Potential Reductions in Greenhouse Gas and Fine Particulate Matter Emissions Using Corn Stover for Ethanol Production in China

: Corn stover is an abundant raw material that can be used to produce ethanol and reduce air pollution. This paper studied the potential reductions in greenhouse gas (GHG) and ﬁne particulate matter (PM 2.5 ) emissions across China if corn stover was used for ethanol production. Field surveys in nine provincial regions were conducted. Life-cycle assessment (LCA) was used to assess the GHG and PM 2.5 emissions from a corn stover based ethanol system. The LCA system boundaries included several process stages from corn planting to ethanol fuel used in vehicles. Corn stover geographical distributions and emission reduction factors were combined. Results showed that the total surplus quantity of corn stover in China was 86.2 million metric tons (Mt) in 2015. It was su ﬃ cient to reach the ethanol production target set by the Chinese government. In the scenario that 38.5 Mt or 44.6% of corn stover surplus were used for ethanol production, the total potential emission reductions were 36.5 Mt CO 2 -eq GHG and 450.9 kt PM 2.5 . Among the 31 provincial regions in China, the reduction potentials varied from 0.001 to 8.9 Mt CO 2 -eq for GHG and from 0.013 to 109.7 kt for PM 2.5 . This study provided useful information to policy makers, researchers and industry managers who work on environmental control and corn stover management. CO 2 -eq and 20.7 t, 0.005%), and Qinghai (7.8 kt CO 2 -eq and 96.3 t, 0.02%), respectively. These results were consistent with the corn stover surplus quantities in each provincial regions because the same reduction factors of GHG and PM 2.5 emissions were used in all the regions.


Introduction
Emissions of greenhouse gas (GHG) are believed to link to climate change and emissions of fine particulate matter (PM 2.5 , particulate matter ≤ 2.5 µm in diameter) are confirmed to relate to smog. Climate change has drawn great concerns around the world [1], including China [2]. The notorious haze issue caused by air pollution also gained much attention in recent years. Pursuing co-benefits in bioenergy production is an effective approach to simultaneously respond to air pollution problems, including GHG emissions [3].
Corn is a staple food in many parts of the world. Its production increased from 8.5 Mt (million metric tons) in 2010/2011 to 11.3 Mt in 2018/2019. The United States, China, and Brazil are the three major corn producing countries in 2018/2019 [4]. Corn stover is a desirable raw material for producing cellulosic ethanol [5][6][7]. Corn stover for ethanol production can provide co-benefits because it can improve energy security and reduce air pollution [8][9][10][11][12][13]. Corn stover for fuel production has a great potential in China because corn made up 39.2% of all cereals in 2017 [14]. The amount of corn stover accounted for 28.0% in 2010 [15] and 24.2% in 2013 of all crop residues in China [16]. A previous study (e.g., abandonment), were included in the questionnaires. The percentage of different corn stover utilizations in 2015 in the sampled provincial regions is shown in Table 1. Table 1. Percentage (%) and number of questionnaires (n) of corn stover field retention (FR), commercial utilization (CU), cooking and heating (CH), burning in field (BF), unplanned utilization (UU), and surplus that could be used for ethanol production (SEP) at different sampled sites in 2015. The SEP at the sampled sites equaled the corn stover burning in field plus the corn stover unplanned utilization. The SEP in other provincial regions not field-surveyed was evaluated according to the administrative division, comprehensive agricultural regionalization information, and data for selected sample sites in China (Table 2). The corn stover was defined to include only cornfield residues. The surplus quantity of corn stover was calculated according to the corn production, factor of cornfield residue on an air-dried basis, and corn stover surplus for ethanol production (Equation (1)):

Sampled
where Q C is the quantity of corn stover surplus, ton; C P denotes the corn production in 2015 [20] with the exception of Hainan Province in 2013 [30], ton; C FRI is the corn stover factor (ratio of corn stover mass to grain yield produced) in 31 provincial regions of China, as described previously [31], dimensionless; and SEP is the corn stover surplus that could be used for ethanol production, %.

