (7.0 MW). The MWE, PAG, and RHG processes have similar SNG compositions and yields. Taking the SNG composition and the HHV and SCE values into account, in the decreasing order of the gas quality, the BPtG processes are as follows: MWE > PAG > RHG > SWE > VPSA. The parameter of P_IN denotes the power storage amount of biomass as an energy carrier. In the decreasing order of P_IN, the BPtG processes are as follows: SWE > MWE > PAG > RHG > VPSA. For a given amount of biomass, the amount of power stored by the SWE process is 4–5 times of those by the MWE, PAG, and RHG processes, and roughly 27 times of that by the VPSA process. This implies that the SWE process has a strong ability to store power while the VPSA process has a weak ability. Regarding a specific project, the BPtG process should be chosen based on the full consideration of the amounts of surplus power and biomass resources.

4.2. Life-Cycle Energy and Exergy Efficiencies

Figure 2a shows in the decreasing order of every energy efficiency indicator (η_En,SP, η_En,CP or η_En,LC), and the processes are as follows: RHG > PAG > MWE > SWE > VPSA. The RHG process always has visible advantages over the other processes, because the resistance heating technology has the highest conversion efficiency. Note that the efficiencies of the SWE process are lower than those of the MWE processes. The reason is that the SWE process generates more H₂ through water electrolysis, and more energy is wasted through the exothermic CO₂ methanation synthesis with H₂. The energy loss caused by the CO₂ separation unit is less than that which occurs in the sufficient water electrolysis and hydrogenation methanation synthesis. Additionally, the VPSA process has the lowest energy efficiencies. The reason is that the heat required by the gasification process is supplied by the exothermic reactions between oxygen and biomass, and more combustible species are oxidized and more heat energy is lost in the cooling and cleaning unit. The difference in the energy efficiencies indicates that the high-efficiency electricity conversion technology plays an important role in the life-cycle efficiency. It is significantly eager to improve the energy efficiency of electricity conversion efficiency to improve competitiveness.

Furthermore, η_En,SP, η_En,CP, and η_En,LC range between 61.8 and 68.2%, 59.0 and 65.1%, and 53.1 and58.6%, respectively. The difference between η_En,CP and η_En,SP is on average 3 percentage points, indicating that the energy consumptions in the stages from cultivation to transportation is relatively small. Additionally, no product has a life-cycle energy efficiency of more than 60%. The difference between η_En,LC and η_En,CP is about 6 percentage points, indicating that the energy loss in the end-use stage is large, and it is valuable to improve the energy efficiency of this stage.

Figure 2b shows that in the decreasing order of every exergy efficiency indicator (η_Ex,SP, η_Ex,CP or η_Ex,LC), the processes are as follows: RHG > SWE > PAG > MWE > VPSA. The RHG process always has visible advantages in exergy efficiency over the other processes, because the exergy loss during electricity conversion is the smallest. Additionally, the VPSA process has the lowest exergy efficiencies. The reasons are similar to those related with energy efficiency. However, according to exergy efficiencies, the SWE process takes second place. The reason is that electricity, as the high-quality energy, accounts for more than 60% of the total input exergy, so the relative change between the total input items is small compared with the relative change between output items.

In detail, η_Ex,SP, η_Ex,CP, and η_Ex,LC range between 57.8 and 65.2%, 57.3 and 64.6%, and 36.4 and 41.1%, respectively. The difference between η_Ex,CP and η_Ex,SP is on average 0.6 percentage points, indicating that the exergy consumptions in the stages from cultivation to transportation are very small compared with the exergy inputs in the production stage. Additionally, no product has a life-cycle energy efficiency of more than 42%. The difference between η_Ex,LC and η_Ex,CP is about 22.6 percentage points, indicating that plenty of exergy loss occurs in the end use of SNG.

The difference between η_Ex,LC and η_Ex,CP is much greater than that between the energy efficiencies (averagely 6 percentage points), which indirectly implies that the SNG should be utilized in cogenerative ways to improve the life-cycle efficiency. For instance, as electricity and heat are the most-used energy in modern life, SNG should firstly be used in the way of combined heat and power. Additionally, every saving measure should not be omitted or ignored. For example, the final pressure of 64 bar is much higher than the operating pressures in a low- and medium-pressure piping net. The previous study indicated that the electricity consumption rate visibly declines with the decrease in methanation pressure [11], and it would also decrease with the decrease in the final pressure. For most scenarios, the energy and exergy efficiencies of BPtG processes will be higher if lower final pressure is employed. From this perspective, the SWE process will not be a suitable option because its p_M is not less than 50 bar [7].

