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How Sustainable is Power Generation in China? A Joint Application of Energy, Emergy and Carbon Accounting

Bo Lou, Sergio Ulgiati, Chaofan Sun

ABSTRACT

The performance of a power generation process is most often evaluated in terms of energy efficiency and emissions. Emissions generated by coal-fired power generation are among the main sources of pollution (CO2, dust and acid gases) in China and other coal-intensive countries worldwide. However, this is only the downstream aspect of a coal plant performance, where focus is most often placed on GHG emissions and carbon footprint. The upstream side, related to the environmental quality of input resources is also of paramount importance and can be dealt with by means of emergy accounting methods to generate environmental footprint indicators. We evaluate in this study both upstream and downstream aspects of a coal-fired power plant, jointly applying energy and emergy methods to a case study of a 2 x 600 MW supercritical power generation plant in China. The investigated power generation system is equipped with dedusting and desulfurization systems, in order to capture most of the pollutants generated by the combustion process. The services needed to abate the remaining fraction of released chemicals are assessed and used for the calculation of energy and emergy performance indicators, with and without including costs and benefits from the dedusting and desulfurization systems. The need for buffer land around the plant to compensate both the CO2 emissions (in the order of 850 g CO2/kwh) and the intensity of the emergy loading is discussed in the paper. A sustainability calculation procedure is developed accordingly.

INTRODUCTION

Use of Coal for Energy in China and Worldwide

Economic growth in China and worldwide requires a continuous input of energy, thus placing a huge load on the still untapped energy resources. Worldwide total primary energy supply (TPES) doubled from 1971 to 2009: about 81% of TPES came from fossil sources while only 19% came from non-fossil ones (IEA, 2011). A large fraction of energy uses in all sectors of national economies is increasingly in the form of electricity, a very fundamental energy carrier. As a consequence, coal still plays an important role in worldwide electricity generation in spite of the huge concerns about atmospheric emissions and contribute to global warming. Coal-fired power plants currently provide 41% of global electricity. In some countries, coal fuels are the dominant source of electricity generation (South Africa, 93%, Poland 92%, Australia 77%, USA 49%, Germany 46%, among others) (WCA, 2010). China is also a major coal user for electricity generation (72.2%, out of a total installed power capacity of 9.6 × 108 kW as of September 2010, and still increasing), due to its large indigenous coal reserves (CEC, 2011). In the year 2010, the Chinese coal consumption was about 1.71 billion ton oil equivalent (BP, 2011), which accounts for about 48% of total coal use worldwide. The verified reserves of Chinese oil and natural gas are instead relatively small (respectively 4.3 and 0.1 billion toe, according to BP 2011), which explains why China is using coal so intensively (WPE, 2012). Economic

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reasons mainly relying on the lower cost of coal compared to oil and gas, strategic reasons based on the fact that coal is a domestic resource for China, and finally industrial reasons linked to the need to support the huge energy demand of the productive sectors, push China to keep a very intensive new coal power plant construction schedule, which is expected to continue in the future, in spite of the fact that China‟s coal was predicted to peak around 2020 by the Energy Watch Group (Zittel et al., 2007). In recent years, the economic development of China and high prices in the world energy market have stimulated further coal use together with the gradual dismission of small coal-fired power units with capacity less than 200 MW in China, so that big and supercritical power generation units (600 MW or more) will be the trend. Yu et al. (2011) investigated the resource consumption and environmental emissions of China‟s coal-fired electricity industry based on three different scenarios of technology innovation in 2007–2030. According to the study, super-critical (SC) and ultra-supercritical (USC) pressure boilers, flue gas desulfurization (FGD) and closed-cycle wet cooling with a high circulation ratio will be the mainstream technologies before 2030, gradually replacing small power plants from the late 2010s. Integrated gasification combined cycle (IGCC) and pressurized fluidized bed combustion combined cycle (PFBC-CC) plants will show a competitive advantage in the late 2020s. Although innovation was proven to be a fundamental step for resource use and environmental performance improvement, these scenarios question the possibility to achieve, based on current policies, the coal consumption intensity control target. Results suggest that air cooling and FGD systems will expand slower than expected, unless higher water prices, SO2 charges and other environmental restrictions are implemented.

