Technical and Economic Assessment of a High-Quality Syngas Production Process Integrating Oxygen Gasification and Water Electrolysis: The Chinese Case.

Natural gas shortage, waste treatment, and renewable energy utilization and storage are the common issues in China. This research aims to evaluate a simple and reliable biomass and power-to-syngas process for high-quality syngas production integrating oxygen gasification and water electrolysis. The technical and economic evaluations are successively performed based on the simulation data. The single-stage and two-stage gasifiers are integrated and compared. The results show that the process with a two-stage gasifier has advantages in both technical and cost performances over that with a single-stage gasifier. Then, the optimal values of the operating variables are determined by investigating their effects on syngas composition and yield and the overall energy and exergy efficiencies. Then, by setting appropriate amounts of water for electrolysis and methanation pressures, the process can generate two typical products: thermochemical synthetic biogas (TSB) or synthetic natural gas (SNG). The overall energy and exergy efficiencies of TSB can reach 74.1 and 69.5%, respectively, while those of SNG can achieve 64.8 and 64.1%, respectively. The unit product cost of TSB is about one third of that of SNG, indicating that TSB makes a profit while SNG results in loss. A sensitivity analysis of the cost indicates that the constraints for TSB are much looser than those for SNG to gain profit. Additionally, this study proposes two indicators to characterize the electricity-storage capacities of biomass and syngas. The indicators show that SNG has big advantages over TSB in the specific capacity of electricity storage, and the capacities of SNG and TSB are approximately 52.0 and 4.7 times of the specific energy density of the lithium-ion battery pack in 2019, respectively. The work could be used as a reference for the countries lacking natural gas and valuable to guide the development of a two-stage gasifier, reduction in equipment cost, and evaluation of electricity-storage performance using biofuels.

Introduction

Natural gas is a desirable clean energy resource for economic and social development, but most of the reserves are owned by a few countries. Like many developing countries, China suffers serious natural gas shortages; the rural areas suffer more due to the domestic imbalance in natural gas distribution and consumption. On the demand side, China has a huge demand on substitute natural gas or other high-quality gaseous fuels, especially in winter. Natural gas is a nonrenewable energy source and it is not the sustainable option in the decarbonization race. Emerging technologies that could curb carbon emissions and offer sustainable ways to future are highly expected. On the resource side, China has abundant agriculture residuals, forestry wastes, and household waste that need to be treated in an environmentally friendly manner.[1] In this context, gaseous biofuels, for example, synthetic natural gas (SNG), have natural advantages in environmental impacts and sustainability.[2]

Traditionally, there are mainly two ways for SNG production from biomass: (i) anaerobic digestion of wet biomass plus biogas upgrading to SNG (including CO2 removal). The anaerobic digestion technology had been commercialized for many years and has several drawbacks such as relatively low yield and energy efficiency, sensitive to reaction temperature and pH, and so forth.[3] (ii) Conventional SNG process, consisting of steam gasification, gas cleaning, methanation, and CO2 separation.[4] Although the conventional SNG process has higher energy efficiencies (up to 74%),[4,5] the high-temperature dual-fluidized bed gasifiers for indirect heating of exothermic steam gasification are complex and difficult to design and operate.[6]

One of the goals that researchers keep on studying is to develop better or novel processes of SNG production. The hydrothermal gasification process (400–450 °C, 25–34 MPa) was developed to produce SNG from wet biomass (algae and manure) with high conversion efficiency.[7,8] So far, some crucial technical barriers still need to be settled such as feeding, salt separation, high-pressure safety and erosion, and so forth.[9,10] The power-to-gas (PtG) concept is one of the recent research hotspots since it converts renewable electrical energy into storable methane via electrolysis and subsequent methanation. PtG is one of the potential technologies to solve large-scale and long-term energy-storage problems as well as reduce CO2 emissions. By far, there are already dozens of laboratory, pilot, and demo plants, of which most are located in Europe.[11] To the best of our knowledge, there are only few studies on the biomass-based PtG process in China until now. In 2020, China announced its ambition to peak its carbon emissions by 2030 and achieve carbon neutrality by 2060. The demands for large-scale electrical energy storage are so urgent. It is meaningful to carry out technoeconomic studies on the PtG process under the Chinese scenario to evaluate the competitiveness and promote the applications.

Considering the abovementioned features of demands, resources, and tendencies in China, this study focuses on the technical route of biomass and renewable PtG integrating oxygen-steam gasification, water electrolysis (WE), and hydrogenation methanation. It is expected to propose a solution to harmonically handle the demands of the clean gaseous biofuel, renewable electricity storage, and waste treatment. As various PtG process configurations can be designed by integrating different feed stocks or unit technologies, the economic assessment is one of the important concerns. Barbuzza et al. found that in Northern Italy, under the scenario of “electricity at zero cost”, the cost of SNG is comparable to the price of the methane for household utilities.[12] In the scenario of California, it was predicted that the projected cost of SNG by PtG in 2050 would be comparable to but still higher than the cost of conventional natural gas if a high capacity factor could be achieved with low electricity costs together with lower electrolyzer capital cost.[13] However, a report by Mesfun et al. showed that under the Swedish scenario, the economic performance of the process using molten carbonate electrolysis cells was better than that of the conventional SNG plant if the electricity costs are lower than a certain value.[14] The minimum selling price of SNG estimated by Menin et al. is more than three times higher than the current subsidized price of grid-quality bSNG produced from waste and byproducts.[15] Taken together, there were different results or requirements on the cost and competitiveness of SNG with WE. The studies indicate that it is necessary to carry out economic assessments based on the local scenario and the ways which can reduce the production cost and improve the competitiveness are of great significance for this technical route. Based on the characteristics of the process and product, either the gasification technology or the quality of the final product might have considerable potential to get close to these aims.

With the abovementioned technical route, the gasification step plays an important role and the gasification technology should be chosen or developed accordingly. Within the framework of SNG production with WE, Gassner and Maréchal compared the performances of directly and indirectly heated gasification systems, as well as the fast internally circulating fluidized bed and circulating fluidized bed.[16,17] However, the optimization or development of the gasifier was not involved. Barbuzza et al. analyzed a PtG system in which hydrogen obtained via electrolysis was used for wood hydrogasification.[12] Recent studies revealed that the staged gasification has some advantages such as low tar content, higher energy efficiency, and so forth.[18,19] It is worth investigating the process integrating staged gasification.