4.3. Life-Cycle Carbon Emission

(1)

Influence of allocation method of cultivation stage

The GWP of the renewable electricity (GWP_RE) from CPCD is 32.58 gCO₂e/kWh for photovolatic power, i.e., 9.05 gCO₂e/MJ (Table S5) [29]. Figure 3 shows the GWP_LC of SNG from five processes without and with CO₂ capture. The life-cycle GWP of natural gas and coal-based SNG were calculated to 65.3 and 243.7, respectively, in gCO₂e/MJ, by combining the production emission data [31] and the combustion emission factors [33]. Figure 3a–c show that the allocation methods of the cultivation stage have significant influences on the GWP_LC of SNG. It sharply declines with the decrease in the GWP of the cultivation stage.

When CO₂ capture is not applied, the GWP_LC of these processes varies between 22.0 and 34.8 gCO₂e/MJ_SNG, 10.5 and 12.1 gCO₂e/MJ_SNG, and 3.2 and 8.3 gCO₂e/MJ_SNG for the energy allocation method, economic allocation method, and excluding cultivation stage, respectively. When the energy allocation method is applied, the GWP_LC of the VPSA process is highest mainly due to its lowest SNG yield. When the other methods are applied, the SWE process has the highest GWP_LC, implying that CO₂ emissions are unavoidable for the utilization of renewable energy without dedicated CO₂ capture. The GWP_LC substantially decreases when the GPW of the cultivation stage is excluded.

When CO₂ capture is applied, the GWP_LC+CP varies between −43.4 and −17.6 gCO₂e/MJ_SNG, −67.7 and −36.7 gCO₂e/MJ_SNG, and −77.2 and −44.1 gCO₂e/MJ_SNG for the energy allocation method, economic allocation method, and excluding cultivation stage, respectively. The GWP_LC+CP of the VPSA process (−77.2–−43.4 gCO₂e/MJ_SNG) is quite abstractive since the SNG can almost totally offset the GWP of natural gas.

Figure 4 further shows the contribution of every stage to GWP_LC. When the energy allocation method is applied, the stages of transportation, construction and dismantling, and end use have little contributions to the GWP_LC of which all the percents are less than 1%. For the SWE process, the GWP_LC is mainly contributed by the SP stage, which is caused by the plenty of renewable power consumed. By contrast, the GWP_LC associated with other processes are mainly contributed by the cultivation stage. The detailed data indicate that the direct greenhouse gas emissions from soil and the indirect greenhouse gas emissions related to electricity, fuels, and materials are the primary causes. The results are highly sensitive to the allocation method for distributing the GWP between grain and straw. The coefficient of the economic allocation method (0.1245) is about a quarter of that based on the energy allocation method (0.4377) [35]. Accordingly, when the economic allocation method is applied, Figure 4b shows that the contribution of the cultivation stage is obviously reduced, especially for the SWE process. However, the results are generally similar to those in Figure 4a. By contrast, when the GWP of the cultivation stage is excluded, Figure 4c shows that the SNG production stage is the major contributor for every process, which is mainly contributed by renewable electricity.

The results based on the energy allocation method are focused on in the following analysis, because the results can be regarded as the conservative and adverse scenario. In the increasing order of GWP_LC, the processes are as follows: VPAS < MWE < PAG < RHG < SWE. SNG without CO₂ capture already has distinct advantages in GWP_LC (22.0–34.8 gCO₂e/MJ_SNG) over natural gas and coal-based SNG (Figure 3a). The evident differences underscore the sustainability of SNG produced by BPtG processes in terms of carbon emissions as well as the promising potential in decarbonization. SNG from the BPtG processes except the SWE with CO₂ capture is much more abstractive because the GWP_LC+CP are negative (−43.4–−17.6 gCO₂e/MJ_SNG), indicating that the SNG can help users achieve negative carbon emissions.

(2)

Influence of renewable power’s GWP

At current, the GWP of wind power (4.02 gCO₂e/kWh, i.e., 1.11 gCO₂e/MJ) is much less than of photovoltaic power [29]. All the GWP_LC with different renewable power and allocation methods are listed in Table S6. Regarding the energy allocation method, Figure 5 shows that the GWP_LC of all SNG declines with the decrease in the renewable power’s GWP. However, only the GWP_LC of SNG from the SWE process obviously falls by 7.2 gCO₂e/MJ_SNG, which accounts for about 30% of its GWP_LC. The GWP_LC of SNG from the VPSA process slightly reduces by 0.7 g/MJ_SNG, posting a 2% decline. Other moderate decreases (2.8–3.2 gCO₂e/MJ_SNG) occur in the MWE, PAG, and RHG processes, resulting in 9–11% declines. About 60% exergy of SNG is converted from renewable power in the SWE process, thus the GWP_LC is more sensitive to the change in the renewable power’s GWP; for the same reason, other G_LC are less sensitive. It should be noted that the GWP_LC of the SWE process is always positive because all the CO₂ is converted into CH₄, and there is no potential to capture CO₂. The results also imply that either renewable power or the SNG from the SWE process can play a part in the decarbonization but cannot achieve the goal of negative carbon emissions. By contrast, with CO₂ storage and utilization, other BPtG processes can help the renewable power be utilized in the negative carbon emission way. That is a great significance of the BPtG processes and can further enlighten the development of the processes of biomass and power to other fuels.