Environmental Problems with Coal Use

In the year 2009, CO2 emissions accounted for about 28.9 Gt, out of which 43.2% from coal, 36.8% from oil, 19.9% from natural gas (IEA, 2011). In the year 1990, these shares were respectively 39.7%, 42.1% and 18.2%, thus showing that coal is now the major driver of CO2 emission increase. Moreover, with reference to the year 1990, global 2009 CO2 emissions were 38.3% higher. The share of 2009 CO2 emissions in China was 83.6% from coal, 13.9% from oil and 2.4% from natural gas. Concerning electricity, one kWh of electricity from coal/peat is responsible for about 901 g CO2 emissions (world average 2007-2009); it is 666 g CO2 emissions from oil and 390 g CO2 emissions from natural gas. In the same period 2007-09, electricity related emissions in China were 898, 572, 422 g CO2/kWh respectively from coal/peat, oil and natural gas. CO2 emissions from combustion are not the only environmental problem related to fossil fuels, and coal in particular. use is responsible of a variety of upstream (extraction and processing) and downstream (burning) environmental burdens, depending on fuel composition, extraction processes, plant typology and efficiency, fuel treatment before burning (EEA, 2008; IEA, 2010). Previous LCA studies by Spath et al. (1999) on different typologies of coal-fired plant (a plant meeting New Source Performance Standards, NSPS, and an advanced plant equipped with a low emission boiler system (LEBS), compared to an average U.S. coal-fired power plant in the year of investigation), highlighted that, apart from the CO2 produced during coal combustion (coal-CO2), large amounts of non-coal CO2 were released by operations related to flue gas clean-up. In particular, limestone production, transportation, and use accounted for about 60% of the total non-coal CO2 emissions, more than twice the CO2 emissions related to transportation of the coal (40% of non-coal CO2), in the average and NSPS systems. In the LEBS system, emissions were decreased by means of the use of a copper oxide sorbent instead of limestone in flue gas clean-up. However, operations associated with the production and use of natural gas to regenerate the CuO sorbent were still responsible for 35% of the total non-coal CO2 emissions. Other major airborne emissions from the system were particulates (mainly from limestone production), SOx, NOx, and CO (mainly from power plant) and CH4 (mainly from coal mine). Babbit and Lindner (2005) performed an LCA study of coal-fired electricity production in Florida, including coal mining, processing and transportation as well as disposal of coal burning waste. About 78% of the air emissions were attributed to carbon dioxide from coal combustion, while coal mining

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and preparation were responsible for large amounts of non-methane volatile organic compounds and methane (over 98%) as well as the most total dissolved solids to water (over 76%). Finally, disposal of combustion products accounted for significant amounts of particulate matter (PM10) to air, total dissolved solids to water, and metals to land. As a consequence, these authors point out that “emissions from the Raw Material Extraction and Material Disposal stages may result in a significant enough negative environmental impact to warrant pursuit of pollution control and prevention opportunities in these stages as well”. More recent LCA studies about coal-fired electricity can be found in the Ecoinvent database (2011), confirming that coal CO2 is always the largest fraction of emissions and that non-coal CO2 and other emissions are also important sources of pollution, especially in the atmosphere.

Setting the Goal of this Paper

Most studies about coal electricity focus on energy conversion efficiency and, from an environmental point of view, on the need to limit the concentration of pollutants released in the generation processes. Small focus has been placed on the relationship between energy conservation and environmental problems, and even less attention was given to the relationship between the sustainability of the economy and the upstream environmental quality of the fossil resource, i.e. the work done by nature to provide the resource itself (emergy method, Odum 1996). What is needed is an integrated approach capable to evaluate a process from both points of view complementing to each other, namely a “user-side” assessment that looks at final efficiency indicators (energy delivered per unit of energy input, and emissions per unit of energy delivered) and a “donor-side” assessment, that considers the work done by nature in providing resources as an important component of sustainability. In order to achieve such integration, we will investigate coal-fired electricity production in China by jointly applying energy, carbon and emergy accounting methods to a modern coal power plant. The procedure will provide a set of performance and sustainability indicators that can be used for evaluation and comparison of Chinese coal power plants as well as any other fossil-fired plant worldwide. The main emissions from coal combustion will be calculated and used to assess the environmental costs of their dilution via the emergy method, according to a dilution volume procedure developed by Ulgiati and Brown (Ulgiati, S. and Brown, M.T., 2002 ). CO2 emissions will be used to calculate the “buffer” area needed for their uptake via photosynthesis, in so generating a carbon/land constraint to coal-fired electricity. Finally, an emergy based indicator, the Emergy Sustainability Index (ESI) will be used to place an additional sustainability constraint, by requiring the ESI of the plant to be not-less-than the ESI of the Chinese economy as a whole (Bo and Ulgiati, 2012), in order to be considered an actual improvement. These two additional constraints, “buffer land” and “plant/economy matching”, shed light on the sustainability of coal-fired electricity in China.

MATERIALS AND METHODS

Description of the Investigated Plant, Coal Composition and Heating Value

The investigated plant is a 5 billion yuan RMB 2×600MW supercritical power generation unit under construction in Guangdong (China), to be fueled by means of Chinese ShenFu bituminous coal. The plant uses electrostatic precipitators to capture and remove coal ash to be used as additive material in downstream cement production. Gas after dedusting is sent to desulfurization devices, based on limestone–gypsum wet flue gas process, to yield a byproduct calcium sulfate dihydrate (plaster) useful as construction material. The calculation procedures of some data and parameters used are listed in the back of the paper.

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Energy, Emergy and Carbon Accounting

Embodied energy accounting

The commercial energy invested into the whole chain of processes that lead from extraction and processing of raw materials and resources to the final product (e.g.: coal-fired electricity) via coal extraction, refining, transport and burning, is calculated by firstly performing an inventory of all input energy and matter flows to the mining and industrial processes. Input flows are then multiplied by suitable energy conversion coefficients which express the unit energy demand „„embodied‟‟ in each flow. Such coefficients are available in published and life cycle assessment literature (Herendeen, 1998; Ecoinvent, 2011). The embodied energy assigned to each input flow (i.e. the commercial energy used up to generate that flow) is calculated according to the following equation:

E = ∑ fi * ei i = 1,..., n Eqn. (1)

th where E is the embodied energy, fi the i input flow of matter or energy and ei the embodied energy coefficient of the ith flow (from literature or calculated in this work). Summing up the embodied energy values of all input flows we calculate the total energy invested into the process, i.e. the total energy cost of producing bioethanol through energy cropping. The boundary of the system can be traced around the final process (power plant, engine, etc) or can be expanded to include the upstream processes for material extraction and refining. The largest possible boundary is placed at the interface of the process with the sources of commercial energy, namely oil, gas and coal reservoirs. In general, the embodied energy method does not include the energy flows received for free from nature without human intervention (e.g.: solar, wind and ).

