Gasification of coal and biomass as a net carbon-negative power source for environment-friendly electricity generation in China

Significance Deploying coal-bioenergy gasification systems with carbon capture and storage (CBECCS) provides a promising opportunity for China to realize its carbon mitigation and air pollution abatement goals simultaneously. We conducted a comprehensive assessment of CBECCS technology for China, with a focus on plant and fuel configurations (e.g., biomass ratios) and economics, as well as CO2 and greenhouse gas emissions and cobenefits for air quality. We find significant opportunities for carbon mitigation with air quality cobenefits from deployment of CBECCS systems in regions that are both rich in crop residues and facing urgent needs to curb serious air pollution. The study thus provides critical information for policy makers seeking to exploit the carbon-negative energy opportunities of CBECCS technology.

RGibbs, is used to simulate gasification of the coal-biomass mixture and combustion of H2. The RGibbs module seeks the chemical equilibrium with minimum Gibbs free energy. Before feeding the coal-biomass into the RGibbs block, the fuels are decomposed into elemental forms through the RYield module based on their component yield specifications (13).
The total heat value of coal and biomass mixtures are assumed to be similar among scenarios in the present analysis. Table S1 Of these chemical reactions, equations (1) and (2) represent strongly exothermic reactions, which provide the energy required for the gasification process. Equations (3) and (4) denote the main chemical reactions contributing to the production of syngas in the gasifier, which are strongly endothermic. Equation (5) represents a slightly exothermic reaction, converting CO and H2O to CO2 and H2, which serves as the main reaction in the next step of the Water Gas Shift (WGS).

S1.2 WGS Process
The raw syngas produced from gasification is further processed through the WGS 6 to enrich the contents of H2 and CO2. The raw syngas is cooled to around 200°C by water quenching in order to facilitate the exothermic reaction of Equation (5) (4,7).
The reaction in the WGS converts the CO and H2O in syngas to H2 and CO2. To maintain a high conversion efficiency (>95%) from CO to CO2, the H2O/CO ratio in the WGS is adjusted to a minimum of 2 (2). The reaction temperature is controlled within 250 ℃ and 300 ℃ to ensure the exothermic conversion proceeds forward to produce CO2 and H2. The heat is recovered and channeled for electric power generation in a Heat Recovery Steam Generator system (HRSG).

S1.3 Acid Gases Removal
A two-stage Rectisol ® process is employed for acid gases removal (AGR). Around 80% of the CO2 can be captured, and virtually all the air pollutants including sulfur dioxide (SO2) and carbonyl sulfide (COS), and Hydrogen sulfide (H2S) can be removed (2,4). The syngas from the WGS process is first cooled through heat exchange with the lower-temperature and H2-rich gas treated previously by the AGR unit, and then flows into two-stage absorbers where H2S and COS are first preferentially removed using a methanol solvent. The sulfur-free syngas passes through the second absorber where CO2 is separated using a chilled methanol solvent. A portion of the CO2-loaded solvent is chilled and sent back to the first absorber, and the rest is pumped to a series of flash drums for CO2 regeneration. Through different levels of decompression, the CO2 stream is passed from high-pressure (HP), medium-pressure (MP) and low-pressure (LP) flash drums and the remaining methanol solvent is first chilled and then sent back to the absorbers. Similarly, the sulfur-rich solvent from the first absorber is heated through a series of flash drums to regenerate the H2S and COS, which can be sent to a Claus plant for further processing to produce sulfur (14)(15)(16)(17). The final hydrogen concentration in the syngas treated by AGR is around 90% by volume in a dry gaseous stream.
The CO2 streams regenerated from the flash drums are compressed to a supercritical condition at 150 bar using a multiple-stage, intercooled compressor. Then the compressed CO2 is transported and injected into suitable saline formations or oil fields for sequestration (1). The major factors that influence the energy consumption during the CO2 capture process include recycled use of the chilled methanol solvent and the compression of CO2 for storage. The heat duty of pre-combustion CO2 capture ranges from 0.7 to 1.2 MJ/kg CO2, much lower than that of post-combustion capture

S1.4 Electricity Generation
In the CBECCS system, electricity is generated by a gas turbine combined with the HRSG. The hydrogen-rich gas from the AGR unit is entrained into an M701G2, a state-of-the-art Mitsubishi gas turbine with a maximum output of 370 MW and a centrifugal compressor pressure ratio of 21:1 (2). The isentropic efficiency of the gas turbine is set at 90% (2,4). Before combustion, the hydrogen is diluted with water steam and nitrogen from the ASU unit to control the hydrogen combustion temperature. The combustion system provides around 235% excess air and limits the flame temperature of the combustor to 1400 ℃ for the gas turbine (2). The stoichiometric flame temperature (SFT) in the combustor is maintained at approximately 2300 K (~2030℃) in order not to exceed NOx emission limits without end-of-pipe de-nitrification systems 8 (18). The exhaust gas from the turbine is sent to the HRSG with a capacity of 170 MW for additional generation of electricity. The isentropic efficiency of the steam turbine of the HRSG is set at 85% (2,4). While a considerable portion of the gross electricity generated by the gas turbine and HRSG (about 23.9-24.2%) must be used for onsite ancillary functions, this leaves more than 75% to be delivered into the electric power grid.

