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Environ. Sci. Technol. 2010, 44, 4026–4032

Life-Cycle Implications of Using Crop Residues for Various Energy Demands in China WEI LU AND TIANZHU ZHANG* Department of Environmental Science & Engineering, State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing 100084, China

Received January 15, 2010. Revised manuscript received April 13, 2010. Accepted April 14, 2010.

Crop residues are a critical component of the sustainable energy and natural resource strategy within a country. In this study, we use hybrid life-cycle environmental and economic analyses to evaluate and compare the atmospheric chemical, climatic, ecological, and economic issues associated with a set of energy conversion technologies that use crop residues for various energy demands in China. Our analysis combines conventional process-based life cycle assessment with economic input-output life cycle assessment. The results show that the return of crop residues to the fields, silo/amination and anaerobic digestion (household scale) offer the greatest ecological benefits, with net greenhouse gas reduction costs of US$3.1/ tC, US$11.5/tC, and US$14.9/tC, respectively. However, if a positive net income for market-oriented operations is the overriding criterion for technology selection, the cofiring of crop residues with coal and crop residue gasification for power generation offer greater economic scope and technical feasibility, with net incomes of US$4.4/Mg and US$4.9/Mg, respectively. We identify that poor economies of scale and the absence of key technologies mean that enterprises that use pure combustion for power generation (US$212/tC), gasification for heat generation (US$366/tC) and large-scale anaerobic digestion for power generation (US$169/tC) or heat generation (US$206/tC) are all prone to operational deficits. In the near term, the Chinese government should also be cautious about any large-scale investment in bioethanol derived from crop residues because, with a carbon price of as high as US$748/tC, bioethanol is the most expensive of all energy conversion technologies in China.

Introduction The concept of social or industrial metabolism focuses on biophysical aspects of the economy and concerns the interactions between societies and their natural environment, whereas principles of sustainability are concerned with a balance of interrelationships among ecological, social, and economic systems (1). In a finite world, the necessary implementation of sustainability principles will determine future trends in social or industrial metabolism; the adoption of alternative energy and resource carriers are inherent in any transitions as a means to solve sustainability crises (2, 3). Since human large-scale consumption of carbon-rich fossil fuels goes against sustainability principles, for example by * Corresponding author phone: 86(10) 62796956; fax: 86 (10) 62796956; e-mail: [email protected]. 4026

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causing fundamental changes in the global climate system and instigating nonrenewable energy crisis (4, 5), a process of substitution of this type of energy carrier has begun. Among a set of possible alternatives to carbon-rich fossil fuels, biomass is considered to be an abundant and potentially carbon-neutral renewable resource for the production of bioenergy and biomaterials, and one that is more technically feasible than many of the alternatives (6, 7). Crop residues are an important category of biomass and global amounts were estimated to be 3945 Tg in 2005 (Supporting Information (SI) Table S2), equivalent to an energy value of 7.4 × 1019 J (8). Numerous competing uses have been developed using conversion technologies to transform crop residues into energy, forage and industrial raw materials (9). Not all crop residues are available for these uses because some of them need to be returned to the soil to reduce erosion and maintain soil fertility (10). By complete utilization of crop residues and their return to the field, it is possible to avoid open-field burning of residue biomass, with its associated environmental impact (11, 12). Studies have been conducted to evaluate the environmental impacts on atmospheric chemistry, climate, and ecosystem ecology of single-crop residue energy conversion technologies, such as direct combustion (13), cofiring (14, 15), gasification (16), liquefaction (17, 18), and anaerobic digestion (19, 20). The economic feasibility of these energy conversion technologies has also been assessed (21-23). Results indicate that these various technologies have different environmental impacts and economic benefits, suggesting that with only limited investment at a regional or national scale a combination of crop residue burning technologies could be utilized to maximize benefits. Clearly, the economic and environmental consequences of the competing uses of crop residues must be assessed objectively with a holistic approach and a longterm perspective. Using China as a case study, we compare the environmental impacts and economic benefits of crop residue energy conversion, including with processing options, by developing a hybrid life-cycle assessment (LCA) framework for this purpose. As a large agricultural country, crop residues in China increased from approximately 146 Tg in 1950 to 682 Tg in 2005 (SI Figure S2). Since 1960, the Chinese government has been conducting long- term and wide-ranging research and development on crop residue energy conversion technologies (24). By 2005, the main uses of crop residues in China were for energy, forage, manure and industrial raw materials, accounting for 24.0, 21.5, 17.2, and 2.9%, respectively. The remaining 34.5% was accounted for by field burning and by a collection-loss component. Then, as now, open field burning of crop residues not only had serious environmental impacts, but also brought on freeway traffic accidents. Since 1992, the Chinese government has enacted several polices to reduce open field burning and to support the commercialization and expansion of biomass energy conversion technologies (25, 26). Although results of some preliminary investigations have shown that certain conversion technologies are economically viable and technically feasible (27-29), an integrated assessment has not been conducted by researchers to allow decision-makers to make a comparison of the economic strengths and ecological benefits of the different technologies that are in use in China today. This may lead to ill-informed investment in the crop residue utilization industry and/or may fail to optimize any environmental benefits due to an unregulated allocation of crop residues to the different bioenergy technologies (30). Thus, studies evaluating the integrated and comparative environ10.1021/es100157e