Emergy accounting

Odum (1988, 1996) introduced the concept of emergy as an expansion of the embodied energy concept over time and quality. Emergy is defined as “the available energy of one kind (generally of the solar type) previously used directly or indirectly to generate a service or a product”. It measures (and compares) the performance and functions of systems based on a common energy metrics, expressed as solar equivalent joules (seJ). The emergy required to generate a unit of output (be it energy, matter, services, currency) is named Unit Emergy Value (UEV) and measured as seJ/unit (seJ/J, seJ/g, seJ/hr, seJ/$). The emergy definition includes both natural processes for resource generation over time and human-dominated activities for resource extraction, manufacturing and delivery. As a consequence, the time involved in resource generation becomes an important parameter for their quality evaluation. The emergy accounting method faces a boundary expansion over time (processes of resource generation) and also over resource category, in that it also includes natural flows (sun, wind, rain, deep heat, gravitational energy) and material flows (mineral ores, metals) that are not explicitly accounted for in the embodied energy method. The main steps of an emergy evaluation of a process include the following: 1) identification of the boundaries of the system or process and drawing of a systems diagram; 2) identification and quantification of the matter, energy and money flows that support the process, including those provided for free by the environment; 3) conversion of the different flows into emergy units by means of suitable conversion factors (UEVs, Unit Emergy Values) with reference – in this paper - to the emergy baseline of 15.83E+24 sej/year (Odum et al., 2000)1; such conversion, the core of the Emergy Accounting approach, is performed by means of the following emergy equation:

Em = ∑ fi * UEVi i = 1, . . . , n Eqn. (2) th where Em is the total solar eMergy supporting the system, fi the i input flow of matter or energy and th UEVi is the Unit Emergy Value of the i flow (from literature or calculated in this work); the calculation procedures according to Equation (2) are generally grouped in a summary Table. 4) Calculation of total emergy U and other performance indicators, among which:

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. EYR, the Emergy Yield Ratio, a measure of the ability of the process to exploit local resources thanks to investments from outside; . ELR, the Environmental Loading Ratio, a measure of the pressure of local and imported nonrenewable investments on local renewable sources; . ESI, the Emergy Sustainability Index, calculated as EYR/ELR, an aggregated measure of benefit and environmental sustainability. Further details on the emergy method and emergy-based indicators can be found in Brown and Ulgiati (2004), Brown (2010) and Ulgiati and Brown (2012). Recent studies that apply the emergy accounting method specifically to power plants are also available (Wang et al., 2006), dealing with power plants in eco-integrated production parks; Häyhä et al. (2011), dealing with electricity generation at national level; Buonocore et al. (2012), dealing with power generation from waste biomass; among others) and can usefully serve as references for further improvement.

Carbon accounting

Activities and processes involving reactions between carbon and oxygen (soil organic matter degradation, fossil fuel burning, metabolism, selected inorganic reactions) release carbon dioxide, thus contributing to its increase in the atmosphere and to global warming and climate change. Fossil fuel based activities (transportation, electricity generation, space heating) are among the most important contributors to CO2 emissions. As a consequence, it may be useful to assess their “carbon footprint”, namely the total amount of carbon or CO2 released in a process or per unit of product or service, in order to implement improvement strategies capable to decrease emissions and global warming potential. In a like manner as with embodied energy and emergy accounting, carbon emissions can be assessed by means of a calculation procedure similar to Equations (1) and (2): MC = ∑ fi * ci i = 1,..., n Eqn. (3) th where MC is the mass of carbon (or CO2) released, fi is the i input flow of matter or energy and ci is the mass of carbon (or carbon dioxide) released for the generation of the flow fi. Similar procedures are implemented for other kinds of airborne and waterborne emissions (heat, NOx, CO, SOx, CH4), in order to be able to calculate the environmental emergy demand for their dilution and abatement. In particular, we calculated the main heat and non-CO2 emissions on a yearly basis; then we estimated the volume of air or water needed for their cooling or dilution down to the biosphere background level (or, at least, down to the threshold required by the enforced laws), by dividing the total amount of emission by this threshold value. The volumes of air or water are multiplied by their average density and converted to mass units; then, their kinetic or chemical energies are calculated and finally converted to emergy by multiplying by suitable UEVs from literature, according to Ulgiati and Brown (2002)

