Assessment Of Advanced Bioethanol Production Potential Under Water And Soil Resource Constraints In China

Water is the basis for the survival of human society,and it is also an important part of crop production and processing.Crop residues based on cellulosic biomass and energy crops planted on marginal land are all "advanced" biomass energy types,which are characterized by not affecting normal food production or competing with cultivated land resources,but also replacing traditional fuels and having the advantage of neutral carbon cycle.However,due to the additional water demand in the production process,it may further aggravate the regional water pressure.As the world’s third largest producer of fuel ethanol,China has the space and advantages to develop its ethanol industry.However,in traditional "advanced" bioethanol energy potential assessment,water resource constraints are often ignored.Therefore,from the perspective of dual constraints of water and soil resources,this study proposed a method to determine the suitability of bioethanol production based on the calculation of bioethanol water footprint and the evaluation of regional water resources pressure degree,and further quantified the bioethanol energy potential and its spatial distribution characteristics of 12 crop residues and 2 energy crops(sweet sorghum and cassava)in China.The main findings are as follows:(1)The bioethanol water footprint of agricultural residues had significant differences.The bioethanol water footprint of beet pulp(1.6m3 /GJ)is the smallest among all crops.As wheat grows in North China where precipitation is relatively low and crop water demand is high,the bioethanol content of wheat stalk is higher than that of other crops,about 16 times that of beet pulp.Except for Tianjin,Yunnan,Xizang,Gansu,Ningxia and Xinjiang,the proportion of blue water in rice stalk is higher than other agricultural residues,ranging from14% to 65%.The bioethanol blue water footprint of most agricultural residues(wheat stalk,rice stalk,corn stalk,millet stalk,sorghum stalk,sunflower stalk and sesame stalk)in Xinjiang region is the highest among all provinces,and the proportion of blue water is more than 45%.(2)The bioethanol water footprint of energy crops was different in different regions.Considering the constraints of water resources,the final suitable marginal land area for sweet sorghum and cassava is 7.19 million hectares and 6.15 million hectares,respectively.The marginal land area of Guangxi,Guizhou and Yunnan provinces is higher than other regions.The low-value bioethanol water footprint of sweet sorghum planted on marginal land was distributed in Yunnan,Sichuan and Guizhou,while the high-value areas were mainly in Shanxi and Hebei provinces.The low bioethanol water footprint of cassava,an energy crop planted on marginal land,is mainly located in Chongqing,Guizhou and Yunnan,while the high bioethanol water footprint is located in Hainan and Jiangxi provinces.Thus sweet sorghum and cassava,two energy crops,are more advantageous in areas with abundant water resources and large marginal land areas,such as Guizhou and Yunnan.(3)In terms of spatial distribution,the theoretical potential,optimal sustainable potential and actual sustainable potential of bioethanol of agricultural residues in China were134.3Mt,67.2Mt and 33.6Mt,respectively,without taking into account water resource constraints.Heilongjiang,Henan and Shandong provinces have the largest potential for bioethanol energy production,accounting for about 28 percent of the country’s total energy potential.Considering the water resources pressure,the theoretical potential,optimal sustainable potential and actual sustainable potential of bioethanol of agricultural residues in China were 68.5Mt,34.3Mt and 17.1Mt,respectively.The bioethanol energy potential of Heilongjiang and Jilin is higher than that of other suitable areas,accounting for about 33% of the total energy potential of the suitable areas.Except for Guangxi,Qinghai and Xinjiang,other regions use wheat,rice or corn residues as the main bioethanol raw materials,and the combined energy ratio of the three is more than 70%.In terms of crop,corn stalk(46.1%),rice stalk(26%)and wheat stalk(16.7%)have the highest energy potential,considering that Shandong and Henan are in areas with serious blue water shortage.Corn straw(41%)and rice straw(42%)accounted for significantly more bioethanol energy than wheat straw(6%)and other straw(11%).(4)The suitable bioethanol potential of sweet sorghum and cassava was 18.7-32.7Mt and 14.0-52.4Mt,respectively,which decreased by 26% and 2% compared with the case without considering water shortage.Sweet sorghum has more potential in Guangxi(5.1-8.9Mt)and Guizhou(4.1-7.0Mt),while cassava has higher bioethanol energy potential in Yunnan(4.2-15.9Mt),Guangxi(2.7-10.2Mt)and Guizhou(2.4-9.1Mt)than other regions.If agricultural residues and energy crops are developed at the same time,bioethanol yields of49.8-153.6Mt can be achieved.(5)The bioethanol potential of agricultural residues in Jilin Province far exceeds the gasoline production in this region.In Guangxi Province,sweet sorghum’s maximum energy potential is 1.7 times that of gasoline production.The highest bioethanol potential of cassava in Yunnan Province could contribute 190 percent of the province’s gasoline production.For regions with no gasoline output,such as Chongqing and Guizhou,developing bioethanol industry can make up the gasoline output gap between these regions and other provinces,and improve the availability of energy.In 2021,China’s gasoline output is 154.57 million tons.Considering the constraint conditions of water and soil resources,agricultural residues,sweet sorghum and cassava energy crops can replace 11.0%-44.3%,12.1%-21.2% and 9.1%-33.9% of the national gasoline output,respectively.If agricultural residues and energy crops are developed at the same time,it is equivalent to contributing32.2% to 99.4% of the national gasoline production demand.In conclusion,under the dual constraints of water and soil resources,the development potential of agricultural residues in Heilongjiang and Jilin provinces is relatively greater.The advantages of agricultural residues production in these regions can be fully utilized,and the resource potential between regions can be fully tapped by constructing large-scale straw fuel ethanol industrial plants.To explore advanced and efficient ethanol treatment and water saving technologies through cooperation with scientific research institutes or professional research institutes in areas with severe blue water shortage.And vigorously develop water-saving agriculture(drip irrigation,micro-irrigation)to reduce local water pressure.Energy crops sweet sorghum and cassava are more suitable for development in regions with abundant water resources such as Guangxi,Yunnan and Guizhou.In order to further increase the potential of bioethanol energy,high yield and high stability varieties should be introduced,and marginal land with low productivity should be fully utilized.