 2010 American Chemical Society

Published on Web 04/28/2010

TABLE 1. Thirteen Crop Residue Energy Conversion Scenarios final use

energy conversion scenario

acronym

power generation

1. 2. 3. 4.

heat drying and pulverization (HDP)fpure combustion HDPfcofiring (15% biomass, 85% hard coal) HDPfgasificationfgas purificationfgas combustion anaerobic digestionfcombined heat and power generation

heat generation

5. heat dryingfpure combustion (dispersive household cookstove) 6. heat dryingfpure combustion (biomass boiler) 7. HDPfgasificationfgas purificationfgas combustion 8. anaerobic digestionfbiogas combustion (household scale) 9. anaerobic digestionfbiogas combustion (large scale) (ADHGls) 10. cellulosic ethanol conversionfE10 blending and distributionfE10 combustion

manure substitution 11. returning to field forage substitution 12. pulverizationfsilo/aminationffeeding papermaking 13. crop residues pulp productionfpaper production

mental and economic benefits of the different conversion technologies are needed to maximize the utilization efficiency of crop residues in China. This paper focuses on qualifying and comparing several environmental and economic issues by assessing the lifecycle of crop residue energy conversion technologies. We consider several important factors that may influence the net SO2, CO, NOx, NMHC, PM10, CO2, CH4, and N2O reduction capacities of crop residue feedstock systems compared to conventional feedstock reference systems. Moreover, we also develop an economic cost assessment procedure to select key crop residue energy conversion technologies for China. Method and Data. Since the 1960s, conventional processbased LCA has been proposed as a means to delineate the major stages and processes involved over the entire life cycle of a product, and to quantify the environmental burden at each stage (31). Yet a process-based LCA may miss significant parts of the environmental interventions in its life cycle inventory (32). Hybrid LCA approaches have been developed by combining process-level data with sector-level inputoutput analysis to describe the complex interdependencies of industries within a national economy (33). Of these, input-output hybrid analysis is based on an environmental input-output framework originally proposed by Leontief (34); a recently developed application is the national economic input-output life cycle assessment (EIO-LCA) model implemented by the Green Design Institute at Carnegie Mellon University (35). The national EIO-LCA model enables an analyst to consider the environmental effects throughout the economy that result from a change in output for a particular industry (36). The life cycle environmental impacts of commercial fertilizer and conventional forage were evaluated using EIO-LCA tables based on the U.S. economy to measure the offsets for crop residue replacement, which are more detailed than those available for the Chinese economy. It was assumed that 2.3% of the emissions, resource-use, and waste from the production of one Mg of nitrogenous fertilizer are saved for every Mg of crop residues used as nitrogenous fertilizer. The same rationale was used to determine the offsets in phosphatic fertilizer and forage production. Process-based LCA was used to measure the environmental impact of the rest of the energy conversion technologies. Meanwhile, cost-benefit analysis was used to assess the economic impacts associated with every stage of the crop residue energy conversion processes (see the Supporting Information for description and assumptions of using the EIO-LCA model and cost-benefit analysis). As this