RESULTS

The systems diagram of the investigated coal-fired plant is shown in Figure 1, where the main input and output flows, components and processes are identified. Input flows are ordered from left to right, clockwise, in order of increasing UEV. Local renewable flows provided for free by nature enter from the left, while products exit to the right. The larger frame identifies the system‟s boundary, placed around the plant and also including some land around, directly occupied by plant‟s facilities or indirectly needed as buffer land. Depending on choices about the size of buffer area, more renewable emergy will have to be included in (direct and indirect) support to the plant and its dynamics. Coal can be included as a local reservoir or an imported flow (as in Figure 1), while all the other flows of energy and materials used in the process are considered as imported from outside the boundary. Coal being “local” means that the plant is located not far from the coal mine, which is not unusual. Coal “imported” means that some transport costs must also be included. Choices about coal (local or imported) affect the indicators in

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Figure 1. Systems diagrams of a coal-fired power plant, showing renewable and nonrenewable input flows, components and sub-processes within the boundary (systems symbols from Odum, 1996). many ways. If coal is local, the EYR and ESI increase, while the opposite is true if coal is imported. When the sustainability constraint ESIplant ≥ ESIeconomy is enforced, also the demand for buffer land is affected, and as a consequence, all the other indicators (ELR, UEV, ED, etc). Of course, energy and carbon indicators are not affected by such choice. We have calculated these indicators for both imported and local coal, for the cases with and without the extraction of co-products ash and plaster, and finally by accounting for the services for dilution or not. The application of Equation 2 to the process starts with the input energy and matter flows listed in Table 1, where each flow is converted to its associated emergy by means of suitable UEVs from literature. All input flows in Table 1 were derived by CEC (2011) and integrated by means of technical calculations from Section 2.1 and governmental environmental databases (WPE, 2012). Items 1 to 5 are material and energy flows related to plant construction, where all inputs have been divided by 30 years of expected plant lifetime. Item 6 is the flow of labor and services needed for construction, converted to emergy by means of the emergy per capita and the emergy/RMB ratios of China (Bo and Ulgiati, 2012). Items 7 to 15 are the main annual input flows to plant operation, including labor and services. In particular, items 9 to 11, ecosystem services for heat and chemical emissions dilution, were treated as described in Section 2.2.3. Items 16 to 18 and items 19 to 23 are additional inputs to ash and sulfur removal respectively. Finally, items 24 to 26 refer to electricity, ash and sulfur products flows. The total emergy for plant construction, power operations, ash and sulfur removal is assigned to the electricity, because all of these are necessary inputs to the final electricity output (electricity generated minus electricity invested for ash and sulfur removal). The calculated UEVs are therefore 3.22E+05 and 2.28E+05 seJ/J with and without L&S respectively. Instead, the emergy assigned to ash and sulfur product flows was calculated as the emergy for removal processes plus a fraction of total operational emergy proportional to the amount of ash and sulfur in coal supplied. These smaller values are used for the calculation of ash and sulfur UEVs with and without L&S.

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Table 1. Emergy account of coal-fired electricity production in China, with ash and sulfur removal(*). Solar Ref. Solar Raw # Item Unit/yr UEV for Emergy amount (sej/unit) UEV (seJ) PLANT CONSTRUCTION PHASE (all input flows divided by estimated plant lifetime, 30 years) 1 Concrete g 1.62E+10 8.53E+08 {1} 1.38E+19 2 Iron and steel for structure g 2.17E+09 4.65E+09 {1} 1.01E+19 3 Insulating materials (plastics and rockwool) g 1.00E+07 9.83E+09 {1} 9.83E+16 4 Copper electric wires g 9.35E+07 9.80E+10 {2} 9.16E+18 5 Petroleum derived fuels and lube oils g 8.69E+13 1.11E+05 {3} 9.64E+18 6 Labor and services for whole plant construction RMB 1.67E+08 9.95E+11 {4} 1.66E+20 PLANT OPERATING PHASE Locally available environmental inputs 7 Solar radiation J 6.26E+19 1.00E+00 {3} 6.26E+19 8 Rain J 2.80E+16 3.05E+04 {3} 8.55E+20 9 Cooling service at condenser (sea water) J 2.84E+16 5.20E+03 {3} 1.48E+20 Indirect environmental inputs from outside the area 10 Cooling service at chimney (heat dilution by wind) J 4.75E+12 2.52E+03 {3} 1.20E+16 11 Dispersal of released chemicals (dilution by wind) J 2.03E+17 2.52E+03 {3} 5.12E+20 Nonrenewable inputs 12 Coal J 5.52E+16 6.63E+04 {5} 3.66E+21 Labor and Services for operational phase 13 Labor Graduated years 6.00E+01 6.57E+16 {4} 3.94E+18 Technical and administrative years 4.00E+01 4.38E+16 {4} 1.75E+18 Unskilled labor years 4.00E+01 2.19E+16 {4} 8.76E+17 14 Labor for plant maintenance years 1.00E+02 4.38E+16 {4} 4.38E+18 15 Services for fuel supply RMB 1.97E+09 9.95E+11 {4} 1.96E+21 De-dusting (ash removal) after combustion 16 Electricity consumption (from plant) J 3.25E+12 3.22E+05 {6} 1.05E+18 17 Steel for structure g 6.48E+09 4.65E+09 {1} 3.02E+19 18 Services RMB 5.17E+05 9.95E+11 {4} 5.15E+17 Desulfurization (sulfur removal from 0.41% to 0.041% for conversion to plaster) 19 Limestone g 3.60E+10 9.50E+09 {7} 3.42E+20 20 Electricity consumption (from plant) J 1.85E+14 3.22E+05 {6} 5.96E+19 21 Steel for structure g 1.97E+08 4.65E+09 {1} 9.15E+17 22 Water (from underground reservoir) J 8.25E+11 5.00E+04 {8} 4.13E+16 23 Services RMB 3.30E+06 9.95E+11 {4} 3.28E+18 Net electricity production 24a Annual net electricity production (with L&S) J 2.36E+16 3.22E+05 {6} 7.58E+21 24b Annual net electricity production (without L&S) J 2.36E+16 2.28E+05 {6} 5.38E+21 25a Ash to cement production (with L&S) g 3.42E+11 2.31E+09 {6} 7.91E+20 25b Ash to cement production (without L&S) g 3.42E+11 1.54E+09 {6} 5.25E+20 26a Sulfur to plaster production (with L&S) g 5.58E+10 9.50E+09 {6} 5.30E+20 26b Sulfur to plaster production (without L&S) g 5.58E+10 8.66E+09 {6} 4.83E+20 (*) (data on a yearly basis; 1200 MW power plant, sited in Guang Dong (China). References for transformities {1} Brown and Buranakarn, 2003. {5} Brown et al., 2011. {2} Cohen et al., 2006. {6} This work, from calculations. {3} Odum, 1996. {7} Odum, 2000. {4} Bo and Ulgiati, 2012. {8} Buenfil, 2001.

