The 24th Conference of the Parties to the United Nations Framework Convention on Climate Change (COP24) was held between 2 and 15 December 2018 in Katowice, Poland. As part of its Paris Agreement commitment, China pledged to peak carbon dioxide (CO2
) emissions around 2030, striving to peak earlier, and to increase the non-fossil share of primary energy to 20% by 2030 [1
]. Further, China’s Energy Supply and Consumption Revolution Strategy (2016–2030) sets goals of capping China’s absolute primary energy consumption at or below 5000 million and 6000 million tons of coal equivalent (Mtce) by 2020 and 2030, respectively [2
]. Yet by the end of 2017, China emitted 28% of the world’s energy-related CO2
emissions, 76% of which were from coal use [1
]. China’s coal-based energy structure is the main cause of the current high-level carbon emissions and air pollution [5
]. The decentralized coal consumption in rural areas accounts for 10% of total coal consumption, equivalent to 400 million tons of standard coal [6
]. Since no dust removal, desulfurization, or other environmental protection measures have been taken, the pollutant discharge is 5–10 times higher than the thermal power loose coal, which is the main factor in pollution [7
]. In addition, the current utilization mode of biomass energy is lagging and the development and utilization of high-tech applications are still low, resulting in the discharge of a large amount of CO2
and soot, which seriously affects the ecological environment and living environment, and has a great potential for improvement [7
To achieve these international and domestic goals, China is considering substituting biomass energy for coal, as coal exerts the most detrimental impact on the environment. In particular, in order to cope with the severe haze in the North China Plain (NCP), since 2016 the Chinese government has promoted the replacement of decentralized coal consumption, coal to electricity, and coal to gas in Beijing, Tianjin, and Hebei. The sudden large-scale increase in gas consumption in winter led to the “gas shortage” in the winter of 2017. As a big agricultural country, straw resources are abundant in China. By 2015, the total amount of straw resources in China had exceeded 1 billion tons, accounting for 20%–30% of the total straw production in the world [8
]. Among them, corn straw resources in the NCP total 80.66 million tons, accounting for 19.55% of the total corn stalk production in China [9
]. By using straw as biomass energy, its efficient and rational use can not only alleviate the energy crisis but also play an important role in environmental protection and sustainable development [10
]. In rural areas, in order to replace traditional fuelwood, coal, and other energy sources, the development of distributed utilization of biomass energy is also an important direction. Especially in areas where agricultural waste resources are abundant [11
], there is a huge space for development and utilization. In the current stock structure, at least 900 million tons of standard coal, such as decentralized burning coal and rural fuelwood, need to be replaced to optimize China’s energy structure [5
]. As renewable and clean energy, biomass energy has attracted more and more attention [12
]. Especially in remote rural areas, exploring biogas or briquetting utilization of biomass energy, supplemented by electric energy (the NCP is dominated by coal power (CP)), is also a beneficial attempt to improve the environment and the living standard of rural residents. At present, the straw utilization schemes suitable for the improvement of rural living energy mainly include biogas (such as centralized biogas production (CBP)) and briquetting (such as straw briquetting fuel (SBF)).
CBP refers to the use of crop straw as the main raw material, anaerobic fermentation through a digester, and the production of high-calorific gas, which is supplied to farmers through a pipe network and used for cooking and other purposes. The raw material is more clean and sanitary, has no odor, can be located near residential areas, and has simple technology and convenient maintenance [13
]. Corn straw has a very strong potential as an energetic material [14
]. This project usually uses natural villages as the unit, and the system scale is hundreds of households. In recent years, the government’s related rural biogas policies are increasingly inclined to promote the development of large and medium-sized biogas projects. The SBF refers to the use of special equipment to make various crop straws, sawdust, sawdust, peanut shells, corn cob, straw, wheat straw, wheat bran, branches, and leaves into rods, granules or blocks under certain pressure and other solid formed fuels. It has good combustion characteristics, lasting firepower, high furnace temperature, and is easy to transport and store.
