Crop residue utilizations and potential for bioethanol production in China

Renewable and Sustainable Energy Reviews 113 (2019) 109288

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Crop residue utilizations and potential for bioethanol production in China Yan Ru Fang a b

a,b

, Yi Wu

a,b

, Guang Hui Xie

a,b,∗

T

College of Agronomy and Biotechnology, China Agricultural University, 100193, Beijing, PR China National Energy R&D Center for Biomass, China Agricultural University, 100193, Beijing, PR China

ARTICLE INFO

ABSTRACT

Keywords: Bioenergy Biomass Bioethanol potential Collectable Straw Sustainability

Crop residue is an abundant and lucrative biomass resource in China, which is expected to decrease its reliance on coal and petroleum through the use of crop residue. This study was conducted with the data of crop production released by the National Bureau of Statistics of China and 1417 valid face-to-face questionnaires on residue utilizations. It was found that the crop residue increased from 725.47 Mt in 2007 to 897.06 Mt in 2016 at the annual rate of 2.63%. It amounted for 897.06 Mt including 781.32 Mt field residue and 115.74 Mt and process residue, respectively in 2016. Field residue retained in field exhibited the highest ratio (35.19%), for bioenergy use was less than 1%. A total amount of available field residue (AFR) for bioethanol was 254.57 Mt in 2016. Maize provided the greatest quantity of AFR (95.00 Mt), followed by rice (78.90 Mt) and wheat (18.89 Mt), sum of three accounted for 76% AFR in China. The largest AFR was found in Heilongjiang (39.79 Mt), followed by Henan (31.03 Mt) and Jilin (22.51 Mt). The density of AFR exhibited high in NEC and CSC and southeast provinces of SWC. The bioethanol potential was 124.3 Mt in 2016, with maize, rice and wheat residues representing 77.8% of the total. The four top-ranked provinces of bioethanol potential were Heilongjiang, Henan, Jilin, and Sichuan. It was suggested that financial and taxation support are necessary to promote cellulosic bioethanol research and industrial use in its early stage.

1. Introduction

USA and Brazil, contributed to at least 84% of the worldwide ethanol production, whereas China's only contribution was only 3% [10]. However, considering China's abundance of feedstock, bioethanol fuel production may be an inevitable long-term energy source for this country [11–13]. Crop residue has high contents of cellulose and hemicellulose, which can be hydrolyzed into fermentable sugars, for conversion into bioethanol. Since crop residue is a key feedstock for bioethanol production, knowing the quantity of this resource input is particularly important. To obtain widespread and efficient use of crop residue in China, the most important task is to evaluate the available crop residue quantity for the entire country and its provincial regions. Specifically, the amount of available crop residue is based on the total crop residue produced and its collectable quantity [14]. The crop residue weight in China has been reported [15–18], with some researchers estimating that the crop residue weight in China was 620 Mt in 2002 [19]; the crop residue weight was estimated to be 630 Mt per year from 1995 to 2005 [20]; later, in 2008, the crop residue weight was reportedly 751 Mt [17]. However, when the same years were assessed [21], different crop residue weights of 641, 649 and 774 Mt were found for the years 2002, 1995 to 2005 and 2008, respectively. A major reason for these disparate quantities for the same years is the different values that were used for

Biomass as a potential low-carbon emitting, renewable energy source, is receiving increasing attention and is being developed around the world [1,2]. Renewables are, by far, the fastest-growing fuel source, as they providing approximately 14% of the world's primary energy [3]. Globally, China is now the largest primary energy consumer and producer, and its rapidly increasing energy demand, especially for coal, petroleum, and other liquids, has made China influential in world energy markets [4]. Furthermore, China is on track to become the world's largest emitter of greenhouse gases [5], and China's carbon emissions increased the most among all countries in 2017 [6]. Due to growing energy consumption and associated environmental consequences of petroleum and other nonrenewable fuel types, bioenergy as a form of renewable energy has received much interest in China [7]. In this context, crop residue is an important and abundant biomass resource in China; if the full potential of crop residue utilization is exploited and realized, China could decrease its reliance on coal and petroleum. Using crop residue for bioenergy production entails the production of gaseous, biomass pellets, solid fuels, and liquid fuels [8]. Among these fuel types, bioethanol could become a popular alternative automotive fuel throughout the world [9]. In 2017, the two largest ethanol producers, ∗

Corresponding author. College of Agronomy and Biotechnology, China Agricultural University, 100193, Beijing, PR China. E-mail address: [email protected] (G.H. Xie).

https://doi.org/10.1016/j.rser.2019.109288 Received 29 December 2018; Received in revised form 22 June 2019; Accepted 18 July 2019 1364-0321/ © 2019 Elsevier Ltd. All rights reserved.

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Y.R. Fang, et al.

List of abbreviations: AFR CSC EC FRI Kt Mt

NC NEC NWC PRI SWC t ha−1 t km−2

available field residue Central-South China East China Field residue index kilo (thousand) metric tons million metric tons

the residue/crop product coefficients (i.e., crop residue index). Thus, a robust crop residue index should be developed and consistently adopted because it is the basic information needed to evaluate the amount of crop residue available for use in bioethanol production. Two previous studies [15,16] estimated the crop residue yields of nine crops in 2005 and 2009, respectively, but did not provide details of the quantity of crop residue for different regions in China. In another study, the crop residue distributions in 2008 and 2009 were analyzed [17], however, the data are already outdated; and the crop residue yields in 2010 were presented for nine crops [18], but not for the whole crop residue types in China. However, all of these studies overlooked the quantity of crop residue available for bioethanol production, which is an important parameter for determining the energy utilization of crop residue, since it is also a key guideline for resolving China's energy deficit and for determining where the bioethanol industry should locate new plants. Additionally, the quantity of crop residue available for bioethanol production serves as critical reference for policy and planning purposes [22]. Townsend et al. [23] analyzed wheat straw availability for bioenergy in England through a postal survey and found that wheat straw used for bioenergy has limitations. In Canada, Li et al. [24] found that the available crop residues for ethanol production averaged 48 Mt per year (for 2001–2010), which obtained by deducting the soil conservation and livestock uses. In summary, the research to date offers no standardized estimation for the amount of available crop residue for bioenergy production nor does it consider its current competitive uses or deduct the uncollectable part from the total crop residue.

North China Northeast China Northwest China Process residue index Southwest China metric tons per hectare metric tons per square kilometer

Therefore, it seems timely and necessary to provide a better understanding of the quantities of biomass supplies so that they can be monitored by including updates of their spatial distributions and temporal variations to meet and accommodate innovative technologies and industry strategies for cellulosic bioethanol development in China. Therefore, this paper's aims were to evaluate the crop residue diversity, quantity, yield, and density as well as the spatiotemporal distribution across China, which was supported by analyses of the collectable and available quantity for bioethanol production of each crop residue type, and calculations of the bioethanol potential of the available quantities of crop residue. This study used the statistical data of crop production across China from 2007 to 2016. The results of this study can be used as a reference for selecting promising sites for use by crop residue-utilization enterprises, and this study provides recommendations regarding which crop residue type to use in different regions as well as policy advice to governments to ensure the sustainable use of crop residue and to maintain regional agricultural production systems. 2. Methods and data sources To assess the potential of bioethanol production from crop residue, the total quantity and availability of crop residue should be determined. The premise behind studying the available quantity of crop residue is to consider both the utilized and collectable quantities. Yearly estimations at the national and provincial region levels were made from 2007 to 2016 for temporal analysis and for 2016 for spatial analysis. In this

Fig. 1. Division of China into six regions (NC: North China, NEC: Northeast China, EC: East China, CSC: Central-South China, SWC: Southwest China, NWC: Northwest China). 2