waste and destination

Ppure Pcofiring Pgasification P&Hdigestion

plant ash, agriculture fly ash, backfill ash, landfill digestate, agriculture

Hstove

plant ash, directly emitted

Hboiler Hgasification Hh-digestion

plant ash, agriculture ash, directly emitted Eigestate, agriculture

Hl-digestion

Digestate, agriculture

Hliquefaction

Distiller’s waste, agriculture

Mreturning Fsilo/amination animal manure, agriculture Ppulping black liquor, alkali recovery

study is highly data intensive and the results are largely predicated by the quality of data, a qualitative and quantitative assessment of each data input is provided in the SI. System Boundary and Assumptions. In this case, 13 crop residue energy conversion technologies were chosen for the hybrid LCA framework, which could be divided into five categories according to their final use. The energy conversion and waste management scenarios are shown in Table 1. For each crop residue energy conversion technology, the common processes include the following stages: feedstock cultivation and collection, transportation, pretreatment, conversion, usage, and waste management. On the basis of the analyses presented here, the initial choice of feedstock selection may result in different environmental and cost impacts through the life cycle of each technology due to different moisture contents, heat values, supply prices, and energy conversion coefficients (37, 38). Our data show that rice straw, wheat stalk, rice husk, corn stover, and fuel wood are all now used as feedstock by different energy conversion technologies in China (see SI Table S15-S21); however, only corn stover is used as a common, technically feasible energy feedstock by all of them. Therefore corn stover was chosen for our analysis to eliminate the uncertainties caused by different feedstocks. The moisture content and heat value of crop residues were assumed to be 15-20% and 16.654MJ/kg (39). The functional unit, to which the total environmental load was related, was chosen as 1 oven-dried (od) Mg of crop residue. During the feedstock transportation phase, it was assumed that all transportation was by 16-tonne diesel truck (the empty return journey was included). Associated transport emissions and the energy input (2.9MJ/tonne/km) were taken from the DEAM database (40). Average transportation distance was calculated as d ) 2R0τ/3, where R0 is the containing radius and τ the tortuosity factor (41). When crop residues arrive at energy conversion plants, pretreatment is needed to reduce their moisture content and improve the conversion efficiency. Compression, drying and pulverization are included in the pretreatment stages. Energy consumption and economic cost of the pretreatment stages are shown in SI Table S7. The processing plants that were selected all use corn stover as feedstock. The energy consumption of the operations was also included in the LCA. For all fuels used in the system, the energy contents were expressed in lower heating values. The electricity used was recalculated to primary energy for an average Chinese electricity production system, which was based on 76% coal-fired thermal power, 23% hydropower VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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and 1% nuclear power. The environmental costs and benefits of energy conversion and end-use were calculated separately. A sensitivity analysis is presented for net carbon fixation per unit corn stover, corn stover yield, removal rate, energy conversion rate, supply price, operation cost, subsidy from government, and capital cost (see SI Tables S22-S34 for a complete breakdown of data inputs and their respective sources). Carbon Storage and Sequestration. Carbon sequestration due to increasing soil organic carbon is an important factor that should be taken into account when estimating the global warming impacts associated with biobased industrial products (42). Removal rate of corn stover, tilling method, and change in land use can influence carbon sequestration rates (43-45). Simulation results from several models show that carbon sequestration rates associated with corn production range from 1.1 × 103 kg/od Mg to 2.5 × 103 kg/od Mg (42, 46, 47). Based on available information on corn yield data, planting area and harvest index (see SI pages S10-S11), we adopted a corn stover carbon sequestration rate of 1.6 × 103 kg/od Mg. Furthermore, using a removal rate of 68.5% and a no-tillage rate of 16%, the average soil carbon sequestration rate of corn stover, being returned to field and no-tillage, was calculated to 400 and 274 kg C/ha/yr respectively (see SI pages S12-S13). Owing to a lack of sufficient data to model the process of land-use change, it was assumed that there was no net carbon flux associated with land-use change. Reference System. Five traditional reference systems were designed to evaluate the effects on emission reductions per unit product by comparing these to the corresponding crop residue energy conversion systems. For the four power generation technologies using crop residues (Ppure, Pcofiring, Pgasification and P&Hdigestion), reference system 1 is a hard coal power plant characterized by an installed electric capacity of 300 MW with an efficiency of 45% (48). For the five heat generation technologies, namely Hstove, Hboiler, Hgasification, Hh-digestion, and Hl-digestion, reference system 2 is a coal-fired heating plant with a heating capacity of 87 MW and a coal consumption rate of 4.7 × 104 Mg/yr. For Hliquefaction, reference system 3 is an oil-based gasoline life cycle system, with gasoline being a combination of conventional and reformulated gasoline. The life-cycle inventory of reference system 3 was calculated using GREET 1.8 (49). Reference system 4 and 5 are the life cycles to produce 1 Mg of fertilizer and one ton of conventional forage, respectively (SI pages S44-S46).