Scope of the Life-Cycle Assessment
A LCA was conducted using the system boundary and flow chart shown in Figure 1. Even though corn stover has traditionally been viewed as wastes, the development of corn stover based ethanol production technologies supported by governmental funding may eventually cause corn stover to evolve into a co-product of corn production. Hence, in this study, the LCA of corn stover based ethanol production started with corn planting, and included corn stover collection, corn stover transport, ethanol production, ethanol transport and use in vehicles ( Figure 1). As part of the LCA, GHG emissions of carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O) were weighed according to their global warming potentials of 1, 25, and 298, respectively, following IPCC's 100-year estimates [32]. It was assumed that carbon in the form of CO 2 from vehicular ethanol combustion originated from biogenic carbon that was derived from corn stover because more than 96% of all carbon in the process entered as biomass feed, with only small amounts of additional carbon coming from glucose (for enzyme production) and fermentation nutrients such as corn steep liquor [33]. Thus, CO 2 emissions from ethanol in the vehicle-use stage were negligible in this study. An FU for treatment comparison in the life-cycle inventory was based on per ton of corn stover.
The GREET (Greenhouse Gases, Regulated Emissions and Energy in Transportation) model developed by the Argonne National Laboratory of the U.S. Department of Energy was used in this study because it was a well-developed LCA tool and included numerous fuel and vehicle cycles [34,35]. For a given transportation fuel or vehicle technology combination, the GREET can be used to calculate life-cycle GHG and PM 2.5 emissions [36].

Corn Planting
In corn planting, the main material input was fertilizer and the average recommended fertilizer application in corn planting was 19.0 kg nitrogen, 9.5 kg phosphorus pentoxide, and 5.3 kg potassium oxide per ton of corn produced. The mass allocation method was used to calculate the partition of fertilizer between corn grain and corn stover [13]. After allocation, the average fertilizer use in corn stover was 9.8 kg nitrogen, 4.9 kg phosphorus pentoxide, and 2.7 kg potassium oxide per ton of dry corn stover. Direct N 2 O emissions from nitrogen fertilization was calculated based on Wang et al. [37], who estimated the N 2 O-N emissions as 2.0% of nitrogen by weight.

Corn Stover Collection and Transportation
To gather data of corn stover collection and transportation, the largest biomass feedstock supplier in City, China, Symbior Biocrude Ltd., was interviewed. During stover collection in Symbior Biocrude Ltd., a grinder attachment to a corn combine first cut the corn stover into pieces of about 10 cm in length. Next, a tractor-pulled raker and baler chopped the corn stover and bundled it into large round bales. On average, a bale of corn stover weighed 0.3 tons with 20.0% moisture content. Fork loaders picked up the bales and placed them on a 13-m-long truck, which could load 52 corn stover bales and consume 30 L of diesel per 100 km travel distance. Table 3 displays the power, efficiency, and diesel consumption data for the machinery used in collection. Table 3. Technical parameters of machinery used in corn stover collection based on the field surveys.

Machinery
Power ( Based on an evaluation of optimized feedstock quality and profits, the company estimated that the maximum transportation distance could be approximately 150 km to 200 km. Therefore, a value of 150 km for transportation distance by heavy-duty truck (13,000 kg payload; 0.3 L per ton-km) was used in this study.

Ethanol Production
The data reported by NREL (National Renewable Energy Laboratory of the U.S.) to analyze ethanol production technology [33] were used, as the second generation ethanol technology has not seen widespread use recently in China. The technology studied by NREL employed dilute acid pretreatment followed by enzymatic hydrolysis and co-fermentation to convert corn stover to ethanol ( Figure 1). First, hydrolysis with dilute sulfuric acid converted most of the feedstock hemicellulose carbohydrates to soluble sugars. Next, the hydrolysate slurry was cooled, and ammonia was added to raise the pH. Then, while the slurry was still at an elevated temperature, enzymatic hydrolysis was initiated. Once the conversion was complete, the slurry was cooled to the fermentation temperature and inoculated with fermentative microorganisms. The fermentation broth was then separated into water, ethanol, and solids. Finally, distillation and molecular sieve adsorption were used to produce 99.5% ethanol. Water was recycled after treatment, and solids were sent to an incinerator to generate heat and electricity. Approximately 68.0% of the electricity generated from the system was used in the plant and the remaining 32% was sold to the grid, providing a co-product credit [33]. A displacement method was used to allocate output between ethanol and the electricity co-product. The key technology input and output data for the ethanol production process are shown in Table 4. These materials were used for microorganism production. All the data are from [33].