Therefore, this paper proposes the following model for improving rural energy structure: In rural areas, it is possible to try and build a new energy source structure based on biomass energy (e.g., CBP, SBF) and electric energy, which form a complementary rural energy structure of a “biogas, briquetting, electricity (BBE)” model to replace the loose coal and original utilization of straw. How feasible is this model? At present, research on this aspect is lacking. This study attempts to use the emergy analysis method to calculate the indicators of CBP and SBF in this respect in order to provide a theoretical basis for the development of the BBE model in rural areas.
Emergy analysis is the most common method for the comprehensive evaluation of biomass energy utilization [17
]. Emergy analysis is an ecological-economic system research theory and method headed by H.T. Odum [21
]. It is a new research method based on traditional energy analysis. It converts all forms of energy into a unified unit: solar joules (sej). Using a uniform energy value standard as a dimension, the different types and incomparable energy in the system are converted into energy values of the same standard to measure and analyze. It combines energy, economy and environment, takes emergy as dimension, and uses solar emergy conversion rate to measure energy, resources, information and services of different nature. It achieves the effect of using the same measurement standard to evaluate system structure, function, and benefit, thus realizing system environmental sustainability and efficiency [22
]. Emergy analysis has been applied to the evaluation of various biomass energy sources [25
The analysis results fully reflect the characteristics of the two straw utilization projects in the BBE model, and the optimal results can be achieved only in the full compliance state. Compared with the emergy analysis of household biogas in Gongcheng County (EYR, 1.10; EIR, 9.57; ESI, 0.10; ELR, 11.06) [22
], all indicators of the CBP project are completely superior to household biogas under full load. Of course, because of different raw materials, the various indexes of SBF are not as good as wood briquetting fuel (EYR, 4.68; ELR, 0.27; ESI, 17.26) [44
], so on the basis of consuming straw, the source of wood waste can be increased. In order to form a complementary rural energy structure of the BBE model, it needs to have strengths and avoid weaknesses in mode selection. Both projects have a low environmental load, which is important for reducing fossil energy consumption, environmental damage and slowing global warming. However, the emergy yields of both projects are low and do not have long-term sustainability. There are interdependencies between economic and environmental sustainability [45
]. Although the indicators of the two projects have their own advantages and disadvantages, they must also be combined with the characteristics of the model itself. The straw collection radius of CBP is small, and many types of agricultural waste can be absorbed [46
]. In addition to straw, based on reasonable site selection, it can also eliminate livestock manure and kitchen waste, which can greatly improve the rural environment [47
]. The biogas directly reaches the farmer’s home, so that the farmers use the pipeline gas to purify the kitchen environment. The application of biogas fertilizer has improved the quality of agricultural products, promoted agricultural production and increased farmer income.
The process of the analysis reveals the disadvantages and improvement measures of the CBP and SBF projects. There is still a lot of optimization space in crop planting, market development, project management, and so on. In the crop planting stage, controlling the emergy input of the planting stage can effectively improve the energy output rate of the two projects. Among them, in the investment in the planting stage, labor cost investment accounted for 60.59% (Table 1
, Table 2
and Table 3
). It can be seen that controlling the labor cost input during the planting stage can most effectively improve the emergy output rate. In terms of market development, the CBP project does not fully exploit the market value of digestate, and there is still great potential for market development and government support. The market development capability of the SBF project also has great development potential. The project can completely develop through the market, release production capacity and achieve great benefits. In terms of project management, through the CBP model, agricultural wastes are resourced and utilized, which not only reduces atmospheric pollution and protects the environment, but also provides clean energy for biogas and returning organic matter to the field, thus promoting the construction of a modern ecological recycling agricultural system [48
]. The advantage of the SBF model is that it overcomes the drawbacks of large biomass volume, is convenient for transportation and storage, has high combustion heat efficiency, and is easy to realize industrialization and large-scale application. Therefore, the material collection radius and product transportation radius can be very large. The use of the corn stalk briquetting fuel plant is expected to play an important role in increasing local resident income, improving rural ecological environments, alleviating energy shortages, guaranteeing energy security, and promoting new rural reconstruction [49
]. Moreover, the use of briquetting fuel is consistent with the long-standing practice of farmers burning solid fuels such as firewood, which can be used as a good substitute for coal and fully meets the requirements of sustainable development. It can be used in a variety of small hot water boilers, home heating stoves or fireplaces, as well as small power generation heating facilities [50
], so it is very convenient to promote and popularize. SBF provides convenient, clean and environmentally friendly energy for rural areas, reducing the damage of vegetation caused by rural fuel consumption; on the other hand, it is an industrial boiler with outstanding energy consumption and emissions as a substitute for coal and heavy oil. The use of it is an important boost to achieve energy conservation and emission reduction.