Judging from the functions of the methanation reaction of CO and CO2, the process can theoretically produce high-quality syngas that has equivalent quality to biogas.[20] Here, the syngas is named thermochemical synthetic biogas (TSB) to distinguish the thermochemical technique from anaerobic digestion. The product in related previous studies is SNG, aiming to eject it into a well-developed gas grid.[11,21] In China, biogas without CO2 removal is widely used for cooking and heating in many places.[22] Accelerated by the “rural revitalization strategy” and rural “coal to gas” policies, the development of biogas has become increasingly popular, and China’s energy sector targets achieving an annual yield of 20 billion m3 of biomethane by 2030.[22] TSB might have considerable potential by directly using as fuel. Then, the feasibility and competitiveness of TSB need to be studied as an alternative option of energy carrier.

Motivated by the abovementioned analysis, the objective of this work is to study a biomass and renewable power-to-syngas process integrating biomass gasification, WE, and hydrogenation methanation. Two types of products, SNG and TSB, are considered and compared. The process is modeled using Aspen Plus software. The composition and yield, energy and exergy efficiencies, electricity-storage capacity, and unit product cost are investigated as the performance indicators. The work would be valuable to guide the development of the gasifier, reduction in equipment cost, and selection of the rational gas quality. The results can be used as references for the areas in other countries lacking natural gas.

Process Simulation and Assessment Methodologies

Process Description

Considering the suitable scale of biomass utilization factory in China,[23] the mass flow rate of biomass was set to 25 t/h as the base value. Wheat straw is chosen as the feedstock as it is an abundant, geographically ubiquitous, and cheap agriculture waste. Table lists the composition (on air-dried basis, ad), lower heating value (LHV), and specific chemical exergy (SCE) of the feedstock. The SCE is calculated based on the composition data.[24]

Table 1

Composition, LHV, and SCE of Wheat Straw

property		value
composition (wt %, ad)
proximate analysis	M	3.3
	FC	18.73
	VM	71.59
	A	6.38
ultimate analysis	C	43
	H	5.36
	N	0.63
	S	0.22
	O	41.11
LHV (kJ/kg, ad)	15,703
SCE (kJ/kg, ad)	17,754

Based on the abovementioned scenario, the plant should be simple, easy, and reliable to construct, operate, and maintain. The biomass and renewable power-to-syngas process is designed and illustrated in Figure . It mainly consists of (i) a gasification system including a water pump (PMG) and heaters for steam generation (HTG) and O2 preheating (HTO); (ii) a WE system; (iii) a producer gas cleaning and cooling (PGCC) system integrating hot gas cleaning devices and several coolers for producer gas (PG) (CLPG1 and CLPG2); and (iv) a methanation synthesis system including a compressor (CMPG), a water pump (PMM), and heater (HTM) and a set of coolers for syngas (CLSG). Other auxiliary devices for heat utilization and organic Rankine cycle (ORC) are integrated in this study but not shown in detail. The CO2 separation is not integrated as a post-upgrading unit since one of the goals is to produce SNG by a simple process. The CO2 separation will obviously increase the complexity of the process and the production cost (about 10–15%) based on the existing biogas upgrading projects.[25]

Figure 1

Process flow sheet of the biomass and renewable power-to-syngas process integrating oxygen gasification and WE.

Gasification System

Gasification is the primary step for high-quality syngas production. The feedstock (mbio) is first dried by low-temperature heat recovered within this process. The dried biomass is converted into PG, tars, residual char, and ash by thermochemical gasification using oxygen and steam as gasifying agents. Prior to gasification, the water for gasification (mWG) and the oxygen (mO) are preheated to 200 °C by the heaters HTG and HTO, respectively. The PG has high contents of H2 and CO, and there are risks of explosion and toxicity hazards. In this study, the gasifier is assumed to be operated at atmospheric pressure and over the gasification temperature (TG) range of 800–1000 °C (Table ).

Table 2

Model Blocks and Operation Parameters of the Major Devices (Underline: Base Value)

device	model	parameter(s)
pyrolysis	Ryield	350 °C, 0.1 MPa
gasification	RGibbs	TG = 800–1000 °C; 0.1 MPa; S/B = 0.4–1; SR = 0.107–0.217
WE	RStoic	80 °C; ηWE = 65–90%, 80%
FBMR	RGibbs	300 °C; pM = 5–70 bar; WR = 1–6

A gasifier is the most crucial equipment in this process. With regard to steam gasification, the gasifier in the form of fluidized bed reactors, especially internally circulating fluidized bed or dual fluidized bed system, is a popular solution.[16,17,26] Since the gasification is an endothermic process, indirect heat supply to gasification is prerequisite to avoid the dilution of PG by nitrogen. When oxygen gasification is involved, a single fluidized bed gasifier or fixed bed gasifier is available because the oxygen gasification is an autothermic process. Additionally, the emerging staged-gasification could be a substitute and even better option.[19] A pilot-scale two-stage gasifier had been successfully applied in oxygen-enriched air gasification.[18] Thus, both single-stage and two-stage gasifiers are studied and compared.

Figure a shows that the conventional single-stage gasifier employs a single fluidized bed or fixed bed as the gasifier. In this case, there is no pyrolyzer and all the pyrolysis and gasification reactions take place in one reactor. All heat recovered from PG (Q1 and Q2) is used to heat the streams such as water, oxygen, and ORC working medium except dried biomass.

Figure 2

Configurations of (a) single-stage and (b) two-stage gasifiers.

Figure b shows that the two-stage gasifier consists of reactors, which are physically separated. The first-stage reactor (pyrolyzer) is operated at about 350 °C and plays the role of the physical zone of heating and pyrolysis to some extent. The second-stage reactor (gasifier) is the physical zone of gasification of the mixtures produced in the first-stage reactor. In this case, the heat recovered by CLSG1 (Q1) is transferred to the first stage to heat the dried biomass.