Ethanol Transport to User Sites and Use in Vehicles
The transportation distance of ethanol to its site of use was estimated to be 150 km using a heavy-duty truck (13,000 kg payload; 0.3 L per ton-km). In this study, a fuel-cell car as used in the GREET model for E100 consumption was selected. According to the GREET, the fuel economy of E100 in a fuel-cell car was 1570.4 J m −1 (7.4 L per 100 km, and 22.2 kg corn stover for 7.4 L).

Potential Reductions in GHG and PM 2.5 Emissions
The total potential reductions in GHG and PM 2.5 emissions from corn stover for ethanol production in China were calculated following Equations (2) and (3), respectively: where Q GER is the total potential reductions in GHG emissions, ton; Q PER is the total potential reductions in PM 2.5 emissions, ton; Q C is the surplus quantity of corn stover available for ethanol production, ton; RF CO 2 is the reduction factor of GHG emissions, kg CO 2 -eq per ton corn stover; RF PM2.5 is the reduction factor of PM 2.5 emissions, kg PM 2.5 -eq per ton corn stover. The reduction factor of GHG (RF CO 2 ) and PM 2.5 emissions (RF PM2.5 ) were calculated by using emissions from a reference case minus emissions from the corn stover-based ethanol production in LCA. The reference case was used as a baseline to assess the GHG and PM 2.5 emission reductions of the ethanol pathway. Corn stover burning in field and vehicles powered by gasoline were assumed in the reference case. As for emissions from corn stover burning in field, the values of 3.5 g of CH 4 , 0.1 g of N 2 O, and 11.7 g of PM 2.5 emissions from 1.0 kg of corn stover field-burning in China were adopted [38,39].
In accordance with life-cycle emissions from corn stover for ethanol production, the CO 2 , N 2 O, CH 4 , and PM 2.5 emissions from diesel consumption during collection, as well as the N 2 O emissions from nitrogen fertilizers were also considered in the pathway of corn stover burning in field. The emissions from gasoline-powered vehicles were calculated based on the blended gasoline stock from crude oil for use in U.S. refineries because the data on emissions from gasoline production in China are not available.

Corn Stover Surplus and Its Spatial Distribution Density
In 2015, the total quantity of corn stover surplus was estimated to be 86.2 Mt in China. Among the 31 provincial regions, those from Northeast to Southwest had higher total surplus quantities than Southeast and Northwest. The three provinces in Northeast region had the highest surplus quantities, which counted for 51.0% (43.9 Mt) of the total corn stover surplus in China. This was because of the highest total corn production (34.5%) and proportion of corn stover surplus for ethanol production (50.9%, Table 1) in Northeast. The Northwest region ranked the lowest, with approximately 2.2 Mt.
Corn stover densities in the Northeast provinces of Jilin, Liaoning and Heilongjiang were relatively high, ranging between 44.4 and 81.6 ton km −2 . They were the lowest in the Western provincial regions of Xinjiang, Qinghai and Tibet, less than 0.3 ton km −2 ( Figure 2). However, the ranking of the total surplus quantities was not necessarily consistent with the ranking of the densities among the 31 provincial regions. For instance, the surplus corn stover in Inner Mongolia ranked the fourth (6.7 Mt), though its density was only 5.5 ton km −2 , ranking 17th among the 31 provincial regions.

Life-Cycle GHG and PM 2.5 Emissions for Corn Stover Based Ethanol
The net emissions for corn stover based ethanol were 245.5 kg CO 2 -eq GHG and 25.7 g PM 2.5 per ton of corn stover (Figure 3). The GHG emissions from the ethanol production stage were the highest and reached 160.3 kg CO 2 -eq (65.3%) because of the use of chemicals and nutrients. The PM 2.5 emissions from the stages of corn planting and corn stover collection were the highest and reached 16.5 g (64.2%) because of the fertilizer use in corn planting and diesel consumption in vehicles. Fertilizer production emits large quantities of air pollutants. Emission credits from co-product could offset part of the air pollution. The emission credits were 82.2 kg CO 2 -eq GHG and 4.8 g PM 2.5 for the corn stover based ethanol production ( Figure 3).
The emission reduction factors were 950.1 kg CO 2 -eq GHG and 11,722.7 g PM 2.5 per ton of corn stover ( Table 5). The GHG emission reductions were mainly due to high emissions from gasoline. The corn stover burning in field emitted the highest PM 2.5 , reaching 11.7 kg per ton of corn stover. The results indicated that the potential reductions in GHG and PM 2.5 emissions were significant if corn stover were used for ethanol production.   2 The reduction factors of GHG and PM 2.5 emissions were calculated by using emissions from the reference case minus emissions from the corn stover-based ethanol production.