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way, estimation of the potential bioethanol production from crop residue was conducted by clarifying the following: (1) study areas and crops; (2) field residue index, process residue index, and the crop yields; (3) utilization of crop residues; (4) collectable coefficients and availability of crop residues for bioethanol production; and (5) bioethanol conversion rate and bioethanol potential.

stubble height, fall-off rate of leaves and branches, ratio of leaf biomass, and loss rate during crop residue collection and storage. According to these findings, the calculated collectable coefficient of crop residue for China is listed in Table 3. According to the utilization of crop residue, the available crop residue for bioethanol production was determined by including the quantity of crop residue burnt in the field and abandoned. Since the portion of crop residue retained and burnt in the field was not removed from the field, this quantity should be included in the stubble weight, while abandoned and competitive uses of crop residue need be removed from the field, as these quantities excluded the stubble weight. Thus, the quantities of residues retained and burnt in the field were calculated by multiplying the total crop residue with the utilization ratio of each, while the remaining quantity of utilization came from multiplying the total crop residue, utilization ratio, and collectable coefficient together. Therefore, available crop residue for bioethanol production was calculated based on the amount of residue burnt in the field as well the collectable quantity of abandoned residue. It means that the total crop residue was deducted from the quantity retained in field and the collectable quantity of competitive uses was the result of the available quantity of crop residue for bioethanol production.

2.1. Study regions and crops A total of 31 provincial regions—22 provinces, 5 autonomous zones, and 4 municipalities in mainland China—were divided into six regions (Fig. 1 [25]): North China (NC), Northeast China (NEC), East China (EC), Central-South China (CSC), Southwest China (SWC), and Northwest China (NWC). For the sake of easing communication, in this study, autonomous zones and municipalities are simply referred as “provinces”. Following the National Bureau of Statistics of China [26], this research studied 16 crops: wheat, maize, other cereals, beans, tubers, cotton, peanut, canola, sesame, other oil crops, jute and ambary, other fibers, sugarcane, sugar beet, and tobacco. 2.2. Field residue index and process residue index In its calculation, crop residue includes both field residue and process residue components, for which annual crop production is multiplied by the values of the field residue index (FRI) and the process residue index (PRI) [17,27], as summarized in Table 1. A total of six types of process residues—rice hull, maize cob, cotton seed hull, peanut husk, sugarcane bagasse and sugar beet bagasse—are produced in the primary manufacturing process [17]. Most crop residue data are presented on an air-dried basis (moisture content is approximately 15%) in this research. Not surprisingly, the quantity of crop residue differs among China's 31 provinces; this disparity is not solely driven by crop production but is also related to the crop FRI and PRI values. Based on the author group's research [28–32], both FRI and PRI were recalculated (Table 2A, Table 2B) for the 16 crops per province to better express the crop residue quantities from 2007 to 2016. The crop yield data were taken from the National Bureau of Statistics of China [26] for 16 major field crops grown in China over the 10-year period. For each crop, the study also calculated the residue yield and distribution density. The residue yield is the crop residue quantity divided by the arable land area, and the distribution density is the crop residue quantity divided by the administrative land area.

2.5. Bioethanol conversion rate and its potential Calculation method of a theoretical bioethanol conversion rate that was based on the cellulose and hemicellulose contents of different crop residues [36–42] (Table 4). The availability of crop residue multiplied by this bioethanol conversion rate equaled the bioethanol potential. 3. Results and discussion 3.1. Production and distribution of crop residue 3.1.1. Changes in crop residue production from 2007 to 2016 The total crop residue at an annual rate 2.63% increased from 725.47 Mt in 2007 to 897.06 Mt in 2016 (Table 5). The yearly quantity of crop residue in China averaged 821.96 Mt, which comprised 711.46 Mt of field residue and 110.50 Mt of process residue from 2007 to 2016 (Table 5). The field residue showed a stable increasing trend, Table 1 Calculations for field residue, process residue, and crop residue in China.

2.3. Utilization of crop residue The field residue utilization percentage were summarized from 1417 valid questionnaires, for which were conducted face-to-face field surveys to interview farmers in 15 provinces during 2015–2016. In questionnaire surveys, all the possible residue utilizations included, i.e. field residue retained in field, burnt in field, and abandoned, and used for animal feed, cooking and heating, paper pulp, electricity generation, other bioenergy, and others. The utilization of other bioenergy use was mainly for biogas production, while a few uses were for biodiesel ethanol, “others” utilization simply means that the crop residue was eventually used but in a way that was unlike the eight other types; a typical example was the use of field residue for house roofing. In this study, the summarized crop residue utilizations were retained in field, burnt in field, abandoned, and competitive uses. 2.4. Collectable coefficient and availability of crop residue for bioethanol production

Residue type

Field residue (FR)

Process residue (PR)

Crop residue (CR)

Rice

APa × FRId

FR + PR

Wheat Maize Other cereals Beans Tubers Cotton

AP × FRI AP × FRI AP × FRI AP × FRI AP × FRI AP × FRI

Peanut Canola Sesame Other oil crops Jute and ambary Other fibers Sugarcane Sugar beet Tobacco

AP × FRI AP × FRI AP × FRI AP × FRI AP × FRI AP × FRI AP × (1–70%b) × FRI AP × (1–75%c) × FRI AP × FRI

rice hull, AP × PRIe N/A cob, AP × PRI N/A N/A N/A seed hull, AP × PRI husk, AP × PRI N/A N/A N/A N/A N/A bagasse, AP × PRI bagasse, AP × PRI N/A

N/A: means not available, no process residue for the crop. a AP: Annual production of a crop (from 2007 to 2016). b Assuming a 70% moisture content for sugarcane. c Assuming a 75% moisture content for sugar beet. d Field residue index. e Process residue index.

The collectable quantity of crop residue refers to the maximum amount of the residue that can be taken from the field and used [33]. Previous research [32,34,35] has studied the major factors associated with the collectable quantity of crop residue, such as crop height, 3

FR FR + PR FR FR FR FR + PR FR + PR FR FR FR FR FR FR + PR FR + PR FR

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Table 2A Values used for Field Residue Index (FRI) and Process Residue Index (PRI) of different field crops in North China (NC), Northeast China (NEC), and East China (EC). Residue type

China

NC

NEC

Beijing Field residue Rice Wheat Maize Other cereals Beans Tubers Cotton Peanut Canola Sesame Other oil crops Jute and ambary Other fibers Sugarcane Sugar beet Tobacco Process residue Rice hull Maize cob Cotton seed hull Peanut husk Sugarcane bagasse Sugar beet bagasse