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Table 2. Performance indices based on energy and carbon accounting, based on input flows from Table 1. Total electrical energy produced per year 2.36E+16 J/yr Total energy invested per year (fuels included) 5.82E+16 J/yr

CO2 released 5.62E+12 G CO2/yr C released 1.53E+12 G C/yr

Dry biomass equivalent to photosynthetic uptake of CO2 3.40E+12 G dry biomass/yr NPP (average) in the area 3.00 T NPP (d.m.)/ha

Set-aside area needed to compensate CO2 emissions 1.13E+06 ha CO2 released/electricity produced 857.71 G CO2/kwh Energy ratio (out/in) 0.41

Table 1 refers to the case of local coal use, ash and sulfur removal, and further dilution of remaining emissions by ecosystem services. Similar procedures from implementation of Equations (1) and (3) provide results listed in Table 2. After calculating the CO2 emissions from coal combustion according to the procedure indicated in Section 2.1, the biomass corresponding to its full photosynthetic uptake is calculated accordingly. Based on an average value of Net Primary Production (NPP) in the region, the set-aside area needed for biomass growth is estimated. Finally, the CO2 emissions per kWh and the Energy Output/Input ratio are calculated. It is important to point out that values in Table 2 are based on nominal 2 x 600 MW power and a total calculated output of 2.36E+16 J/yr. A comparison of the different results depending on the choices about coal (local or imported), about extraction of co-products, and about inclusion of ecosystem services is shown in Tables 3 to 6. All Tables show how results are affected by the inclusion of co-products and ecosystem services for dilution/abatement of emissions. Buffer areas in Tables 3 and 5 are calculated based on set-aside area needed for photosynthetic uptake of CO2, while buffer areas consistent with ESI constraints (ESIplant ≥ ESIeconomy) are considered in Tables 4 and 6. Moreover, Tables 3 and 4 are calculated considering coal as a local resource (extracted within the boundary of the system), whereas Tables 5 and 6 are based on the assumption that coal is imported. Results in Tables 3 to 6 must be properly interpreted. First of all, let‟s point out that the indicators based on the assumption of local coal always have higher EYR and therefore higher ESI (Table 3 versus Table 5 and Table 4 versus Table 6). Instead, their ELR and CO2 emissions per kWh are not affected. Let‟s then consider only Tables 3 and 5. Options A and B are presented, where ecosystem services for dilution of emissions are not accounted for. This is the “lower bound” case, when too many sources of combustion are concentrated in the same area and the ecosystem services available in the surrounding region are not sufficient to abate or dilute the heat and the emissions. Therefore, the emergy of ecosystem services was not included in the calculation procedure of the indicators. Instead, options C and D refer to the ideal case in which the investigated plant is the only source of pollution in the area and therefore the available ecosystem services are effective in the abatement or dilution of emissions. If ash and sulfur are removed before flue gases are released, residual emissions are less and less ecosystem services are needed (Option C versus option D in Tables 3 and 5). As a consequence, option C is always more sustainable than option D (ESIC > ESID), since less emissions need to be treated. When emergy costs of dilution are not accounted for (options A and B), the advantage of less emissions to be treated does not emerge in the calculation of indicators; as a consequence, option A seems more sustainable than option B in Tables 3 and 5, due to the small resource investment needed in option B for ash and sulfur removal, not compensated by the smaller demand for ecosystem services as in option C.