WE System

The WE system converts water (mWE) into highly pure O2 and H2 by renewable electricity. The exact amount of O2 is piped into the gasifier to operate the gasifier at the expected TG. The surplus O2, if there exists, is sold to outside users. By comparison, all the H2 is piped into the methanation reactor. Currently, the operating temperature of alkaline electrolysis cells (AECs) and proton exchange membrane electrolysis cells (PEMECs) varies from 50 to 80 °C, while that of solid oxide electrolysis cells (SOECs) varies from 650 to 1000 °C. The operating pressures of AEC and SOEC do not exceed 30 bar, while that of PEMECs can be up to 200 bar.[27] The efficiency of the electrolysis system ranges from 62 to 90% based on HHVH.[28]

PGCC System

A series of heat exchangers are arranged in the PGCC and recover the heat of PG from gasification at required temperatures. First, high-temperature PG generated is first cooled to about 300 °C using the cooler CLPG,1 and simultaneously plenty of heat (Q1) is recovered. For a two-stage gasifier, Q1 is used to heat dried biomass in the first-stage reactor; while for the single-stage gasifier, Q1 is used to heat other streams except dried biomass, as shown in Figure a.

The PG is purified using the hot gas cleaning technology, which integrates particle removal with candle filters or electrostatic precipitators, catalytic cracking of the tars, and high-temperature adsorption of other contaminants. Subsequently, the hot PG is cooled to 40 °C using a set of coolers (CLPG,2) and partial vapor in PG is condensed and removed. In this step, the heat is recovered from PG with temperature above 80 °C.

Methanation Synthesis System

In the methanation reactor, CO and/or CO2 with H2 are converted into CH4 and H2O depending on the methanation reactions and water–gas shift reaction (WGSR)[20]

Besides, carbon formation may also occur[20]

Thus, a certain amount of water (mWM) is added into the methanation reactor to suppress carbon formation. Prior to methanation, the water for methanation (wM) is heated to 200 °C.

The stoichiometric coefficients of R and R allow determining the amount of H2 that is needed to completely reform CO and CO2 into CH4. Here, the stoichiometric number (SN) of the incoming gas stream is defined in order to characterize the achievable methane yield. The amount of H2 generated by WE can vary extensively. Two SNs are defined to represent the characteristics of the mixed gas (PG and H2 streams), that iswhere mH and mH are the molar flow rates of H2 in PG and that generated by WE, respectively. mCO,PG and mCO are the molar flow rates of CO and CO2 in PG, respectively. When SN1 is less than 1, WGSR plays an important role in syngas composition. To obtain a highly pure methane stream, SN2 should be close to unity.

Methane synthesis of a H2/CO/CO2 mixture is highly exothermic and its reactor design is critical with regard to temperature control. The single isothermal once-through fluidized bed methanation reactor (FBMR) is adopted in this process configuration, which allows for simultaneous methanation and WGSRs.[29] Lower methanation temperature favors the thermodynamic performances of the process, and the FBMR is assumed to operate constantly at 300 °C and over the methanation pressure (pM) range of 5–70 bar (Table ). The heat released by exothermic reactions (Q3) is also recovered to heat water, oxygen, or ORC working medium. Finally, the syngas, the output of methanation reactions, is cooled to the required temperature using the cooler CMSG, and a drier may be installed as needed. It also removes most vapor in syngas and recovers plenty of sensible heat (Q4) with temperature above 80 °C.

Energy Integration

There are mainly four heat streams (Q1–Q4) that can be revered and utilized. The heat energies, either Q1–Q4 of the process with a single-stage gasifier or Q2–Q4 of the process with a two-stage gasifier, are first used to preheat the gasifying agents (mWG and mO) and water for methanation synthesis (mWM). The surplus recovered heat energies are used to generate electricity using the ORC technology to compensate the electricity of the whole process. Finally, the gap between the total power consumption (PT) and power generated by ORC (PORC) is filled by the external renewable power (PRE).

Process Simulation

The modeling of the process is based on our relevant previous works including conventional SNG process,[26] chemical looping gasification,[30] and staged gasification,[19] where we had described the simulations of the steam gasification, PGCC, and methanation in detail. Briefly, the models for reactors are listed in Table . Based on the Aspen Plus platform, the PR-BM method is applied as the base property method and STEAM-TA is employed as the free-water method. The gasifier is simulated by resolving into pyrolysis and gasification processes, which are simulated using RYield and RGibbs models, respectively. The RGibbs model is restricted by chemical equilibrium with the temperature approach. ξ It is assumed that all reactors are operated under a steady state, and residence times are long enough for the reactions to reach chemical equilibrium. The influences of particle sizes of feedstock are not considered. ξ The reactor temperature and pressure are taken as uniform, and the pressure loss of the system is set to be 3 kPa. Ash in biomass does not participate in any chemical reactions. The carbon conversion efficiency of the feedstock is 99%. The heat loss of gasification is assumed to be 3% of the input energy of biomass (based on LHV). A design-Spec is set to calculate the amount of water for electrolysis (mWE) at a given TG based on energy conservation. The simulation of oxygen-steam gasification is verified based on experimental data at the pilot scale reported by Barisano et al.[31]Table S1 indicates that the differences between simulation values and average experimental vales of the major components (in vol %) are less than 4 percentage points.

With respect to PGCC, besides Heater models for simulation of coolers, a Sep model is applied to simulate the separation procedure in PGCC. A pressure drop of 5 kPa in PGCC is assumed to estimate the energy and exergy losses.[26] To simplify the simulation and protect the downstream compressor, the separation efficiency for ash and char is assumed to be 100%.

The FBMR is simulated based on the RGibbs model and both phase and chemical equilibriums are calculated. The pressure drop in the reactor is assumed to be 2 kPa. The verification of the simulation of the methanation at pM of 60 bar is verified using composition data of PG and SNG reported by Sarić et al.[32]Table S2 indicates that all the simulation values are within the ranges of the reference values.

The model blocks and operation parameters of other auxiliary devices are listed in Table S3. Heat exchangers are modeled with a pressure drop of 0.5 kPa and an energy efficiency of 95%. Pumps are operated with an isentropic efficiency of 85% and a mechanical efficiency of 98%. The multistage compressor is simulated using the polytropic model using the ASME method. The net efficiency of ORC is set as 25%.