4.4. Equivalent Unit Production Cost

The feedstock cost (C_FS) studied in this work varies from −50 to 300, in CNY/t (the US dollar to Chinese Yuan exchange rate was 7.0721). The C_FS of −50 CNY/t is considered for the biowaste scenario with subsidies. In 2020, the electricity price for electricity storage at the generating side varied from 0.1 CNY/kWh to 0.95 CNY/kWh in China [36,37]. Thus, the electricity cost (C_RE) considered in this study ranges from 0.1 CNY/kWh to 0.9 CNY/kWh, and the C_RE of 0.3 CNY/kWh is set as the basic scenario to make sure that the process will be profitable even without the subsidy for the electricity cost.

(1)

Cost under Baseline Scenario

At the baseline, i.e., C_FS = 300 CNY/t and C_RE = 0.3 CNY/kWh, the top four constituents of eUPC are always feedstock cost, electricity cost, operation and maintenance (O&M), and depreciation (

Table 2. Contributions of top four constituents of unit production costs.

BPtG Process	SWE	MWE	VPSA	PAG	RHG
ECT (106 CNY)	137	118	72	94	94
TCI (106 CNY)	391	337	206	268	268
eUPC (CNY/Nm3)	3.475	2.719	2.025	2.499	2.400
eUPC contribution (%)
Feedstock cost	10.2	25.4	47.1	27.6	28.8
Electricity cost	78.8	44.5	13.2	43.9	41.7
O&M	5.5	5.6	20.0	6.1	6.4
Depreciation	4.3	9.1	10.4	7.9	8.2

). However, the orders of these items are not the same. The electricity cost (0.3 CNY/kWh) is 0.083 CNY/MJ, while the feedstock cost (300 CNY/t) is 0.017 CNY/MJ. The electricity cost is about 4.9 times the feedstock cost. Even when the electricity cost is 0.1 CNY/kWh, the electricity cost per MJ (0.028 CNY/MJ) is about 1.6 times the feedstock cost per MJ. Thus, it can be speculated that the eUPC rises with the increase in the share of the electricity in the total inputting energy, but declines with the increase in the share of biomass in the total inputting energy.

(2)

Influences of Feedstock and Electricity Costs

The natural gas price at the gate station and selling price in Eastern China varies from 2.1 to1 2.8, respectively, in CNY/Nm³. To clarify the cost competitiveness of SNG, this study employed the intermediate value of the above prices as a critical cost, i.e., 2.5 CNY/Nm³. If the eUPC of a product is lower than the critical cost, the product and the project are profitable.

Figure 6 shows that for the given plant scale and feedstock cost, the eUPC proportionally increases with the increase in electricity cost. Figure 6a shows that when the feedstock cost is 300 CNY/t, there exists a critical C_RE for every BPtG process, which increases in the following order: SWE (~0.20 CNY/kWh) < MWE (~0.25 CNY/kWh) < PAG (~0.30 CNY/kWh) < RHG (~0.33 CNY/kWh) < VPSA (~0.82 CNY/kWh). The result indicates that the SWE processes may not achieve or sustain profitability, since the C_RE mainly varies between 0.2 and 0.5, in CNY/kWh. The result also implies that the VPSA process has the highest resistance to the fluctuation of C_RE.

Additionally, when the C_RE is higher than about 0.15 CNY/kWh, the eUPC decreases in this order: SWE > MWE > PAG > RHG > VPSA, and most eUPCs are greater than 2 CNY/Nm³. It should be noted that when the C_RE is lower than 0.15 CNY/kWh, the eUPC of SNG from the SWE process is lowest, because the most energy of SNG is converted from renewable power, and its eUPC is sensitive to C_RE as is indicated by the slope in Figure 6a. Considering the potential fluctuation in electricity cost for electricity storage, C_RE is probably higher than 0.15 CNY/kWh, and then the SWE process is uncompetitive.

Figure 6b shows that when the feedstock cost is −50 CNY/t, the eUPC also proportionally increases with the increase in electricity cost. There exists a critical C_RE for various BPtG processes except the VPSA process, which increases in the following order: SWE (~0.24 CNY/kWh) < MWE (~0.45 CNY/kWh) < PAG (~0.52 CNY/kWh) < RHG (~0.57 CNY/kWh). Obviously, the critical C_RE with a C_FS of −50 CNY/t is greater than that with a C_FS of 300 CNY/t for a given process, indicating that the C_FS has a major influence on eUPC and the profitability of such a project. The SNG produced by the VPSA process always has the lowest eUPC and is always profitable with the given electricity cost. The reason is that the most energy of SNG produced by the VPSA process is converted from biomass and the eUPC mainly depends on C_FS. However, it should be noted that the SNG from the VPSA process has poor performances in energy efficiency.