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Table 3. Indicators of coal-fired electricity generation in China, with and without accounting for co- products and ecosystem services, assuming coal as a local resource. B. C. D. A. Electricity Electricity & Co- Electricity & Electricity Indicator Note (without products (without Co-products (with dilution) dilution) (with dilution) dilution) with L&S 2.81E+05 3.00E+05 3.22E+05 3.22E+05 UEV electricity (seJ/J) without L&S 1.91E+05 2.09E+05 2.31E+05 2.31E+05 with L&S 6.68E+21 7.07E+21 7.58E+21 7.58E+21 U (seJ/yr) without L&S 4.54E+21 4.92E+21 5.44E+21 5.44E+21 EYR 3.09 2.77 2.47 2.02 ELR 6.83 7.27 7.87 10.33 ESI= EYR/ELR 0.45 0.38 0.31 0.20 ED (seJ/m2) 5.90E+11 6.23E+11 6.68E+11 7.78E+11 Buffer land radius for photosynthetic 42.46 42.50 42.50 42.46 CO2 uptake (km) CO2 emissions (g/kWh) 849.22 857.71 857.71 849.22

Table 4. Indicators of coal-fired electricity generation in China, under an emergy-based sustainability constraint (ESIplant ≥ ESIeconomy), considering coal as a local resource. A. B. C. D. Electricity Electricity & Co- Electricity & Electricity Indicator Note (without products (without Co-products (with dilution) dilution) (with dilution) dilution) with L&S 2.83E+05 3.07E+05 3.33E+05 4.09E+05 UEV electricity (seJ/J) without L&S 1.93E+05 2.16E+05 2.42E+05 3.19E+05 with L&S 6.72E+21 7.25E+21 7.86E+21 9.72E+21 Total emergy, U (seJ/yr) without L&S 4.58E+21 5.10E+21 5.71E+21 7.58E+21 EYR 3.10 2.81 2.59 2.22 ELR 6.56 6.02 5.51 4.77 ESI= EYR/ELR 0.47 0.47 0.47 0.47 ED (seJ/m2) 6.59E+11 5.29E+11 4.91E+11 3.97E+11 Buffer land radius (km) for photosynthetic 43.35 45.15 50.79 62.46 CO2 uptake CO2 g/Kwh 849.22 857.71 857.71 849.22

Let‟s now consider Tables 4 (coal local) and 6 (coal imported). In these Tables we have assumed a sustainability constraint that equates the ESI of the plant to the ESI of the Chinese economy (ESIChina= 0.47), in so imposing that the plant at least does not worsen the global sustainability of the country. In the calculation procedures, this is obtained by increasing the buffer land around the plant. In so doing, more renewable emergy is assigned to the plant surroundings in the form of additional set-aside land and therefore higher EYRs and lower ELRs are obtained until the ESI becomes equal to the chosen value. In Tables 4 and 6 this choice translates into a larger radius of the surrounding land assigned to the plant (i.e. not used for any other productive installation). This buffer land is much higher than the one only based on photosynthetic CO2 uptake as in Tables 3 and 5, because of the fact that emergy based indicators are more comprehensive than just carbon indicators and account for the totality of ecosystem services, not only for CO2 uptake.

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Table 5. Indicators of coal-fired electricity generation in China, with and without accounting for co- products and ecosystem services, considering coal as imported. A. B. C. D. Electricity Electricity & Co- Electricity & Electricity Indicator Note (without products (without Co-products (with dilution) dilution) (with dilution) dilution) with L&S 2.81E+05 3.00E+05 3.22E+05 3.22E+05 UEV electricity (seJ/J) without L&S 1.91E+05 2.09E+05 2.31E+05 2.31E+05 with L&S 6.68E+21 7.07E+21 7.58E+21 7.58E+21 U (seJ/yr) without L&S 4.54E+21 4.92E+21 5.44E+21 5.44E+21 EYR 1.15 1.14 1.13 1.10 ELR 6.83 7.27 7.87 10.33 ESI= EYR/ELR 0.17 0.16 0.14 0.11 ED (seJ/m2) 5.90E+11 6.23E+11 6.68E+11 7.78E+11 Buffer land radius (km) for 42.46 42.50 42.50 42.46 photosynthetic CO2 uptake CO2 emissions (g/kWh) 849.22 857.71 857.71 849.22

Table 6. Indicators of coal-fired electricity generation in China, under an emergy-based sustainability constraint (ESIplant ≥ ESIeconomy), considering coal as imported. A. B. C. D. Electricity Electricity & Co- Electricity & Electricity Indicator Note (without products (without Co-products (with dilution) dilution) (with dilution) dilution) with L&S 3.31E+05 3.51E+05 3.79E+05 4.57E+05 UEV electricity (seJ/J) without L&S 2.41E+05 2.60E+05 2.88E+05 3.67E+05 with L&S 7.86E+21 8.26E+21 8.93E+21 1.09E+22 Total emergy, U (seJ/yr) without L&S 5.72E+21 6.12E+21 6.78E+21 8.71E+21 EYR 1.35 1.33 1.35 1.35 ELR 2.87 2.83 2.88 2.85 ESI= EYR/ELR 0.47 0.47 0.47 0.47 ED (seJ/m2) 2.91E+11 3.04E+11 2.92E+11 2.65E+11 Buffer land radius (km) for 65.57 65.81 70.26 80.80 photosynthetic CO2 uptake CO2 g/Kwh 849.22 857.71 857.71 849.22