Energy and Exergy Analyses

To evaluate the technical competitiveness of the process, the following parameters are focused on composition, yield, LHV, and SCE of the syngas and overall energy and exergy efficiencies of the processwhere mlSG is the mole flow rate of syngas, in kmol/h and Enbio and Exbio are the energy and exergy flow rate of biomass, respectively, in MJ/h. PRE is the electrical power supplied by external renewable sources, in kW, which is equal to the total electrical power consumed by the process (PT) minus the power generated by the surplus recovered heat using ORC (PORC).

The following variables are studied: TG, stoichiometric ratio of O2 (SR), steam-to-biomass ratio (S/B), water ratio for electrolysis (WR), electrolysis efficiency (ηWE), and pM. The SR is defined as the mole ratio between the amount of actual oxygen fed in and the stoichiometric amount of oxygen required for gasification. The S/B is defined as the mass flow rate of steam fed to the gasifier divided by the mass flow rate of biomass (as-received basis). The WR is defined as given below:where mWE is the actual amount of water for electrolysis to generate H2 and O2 and mWE,G is the amount of water for electrolysis corresponding to the amount of oxygen that just satisfies the demand of biomass gasification under the given conditions. When WR is greater than 1, the surplus O2 is sold to external users, while all H2 is added into the methanation reactor. The ranges and base values (marked by underline) of the key variables are summarized in Table .

Economic Assessment

The economic assessment aims for determining costs of production for typical syngas and comparing these process configurations. To achieve this goal, this study employs the cost model (Table S4) including total capital investment (TCI), total production cost (TPC), and unit product cost (UPC).[33,34] TCI includes fixed capital investment (FCI) and working capital (WC). Also, FCI consists of indirect and direct investments that have to be estimated first. For this purpose, all main equipment components have to be designed according to the mass and energy flows and their equipment costs have to be gathered.

Equipment Cost and Fixed Capital Investment

Among the components, equipment costs are the key data to estimate TCI, which are expressed as given belowwhere PCref and ECref are the production capacity and equipment cost of the reference case, respectively. PC and EC2019 are the production capacity and equipment cost of the planning plant in 2019, respectively. n refers to the scale exponent and CEPCI refers to the chemical engineering plant cost index, which is developed to refresh EC from a past date to present time. Based on the values of PCref, ECref, CEPCIref, and n in Table S5,[34,35] the equipment costs of the devices are calculated using the simulated technical data. The systems of gasifier, methanation, and WE include both the reactors and pumps. Based on the equipment cost, the other investment indicators are estimated based on the TCI model presented in Table S4.

Total Production Cost

Table S6 presents the TPC structure on an annual basis.[34] It primarily consists of raw material, utilities, maintenance and operating, depreciation, plant overhead cost, administrative cost, distribution, and selling cost. In China, the cost of agriculture and forest residuals (Cbio) generally varies from 100 to 450 CNY/t. When household waste is processed as feedstock, Cbio can be negative values with subsidies (as low as −60 CNY/t). As one of the goals of this process is to store surplus renewable electricity, considering the grid purchase prices of solar and wind powers in different areas in China, we assume the electricity cost (CEL) varies from 0.1 to 0.45 CNY/kWh with a base value of 0.3 CNY/kWh. Other costs such as catalysis, water, and operating laborer are listed in Table S6. Additionally, under conditions of 4% salvage value and 20 years lifetime, depreciation is calculated according to the straight line method.

Unit Product Cost

UPC is calculated based on TPC and annual yield of the product. It should be noted that in the case of SNG, plenty of byproduct O2 is generated by WE. The O2 is assumed to be sold at the original state. The pressurization and liquefaction are carried out by external users. The cost of oxygen continuously reduced in recent years.[36] In 2021, oxygen cost of a newly built project using vacuum pressure swing adsorption in Hunan province, China, is as low as 0.2 CNY/m3,[37] which is adopted as the selling price in this study.

Process Performances

Comparison between Single-Stage and Two-Stage Gasifiers

In actual operation, the gasification temperature is determined by the stoichiometric ratio. In order to demonstrate the differences between single-stage and two-stage gasifiers, the SR value is calculated in reverse by the energy conservation at the given TG. The result in Figure shows that SR proportionally increases with TG. As the oxygen-steam gasification is an autothermic process, more heat is required at higher TG, and the adjustment measure is to increase the amount of oxygen to combust more biomass and release more heat. The SR for single-stage gasification varies from 0.159 to 0.217, which is consistent with the data in previous studies.[38,39] This reveals that the gasification simulation in this work is correct and reliable. Figure also shows that the SR of a single-stage gasifier is about 1.5 times of that of a two-stage gasifier at the same TG over the studied range. When ηWE is a constant and no surplus O2 is generated (WR = 1), the electrical power consumed by WE (pWE) is proportional to the flow rate of O2. Thus, the pWE for the operation of a single-stage gasifier is about 1.5 times of that of a two-stage gasifier, which is also confirmed by the case presented in Figure .

Figure 3

Relation between SR and TG.

Figure 4

Comparison of power consumptions between the processes with single-stage and two-stage gasifiers.

Table presents the compositions of PG and syngas, as well as the energy and exergy efficiencies of the processes with single-stage and two-stage gasifiers. Although there is a big difference in SR, the PG composition with a single-stage gasifier is similar to that with a two-stage gasifier. The differences in syngas composition and yield with the two types of gasification can be ignored due to the adjustment by methanation reactions and so are the differences in LHV and SCE of syngas, which are determined by the composition.