Tianjin

Hebei

Shanxi

0.99 1.33 1.05 1.90 1.49 0.51 3.98 0.96 2.59 2.78 2.48

1.04 1.26 1.34 1.21 0.99 0.96 1.90 1.90 1.50 1.50 0.40 0.40 3.63 3.63 0.83 0.83 N/A N/A N/A N/A 2.48 2.48

0.90 1.27 1.03 1.90 1.50 0.40 3.63 0.83 2.30 2.62 2.48

1.82

N/A

N/A

1.82 N/A

4.23 0.26 0.23 0.65

N/A N/A N/A N/A

N/A N/A N/A N/A

Inner Mongolia

Liaoning

Shanghai

Jiangsu

4.23 4.23 N/A N/A 0.23 0.23 0.69 0.69

N/A 0.26 N/A N/A

4.23 0.26 N/A 0.69

4.23 0.26 N/A 0.69

0.20 0.18 0.47

0.20 0.17 0.47

0.20 0.20 0.18 0.20 0.47N/A

0.23 0.24 0.47

0.19 0.24 0.47

0.23 0.24 0.47

0.23 0.24 0.47

0.23 0.24 0.47

0.24 0.24 0.47

0.28 0.20

0.28 0.28 0.25 0.28 0.28 N/A N/A N/A N/A N/A

0.28 N/A

0.28 0.28 N/A N/A

0.28 0.20

0.28 0.20

0.28 0.20

0.28 0.20

0.28 0.20

0.28 0.30 0.20 N/A

0.05

N/A

N/A

0.20 0.14 0.47

0.05

0.20 0.17 0.47

0.05

0.05

0.05

0.05

0.05

N/A

N/A

N/A

1.82 N/A

0.98 1.41 0.91 1.90 1.67 0.50 4.65 1.22 2.66 2.96 2.48

Shandong

N/A N/A 0.23 0.69

4.23 N/A 4.23 N/A N/A N/A 0.23 0.23 0.23 0.69 0.69 0.69

1.08 1.39 0.89 1.90 1.67 0.55 4.65 1.04 2.66 2.96 2.48

Jiangxi

N/A

1.82

1.03 1.16 0.97 1.90 1.67 0.51 4.65 1.22 2.66 2.96 2.48

Fujian

N/A

N/A

1.01 1.25 0.93 1.90 1.67 0.51 4.65 1.22 2.66 2.96 N/A

Anhui

N/A

N/A

1.17 1.47 0.97 1.90 1.67 0.51 4.65 1.22 2.66 2.96 2.48

Zhejiang

N/A

0.20 0.17 0.47

0.98 1.27 1.00 1.90 1.42 0.57 3.63 0.83 2.30 2.62 2.48

Heilongjiang

1.21 1.13 0.89 1.90 1.67 0.51 4.65 1.22 2.66 N/A 2.48

0.20 0.17 0.47

0.79 1.17 1.31 1.90 1.50 0.59 3.63 0.83 2.30 2.62 2.48

Jilin

0.98 0.87 1.30 1.09 1.07 1.15 1.90 1.90 1.65 1.24 0.57 0.57 3.63N/A 0.83 0.83 N/A 2.30 2.62 2.62 2.48 2.48

0.22 0.19 0.47

0.95 1.30 1.15 1.90 1.50 0.40 3.63 0.83 2.30 2.62 2.48

EC

1.22 1.44 0.93 1.90 1.50 0.40 3.66 0.86 2.30 2.62 2.48

1.82 N/A

4.23 N/A 4.23 N/A 0.26 0.26 0.26 N/A N/A N/A N/A 0.23 0.70 0.69 0.69 0.69

N/A

N/A

N/A

0.20 0.15 0.47

0.05

N/A: means not available, because of no statistics for crop production according to the China Statistical Yearbook [26].

Table 2B Values used for Field Residue Index (FRI) and Process Residue Index (PRI) of different field crops in Central-South China (CSC), Southwest China (SWC), and Northwest China (NWC). Residue type

CSC Henan

Field residue Rice Wheat Maize Other cereals Beans Tubers Cotton Peanut Canola Sesame Other oil crops Jute and ambary Other fibers Sugarcane Sugar beet Tobacco Process residue Rice hull Maize cob Cotton seed hull Peanut husk Sugarcane bagasse Sugar beet bagasse

0.92 1.34 1.05 1.90 1.50 0.40 3.34 0.83 2.30 2.62 2.48 1.82

SWC Hubei 0.91 1.44 0.95 1.90 1.67 0.50 4.65 1.22 2.66 2.96 2.48 1.82

Hunan

Guangdong

Guangxi

Hainan

NWC

Chongqing

Sichuan

Guizhou

Yunnan

Tibet

Gansu

Qinghai

Ningxia

Xinjiang

1.01 0.89 1.27 1.32 0.91 1.08 1.90 1.90 1.50 1.50 0.71 0.59 N/A 3.63 N/A 0.83 2.30 2.30 N/A 2.62 2.48 2.48 N/A N/A

0.80 1.31 1.10 1.90 1.50 0.59 3.63 0.83 2.30 N/A 2.48 N/A

N/A 1.36 1.08 1.90 1.50 0.71 N/A N/A 2.30 N/A 2.48 N/A

0.94 1.12 1.21 1.90 1.50 0.59 N/A N/A N/A N/A 2.48 N/A

0.70 1.41 1.14 1.90 1.46 0.59 3.95 0.83 2.30 2.62 2.48 N/A

N/A N/A N/A N/A

4.23 0.26 N/A 0.69

4.23 N/A N/A N/A 0.23 0.23 0.69 0.69

N/A N/A N/A 0.69

4.23 N/A 0.23 0.69

0.23 0.17 N/A

0.20 0.17 0.47

0.20 N/A 0.17 0.17 0.47 N/A

0.20 0.17 N/A

0.20 0.17 0.47

0.28 0.20

0.93 1.01 1.43 1.32 0.93 0.89 1.90 1.90 1.67 1.67 0.50 0.55 4.65 N/A 1.22 1.22 2.66 2.66 2.96 2.96 2.48 N/A 1.82 1.82

1.04 1.27 0.90 1.90 1.67 0.55 4.65 1.22 2.66 2.96 2.48 1.82

1.14 0.86 N/A 1.12 0.90 0.93 1.90 1.90 1.67 1.67 0.55 0.47 N/A N/A 1.22 1.22 N/A 2.66 2.96 2.96 N/A 2.48 1.82 N/A

0.85 1.16 0.95 1.90 1.67 0.47 4.65 1.22 2.66 2.96 2.48 1.82

1.08 1.34 0.90 1.90 1.67 0.47 4.65 1.22 2.66 N/A 2.48 N/A

1.08 1.25 0.89 1.90 1.67 0.47 N/A 1.22 2.66 N/A 2.48 N/A

4.23 4.23 4.23 N/A 0.26 0.26 0.26 0.26 N/A N/A N/A N/A 0.69 0.69 0.83 0.69

4.23 0.26 N/A 0.69

4.23 4.23 0.26 0.26 N/A N/A N/A 0.69

4.23 0.26 0.23 0.69

4.23 0.26 N/A 0.69

4.23 0.26 0.23 0.51

0.24 0.23 0.24 0.24 N/A N/A

0.23 0.23 0.47

0.23 0.26 0.47

0.23 0.23 N/A

0.28 0.20

0.28 0.20

0.28 0.20

N/A N/A

0.05

N/A

0.20 0.14 0.47

0.24 0.24 0.47

0.22 0.24 0.24 0.24 0.47 N/A

0.23 0.24 0.47

0.28 0.20

0.28 0.20

0.28 0.20

0.28 0.20

N/A

N/A

N/A

0.31 0.20 N/A

N/A

0.28 0.20 N/A

0.28 0.20 N/A

0.05

N/A

Shaanxi

N/A

N/A: means not available, because of no statistics for crop production according to the China Statistical Yearbook [26]. 4

0.28 N/A N/A N/A 0.05

0.05

N/A N/A

0.28 N/A

N/A

0.05

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reports and this study confirms the substantial increase in crop residue production has lasted in the country for 63 years since 1952. The major field residues came from rice, wheat, and maize, the sum of which contributed to an annual average of 78% of total field residue between 2007 and 2016, according to this study (Table 5). This result agrees with the finding regarding the total rice, wheat and maize residues, which account for 75.7% of the total residue for all field crops [43]. The major process residues was composed of rice hull, maize cob, and sugarcane bagasse, the sum of which accounted for 93% of the total process residues annually in China from 2007 to 2016. Rice hull exhibited the highest annual average process residue (44.03 Mt), followed by maize cob (34.91 Mt) and sugarcane bagasse (23.71 Mt) throughout the 10-year period. A few previous studies reported that process residue was as well mainly composed of rice, maize and sugarcane. Bi [34], who reported that China produced 40.30 Mt of rice hull, 34.84 Mt of maize cob, and 29.80 Mt of sugarcane bagasse in 2008, and 34.38 Mt of rice hull, 25.72 Mt of maize cob, and 18.81 Mt of sugarcane bagasse annually during 2007–2009 according to Guo et al. [31]. These findings indicate that major components of the process residue are in agreement with this study in a great extent, although the pattern changes with time. In these two previous reports, the sum percentage of rice hull, maize cob, and sugarcane bagasse to the total process residue were 89.04% [31] and 95.98% [34], which is similar with the finding of this study (93%).