Results and Discussion Environmental Impact Analysis. The results for atmospheric emissions and energy consumption associated with crop residues energy conversion scenarios are presented in Figure 1. It can be seen that for P&Hdigestion and Hdigestion either household-scale or large-scale uses generate the lowest amounts of atmospheric emissions when corn stover is used to generate power or heat. However, it seems that these amounts are still higher when compared with that of Mreturning. Only 0.26 kg SO2, 0.17 kg CO, 0.27 kg NOx, 0.05 kg NMHC and 0.06 kg PM10 are generated to return 1 Mg of corn stover to the field. Furthermore, Fsilo/amination can also be seen to perform well in alleviating the environmental impacts of crop residues utilization. Moreover, when burning 1 od Mg corn stover for power generation, Ppure and Pcofiring generate 2.55 and 2.82 kg SO2, respectively, which is comparable to that of Hstove (2.52 kg). However, only 0.66 kg SO2, 1.17 kg NOx and 0.06 kg PM10 are generated through Pgasification. This is because the waste heat recovered accounts for 70% of energy consumption in the pretreatment phase, while energy consumption in the pretreatment phase accounts for 24-32% of total energy input during power generation. 4028

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Conversely, direct burning for heat generation with either traditional or improved cookstoves produces CO emissions as high as 75.6 kg/od Mg and 38.8 kg/od Mg, respectively. In rural areas of China, the time used in heat drying of crop residues is too short to decrease their moisture content. In fact, even with the same type of cookstove, an increase in moisture content of the fuel results in an increase in the emission factor of CO (50). Furthermore, the results show that atmospheric emissions, such as SO2 (5.16 kg/od Mg), NOx (7.52 kg/od Mg) and PM10 (0.58 kg/od Mg), of biofuel life cycle systems are relatively high compared to those of oil-based gasoline life cycle systems. Due to a large amount of atmospheric emissions, such as NOx, SO2, and PM10, from steam production in the ethanol conversion stage, the biofuel life cycle system may also contribute similarly to the acidification potential (AP) and photochemical oxidation (POCP). Global Warming Potential (GWP). It is generally agreed that the use of biofuels and bioproducts reduces greenhouse gas emissions because of the carbon neutral status of biomass (9). Within Mreturning and Fsilo/amination, the maximum reduction in GWP would be 1.64 × 103 kg CO2 E (carbon dioxide equivalent)/od Mg and 1.15 × 103 kg CO2 E/od Mg. Another benefit of Mreturning is that it increases the level of soil carbon, which helps to avoid soil erosion; Sheehan et al. (47) modeled this increase to be 32% over a 90-year period. Additionally, Ppure and Hliquefaction show an increase in GWP of 319 kg CO2 E/od Mg and 336 kg CO2 E/od Mg when the carbon sequestration rate of corn production is 1612 kg/od Mg. We estimated that producing 1 Mwh of electricity with corn stover causes a GWP reduction of 4.30 × 102 to 1.28 × 103 kg CO2 E, whereas producing 1 GJ of heat causes a reduction of between 6.73 × 101 and 3.21 × 102 kg CO2 E. In contrast, the pulping of corn stover for paper manufacture adds up to 2.70 × 103 kg CO2 E/tonne of GWP, which is two times higher than pulping with waste paper. Aggregation of Economic, Energetic, and Ecological Issues. This analysis has considered the capital, raw material, operational, and transportation costs of the individual energy conversion schemes. These costs associated with the life cycle of corn stover are summarized in SI Table S10. It should be noted that the supply price of corn stover is different for each technology because of the lack of a uniform pricing system due to regional difference. The total current value of each energy conversion scheme was calculated by assuming an annual discount rate of 7% and different project life spans (SI Tables S8-S10). Comparison of the four power generation scenarios which integrate economic and environmental perspectives shows that, even without government subsidy, Pcofiring offers the greatest ecological efficiency in terms of a reduction of carbon emissions, with a net cost of US$101/tC with a biomass price of approximately US$19.5/Mg in 2005. If costs of NOx and SOx emissions are taken into account, cofiring could reduce CO2 emissions from the coal-fired electricity generation sector at a carbon price of about US$112/tC, which is double that of the US$50/tC presented by Robinson et al. (14). Due to a lower unit capital cost and operational cost, Pgasification has a potential role in making use of crop residues as it has a net GHG reduction cost of US$124/ tC if a subsidy of US$20.4/Mg is provided. Conversely, although showing the best performance with regards to environment impacts, P&Hdigestion in the near-term does not show a very cost-effective use of biomass to offset coal generated electricity with a net cost of US$169/tC. For heat generation scenarios, the key finding of this analysis is that none of Hboiler, Hgasification, Hl-digestion, or Hliquefaction was economically viable for GHG reduction due to high unit costs (US$315/tC, US$366/tC, US$206/tC, and US$748/tC, respectively). Of these scenarios, Hh-digestion shows a clear advantage as a method in reducing GHG emissions at a