Table 3

Technical Performances with Single-Stage and Two-Stage Gasifiers

parameter	single-stage	two-stage
PG Composition (vol %, db)
CH4	9.9	9.3
H2	31.3	35.4
CO	34.1	36.4
CO2	24.2	18.4
Syngas Composition (vol %, db)
CH4	47.5	47.6
H2	3.6	3.4
CO	0.03	0.03
CO2	48.1	48.1
YSG (Nm3/kg)	0.839	0.838
ηen (%)	68.4	74.1
ηex (%)	64.6	69.5

Table shows that at a TG of 800 °C, ηen and ηex with a single-stage gasifier are about 6 and 5 percentage points lower than those with a two-stage gasifier, respectively. The Q1 accounts for approximately 9% of the total energy input of the feedstock. Thus, the oxygen required by a single-stage gasifier is much more than that by a two-stage gasifier, which is represented by the values of SR (Figure ). Figure further indicates that most electricity (>90%) is consumed by the WE system. As WE has a significant energy loss (20% in the basic scenario), the ηen and ηex with a single-stage gasifier are obviously lower than those with a two-stage gasifier, respectively. Thus, the following study is mainly carried out based on a two-stage gasifier.

Effects of TG

The effects of TG on PG composition had been investigated by previous studies and the variations had been figured out.[39] Then, only properties of syngas are focused on in the following analysis. Table indicates that for the process integrating oxygen gasification and WE, when TG rises from 800 to 1000 °C, SN1 decreases very slightly while SN2 increases visibly. Even so, the composition and yield of syngas (TSB) vary very little with TG, that is, the results driven by the methanation-related reactions (R1–R3). This result is beneficial to the composition control in operation. As TG is a crucial operating parameter to tar reduction, the TG in such a process can be appropriately improved, which causes little variation in syngas composition.

Table 4

Properties of PG and Syngas Over the TG Range of 800–1000 °C

TG (°C)	800	850	900	950	1000
SN1	0.519	0.518	0.518	0.517	0.517
SN2	0.310	0.326	0.341	0.352	0.364
Syngas Composition (vol %, db)
CH4	47.6	47.6	47.6	47.6	47.6
H2	3.4	3.5	3.5	3.5	3.4
CO	0.03	0.03	0.03	0.03	0.03
CO2	48.1	48.1	48.1	48.1	48.1
YSG (Nm3/kg)	0.838	0.838	0.838	0.839	0.839

The combination of Tables and 4 shows that the syngas has very lower CO content (<1 vol %) and medium HHV, which meets the requirements of city gas by the Chinese technical standard.[40] Also, it is interesting that the compositions of syngas with a WR of 1 at various TG are similar to that of biogas[41] or crude SNG.[42] Therefore, the syngas is named TSB, that is, TSB, which can be directly used without upgrading to SNG in rural areas.

Additionally, Figure shows that both ηen and ηex gradually reduce by 4 percentage points with the increase in TG from 800 to 1000 °C. This can be explained by the variation in electricity consumption. The SR proportionally increases with TG, which causes increases in the amount of water for electrolysis, electrical power, and energy loss during electrolysis. This finally results in the decreases in energy and exergy efficiencies. In brief, the abovementioned variations demonstrate that lower gasification temperature is favorable on the premise that the requirements of gasification are satisfied, such as carbon conversion, tar content, and so forth.

Figure 5

Variations of ηen and ηex with TG.

Effects of S/B

S/B has significant influence on PG composition, which had been sufficiently invested by previous studies.[43,44] This study only focuses on the effects on syngas. The results indicate that S/B has little impact on both composition and yield of the syngas due to the function of hydrogenation methanation. Figure shows that the ηen and ηex decline slightly over the S/B range of 0.4–1.0. These implies that the S/B is a minor variable for this process. Briefly, smaller S/B is favorable on the condition that the requirements of gasification are satisfied. This result can guide the selection of gasifier.

Figure 6

Variations of ηen and ηex with S/B.

Effects of ηWE

Apparently, ηWE only has influences on the process efficiencies. Figure shows that when HHV-based ηWE increases from 65 to 90%, ηen and ηex gradually improve from 69.9 to 75.2% and from 65.9 to 70.3%, respectively. As indicated by the cases in Figure , the electricity for water electrolysis accounts for more than 90% of the total electricity consumption of the process. Therefore, the efficiency of water electrolysis has direct and significant influence on the overall process efficiencies. One should pay attention to this variable and the improvement in ηWE is highly expected.

Figure 7

Variations of ηen and ηex with ηWE.

Effects of WR and pM

The amounts of water for electrolysis and H2 added into the FBMR increase proportionally with the increase in the amount of WR. Figure indicates that at a WR of about 3.5, SN1 is equal to 1. When WR is less than 3.5, that is, SN1 < 1, the amount of H2 is not enough to covert CO in product gas. Then, WGSR plays an important role in the methanation synthesis. It coverts CO into CO2 and generates H2 from H2O for CO methantion R. While SN2 reaches 1 at a WR of about 7, which means that CO and CO2 can be sufficiently converted to CH4 to obtain syngas with a high concentration of CH4. In detail, the effects of WR on syngas composition are as shown in Figure a–c.

Figure 8

Variations of SN1 and SN2 with WR.

Figure 9

Variations of syngas composition with WR: (a) CH4, (b) CO2, and (c) H2.

Figure a shows that CH4 concentration first increases, then declines with the increase in WR, and reaches the maximum values around a WR of 7 (i.e., SN2 ≈ 1). At the ascent stage, with the increase in H2 added into the FBMR, the methanation reactions R and R shift toward the right, that is, CH4 side. As a result, CH4 concentration continuously increases, while CO2 concentration steadily declines (seeing Figure b). At this stage, most H2 is converted into CH4 and its concentration maintains at low levels (<10%, seeing Figure c). When WR exceeds the critical value of 7 (i.e., SN2 > 1), CO and CO2 have been depleted, then CO2 concentration remains in extremely low levels (seeing Figure b), and little CH4 could be further generated. At this stage, the addition of H2 only dilutes CH4 concentration. Hence, as shown in Figure a,c, CH4 concentration reduces obviously while H2 concentration improves sharply, respectively.

Figure also shows the variations of syngas composition with pM. The CH4 concentration in natural gas commonly exceeds 85 vol %. According to the Chinese technical standard “coal-based SNG”,[45] H2 and CO2 concentrations should not exceed 5 and 3 mol %, respectively. The effects of WR and pM clearly indicates that only a combination of WR and pM with their narrow ranges can directly result in SNG composition without post CO2 separation. In this study, the conditions for SNG production are as follows: WR is around 7 and pM is not less than 50 bar. Figure also indicates that by varying WR, this process can generate all the types of syngas with a quality between TSB and SNG. Thus, the process has a powerful flexibility in the final product, and TSB and SNG are only two typical products.