Potential Reductions in GHG and PM 2.5 Emissions under Four Scenarios
According to the Medium and Long-Term Development Plans for Renewable Energy released in 2007 in China [40], looking toward a target of 10.0 Mt ethanol production by 2020, the corn stover demand could be 38.5 Mt, or 44.6% of corn stover surplus (Scenario B, Table 6). Under this scenario, the potential reductions in emissions were 36.5 Mt CO 2 -eq GHG and 450.9 kt PM 2.5 . By comparison, under Scenario A, which assumed half the ethanol production (5.0 Mt) of Scenario B, the potential reductions were 18.3 Mt CO 2 -eq GHG and 225.4 kt PM 2.5 . Under Scenario C, which assumed 50.0% higher ethanol production than Scenario B, the potential reductions were 54.8 Mt CO 2 -eq GHG and 676.3 kt PM 2.5 . Finally, when it was assumed that all the corn stover surplus was used to produce ethanol (Scenario D), ethanol production could reach 22.4 Mt and the potential mitigations were 81.9 Mt CO 2 -eq GHG and 1010.4 kt PM 2.5 . The results indicated that the potential GHG and PM 2.5 emission reductions were sensitive to the quantities of corn stover surplus for ethanol production.  [40]. Scenario A was for half of the ethanol target of Scenario B. In Scenario C, the ethanol target was 50% higher than Scenario B. Scenario D was based on all the corn stover surplus quantity in this study.

Potential Reductions in GHG and PM 2.5 Emissions in a Provincial Regional Context
Based on Scenario B, the potential reductions in GHG and PM 2.5 emissions in 2015 were the highest in Northeast China, i.e., 8889.3 kt CO 2 -eq and 109,680.5 t for Heilongjiang (24.3%), 6612.6 kt CO 2 -eq and 81,589.3 t for Jilin (18.1%), and 3125.8 kt CO 2 -eq and 38,566.9 t for Liaoning (8.6%), respectively (Table 7). In contrast, the potential reductions in GHG and PM 2.5 emissions, and the weight in the country (%) were the lowest in Shanghai (1.1 kt CO 2 -eq and 13.3 t, 0.003%), Tibet (1.7 kt CO 2 -eq and 20.7 t, 0.005%), and Qinghai (7.8 kt CO 2 -eq and 96.3 t, 0.02%), respectively. These results were consistent with the corn stover surplus quantities in each provincial regions because the same reduction factors of GHG and PM 2.5 emissions were used in all the regions. Table 7. Geographical distributions of corn stover surplus, availability for bioethanol production, and potential total emission reductions in GHG (Q GER ) and PM 2.5 (Q PER ) from corn stover-based bioethanol compared with the reference case in different regions and provinces in 2015.  3 The ratio is percentage (%) of each provincial region in the total (China). The ratio is the same for surplus quantity, available quantity, Q GER and Q PER within the same columns.

Region and Province 1 Surplus Quantity (kt) Available Quantity (kt) 2 Q GER (kt CO 2 -eq) Q PER (t) Ratio (%) 3
The potential reductions in emissions exhibited variations in regional distribution between 939.6 kt to 18,627.7 kt CO 2 -eq GHG, and 11,592.7 t to 229,836.7 t PM 2.5 , in the order of: Northwest China (2.6%) < East China (5.9%) < Central-South China (11.5%) < Southwest China (12.5%) < North China (16.6%) < Northeast China (51.0%). These results indicated that the potential of GHG and PM 2.5 emission reductions was especially high in Northeast China if corn stover was used for ethanol production.