Table 3 Collectable coefficient values of the field residue types in China. Residue type

Collectable coefficient

Residue type

Collectable coefficient

Rice Wheat Maize Other cereals Beans Tubers Cotton Peanut

0.79 0.70 0.88 0.84 0.54 0.76 0.88 0.83

Canola Sesame Other oil crops Jute and ambary Other fibers Sugarcane Sugar beet Tobacco

0.65 0.84 0.85 0.87 0.86 0.70 0.75 0.93

Table 4 Theoretically based bioethanol conversion rate of the field residue types in China. Residue type

Conversion rate (g kg1 )

Residue type

Conversion rate (g kg1 )

Rice Wheat Maize Other cereals

521.09 487.75 487.89 498.91

460.20 492.19 492.19 481.14

Beans Tubers Cotton Peanut

363.51 561.20 442.65 342.54

Canola Sesame Other oil crops Jute and ambary Other fibers Sugarcane Sugar beet Tobacco

481.14 439.62 436.10 401.59

3.1.2. The state–of–the-art of crop residue For the current state, the total crop residue amounted to 897.06 Mt, which consisted of 781.32 Mt of field residue and 115.74 Mt of process residue in 2016 in China (Table 6). Because the crop production data was released by the official statistics administration after a duration of some 2–3 years, the data reported in this study was the latest. The production of crop residue in 2016 has not been reported in the previous literature to the best of our knowledge. The total mass of all crop residue was 875.33 Mt in 2015 in China, according to this study. It includes 757.79 Mt of field residue and 117.54 Mt of process residue. For the same year, Cao et al. [45] reported residue production of 674.85 Mt, including 621.81 Mt from field crops and 53.04 Mt from fruits and vegetables. Its field residue from

while the process residue and total crop residue also increased, with some fluctuation evident over the 10-year period. There was a near 24% increase from 2007 to 2016 in the total crop residue, while the field residue and process residue increased by 25% and 18%, respectively. Assessment on crop residue in China has been well documented in the previous studies. Bi [34] reported that crop residue increased from 214.35 Mt in 1952 to 771.35 Mt in 2008. Jia et al. [43] reported that it increased from 356.40 Mt in 1978 to 819.70 Mt in 2014 in China. As well, the annual production of crop residue was reported as 532.99 Mt in 2007 by Ji [18], 750.36 Mt in 2008–2009 by Wang et al. [17], 846.00 Mt in 2012 by Li et al. [44]. The findings of these previous

Table 5 Estimates of field residue and process residue resources in China from 2007 to 2016. Residue type

2007 (Mt)

2008 (Mt)

2009 (Mt)

2010 (Mt)

2011 (Mt)

2012 (Mt)

2013 (Mt)

2014 (Mt)

2015 (Mt)

2016 (Mt)

Field residue Rice Wheat Maize Other cereals Beans Tubers Cotton Peanut Canola Sesame Other oil crops Jute and ambary Other fibers Sugarcane Sugar beet Tobacco Process residue Rice hull Maize cob Cotton seed hull Peanut husk Sugarcane bagasse Sugar beet bagasse Total crop residue

627.03 182.87 145.60 159.71 9.97 25.87 14.01 30.19 12.43 27.40 1.55 3.76 0.18 2.66 8.81 0.50 1.54 98.44 41.05 27.18 3.58 3.65 22.59 0.40 725.47

665.09 188.39 149.80 174.86 9.58 30.26 14.93 29.65 13.69 31.34 1.63 6.43 0.15 2.28 9.68 0.56 1.84 104.88 42.32 29.76 3.52 3.99 24.83 0.45 769.96

665.49 191.53 153.43 172.55 8.47 28.69 15.01 25.33 14.12 35.44 1.73 6.34 0.13 1.32 9.02 0.40 1.99 103.12 43.05 29.52 3.00 4.11 23.12 0.32 768.61

678.31 191.80 153.61 187.16 7.87 28.15 15.65 23.81 15.01 33.93 1.63 7.41 0.12 1.05 8.64 0.52 1.93 104.86 43.12 31.98 2.80 4.37 22.16 0.42 783.17

709.59 196.63 156.48 204.44 8.70 28.43 16.51 26.41 15.41 34.88 1.69 7.41 0.14 0.93 8.93 0.60 2.01 109.83 44.23 34.65 3.10 4.48 22.89 0.48 819.43

732.28 199.80 161.34 218.31 7.81 25.95 16.57 27.44 16.02 36.36 1.78 7.51 0.12 0.81 9.60 0.66 2.18 115.10 44.95 37.12 3.21 4.66 24.62 0.53 847.38

745.58 199.12 162.84 233.05 7.63 24.11 16.74 25.28 16.28 37.55 1.73 7.73 0.11 0.71 10.00 0.52 2.18 117.96 44.79 39.42 2.96 4.74 25.64 0.42 863.54

752.15 201.88 168.60 230.54 8.25 24.44 16.75 24.70 15.94 38.37 1.75 7.91 0.10 0.74 9.80 0.45 1.93 117.52 45.44 39.09 2.90 4.61 25.12 0.36 869.67

757.79 203.61 174.07 240.03 0.00 24.01 16.66 22.41 15.95 38.79 1.78 8.33 0.10 0.67 9.12 0.45 1.82 117.54 45.83 40.73 2.63 4.59 23.39 0.36 875.33

781.32 202.42 172.29 233.93 8.79 25.94 16.83 21.07 16.70 37.78 1.76 9.50 0.10 0.84 29.59 2.15 1.64 115.74 45.54 39.69 2.49 4.83 22.76 0.43 897.06

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Table 6 Spatial distribution of the crop residue in the six regions and 31 provinces of China in 2016. Region and province

NC Beijing Tianjin Hebei Shanxi Inner Mongolia NEC Liaoning Jilin Heilongjiang EC Shanghai Jiangsu Zhejiang Anhui Fujian Jiangxi Shandong CSC Henan Hubei Hunan Guangdong Guangxi Hainan SWC Chongqing Sichuan Guizhou Yunnan Tibet NWC Shaanxi Gansu Qinghai Ningxia Xinjiang Total

Field residue

Process residue a

Total crop residue a

Amount (Mt)

Percent (%)

Amount (Mt)

Percent (%)

Amount (Mt)

Percenta (%)

98.90 0.56 2.17 41.07 15.35 39.75 124.07 21.10 39.08 63.89 191.38 1.23 47.52 8.54 42.96 7.01 23.90 60.21 203.95 78.74 34.43 34.27 18.36 35.60 2.55 90.96 10.96 38.44 13.72 25.95 1.88 72.07 15.35 14.24 1.88 4.47 36.13 781.32

12.66 0.07 0.28 5.26 1.96 5.09 15.88 2.70 5.00 8.18 24.49 0.16 6.08 1.09 5.50 0.90 3.06 7.71 26.10 10.08 4.41 4.39 2.35 4.56 0.33 11.64 1.40 4.92 1.76 3.32 0.24 9.22 1.96 1.82 0.24 0.57 4.62 100

8.99 0.07 0.24 3.12 1.49 4.07 21.11 3.64 6.56 10.91 21.80 0.19 4.40 1.57 4.66 1.28 5.13 4.57 41.74 5.08 5.11 6.32 6.09 18.34 0.80 16.29 1.84 5.62 2.09 6.73 0.01 5.82 1.14 0.96 0.03 0.49 3.20 115.74