Figure a,b shows the variations of ηen and ηex with WR, respectively. With the increase in WR, both ηen and ηex first decrease and then increase at a given pM. They reach minimum values around a WR of 7 (SN2 ≈ 1). These variations are just contrary to those of CH4 concentration (Figure a); however, the reasons are the same. The “decrease” is caused by the characteristic of exothermic reactions R and R, which inevitably result in energy or exergy losses. By comparison, the “increase” is caused by the added H2 that cannot further react due to the absence of reactants CO and CO2. Both the maximum values of ηen and ηex are at a WR of 1, which means that the electrolysis rate is just sufficient to satisfy the O2 demand of the gasifier. The condition for maximum ηen and ηex is the same as that of the SNG process integrating WE and CO2 separation.[16] However, the products are quite different. Additionally, ηen and ηex gradually decrease with the increase in pM, as the increase in pM favors the exothermic methanation synthesis.

Figure 10

Variations of ηen and ηex with WR: (a) ηen and (b) ηex.

In short, lower WR and pM are beneficial to the energy and exergy efficiencies; however, they lead to the syngas that is equivalent or similar to TSB. Thus, there is a contradiction between composition and efficiencies. Most rural areas in China lack natural gas facilities. With the matched newly construction of facilities, both TSB and SNG would be feasible to produce, pipe, and use. Then, the following cost assessment includes both types of products.

Performance of Typical Products and Electricity Storage

Guided by the abovementioned results, Table summarizes the technical performances of four cases with the combinations of gas types (TSB and SNG) and gasifier types (single-stage and two-stage). The operating parameters for TSB production are less or lower than those for SNG. WR changes according to gasifier type and gas type. As discussed above, in the case of TSB, ηen and ηex with a two-stage gasifier are obviously higher than those with a single-stage gasifier, respectively. However, in the case of SNG, the differences in the efficiencies with two types of gasifiers are much smaller. The reason is that in the case of SNG, PRE is about 1.6–and 1.8 times of those of the energy input of biomass (Enbio or Exbio, Table ). The efficiencies are dominated by PRE as one term in the denominator of eqs and 6. The recovered heat Q1, about 9% of the energy input of biomass, apparently has little contributions to both energy and exergy efficiencies.

Table 5

Technical Performances of TSB and SNG Produced by Single-Stage and Two-Stage Gasifiers

gas type	TSB	TSB	SNG	SNG
gasifier	single-stage	two-stage	single-stage	two-stage
SR	0.159	0.107	0.159	0.107
WR	1	1	4.92	6.81
pM	5	5	70	70
Syngas Composition (vol %, db)
CH4	47.47	47.62	93.67	93.72
H2	3.61	3.41	3.51	3.35
CO	0.03	0.03	0.001	0.001
CO2	48.09	48.14	2.03	2.14
N2	0.60	0.60	0.60	0.60
HHV (MJ/Nm3)	19.33	19.36	37.67	37.67
LHV (MJ/Nm3)	17.39	17.43	33.92	33.92
SCE (MJ/Nm3)	18.00	18.03	35.13	35.13
Y (Nm3/kg)	0.839	0.838	0.839	0.837
ηen (%)	68.4	74.1	62.5	64.8
ηex (%)	64.6	69.5	62.0	64.1
Enbio (MWLHV)	109.0	109.0	109.0	109.0
Exbio (MWSCE)	123.3	123.3	123.3	123.3
PRE (MWel)	39.1	27.7	206.8	197.2
GSC (kWh/kg)	1.57	1.11	8.27	7.89
NSC (kWh/kg)	0.98	0.75	8.52	8.32
ECT (million CNY)	71.82	70.85	138.42	140.19
ECWE (million CNY)	25.77	19.75	74.96	71.44
H2 (Nm3/h)	9856	6628	48522	45163
UPC (CNY/Nm3)	1.267	1.130	3.879	3.746

Table also shows that the TSB process has higher energy and exergy efficiencies compared with the SNG process. From this perspective, TSB is the better option than SNG. Additionally, the energy and exergy efficiencies of the conventional SNG process with sufficient optimization can reach up to 73.9[5] and 71.8%,[46] respectively, which are approximately 10 and 8 percentage points higher than those of this process for SNG production (64.8 and 64.1%). However, TSB is generated as the final product; this process has comparative energy and exergy efficiencies (74.1 and 69.5%). The comparison of TSB and SNG with conventional SNG demonstrates the advantages of TSB in the aspect of efficiencies. Additionally, the yields of TSB and SNG are around 0.838 Nm3/kg, which are quite greater than that of conventional SNG from wheat straw (0.282–0.334 Nm3/kg[23,26]). The difference can be regarded as the indirect manifestation of stored electricity.

One of the purposes of a PtG process is to store the renewable electricity. Thus, the performance of electricity storage should be concerned. Inspired by the specific energy density of battery (in Wh/kg), we propose the concept of specific capacity of electricity storage for such processes. Because feedstock and product differ greatly from each other in physical and chemical properties, two indicators are defined to evaluate the performances of electricity storage of both biomass and syngas. First, the gross-specific capacity of electricity storage (GSC, in kWh/kg) is defined as the amount of the renewable electricity inputted and converted by a specific biomass and renewable power-to-syngas process relative to 1 kg of the biomass feedstock, and it can be calculated as followswhere mbio is the mass flow rate of biomass, in kg/h. It can be applied to determine how much biomass is required or should be reserved for a renewable electricity-storage plant. In turn, it can be used to determine how much renewable electricity can be stored based on the local biomass resource.