FIGURE 1. Atmospheric emissions from different crop residue energy conversion scenarios for one Mg of corn stover. Ppure represents a 6 WM pure combustion power plant with a power generation efficiency of 20%. Pcofiring represents a 140 MW cofiring power plant with a power generation efficiency of 30%. Pgasification represents a 4 MW gasification power plant with a gasifier efficiency of 78% and power generation efficiency of 28%. P&Hdigestion represents a biogas power plant with gas turbines >1 MWelectr and with a conversion efficiency of 0.85 divided between electricity (0.40) and heat (0.45). Hstove (TSC) and Hstove (ISC) represent heat generation with traditional stover cookstove and improved cookstove, respectively. Hboiler represents heat generation with biomass boiler with an energy utilization efficiency of 60% and an energy conversion efficiency of 14.3%. Hgasification represents a biomass gasification station with a gas storage capacity of 300 m3. Hh-digestion and Hl-digestion represent a household-scale biogas digester and a large-scale biogas plant. Hliquefaction represents a cellulose ethanol plant with a production capacity of 3000 Mg/yr. Mreturning and Fsilo/amination represent crop residues returned directly to the field and fed to animals, respectively. Bopen field represents corn stover open field burning. carbon price of US$14.8/tC, if commercial viability and profit are not the main factors to be considered. Due to the relatively low investment required and the proven technology, the construction and use of household biogas digesters has been widely popular in rural areas of China (51). Our analysis also shows that the low capital costs and relatively high ecological efficiency of Mreturning (US$3.1/tC) and Fsilo/amination (US$11.5/ tC) make them competitive options among several other advanced biomass technologies that are proposed. Moreover, although it seems that the carbon prices of Hstove (TSC) (US$30.7/tC) and Hstove (ISC) (US$20.6/tC) are lower, we do not suggest the encouraged usage of these stoves due to their negative environmental impacts. Selection of the correct conversion technology can also be aided by performing net energy value (NEV) analyses. In our study, positive net energy outputs could be achieved only under the scenarios of Pcofiring (5.31 × 102 MJ/od Mg), Hgasification (3.47 × 103 MJ/od Mg), Hh-digestion (3.13 × 102 MJ/od Mg), Hl-digestion (1.50 × 101 MJ/od Mg), and Hliquefaction (8.99 × 102 MJ/od Mg). Other technologies could not yield positive net energy outputs due to poor energy conversion and low utilization efficiencies.