Furthermore, Figure 6c shows that the eUPC proportionally increases with the increased feedstock cost. The eUPC always decreases in the following order: SWE > MWE > PAG > RHG > VPSA. When C_RE is 0.3 CNY/kWh, the SWE process cannot make a profit, while other processes can make a profit in most cases. Figure 6 also indicates that the eUPC of the RHG process is always slightly lower than that of the PAG process. The comparison of eUPCs between the PAG and RHG processes implies that the difference between the electricity utilization efficiency has a visible and considerable effect.

To sum up, from the perspective of the eUPC, the VPSA process is a preferential process, followed by the RHG process, the PAG process and the MWE process, while the SWE process is not recommended unless the electricity cost is very low.

(3)

Influences of Carbon Trading of Separated CO₂ Stream

When the separated CO₂ stream is sold on the carbon trading market, the income can offset a part of the production cost. For the same reason as mentioned above, this is not applicable to the SWE process.

Table 3. Comparison between eUPC and eUPC_+CP.

BPtG	Relative Change (%)
CFS (¥/t)	300	300	−50
CRE (¥/kWh)	0.9	0.3	0.1
MWE process	−1.3	−2.6	−6.7
VPSA process	−4.8	−6.1	−18.2
PAG process	−1.5	−2.8	−7.8
RHG process	−1.6	−2.9	−8.1

shows that under the boundary conditions, the relative changes between eUPC_+CP and eUPC vary from −6.7% to −1.3%, −18.2% to −4.8%, −7.8% to −1.5%, and −8.1% to −1.6% for MWE, VPSA, PAG, and RHG processes, respectively. Under the basic scenario, the eUPC_+CP of the MWE, VPSA, PAG, and RHG processes is 2.649, 1.901, 2.429, and 2.330, respectively, in CNY/Nm³; correspondingly, the relative change is −2.6%, −6.1%, −2.8%, and −2.9%, respectively. The results indicate that the carbon trading income plays visible positive effects on the production cost, especially for the VPSA process, and it should be seriously taken into account if the carbon trading market is available.

Finally, it should be stated that the results of the eUPC demonstrate the economic competitiveness of SNG compared to natural gas but fail to directly show the electricity storage cost and its competitiveness. The related performances deserve further investigations.

5. Conclusions

Five BPtG processes integrated with SWE, MWE, VPSA, PAG, and RHG were studied from the perspectives of life-cycle energy and exergy efficiencies, global warming potential, and equivalent unit production cost. The main findings are as follows.

The life cycles of the energy and exergy efficiencies of SNG from those processes range between 53.1 and 58.6% and 36.4 and 41.1%, respectively. The RHG process always has the highest energy and exergy efficiencies, while the VPSA process always has the lowest efficiencies. The SWE process has the strongest ability in storing power while the VPSA process has the weakest ability.

The allocation methods of the cultivation stage have significant influences on the life-cycle carbon emissions. Conservatively based on the energy allocation method, the life-cycle carbon emissions without or with CO₂ capture range from 22.0 to 34.8 gCO₂e/MJ_SNG and from −43.4 to −17.6 gCO₂e/MJ_SNG, respectively. The carbon emission data underscore the sustainability of the SNG from BPtG processes in terms of carbon emissions as well as the promising potential in decarbonization.

The VPSA process has the lowest eUPC, while the SWE process has the highest eUPC. Both feedstock and electricity costs have significant influences on eUPC and the profitability of these projects. Considering the fluctuation of the electricity cost, the VPSA process can be nearly always profitable; the PAG, RHG, and MWE processes are probably profitable. By contrast, the SWE process is only competitive when the electricity cost is very low.

Finally, the RHG, PAG, and MWE processes are recommended for their compromise of multi-perspective performances, achievable negative carbon emissions, and potential flexible applications. The SWE process has no obvious advantages in carbon emission and economic competitiveness among these processes. It is recommended only when the electricity cost is very low and the electricity storage demand is huge. The VPSA process is recommended when users are very sensitive to the SNG price and the electricity storage is a secondary concern.

The direct use of renewable power can play a part in decarbonization but cannot achieve the goal of zero or negative carbon emissions. By contrast, as the highly pure CO₂ stream is generated, the BPtG processes with CO₂ capture can help the renewable power be utilized in a negative carbon emission way. That is a great significance of the BPtG processes. In the future, biomass and power to other fuel processes are worthy of study.