S2 Evaluation of the potential and cost for crop-residue biomass in China
To evaluate the potential of crop residues, we first determine the amount of biomass produced annually and estimate then the amount that could be feasibly collected and utilized to produce electricity. and existing published reports (19)(20)(21).

S2.1 Potential for crop residues
Crop residue refers to the biomass left over after harvesting and processing of crops such as corn, rice and wheat, of which the availability was estimated on the basis of agricultural product yield (22). Following the methods reported by earlier studies (23)(24)(25), we evaluated the residue production for each province in China based on the residue to grain index (R/G index). Using the crop production data derived from the NBSC in 2016 (26), we estimated the total dry-basis crop residue production rate at 930 million tonnes per year in China, of which approximately 465-651 million tonnes per year are assumed to be sustainably recoverable for energy use, with an energy value of 6.89-9.65 EJ/year (27)(28)(29). It is noteworthy that the production of crop residues

S2.2 Cost of crop residues
In the present analysis, we assume that each CBECCS system would require 10-24 collection stations to process sufficient crop residues with an average collection radius of 10 km. The prices of crop residues ( ) are determined primarily by collection costs ( ), processing costs ( ) and transportation costs ( ), as follows.

S2.2.1 Collection cost
Following the methods of an earlier study by Zhang et al. (35), the total cost for collecting crop residues, , can be calculated by: With terms defined as follows. We assume the average distance of crop residues from their collection stations, R is 10 km, in which we further assume that the transport distance (d1) is 1.5 times the direct distance (r) to take account of zig-zags of roads between the residue supplies and collection stations; D refers to the density of crop residues; the price of purchasing straw, =150.0 RMB/t; the cost of labor for shipping straw =40.0 RMB/t; the quantity of diesel consumed by a full-loaded vehicle, =0.063 L/(km•t), as summarized in

S2.2.2 Cost of Processing and storage in collection station
The cost of processing and storage, (RMB/t), is calculated by: Here, we adopted the cost parameters from a study by Zhang et al.

S2.2.3 Transportation cost from collection station to CBECCS plants
The cost of delivery of crop residues from the collection stations to CBECCS plants is expressed as (RMB/t): Based on the same study (35), we assume that the distance for transporting crop residues, d2=70 km; the cost of labor for shipping crop residues

S2.2.4 Price of crop residues at CBECCS power plants
The price at the power plant, , is:

S3.1 Goal and scope
The scope for the life-cycle analysis (LCA) includes all operations required for the production of biomass, coal mining and processing, coal and biomass transportation and maintenance, as well as the operation of the CBECCS system. Here we take wheat straw as a representative case for the LCA evaluation of GHG emissions from use of crop residues for energy in China. The production of auxiliary equipment contributes negligible emissions compared to those of other included sources and are not taken into account the present analysis (39). The reference unit selected here is one kWh of generated electricity delivered to the grid. In addition, the R/G index on an energy basis is set as 1.2 following current literature (40).

S3.2.1 Direct and indirect GHG emissions of crop (wheat) production
The major direct GHG emissions associated with wheat production involve CO2 13 emissions associated with the loss of soil organic carbon (SOC) and N2O emissions resulting from application of nitrogen fertilizer. Using an SOC of 377.2 kg/(ha·yr), the average value for the three largest wheat-producing provinces (Hebei, Henan and Shangdong) in China (26,41,42), we estimate the direct CO2 emission factor for wheat production as 0.31 t CO2-eq /t wheat. According to IPCC 2006 Guidelines, the N2O emissions can be evaluated by using a combination of factors including nitrogen fertilizer usage for wheat cultivation (608.6 kg N fertilizer /(ha·yr)) (43), the nitrogen content of nitrogen fertilizer of 300 g/kg, the IPCC default value of conversion factor from nitrogen content to N2O emissions (0.01 kg N2O/kg N) in dry land, together with the wheat production per hectare crop field (43)(44)(45). The N2O emissions associated with wheat planting are estimated at 0.39 g/kg wheat, equivalent to 0.12 t CO2-eq /t wheat.
The direct GHG emissions associated with crop production are summarized in Table   S11.
Indirect emissions refer to emissions resulting from the production of materials and energy required for agricultural activities, which include fertilizers, pesticides, equipment and diesel, as summarized in Table S11 (46)(47)(48). This study mainly considers three major GHGs associated with crop-residue biomass, namely CO2, CH4 and N2O.
For global warming potential on a timescale of 100 years, 1 g of CH4 and 1 g of N2O are equivalent to 25 g and 298 g of CO2, respectively (48,49).