7.77 0.06 0.20 2.70 1.29 3.52 18.24 3.14 5.67 9.43 18.83 0.17 3.80 1.35 4.02 1.10 4.43 3.95 36.06 4.39 4.42 5.46 5.26 15.85 0.69 14.07 1.59 4.85 1.81 5.82 0.00 5.03 0.98 0.83 0.03 0.42 2.77 100

107.90 0.64 2.41 44.19 16.84 43.82 145.18 24.74 45.64 74.80 213.17 1.42 51.92 10.11 47.61 8.28 29.04 64.78 245.69 83.82 39.54 40.59 24.45 53.95 3.34 107.24 12.80 44.06 15.81 32.68 1.89 77.88 16.49 15.20 1.91 4.96 39.33 897.06

12.03 0.07 0.27 4.93 1.88 4.88 16.18 2.76 5.09 8.34 23.76 0.16 5.79 1.13 5.31 0.92 3.24 7.22 27.39 9.34 4.41 4.52 2.73 6.01 0.37 11.96 1.43 4.91 1.76 3.64 0.21 8.68 1.84 1.69 0.21 0.55 4.38 100

NC: North China, NEC: Northeast China, EC: East China, CSC: Central-South China, SWC: Southwest China, NWC: Northwest China. a Residue amount produced in each province as a percentage of each type of China's total residue value.

field crops was lower than this study due to its lower FRI values of wheat, rice, other cereals, and cotton. The FRI values are the only factor to affect the accuracy of field residue assessment for specific crops, production data of which could be collected from the official information administration. The researcher group of this study was the only one to determine FRI values with the latest field crop data and enough sample size for each province of China in the earlier report published in 2013 [17]. For this study, the FRI values of all the field crops were updated according to the latest crop production progress in each province. The total mass of all crop residue was 869.67 Mt in 2014 in this study. The finding was 99.39 Mt or 13% higher than the crop residue of 770.28 Mt reported by He [32] for the same year. This was because He [32] calculated the field residue with much lower FRI values of maize, sugarcane, and sugar beet. For example, He [32] made a mistake to use harvest index (0.46) as FRI (1.17) for maize. As a result, He [32] underestimated field residue of maize, which production was the largest one among all the crops in China.

CSC > EC > NEC > SWC > NC > NWC. The order of the total crop residue in Ref. [45] was the same as reported in this study, which studied the crop residue in 2015 in China. The decreasing orders were EC > CSC > NEC > SWC > NC > NWC and CSC > EC > SWC > NEC > NC > NWC for the field residue and process residue, on average, for 2008 and 2009, respectively [17]; the major reason for the slight differences reported in this study may be due to the different data that were taken from the different research years. This study reveals that the greatest quantities of crop residue and field residue were found in Henan province, accounting for 9.34% and 10.08% of the total residue in 2016 in China, respectively. However, the largest amount of process residue was produced in Guangxi, which accounted for 15.85% of the total process residue. Beijing had a total crop residue of less than 1 Mt, which corresponded to only 0.1% of China's total value. The study of [43] found that the largest quantity of field residue was produced in Henan, and [43] showed that Guangxi produced the greatest quantity of process residue, mainly the sugarcane bagasse, as in the present study. Rice and wheat straw were mainly found to be distributed in EC and CSC, whereas maize residue was mostly found in NEC and NC in 2016 according to the present study. Yang et al. [46] also reported that maize residue was mainly distributed in NEC and NC from 2008 to 2010. The residue of peanut, canola, sugarcane, and sesame crops were primarily distributed in CSC, with cotton straw mainly found in NWC, and SWC harbored the most tobacco residue among the six regions.

3.1.3. Spatial distribution of crop residue in regions and provinces The mass of the total crop residue varied between 77.88 and 245.69 Mt among six regions in China, with a decreasing order of CSC > EC > NEC > SWC > NC > NWC in 2016 (Fig. 2). The mass of the field residue exhibited the order of EC > CSC > NEC > NC > SWC > NWC, and the mass of process residue had the order of 6

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Fig. 2. Crop residue production in six regions of China in 2016 (including field residue and process residue). NC: North China, NEC: Northeast China, EC: East China, CSC: Central-South China, SWC: Southwest China, NWC: Northwest China.

Maize, wheat and cotton residues were the main residue types in the NWC region, which was same as in Ref. [47]. Rice field residue exhibited a high yield in NEC, topping the yield of

all crops in the six regions, except that of NWC, where the highest field residue yield was maize (Table 7). The yields of all the process residues were < 2 t ha−1. Maize had the highest density of field residue in NC,

Table 7 Crop residue yields and densities in China's six regions in 2016. Residue type

Field residue Rice Wheat Maize Other cereals Beans Tubers Cotton Peanut Canola Sesame Other oil crops Jute and ambary Sugarcane Sugarbeet Tobacco Process residue Rice hull Maize cob Cotton seed hull Peanut husk Sugarcane bagasse Sugarbeet bagasse

NC

NEC

EC

CSC

SWC

NWC

Yield

Density

Yield

Density

Yield

Density

Yield

Density

Yield

Density

Yield

Density

(t ha−1)

(t km−2)

(t ha−1)

(t km−2)

(t ha−1)

(t km−2)

(t ha−1)

(t km−2)

(t ha−1)

(t km−2)

(t ha−1)

(t km−2)

6.85 6.06 6.49 2.37 2.68 1.92 4.94 2.46 2.74 1.67 5.83 0.81 N/A 1.90 1.50

3.45 37.77 60.82 1.12 1.89 0.97 2.62 1.28 0.26 0.03 2.17 0.00 N/A 0.33 0.01

7.57 4.53 7.24 5.46 2.73 3.12 4.92 2.35 6.87 4.51 6.08 N/A N/A 2.76 1.85

35.65 0.29 111.59 0.27 7.37 1.62 0.00 2.45 0.01 0.03 1.14 N/A N/A 0.07 0.10

8.11 5.45 4.86 6.52 3.72 2.22 5.26 3.95 5.39 3.89 5.90 3.03 4.48 0.15 1.58

86.66 58.61 22.08 3.32 4.28 2.37 2.82 4.19 6.73 0.23 0.07 0.00 0.54 0.00 0.19

6.41 4.16 4.58 5.27 3.47 1.92 4.58 3.64 4.35 4.31 4.32 6.83 6.13 N/A 1.27

64.39 52.41 24.79 0.53 2.56 3.17 1.57 7.78 12.15 1.19 0.08 0.06 19.97 N/A 0.43

6.27 4.40 4.86 4.75 3.33 2.20 2.68 2.07 5.51 1.73 5.33 1.18 12.83 2.69 1.26

25.21 4.05 15.89 1.39 4.57 6.63 0.02 0.92 9.38 0.05 0.11 0.00 2.82 0.00 0.62

6.48 5.36 7.19 4.41 2.70 2.23 6.14 3.47 4.16 2.89 5.76 N/A 5.58 8.51 2.31

2.64 9.98 17.30 1.30 0.82 1.80 1.86 0.09 1.51 0.04 1.93 N/A 0.00 0.17 0.05

1.40 0.98 0.64 0.80 N/A 1.13

0.60 9.30 0.34 0.40 N/A 0.07

1.63 1.22 0.64 0.78 N/A 1.61

7.77 18.75 0.00 0.81 N/A 0.01

1.59 1.17 0.55 0.97 9.36 0.32

16.88 4.07 0.33 1.26 0.42 0.00

1.48 1.05 0.50 0.90 13.11 N/A

15.05 4.13 0.17 2.24 15.36 N/A

1.47 1.17 0.27 0.47 9.87 0.54

6.17 4.12 0.00 0.21 2.17 0.00

1.58 1.06 0.77 1.15 4.29 1.70

0.58 2.52 0.22 0.03 0.00 0.03

N/A: means not available because there were no statistics for crop production according to the China Statistical Yearbook [26]. NC: North China, NEC: Northeast China, EC: East China, CSC: Central-South China, SWC: Southwest China, NWC: Northwest China. 7