Second, the net specific capacity of electricity storage (NSC, in kWh/kg) is defined as the amount of the equivalent renewable electricity embedded in 1 kg of the biofuel, and it can be calculated as followswhere mSG is the mass flow rate of the product, in kg/h. ExSG is the chemical exergy flow rate of syngas, in MJ/h. Here, SCESG is SCE of the syngas, in MJ/kg. The numerator of eq denotes the renewable electrical power that is veritably and availably embedded in the biofuel. As the energy content of biofuel is converted from that of biomass and renewable electricity, we assume that the proportion of the embedded renewable electricity in the energy content of biofuel is the same as that of renewable electrical power (3.6PRE) in the total energy input (Exbio + 3.6PRE) for the whole conversion process. Among the three thermodynamic parameters of biomass, that is, LHV, HHV and SCE, we suggest using chemical exergy to quantize the energy content. The reason is that chemical exergy represents the maximum work that can be obtained, of which the energy quality is equivalent to that of electricity. This indicator can be used to evaluate or compare the electricity-storage abilities of different biofuels or process configurations.

Obviously, GSC and NSC are closely relevant to the thermochemical properties of the product (i.e., the energy carrier). Table shows that GSC (7.89 kWh/kg) and NSC (8.32 kWh/kg) via SNG are 5–7 times of those via TSB (1.11 and 0.75 kWh/kg, respectively). From this point of view, SNG is a preferred energy carrier due to its larger GSC and NSC. Additionally, GSC and NSC are also related to the process configuration and its efficiency. For example, when the final storage carrier is fixed, either TSB or SNG, one should choose the process configuration with lower GSC and NSC because that process has a higher efficiency and a less energy or exergy loss. However, this result may result in confusion. Here, we recommend using GSC and NSC only to quantify and compare the storage abilities of different types of feedstock or biofuels rather than to measure the process efficiencies. The conversion performances could be thoroughly evaluated by the energy and exergy efficiencies.

Table shows that the NSC via TSB (0.75 kWh/kg) is less than the GSC (1.11 kWh/kg); however, the NSC via SNG (8.32 kWh/kg) is greater than the GSC (7.89 kWh/kg). This is caused by the big difference in the amount of electricity consumed by the WE system. The specific energy density of the lithium-ion battery pack in Telsa electrical vehicles reaches up to 0.160 kWh/kg in 2019.[47] The GSC and NSC via TSB are 6.9 and 4.7 times of the specific energy density of the lithium-ion battery pack, respectively, while those via SNG are up to 49.5 and 52.0 times, respectively. Thus, TSB and SNG are excellent energy carriers for large-scale and long-term electricity storage compared with lithium-ion batteries. With GSC and NSC, we can separately or synthetically evaluate and compare various biofuels and process configurations that play a role in renewable electricity storage in future studies.

Economic Assessment

Costs of TSB and SNG

The equipment costs of the main devices are calculated using the simulated technical data and the equipment values are listed in Table . Take the process with a two-stage gasifier as an example, the scale of WE for SNG production is about 6.8 times of that for TSB. When SNG is chosen as the product, EC of WE is the most expensive equipment. The industry of WE for renewable electricity storage is in the early stage in China and its equipment cost will reduce dramatically in future with the expansion in the industrial scale.

Table 6

Summary of PC and EC for SNG Production with Single-Stage and Two-Stage Gasifiers

		PC		EC2019 (million CNY)
system	benchmark	single-stage	two-stage	single-stage	two-stage
ECG	biomass (t/h)	25	25	15.02	22.53
ECPGCC	PG (t/h)	29.7	28.9	13.60	13.31
ECCM	PG (t/h)	29.7	28.9	7.42	7.29
ECM	syngas (t/h)	34.1	33.0	14.41	14.09
ECHTs	water (t/h)	150.3	155.2	0.84	0.86
ECWE (SNG)	H2 (Nm3/h)	48522	45163	74.96	71.44
ECORC (SNG)	power output (MW)	15.2	12.5	12.15	10.66

The equipment cost of the four cases is summarized in Table . Compared with the process with a two-stage gasifier, the EC of the gasifier of the process with a single-stage gasifier decreases dramatically, while the EC of WE increases obviously and the ECs of other systems increase. As a result, the total equipment cost (ECT) with a single-stage gasifier is slightly greater than that with a two-stage gasifier for the same product, although the single-stage gasifier is much cheaper. The reason is that the single-stage gasifier with higher SR needs a larger scale of electrolysis device. For example, in the case of SNG, the ECWE with single-stage and two gasifiers is 74.96 and 71.44, respectively, in million CNY. The flow rates of PG and syngas with a single-stage gasifier also increase because more oxygen is added into the gasifier, which leads to small increases in equipment cost of the related auxiliary devices (Table ). The abovementioned results indicate that from the perspective of the total equipment cost, the process also should integrate the two-stage gasifier. The comparison between ECWE of TSB and SNG shows that the ECWE of SNG is so large that it is similar to the ECT of TSB. As other devices are relatively mature in technology and cost and the industry of WE for renewable electricity storage in China is very weak now, reducing ECWE is the potentially promising way to reduce the total equipment cost.

The typical selling price of natural gas in Eastern China is about 2.40 CNY/Nm3. It can be regarded as the SNG’s critical cost. If the UPC of SNG is lower than the critical cost, the project is profitable. A survey released in 2018 shows that the prices of biogas in different provinces in China range from 1.01 to 1.48 with an average value of 1.26, in CNY/m3.[48] We assume that the critical cost of TSB is 1.26 CNY/Nm3 in the following discussion. Table shows that the UPCs of SNG (3.746 CNY/Nm3) are dramatically higher than its critical cost (2.40 CNY/Nm3). It implies that the production of SNG is unprofitable at the base scenario. By comparison, the UPCs of TSB (1.130 CNY/Nm3) are lower than its critical cost (1.26 CNY/Nm3), which can make a profit.

The composition of the UPCs are further analyzed to determine the biggest component and improve the cost competitiveness. Figure a indicates that CEL is the biggest component in SNG’s UPC (74.1%), and the following component is Cbio (9.4%). Figure b indicates that the distribution of TSB’s UPC is more even. However, the same result is that CEL and Cbio are the top two components in TSB’s UPC (35.1 and 31.7%, respectively). Thus, the influences of electricity cost and biomass cost are focused on in the following sensitivity analysis. Additionally, the result also shows that the revenue by selling oxygen at a price of 0.2 CNY/m3 has a small impact on the TPC and UPC.

Figure 11

Composition of UPC: (a) SNG and (b) TSB.