Discussion
In 2015, the total corn stover surplus for bioethanol production reached 86.2 Mt. In the near future, this quantity could be higher because the proportion of corn stover for cooking and heating could be added. The use of corn stover for cooking and heating would likely decrease with the recent development of more energy options in China (e.g., coal and natural gas).
In addition, even though the Chinese government issued seven regulations between 2008 and 2015 to ban the crop residues burning in field [41], corn stover burning in field still remained widespread. As shown in this study, the average corn stover burning in field reached 34.9% of the total corn stover in 2015. Wang and Wang [42] also reported that 26.0% of crop residues were burned in field in China in 2008 and 2009.
The results of this study suggested that regulations are needed to combine the ban of burning corn stover in field with the encouragement of corn stover commercial utilization, e.g., bioenergy production. This is expected to greatly decrease the corn stover burning in field. This approach may also be applicable to other countries such as India, where more than half of the crop straw was burned openly in field [43]. It was also found in this study that the percentage of corn stover burning in field was related to the regulations on crop residue management. Chen et al. [29] reported that the regulations in South and East regions in China were not always available, and in this study, the percentages of corn stover burning in field were higher in these regions (40.4-71.1%).
In this study, the overall life-cycle GHG emissions per unit of ethanol produced from corn stover were 245.5 kg of CO 2 -eq ton −1 , 738.7 g CO 2 -eq L −1 , or 34.7 g CO 2 -eq MJ −1 . The 34.7 g CO 2 -eq MJ −1 was similar to the value of 38.0 g CO 2 -eq MJ −1 , which was also based on the NREL bioethanol production process [44]. However, it was lower than the 65.3 g CO 2 -eq MJ −1 reported by Zhao et al. [13] because the fertilizer quantity in this study was lower than in previously published studies. The 738.7 g CO 2 -eq L −1 in this study was higher than a previous estimate of 330 g CO 2 -eq L −1 [27] because of the inclusion of GHG emissions from fertilizer use in field in this study. The results demonstrated that chemical fertilizer use in the cornfield had an obvious effect on life-cycle GHG emissions. Reducing the use of chemical fertilizer or replacing it with organic fertilizer, e.g., animal manure, could decrease the life-cycle GHG emissions from bioethanol production using corn stover. It is also worth noting that the major source of GHG emissions was from the stage of ethanol production because of the use of a large amount of chemicals and nutrients. This finding was consistent with McKechnie et al. [44] and Zhao et al. [13], whose studies were based on the same ethanol conversion process as in this study.
Since 2014, the National Development and Reform Commission of China has been regulating the assessments and interventions for the control of CO 2 emissions in each province [45]. Ultimately, the control of CO 2 and PM 2.5 emissions is expected to have dramatic environmental and social impacts in China. This study combined corn stover geographical distributions and its reduction factors based on LCA, in order to assess the potential reductions in GHG and PM 2.5 emissions across China. It could be a reference to estimate the total environmental impacts from organic waste utilization in other countries. It would encourage policy makers, researchers and industry managers to promote the commercial development of corn stover for bioenergy use. Its development could not only decrease the pollutant emissions and provide the service of disposing the corn stover as agricultural waste, but also supply the lots of job opportunities and promote the local economic development.

Conclusions
This study revealed that the total surplus quantity of corn stover reached 86.2 Mt in 2015 in China. However, geographical distributions of the corn stover surplus among 31 different provincial regions varied greatly. The use of corn stover for ethanol production had great potential to reduce GHG and PM 2.5 emissions, especially in Northeast China. The potential reductions from 38.5 Mt corn stover, or 44.6% of corn stover surplus, to produce 10 Mt ethanol were 36.5 Mt CO 2 -eq GHG and 450.9 kt PM 2.5 . Chemical fertilizer application in the stage of corn planting had a negative effect on life-cycle GHG emission reductions. A large amount of chemical and nutrient use in the stage of ethanol production also greatly decreased the benefit of GHG emission reductions. New regulations that could combine the ban of in-field corn stover burning with the encouragement of corn stover ethanol production could lead to more GHG and PM 2.5 emission reductions.

Conflicts of Interest:
All authors declare no conflict of interest.