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NEC, and NWC but was displaced by the rice field residue of the other three regions (Table 7). In NEC, the density of maize residue was 111.59 t km−2, which was the highest found for any crop residue across all of China. With respect to the density of the process residue, those of rice hull in EC and CSC, maize cob in NEC, and sugarcane bagasse in CSC were > 15 t km−2, while for other process residues, they were < 10 t km−2.

that Heilongjiang. Beijing had the least collectable quantity of crop residue from 2007 to 2016. The collectable quantity of crop residue in the six regions in 2016 is illustrated in Fig. 4. This value was highest for Henan, at almost 60 Mt, followed by Heilongjiang, at more than 50 Mt. Shandong was ranked third, with 46.37 Mt of collectable quantity of crop residue. In terms of crop type, the greatest quantity of collectable crop residue came from maize followed by rice and wheat, which when pooled, accounted for 79.51% of the total crop residue in China. The total collectable residue of maize, rice and wheat accounted for 79.96% of that found in Ref. [18], which was same as in this study. For rice residue, its highest collectable quantity was in Hunan Province, at approximately 19.1 Mt, followed by Jiangsu, Heilongjiang, and Jiangxi, at > 15 Mt each. Sun et al. [51] studied the collectable quantity of crop residue in Jiangsu, which was 16.3 Mt in 2014 and had the same collectable coefficient as in the present study. Wheat straw in Henan has the greatest collectable quantity, at 32.5 Mt, which was also the highest quantity of collectable residue of all studied crops found in China. Heilongjiang had the maximum collectable quantity of maize residue, at more 31 Mt, followed by Inner Mongolia, with almost 25 Mt. Generally, the collectable quantity of cotton residue was small in China, peaking at 12.5 Mt in Xinjiang, followed 1.77 Mt in Shandong and no more than 1 Mt for all remaining provinces combined, with almost half of them having no cotton residue for collection. Guangxi had the largest collectable quantity of sugarcane residue (almost 14 Mt), followed by Yunnan (3.16 Mt) and Guangdong (2.69 Mt). The rest of the pooled provinces had under 0.5 Mt of sugarcane residue (near zero in 16 provinces). Ji [18] also gave a collectable coefficient for the crop residue collectable quantity calculation; however, the coefficient only showed the crop group; for example, wheat, maize and rice, and so on had the same coefficient due to these crops belonging to the cereal group, and no signal crop had a signal collectable coefficient, as in this study.

3.2. Utilization and the collectable crop residue 3.2.1. Utilization of crop residue The utilization of field residue exhibited a pattern of residue retained in the field (35.19%), burnt in the field (25.20%), abandoned (8.93%), feed (20.89%), cooking and heating (5.84%), paper pulp (0.44%), electricity (0.55%), other bioenergy use (0.13%) and others (2.83%) for all crops in 2016 (Table 8). Based on this study, the proportion of cooking and heating was lower than found in Ref. [48], which reported that the value was 20% and was unreliable. One plausible explanation is that the economic development and living conditions improved, which directly affected crop residue utilization; for example, the amount of residue used in households for cooking and heating would decrease as farmers gain a greater income and better quality of living. The National Development and Reform Commission [49] reported a similar result as in the present study, in that the total crop residue retained in the field was 43.2% and used in feed was 18.8% in 2015; these data are consistent with those reported in this research. The utilization ratio of different crop residues in China showed that more than 40% of residues of rice, wheat, tubers, and sugar beet were retained in the field, and the corresponding ratios for the other crop residue types were > 30% (Table 8). The rice, maize, cotton, and canola residue burnt ratios were all > 30%, and the burnt rate of other crop residue exceeded 18%. The crop residue for heating and cooking was still found and was important. However, the crop residue used for paper pulp, electricity generation, and other bioenergy production was generally very low for all crops. The residue of maize as feed was 25.68% (Table 8); Shi et al. [50] found a similar value of 29.15%.

3.3. Availability of crop residue for bioethanol production 3.3.1. Distribution of available crop residue The available quantity of crop residue for bioethanol production was estimated as 231.5 Mt annually from 2007 to 2016 (Fig. 5). Clearly, this value was greatest in Heilongjiang and Henan. From 2007 to 2011, the maximum amount of available crop residue was in Henan, but switched to Heilongjiang from 2012 to 2016. The total available crop residue of these two provinces accounted for 26% (55.03 Mt) and 28% (64.10 Mt) of the total available weight in China from 2007 to 2011 and 2012 to 2016, respectively. In addition to Henan and Heilongjiang,

3.2.2. Collectable quantity of crop residue As Fig. 3 shows, over the period of 2007–2016, all provinces showed a similar variation in this key parameter, except Heilongjiang. The total collectable crop residue sustained yearly increases corresponding to the increase in growth. In particular, the highest collectable quantity of crop residue was in Henan, except for 2013–2015, when the highest collectable quantity of crop residue in Henan was slightly surpassed by Table 8 Utilization and ratio values of different field residue types in China. Crop residue type

Retained in field (%)

Burnt in field (%)

Abandoned (%)

Feed (%)

Cooking and heating (%)

Paper pulp (%)

Electricity generation (%)

Other bioenergy use (%)

Others (%)

Rice Wheat Maize Other cereals Beans Tubers Cotton Peanut Canola Sesame Other oil crops Jute and ambary Other fibers Sugarcane Sugar beet Tobacco Total

43.67 40.49 30.23 38.13 39.12 42.48 31.30 30.14 30.62 30.38 30.38 30.14 30.14 36.63 42.48 36.63 35.19

34.68 18.27 30.94 27.96 19.76 18.89 31.38 22.03 32.81 27.42 27.42 22.03 22.03 24.37 18.89 24.37 25.20

3.49 2.46 4.77 3.57 1.82 10.29 11.23 12.09 9.38 10.74 10.74 12.09 12.09 13.89 10.29 13.89 8.93

12.85 32.41 25.68 23.65 26.14 20.14 17.48 20.79 18.10 19.44 19.44 20.79 20.79 18.20 20.14 18.20 20.89

3.02 3.44 6.89 4.45 10.51 6.82 6.99 4.39 7.72 6.05 6.05 4.39 4.39 5.76 6.82 5.76 5.84

0.99 1.06 0.34 0.80 0.44 0.38 0.40 0.24 0.38 0.31 0.31 0.24 0.24 0.31 0.38 0.31 0.45

0.16 0.53 0.58 0.42 1.74 0.59 0.46 0.38 0.59 0.49 0.49 0.38 0.38 0.49 0.59 0.49 0.55

0.12 1.08 0.00 0.40 0.05 0.04 0.06 0.03 0.04 0.03 0.03 0.03 0.03 0.03 0.04 0.03 0.13

1.02 0.26 0.57 0.62 0.42 0.37 0.70 9.91 0.36 5.14 5.14 9.91 9.91 0.32 0.37 0.32 2.83

8

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Fig. 3. Changes in the annual value of the collectable amount of crop residue in 31 provinces of China from 2007 to 2016.