Sensitivity Analysis

In order to assess the effect of the most influential variables on UPC, a sensitivity analysis is carried out by changing the variables while the others are held at their base values. Based on the abovementioned results, the sensitivity analysis focuses on the following important variables, plant scale (S), ECWE, CEL, and Cbio. The fluctuation of the variables is specified with the reference data from the similar or related cases.[23,49] It is assumed that in near future, with the rapid development of WE, approximately 50% reduction in the manufacturing sector is achievable.[13] The ranges of these variables are as given below: S = 5–40 t/h (20–160%); ECWE = 50–100%; CEL = 0.1–0.45 CNY/kWh (33.33–150%); and C = -60–450 CNY/t (−20 to 150%). The variations of UPC with these variables are as shown in Figure .

Figure 12

Sensitivity analysis of UPC: (a) SNG and (b) TSB.

Figure a presents the sensitivity results about SNG’s UPC. When the plant scale increases from 20% (5 t/h) to 160% (40 t/h), the UPC decreases rapidly at first and then slowly. The reasonable plant scale should not be less than 60% of the base value (15 t/h). Figure a also shows that the UPC has little sensitivity toward ECEL judging from the slopes of the curves. The UPC proportionally increases with the increase in either CEL or Cbio. CEL is the most influential variable for SNG’s UPC. Nevertheless, it should be noticed that Cbio can be negative values, and the cost competitiveness will dramatically rise under that condition. The UPC of SNG at the base scenario is much higher than its critical cost (2.40 CNY/Nm3) as indicated by the red dashed line in Figure a. The SNG project would be profitable when CEL is less than 50% of its base value (about 0.15 CNY/kWh). In 2019, the basic price of residential electricity is about 0.6 CNY/kWh, while industrial electricity price is greater than 0.8 CNY/kWh. There is a big economic challenge to operate the SNG project at a CEL of 0.15 CNY/kWh.

Figure b shows that the sensitivity results of TSB are similar to those of SNG. Compared with the results of SNG, the UPC of TSB is much less sensitive to CEL, while it shows slightly little sensitivity toward the remaining variables. As indicated by the red dashed lines in Figure b, the UPC of TSB at the base scenario is visibly lower than its critical cost (1.26 CNY/Nm3). TSB as the product is more likely to be profitable when variables fluctuate in the ranges. Unless the plant scale is too small (approximately < 50% of base value, i.e., 12.5 t/h), the UPC of TSB would not exceed its critical cost by varying a variable within the specified range.

Figure focuses on the effects of combined CEL and Cbio on UPC, while other variables are fixed at their base values. The UPC on the red line is constantly 2.40 CNY/Nm3 for SNG (Figure a) or 1.33 CNY/Nm3 for TSB (Figure b). The combinations of CEL and Cbio corresponding to the points in the gray areas can make a profit. The comparison between Figure a,b clearly indicates that TSB has much wider ranges of CEL and Cbio, as well as their combination to gain profit. To sum up, the UPC analysis confirms the advantages of TSB over SNG in the aspect of cost.

Figure 13

Effects of CEL and Cbio on UPC: (a) SNG and (b) TSB.

Conclusions

In this work, a biomass and renewable power-to-syngas process for high-quality syngas production integrating oxygen gasification, water electricity, and methanation without CO2 separation is simulated and investigated from technical and economic aspects. For the first time, the work investigated the process configuration integrating a two-stage gasifier, considered TSB as a new type of product, proposed gross and net specific capacity of electricity storage, and evaluated the unit production costs under the Chinese scenario. The main findings of the current study are presented below:

Compared with the single-stage gasifier, the two-stage gasifier utilizes the high-temperature part of heat in PG more sufficiently and operates with a smaller SR, which results in a smaller scale of the matched WE system. The process configuration with a two-stage gasifier has advantages of about 5 percentage points in energy and exergy efficiencies over that with a single-stage gasifier. It also owns lower total equipment cost and unit production cost mainly contributed by the WE system. Reducing equipment cost of electrolyzer is the potentially promising way to reduce the total equipment cost.

WR is an important parameter significantly affecting both technical and economic indicators of this process. The process generates TSB at a WR of 1 or SNG at a WR around 7. Only with the proper combinations of WR (∼7) and pM (>50 bar), the process can directly generate SNG that meets the Chinese technical standard, which is a valuable reference for other countries with different limiting values. The adaptive pM of the FBMR in this process could be several times higher than that in the conventional SNG production process with CO2 separation.[50]

The process can generate two typical products: TSB and SNG. At the base scenario, TSB has very high energy and exergy efficiencies (≤74.1%) and profitable UPC (1.130 CNY/Nm3), while SNG has relatively low efficiency (≤64.8%) and unprofitable UPC (3.746 CNY/Nm3). TSB is profitable over wider ranges of the sensitivity variables, while SNG can only gain profit with very low biomass cost and/or electricity cost.

Two indicators GSC and NSC are proposed by this work to evaluate the electricity-storage capacities of biomass and biofuels, respectively. SNG only has significant advantages over TSB in capacities of electricity storage. The GSC and NSC via TSB are 6.9 and 4.7 times of the specific energy density of the lithium-ion battery pack, respectively, while those via SNG are up to 49.5 and 52.0 times, respectively. From this perspective, both TSB and SNG are excellent energy carriers for large-scale electricity storage compared with lithium-ion batteries. In future, we can employ the proposed indicators to compare the competitiveness of a biofuel project with that of a battery project for electricity storage.

As both biomass and renewable power are sustainable and relatively clean, this process with a two-stage gasifier is a promising and efficient pathway to generate sustainable gaseous fuels, store renewable power massively, and utilize biomass and wastes. The quantitative assessments on sustainability and environmental impacts deserve further investigation including the comparison between TSB and SNG. Although some pilot-scale two-stage gasifiers had been studied,[18,51] there is still much work that needs to be done for scale up and commercialization. Additionally, other technologies driven by renewable power and having connection with oxygen and/or steam gasification are also worth investigating in future, such as low-power oxygen production techniques. Similarly, two-stage gasifier and alternative technologies driven by renewable power may benefit other power-to-X processes using biomass as a carbon source, for example, bio-methanol, bio-dimethyl ether, and so forth, which are recommended for further efforts.