DISCUSSION

The question if a fossil-fueled power plant is sustainable is a trivial one. Reliance on nonrenewable and polluting sources of energy cannot be a suitable sustainability requisite. However, we may at least explore the conditions that would make such energy generation more sustainable than previous patterns and society in general. First of all, can we consider the investigated plant representative of electricity generation in China? The likely answer is yes. In fact, coal supports 72% of total Chinese electricity generation; moreover, the plant energy and carbon performances calculated in Table 2 are very similar to the average values in China (IEA, 2010; IEA, 2011). An important component of sustainability, beyond the absolute nonrenewability of the fuel, is the level of process emissions and the energy and environmental costs of their uptake or dilution. Concerning costs, we focused on CO2 photosynthetic uptake, on ash and sulfur removal from flue gases in order to minimize emissions, and finally on dilution of remaining emissions by ecosystem

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services. Assuming that released CO2 can be absorbed by dedicated tree plantations in set-aside areas translates into a high buffer land demand, void of any other sources of combustion. The portion of land to be set-aside is quantified as a virtual circular area around the plant, with a radius of about 42 km, equivalent to 1.1 million ha. The different alternatives about ash and sulfur removal and about final dilution of remaining emissions do not affect the buffer area to a significant extent (Table 3). The assumption made in Table 5, namely coal imported from a relatively small distance instead of being local, does not affect the calculated radius, being coal transport energy only a very small fraction of total energy in coal (more or less in the order of 5%). Setting aside 1.1 million ha to be converted to tree plantation is not a minor constraint and needs to be carefully planned in order not to prevent fertile land use cropping for food as well as in order to provide additional living and business opportunities at low energy intensity. Considering the large number of coal-fired power plants in China, it is unlikely that this can be a suitable solution in the long run, but could become at least a partial solution during the much needed transition to carbon-free power for China. Ash and sulfur removal cannot be avoided, in spite of their technological and energy costs, until coal-fired electricity remains the main power alternative in China. If an integrated industrial system is designed, centered around an operating coal power plant, it might provide valuable input materials for the construction and chemical industries (cement and sulfur compounds), in addition to the potential use of the discarded heat from cooling devices. Distance is, in this case, a limiting constraint: ash, sulfur and heat cannot be transported to long distances, not to face huge energy expenses for transportation. The dilution of the remaining emissions from plant operations, namely heat, SO2, NOx, CH4, unburnt hydrocarbons, CO, residual ash and particulate (PM2.5 and PM10) places an additional constraint related to the availability of ecosystem services and their emergy cost. As shown in Table 1, items 9 to 11, more emergy is needed in the form of renewable ecosystem services “imported” from the surrounding area, thus decreasing the EYR and increasing the ELR of the process. If ecosystem services are “used” to support one process, they are no longer available for other nearby processes (e.g. further cooling of another heat emission), which places a constraint on the number of emission sources that can be supported by a given area. Therefore, only a small number of high-emission processes can be located in a region, not to overload its carrying capacity. Once the emergy of ecosystem services locally available is known, the number of acceptable pollution sources can be easily calculated. How telling are Tables 3 and 5 in this regard? The most sustainable solution, within the framework of coal-fired power generation, is the one capable to generate electricity, ash and sulfur (and residual heat) as co-products, by also allowing the needed dilution of all emissions. This is not an easy solution: optimum dilution is impossible if most ash and sulfur are not preliminary removed, and if other pollution sources are located nearby, thus increasing the load on the local ecosystem services. The differences between the different options (all of which possible, in principle) can be easily assessed by comparing their indicators in Tables 3 and 5. Results do not suggest coal-fired plants as a sustainable option for Chinese economy. To make these plants environmentally sustainable would require a huge portion of buffer land to be devoted to CO2 uptake or to ESI increase and a huge environmental support in terms of ecosystem services for dilution of emissions. Both land and ecosystem services are limited, and the situation is made even worse by the presence of other pollution sources in the same areas. Results, however, identify two major options that might help the transition towards renewable energy: increasing CO2 capture thanks to set aside reforestation areas (different than arable land for food) and increasing multi-product patterns and integrated production networks, in order to extract more products (heat, chemicals, construction materials) other than electricity out of power plant operation. In addition to the achievement of better environmental conditions, an integrated network capable to generate additional products would also save the energy required for their production in specifically dedicated processes (not accounted for as a saving in the present study) (Ulgiati et al., 2007). Outside of these constraints and opportunities, coal-fired electricity generation is not sustainable and places huge concerns on the sustainability of the Chinese economy as well.

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CONCLUSIONS

A 2 x 600 MW coal-fired plant in Guang Dong (China), representative of the most recent supercritical power generation in China, was investigated by means of an integrated approach based on embodied energy, emergy and carbon accounting. Constraints based on carbon uptake and emergy sustainability index (ESI) translated into the need to set-aside a buffer land from 1 million to 4 million ha, respectively, to uptake CO2 emissions (lower estimate) and balance plant unsustainability (higher estimate). Such a huge land demand is unlikely to be a valid strategy to offset the pollution generated by the large and increasing number of coal plants in China. As a consequence, carbon-based energy patterns do not seem a sustainable strategy for future environmentally sound energy generation in China. Set-aside land might however provide a temporary strategy towards renewable energy patterns in China. Results show that removal of ash and sulfur for use in other processes as well as the use of co-generated heat are likely to decrease the need for ecosystem services associated to the dilution and cooling of emissions, thus increasing the sustainability of the plant and suggesting integrated eco- industrial networks as a second temporary strategy over the transition to sustainable and carbon-free energy. No doubt, however, that results place a limit to the number of fossil-fired power plants that are acceptable and sustainable in China, no matter the fact that coal is a cheap domestic resource. Finally, the integrated energy, emergy and carbon accounting approach proves to be a suitable tool for investigation, comparison and design of power plant alternatives and their planning to meet the future energy demand of the Chinese society.