Fig. 4. Collectable quantity of crop residue production in 31 provinces in 2016 (“Oil crops” includes peanut, canola, sesame, and other oil crops. “Other crops” includes other cereals, beans, tubers, jute and ambary, other fibers, sugar beet, and tobacco).

more than 15 Mt (average value from 2007 to 2016) of the quantity of available crop residue could be found in Jilin, Sichuan, Hunan, and Guangxi. In stark contrast, Shanghai, Beijing, Qinghai, and Tianjin each had a low quantity of available crop residue, which were all lower than 0.5 Mt for 2007–2016. In this study, the quantity of crop residue available for bioethanol

production was calculated by summing the burnt and collectable weights of the abandoned part, not unlike the calculations by Ref. [52], which also considered the burnt crop residue and its discarded quantity. Considering rural economic development and improvements in rural citizens’ living standards, household use of crop residue for cooking and heating should be added to the quantity of residue available for 9

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Fig. 5. Changes in the annual available amount of crop residue for bioethanol production in 31 provinces of China from 2007 to 2016.

bioethanol conversion in future years. For 2016, the total field residue available for bioethanol conversion was 254.57 Mt in China, and the amount of available field residue in the six regions is shown in Fig. 6. This value was highest in Heilongjiang (with almost 39.79 Mt), followed by Henan and Jilin, with 31.03 and 22.51 Mt, respectively. Shanghai and Beijing had little total available

crop residue, at 0.1 and 0.2 Mt, respectively. Yang et al. [53] also reported that Henan and Shandong each had a high available quantity of crop residue for bioenergy production in 2007. The highest available residue found was maize, with 95 Mt of it in China, which accounted for 37% of the country's total available residue, followed by rice residue (78.90 Mt), at only 16.1 Mt less than the maize

Fig. 6. Available amount of crop residue in China in 2016 (“Other crops” includes other cereals, beans, tubers, jute and ambary, other fibers, sugar beet, and tobacco. “Oil crops” includes peanut, canola, sesame, and other oil crops). 10

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Fig. 7. Distribution density maps of the availability of crop residue for bioethanol production in China in 2016 (province name see Fig. 1. “Other crops” includes other cereals, beans, tubers, jute and ambary, other fibers, sugar beet, and tobacco. “Oil crops” includes peanut, canola, sesame, and other oil crops).

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residue value. The third largest available crop residue was wheat, but only 18.89 Mt was found, corresponding to only 7.4% of total available crop residue. Hence, the crop residues of rice, wheat, and maize were still the major feedstock available for bioethanol production, representing 76% of the entire available crop residue in China. The highest quantities of available rice and maize residues were found both in Heilongjiang, at 18.35 and 19.28 Mt, respectively, while Henan had the highest value of wheat residue, at 12.79 Mt. Most of the available cotton residue was found in Shandong, and most of the available sugarcane residue was found in Guangxi (12.16 Mt).

following: CSC > NEC > SWC > NC > EC > NWC. Importantly, in the NC, NEC, and NWC regions the greatest amount of bioethanol was produced by maize residue, whereas in the remaining three regions, rice residue was the major feedstock. The bioethanol production potential varied between 0.041 and 19.81 Mt in the 31 provinces in 2016. Heilongjiang, Henan, Jilin, and Sichuan exhibited the highest level of bioethanol potential varied between 9.48 and 19.81 Mt. The sum of bioethanol potential in the four provinces accounted for 44.63% of the total, in which Heilongjiang was the highest (15%). Shanghai and Beijing exhibited the lowest level of 0.041 Mt and 0.06 Mt, respectively. A total of 96.7 Mt bioethanol came from maize, rice and wheat residues (a total of 486.4 Mt of available quantity), which accounted for 77.8% of the total bioethanol production potential in 2016 (Table 9). The available residues of maize, rice and wheat were also the major feedstock for bioethanol production in 2005 in Ref. [54]. Maize residue offered the largest bioethanol production potential, at 46.35 Mt, or 37.3% of China's total bioethanol potential. Ranking second in bioethanol potential was that from rice residue, at 33.1% of the national total. The bioethanol potential from maize and rice residues were greatest in Heilongjiang, at 9.4 and 9.6 Mt, respectively. Henan had the largest bioethanol potential from wheat residue, at 6.2 Mt, followed by Hubei Province. With respect to cotton and sugarcane residues, their bioethanol potential peaked in Shandong and Guangxi, respectively. Bioethanol annual consumption reached over 6 Mt in 2020, which was the bioethanol project goal of China 13th Five-Year Plan [55]. Only the signal residue of maize in China can produce 8-fold greater bioethanol utilization than the target of 2020. Therefore, crop residue is a steady feedstock for bioethanol production in China [54].

3.3.2. Distribution density of available crop residue for bioethanol production The distribution density of the crop residue available for bioethanol production in 2016 exhibited high in NEC and CSC and southeast provinces of SWC (Fig. 7). The highest density of available rice residue found in Hunan, followed by Heilongjiang, Hubei, Jilin, and Liaoning; that is, across China, NEC had the greatest density of available rice residue. The province of Henan had the highest density of available wheat residue, which was close to zero in nine provinces. The top four provinces regarding the available maize residue density were, in descending order, Jilin > Henan > Liaoning > Heilongjiang, whereas 10 provinces each had a density of < 1 t km−2. The available residues of cotton and sugarcane reached their highest density in Shandong and Guangxi, respectively. The top five provinces, ranked in descending order of available total crop residue were Henan > Jilin > Liaoning > Heilongjiang > Guangxi; however, in Tibet and Qinghai, the corresponding values were < 0.5 t km−2. The amount of crop residue available for bioethanol differs among the crop types, planting regions, plant structures, seasonal variations, and harvesting methods used [43,53].

3.5. Practical implications of this study

3.4. Bioethanol potential

According to this study, crop residue exhibited high potential for bioethanol production due to its available amount, most of which was assumed to be part of residue burnt in the field. Cellulosic bioethanol produced from crop residue has also been recognized a highly promising way to reduce fossil energy consumption and greenhouse house gas emissions [56,57]. Several technological pathways of commercial conversion have been developed and demonstrated [58], and further innovations are being researched and intensively demonstrated [59,60]. On the other hand, crop residue management causes great concern due to the large quantity being open burnt in the field, which had significant impacts on air pollution, climate change and potential human health in China between 1996 and 2015 [61–63]. The direct reason for farmers to burn residue in the field is that the residue cannot

Theoretical bioethanol production of crop residue followed a smooth, upward-sloping trend for the 2007–2016 period (Table 9), and was 28% higher in 2016 than in 2007. Rice residue generated the greatest amount of bioethanol production from 2007 to 2010, after which maize residue produced the greatest yield from 2011 to 2016. The total predicted bioethanol production from rice, wheat, and maize residues, as a percentage of China's total, ranged from 76% in 2008 up to 81% in 2015. The spatial distribution of bioethanol production in China in 2016 is summarized in Table 10. The expected bioethanol yield totaled 124.3 Mt, and its value per region ranked in descending order, as

Table 9 Theoretical production of bioethanol produced from different crop residue types in China in the period 2007–2016. Residue type

2007 (Mt)

2008 (Mt)

2009 (Mt)

2010 (Mt)

2011 (Mt)

2012 (Mt)

2013 (Mt)

2014 (Mt)

2015 (Mt)

2016 (Mt)

Rice Wheat Maize Other cereals Beans Tubers Cotton Peanut Canola Sesame Other oil crops Jute and ambary Other fibers Sugarcane Sugar beet Tobacco Total

35.05 8.21 31.88 1.32 1.31 2.09 3.71 1.35 4.76 0.24 0.60 0.02 0.35 6.17 0.08 0.33 97.47

36.53 8.28 35.06 1.27 1.68 2.20 3.59 1.48 5.48 0.26 1.03 0.02 0.29 6.94 0.08 0.39 104.57

36.96 8.38 34.43 1.12 1.57 2.27 3.11 1.51 6.30 0.27 1.01 0.01 0.19 6.45 0.06 0.42 104.06

37.84 8.28 37.50 1.04 1.55 2.33 2.85 1.58 6.12 0.26 1.18 0.01 0.15 6.21 0.08 0.42 107.40

39.31 8.55 40.86 1.15 1.53 2.43 3.05 1.60 6.48 0.27 1.18 0.01 0.14 6.47 0.09 0.44 113.56