S3.2.2 GHG emissions during collection and transportation of crop residues
Crop residues on farm land would be collected and transported to collection stations, and after initial processing, they would be finally delivered by trucks to 14 CBECCS power plants. The diesel consumption rate of trucks is listed in Table S9, and the associated GHG emissions during collection and transportation of biomass are summarized in Table S10. The present analysis differentiates the GHG emissions from the loaded and unloaded modes of trucks during their round trips.
In addition, biomass is lost (wasted) during collection, storage and transportation and this would incur an increase of pre-combustion emissions per unit delivered biomass. The overall loss rates reported in literature vary from 3% to 10.9% during transportation and storage, relative to the initial dry matter (50,51). In the present analysis, we adopted a loss rate of 5% of the crop residue under transport and storage conditions in the LCA of GHG emissions. As calculated in Table S11, the overall precombustion GHG emissions of crop residue (wheat) would increase to 15.6 g CO2eq/MJ including consideration of the losses.

S3.3 Upstream GHG emissions of coal
The upstream GHG emissions for coal delivered to power plants include not only the CO2 emissions associated with the energy consumed in the mining, extraction and transportation processes, but also the methane released during mining and extraction (52 (39). As calculated in Table S12, a value of 9.5 g CO2-eq/MJ is adopted for pre-combustion GHG emissions for coal in the present analysis, compared to a the range of 7.3 g CO2-eq/MJ to 29.0 g CO2-eq/MJ reported in existing literature (52,56).

S3.4 Combustion GHG emissions
The combustion of crop residues in CBECCS systems is deemed not to contribute additional CO2 emissions, as their organic carbon was derived from CO2 in the atmosphere through photosynthesis in the growing process.
Carbon emissions from coal combustion are calculated per kWh of electricity generation. The data of net generating efficiency, emissions before flue-gas cleaning systems, and the amounts of flue gas for a full load of coal fired power plants are derived from operational reports and feasibility studies of currently operating power plants (39).
As summarized in Table S13, the operational data for IGCC and CBECCS systems used in this study are derived from the Tianjin IGCC power plant (39) . The CO2 emissions per kWh electricity generated by the CBECCS systems vary also as a function of biomass ratios and the associated net efficiency for electricity generation, and are estimated through the Aspen Plus simulation.

S3.5 GHG emissions associated with plant construction
The power plant construction data are collected from the Thermal Power Engineering Design Handbook (39,57). The IGCC construction data are collected from U.S. National Energy Technology Laboratory reports (39,58). Here, we assume that the carbon emissions associated with CCS facility construction are the same as those of the IGCC system on the basis of per capital investment. With assumptions of plant lifetimes of 35 years and capacity factors of 80%, the CO2 emissions associated with plant construction are 0.55 g CO2-eq/kWh, 0.81 g CO2-eq/kWh, 0.88 g CO2-eq/kWh respectively for IGGC, supercritical pulverized coal plants (SC-PC), and CBECCS systems (see Table S13).

S4 Economic analysis
The levelized cost of electricity generation (LCOE) using the CBECCS systems is evaluated using a cash flow model with considerations of overnight capital investment, operational and maintenance (O&M) costs, and expense for fuel (14). The transport and storage (5). The construction period is assumed to be three years, with the first, second and third years allocated respectively with 20%, 60% and 20% of the total capital investments (see Table S14).  (13) and (14): where is the total annual cost; (US$) is the capital cost of the plant; is the discount rate; n (yr) is the operational lifetime; t (yr) is the construction time;

S5 Potential co-benefits to air pollution
The benefits of CBECCS system in reducing the emissions of air pollutants come from two aspects: one is rooted in the reduction of open field (OBB) and domestic (DBB) biomass burning, and the second is associated with the emissions mitigated through the CBECCS process.

S5.1 Biomass burning
Open where refers to the air pollutants emitted from the direct burning of crop residues; is the total crop yield in China (as discussed in section S2.1 on biomass distribution); C is the R/G index for crops residues (Table S7) Table S15. When the crop residues are used in CBECCS systems, air pollutant emissions from OBB discussed above would be avoided.
In addition, the demand for crop residues of CBECCS system is expected to reduce the burning for household use (i.e., DBB), contributing to an additional reduction in air pollutants should crop residues be replaced by clean energy. Here, the burning ratio of DBB is chosen as 40%.

S5.2 CBECCS system
In contrast to traditional pulverized coal-fired (PC) power plants, nearly all of the particulates, mercury, and compounds containing nitrogen or sulfur can be removed from syngas before combustion in CBECCS systems, offering an effective way to reduce emissions of air pollutants (14). As a result, per kWh emissions of SO2, NOx, PM2.5 and black carbon (BC) from a CBECCS plant are significantly lower compared to those of traditional PC plants (Table S16). The emission reduction in air pollutant k by CBECCS system ( ) can be calculated as follows: where (g/kWh) and  Table S18), namely Huabei, Dongbei, Yuwan, Ordos, Jianghan-Dongting, Sichuan and Xinjiang basins (see Table S18). Huabei and Yuwan basins, covering the same Hebei, Henan, Shandong and Anhui provinces that produce a lot of crop residues, have abundant sequestration capacities of CO2, which are estimated at 264 Gt and 186 Gt.
Given these capacities, the deployment of 116 CBECCS power plants in the proposed 21 provinces theoretically would not face limits of carbon storage capacity for more than 2000 years.
The highest pollution emissions occur in the eastern part of China, particularly in the North China Plain and the Yangtze River Delta (71). As shown in Table S17,            Table S11 GHG emissions in agricultural processes and transportation.