Likewise, an economic analysis shows that, even with a subsidy of US$26.3/Mg from the Chinese government, Hliquefaction only attained a net income of US$1.1/od Mg due to a techno-economic barrier to produce bioethanol from crop residues within China. Compared with an ethanol yield of 340 L/od Mg presented by Spatari et al. (18), the conversion efficiency of up to 195 L/od Mg for corn stover-derived ethanol in China needs to be improved upon to reduce the total cost. Similarly, a large number of Hgasification enterprises run at a loss (-US$50/Mg) due to poor economies of scale and a variable supply price for gas, which in 2005 fluctuated between US$0.0244/m3 and US$0.0513/m3. Only Pcofiring, Pgasification, and Fsilo/amination attain a relatively large net positive income of up to US$4.4/Mg, US$4.9/Mg, and US$3.2/Mg, respectively. Sensitivity Analysis and Policy Implications. Since the results of our analyses of the life-cycle inventory and economic costs are quite sensitive to changes in several factors, such as crop yield, net carbon fixation, removal rate, energy conversion efficiency, feedstock supply price, operational costs, capital costs, and subsidies (18, 52), a sensitivity analysis was conducted to analyze the extent of VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Sensitivity analysis of factors related to reduction of global warming potential (GWP) (a-d) and total cost (e-h). Part (a), for example, shows that if net carbon fixation per unit corn increases by 10%, the amount of GWP reduction of Ppure will increase 37%. uncertainty of these factors affecting the GWP reduction and total costs. In Figure 2, it can be seen that the variation in 4030

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net carbon fixation, removal rate, and energy conversion efficiency significantly impact GWP reduction, except for that

of crop yield. Variations of up to (10% in crop yield cause changes in GWP of (1%. Corn stover supply price (varying by (10%) affects the total cost of Pgasification the most, with an uncertainty of (11%. Meanwhile, total costs of Mreturning and Hliquefaction are largely dependent on the operational costs, and total cost of Hh-digestion mainly fluctuates with the changes in subsidies and capital costs. Moreover, uncertainties about the economies of scale were found to strongly affect costs and cause slight increases in the life cycle of CO2 emissions within Pgasification, Hgasification, and Hl-digestion. The biomass blending ratio (BBR) also shows a significant effect on CO2 emissions of Pcofiring. Above all else, the four factors that should be considered by policy-makers in China over decisions about which energy conversion technologies should be adopted in the near-term for the generation of bioenergy or bioproducts are (i) environmental impact, (ii) greenhouse gas emissions, (iii) net energy value, and (iv) economic viability. In our analysis, Pcofiring was the best choice for crop residues utilization as it performs well on all four counts. If commercial operation and financial profit are not the main factors to be considered, Hh-digestion could be used to reduce GHG emissions at a relatively low carbon cost, while generating a considerable net energy output. We also identified that if energy value is not of paramount concern, then Mreturning and Fsilo/amination become attractive options as they are the most environmentally friendly among the conversion technologies, and the latter also has a positive economic benefit. Additionally, based on our results, we suggest that it is not advisible to propose large-scale investment in liquefaction technology with crop residues as feedstock as this technology is still in its infancy, and consequently at present it offers poor financial return and only a low potential for carbon reduction. Technological innovation and economies of scale should be developed not only for Hliquefaction but also for Ppure and Hgasification. It can be expected that total costs will be reduced greatly if market-oriented forces operate in the conversion technology sector. Another means to reduce costs in the early stages of the crop residue industry is for the state to provide economic incentive policies, such as subsidies and tax credits. These should be implemented and maintained in such a way as to ensure the sustainable development of the different technologies which underpin the energy conversion industry in China. In this study, integrated environmental and economic assessment methods were used to evaluate the ecological and economic performance of 13 crop residue conversion technologies. Since this analysis is based on static baseline data from 2005, there is a need for multitime frame analysis to simulate the dynamic GWP reduction processes, with a further consideration on technological improvements and land-use change. Such analyses could also help inform China’s energy strategy and policy goals in relation to the utilization and structuring of crop residue conversion technologies.

Acknowledgments We gratefully acknowledge the comments and suggestions from Dr. Ming Xu and two anonymous reviewers. This work was funded by a National Key Technologies R&D Program of China-Common Technology Development and Applications of Urban Circular Economy in Suzhou (2006BAC02A1).

Supporting Information Available Detailed discussion of methods and assumptions, quantitative results of life-cycle assessment and cost-benefit analysis, and more figures and data. This material is available free of charge via the Internet at http://pubs.acs.org.

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