40.04 8.66 43.59 1.03 1.30 2.46 2.76 1.65 6.73 0.28 1.20 0.01 0.12 6.98 0.10 0.48 117.39

40.11 8.71 46.42 1.01 1.19 2.49 2.34 1.67 7.02 0.27 1.23 0.01 0.10 7.28 0.07 0.47 120.41

40.65 8.98 45.62 1.09 1.26 2.53 2.11 1.65 7.25 0.28 1.26 0.01 0.10 7.15 0.06 0.42 120.43

40.93 9.31 47.87 0.00 1.20 2.55 1.79 1.65 7.39 0.28 1.33 0.01 0.09 6.64 0.06 0.39 121.49

41.12 9.22 46.35 1.16 1.35 2.57 1.46 1.69 7.30 0.28 1.52 0.01 0.10 9.62 0.18 0.35 124.27

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Table 10 Theoretical production of bioethanol from available crop residues in the six regions and 31 provinces in China in 2016. a

Region and province

Rice (Kt)

Wheat (Kt)

Maize (Kt)

Oil crops (Kt)

NC Beijing Tianjin Hebei Shanxi Inner Mongolia NEC Liaoning Jilin Heilongjiang EC Shanghai Jiangsu Zhejiang Anhui Fujian Jiangxi Shandong CSC Henan Hubei Hunan Guangdong Guangxi Hainan SWC Chongqing Sichuan Guizhou Yunnan Tibet NWC Shaanxi Gansu Qinghai Ningxia Xinjiang China

240.49 0.25 32.54 101.38 0.95 105.37 14839.79 2246.31 3032.07 9561.42 2114.49 29.41 656.49 184.87 442.90 154.37 615.19 31.25 15132.76 1070.07 3439.22 5404.28 2399.83 2458.97 360.39 8494.89 1190.61 3871.56 1285.64 2145.64 1.45 293.60 131.29 4.08 0.00 66.88 91.34 41116.02

0.00 0.00 0.00 0.00 0.00 0.00 46.45 3.75 0.17 42.53 475.04 0.97 116.48 2.25 114.54 0.05 0.26 240.48 7084.85 6240.41 830.76 11.36 0.53 1.79 0.00 1143.53 29.60 747.47 124.41 196.65 45.40 465.81 187.37 111.85 14.36 23.50 128.73 9215.69

7398.01 56.37 149.71 2303.15 1315.70 3573.08 21087.21 3811.45 7868.58 9407.18 1767.21 1.33 159.31 19.98 314.66 13.88 8.45 1249.60 9836.67 6700.58 1142.28 823.64 297.28 872.88 0.00 5664.39 709.89 2138.54 870.56 1938.61 6.80 596.28 177.55 184.11 5.88 117.66 111.08 46349.78

1088.08 1.04 4.68 255.71 55.74 770.90 211.06 49.11 92.63 69.32 1498.14 1.85 271.52 62.66 438.15 47.51 250.35 426.09 4001.16 784.33 1358.36 1735.24 101.65 11.73 9.85 3370.06 417.43 1848.24 672.65 393.62 38.13 618.52 96.17 196.19 29.33 56.96 239.87 10787.02

Cotton (Kt)

Sugarcane (Kt)

Other crops (Kt)

248.58 0.04 17.37 223.31 7.70 0.16 0.09 0.09 0.00 0.00 735.09 0.32 68.52 15.33 171.32 0.08 68.01 411.52 358.67 67.58 174.87 113.88 0.00 2.34 0.00 9.34 0.00 8.21 1.13 0.00 0.00 107.41 12.99 7.64 0.00 0.00 86.78 1459.19

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 101.71 0.29 4.71 32.44 10.58 19.34 34.35 0.00 6459.29 14.54 23.13 34.59 916.22 5344.10 126.72 3060.66 5.07 53.89 128.15 2873.56 0.00 0.07 0.07 0.00 0.00 0.00 0.00 9621.72

287.90 0.17 0.54 86.40 61.93 138.86 968.06 96.75 139.87 731.43 563.74 6.61 214.21 55.68 46.09 102.15 56.60 82.41 696.95 156.87 125.72 161.81 154.44 75.92 22.18 2443.66 538.54 811.34 420.47 486.89 186.41 761.97 142.42 311.12 62.83 53.79 191.81 5722.27

b

Total (Kt) 9263.06 57.87 204.84 2969.95 1442.01 4588.38 37152.67 6207.46 11133.33 19811.88 7255.42 40.78 1491.25 373.21 1538.23 337.37 1033.22 2441.36 43570.35 15034.38 7094.34 8284.81 3869.94 8767.73 519.14 24186.53 2891.14 9479.26 3503.01 8034.96 278.18 2843.66 747.86 815.01 112.40 318.79 849.61 124271.69

NC: North China, NEC: Northeast China, EC: East China, CSC: Central-South China, SWC: Southwest China, NWC: Northwest China. a Oil crops include peanut, canola, sesame and other oil crops. b Other crops include other cereals, beans, tubers, jute and ambary, other fibers, sugar beet, and tobacco.

be used for a satisfactory positive economic benefit; whereas burning was the easiest way to clean the field for next cropping without any cost. Currently, to prevent open burning of residue, governments in China have released various policy measures, which heavily primarily rely on prohibition and penalties [64]. However, unless a large industry is well developed to find a solution for crop residue with a proper value chain to satisfy economic requirements, farmers have no way to remove the extra residue from the field, except to burn it in the field. To reduce open burning of residue and increase its environmental benefits, it is of essential importance that cooperative policy makers from multiple administrative departments stimulate the development of cellulosic ethanol development in scientific areas [65] and industrial areas [56]. The Chinese government has been ambitious in planning how to apply ethanol-blend gasoline across the whole country by 2020. One of the strategies for this plan is to develop commercial cellulosic bioethanol for long-term development. Form 2014 introduced a series of financial and taxation policies, including a subsidy of 800 CNY/toms cellulosic ethanol. However, support for this policy is decreasing, and the industry cannot survive under the current market conditions, which indicates that relatively high financial and taxation support are necessary to promote cellulosic bioethanol research and industrial use in its early stage.

4. Conclusions China has large amounts of crop residue, which increased at an annual rate of 2.63% from 2007 to 2016. Its annual average production was 821.96 Mt composing of 711.46 Mt field residue and 110.50 Mt process residue during the 10-year period. The residue produced from maize (30.50%), rice (27.64%) and wheat (19.21%) accounting for 77.35% of the total crop residue in 2016. The sum of field residue retained in the field (35.19%), burnt in the field (25.20%), and use as animal feed (20.89%) accounted for 81.28% of the total field residue; whereas amount of 0.68% field residue was used for energy production in 2016. The greatest quantity of collectable crop residue was produced from maize (30.51%), followed by rice (30.51%) and wheat (17.37%), sum of which representing 78.39% of total. Henan province had the highest collectable quantity (59.45 Mt), followed by Heilongjiang (51.71 Mt) and Shandong (47.18 Mt). However, the highest value of available crop residue was in Heilongjiang (with almost 40 Mt), followed by Henan (31.03 Mt) and Jilin (22.51 Mt) in 2016. The descending order of the top five provinces regarding available total crop residue density was Henan > Jilin > Liaoning > Heilongjiang > Guangxi. The total theoretical bioethanol yield was 124.3 Mt, of which, 96.7 Mt came from maize, rice and wheat residues in 2016. The highest theoretical quantity of bioethanol production was found in Heilongjiang, followed by Henan, Jilin, and Sichuan. The major crop residue for bioethanol production in Heilongjiang and Sichuan was rice, in Henan and Jilin it was maize, the highest quantity of 13

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wheat residue for bioethanol conversion was in Henan. Therefore, this study recommends Heilongjiang, Henan, Jilin, and Sichuan as priority production locations, with maize, rice and wheat residues as promising feedstock for the bioethanol industries.

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