ACKNOWLEDGEMENTS The Authors gratefully acknowledge the support received from Projects of Guangdong Natural Science Foundation (S2013010016748) and Green Energy Technology Key Laboratory of Guangdong Province（2008A060301002）and Key Laboratory of Efficient and Clean Energy Utilization of Guangdong Province (KLB10004).

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CALCULATIONS, NOTES AND APPENDICES a) An exchange rate US$/RMB Yuan= 6.56 was used. b) Coal consumption was estimated as 2.53 million t/yr. The coal LHV (equal to the HHV minus the heat loss due to water evaporation) is 21.80 MJ/kg. Therefore, the plant receives 5.52E+16 J/yr as input energy. c) Each boiler unit generates 1,913 t/hr of steam; the process requires the addition of 3% by weight of steam deionized water. Since boilers operate 5,500 h/yr, 631,290 t/yr of deionized water are needed. d) The elemental content of coal as received at the power plant is: Ash=15％，Water=13％，Carbon=57.33％， Hydrogen =3.62％，Oxygen =9.94％，Nitrogen=0.70％，Sulfur=0.41％. The complete combustion of 1 kg of coal requires a theoretical volume of air as: 1 C H S O V 0  (1.866 ar  5.55 ar  0.7 ar  0.7 ar ) 0.21 100 100 100 100 and an actual volume as: Vk=  V° (=1.2) translating into an actual combustion air input of 2.25E+10 g/yr. e) A cooling water demand of 630,000 t/yr was estimated, based on a cooling water flow of 34.3 m3/s. At a treatment cost of about 4.5 RMB/t, cooling will cost 2.84E+6 RMB/yr, equal to 4.33E+05 US $/yr. f) During one year of operating time, the plant will theoretically generate a yield Y as: Y=2*600 MW*5500 hrs/yr= 2*600,000 kW*5500 hrs/yr= 6.6E+09 kWh/yr= =2.38E+16 J/yr g) The dedusting system consumes 903,000 kWh/yr, equal to 3.25E+12 J/yr. The desulfurization consumes 51.33E+06 kWh/yr, equal to 1.85E+14 J/yr. h) Based on the mass of coal used and its sulfur fraction (0.41% by weight), the available mass of S in coal is 1.04E+10 g/yr, which translates into 2.08E+10 g/yr of SO2, according to the reaction: S + O2= SO2 Eqn. (4) i) The SO2 generated is converted into calcium sulfate dihydrate according to the desulfurization reaction: 2 SO2 + 2 CaCO3 + 4 H2O + O2 = 2 CaSO4 * 2 H2O + 2 CO2 Eqn. (5) l) About 3.24E+10 g of CaCO3 and 825,000 t of H2O are needed every year for desulfurization (Eqn. 5). If limestone contains 90% CaCO3, the needed quantity of limestone will be 3.602E+10 g/yr. The price of limestone is 150 RMB/t (22.89＄/t), equal to a total cost of 8.24E+05＄/yr for limestone purchase. The price of water is 2.5 RMB/t (0.38＄/t), equal to a total water cost of 3.14E+05＄/yr. m) From the desulfurization reaction, a needed volume of O2 is 3.63E+06 m3/yr can be calculated. Since O2 is 21% of air volume fraction (m3/m3), this translates into an air input to desulfurization equal to 2.24E+10 g/yr. n) The net annual electricity output of coal-fired power generation with dedusting is calculated as the annual nominal output of coal-fired power generation system minus the annual electricity consumption of dedusting system (= 2.38E+16 J/yr - 3.25E+12 J/yr)= 2.379E+16 J/yr. o) The annual electricity output of coal-fired power generation with dedusting and desulfurization systems is calculated as the annual nominal output of coal-fired power generation system minus the annual electricity consumption of dedusting system minus the annual electricity consumption of desulfurization system (= 2.38E+16 J/yr - 3.25E+12 J/yr - 1.85E+14 J/yr)= 2.36E+16 J/yr.

1 Prior to 2000, the annual emergy driving the geobiosphere was calculated as 9.44E+24 seJ/yr (Odum, 1996) as the sum of solar radiation, deep heat and tidal momentum (calculated as solar-equivalent amounts)(Odum et al,2000) recalculated the total emergy baseline as 15.83E+24 seJ/yr in order to include the co-action of solar, gravitational and geothermal sources. Previously calculated UEV values must be multiplied by 1.68 (the ratio of 15.83/9.44) for conversion to the new baseline. Brown and Ulgiati (2010) refined the calculation to 15.2E+24 seJ/yr, based on updated values and conversion of energy to exergy units. The emergy baseline is the reference for all main biosphere-scale processes, the UEV of which are calculated also under the assumption to put the UEV of solar radiation equal to 1 seJ/J. All other UEVs of human-dominated processes are calculated accordingly, as ratios of the needed emergy input flows to the output flow(s). In this paper we refer to the Odum et al. (2000) baseline (all previous data converted accordingly).

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