The government needs to provide corresponding support based on the strengths and weaknesses of the project. The government plays an essential role in the diffusion of improved technologies [30
]. In Italy, the wood-pellet market serves more than 15% of the nation’s apartments. In Austria, government grants that cover 30% of the investment cost have encouraged companies and homeowners to install biomass heating systems that burn wood chips or pellets, so that biomass fuels one-third of the heating market [51
]. Renewable energy technologies in Germany still enjoy a privileged position regarding their diffusion and marketing [52
]. At present, in addition to subsidies for the price of biomass power generation in China, other biomass utilization projects are handled by local governments [53
]. In the two typical cases selected, there are still many unreasonable places in everything from construction to operation. Product promotion and promotion are not in place, and government policy support is not in place, resulting in serious waste of production capacity. Subsidies for modern equipment purchase, policies of effective and renewable energy technologies, and educational investment in rural areas, may help for a positive transition in energy consumption for rural households [54
]. It is fully capable of fully releasing production capacity, better-improving the eco-economic benefits and sustainable development capabilities of the two models, and improving the competitiveness of rural renewable energy utilization. In the end, it meets the requirements of the new socialist countryside construction and ecological civilization construction that the Chinese government is vigorously promoting [55
The development of complementary integration based on local conditions is an important measure to optimize the energy consumption structure in rural areas, improve the ecological environment. The development and utilization of agricultural biomass energy and its industrial development not only have a good economic driving effect, but also can optimize the rural energy structure, adjust the structure of agricultural production, and improve the quality of life of farmers. First, in order to ensure production capacity, it is necessary to fully understand the market demand of the product. Wang et al. (2019), according to a survey, found the willingness to buy for CBP products is $
10.1 per capita, and analyzed the factors affecting residents’ willingness to buy [57
]. It is also necessary to adapt the needs of farmers to the local conditions and respect farmers’ living habits, improve the accessibility and affordability [58
]. Second, through the biogas and briquetting of biomass energy, and the integration and utilization of electricity coupled with other renewable energy sources such as solar energy, one can better optimize the connotation and applicability of the BBE model [59
]. Third, combined with the project’s own attributes and choosing the most suitable biomass energy use in rural areas, the optimal path of domestic energy use in rural areas is sought under the premise of economic and environmental sustainability. In the end, the BBE model returns the waste to the field, reduces environmental pollution, reduces the use of chemical fertilizers and pesticides, reduces production costs, improves the quality and output of agricultural products, and promotes the benign development of recycling agriculture [60
]. Through the improvement of the quality of life of farmers, the improvement of rural living conditions will enable the real sustainable development of agriculture.
In order to form a complementary rural energy structure of the BBE model, it needs to have strengths and avoid weaknesses in its mode selection. The process of the analysis reveals the disadvantages and improvement measures of the two modes. Both projects have high renewability and low environmental load. The two models are in full compliance with the environmental protection requirements of the current rural ecological civilization construction in terms of environmental indicators. However, it does not have long-term sustainability because economic indicators and sustainability indicators are not good. Especially in the case of severe insufficient market demand, capacity cannot be released and the economic and sustainability of the two models are relatively poor, but with the release of the production capacity, economics and sustainability are gradually improving. The two models still have great potential for optimization and improvement. The government needs to provide corresponding support based on the strengths and weaknesses of the project. From the perspective of economic value, a certain gap needs to increase government intervention and support. The development of complementary integration based on local conditions is an important measure to optimize the energy consumption structure in rural areas and improve the ecological environment.
However, this study has several limitations that need attention. First, there are many types of projects, and the typicality of the project has certain limitations. Secondly, according to the research projects in Hebei Province, they have certain limitations to estimate the whole NCP. Third, the utilization of straw resources is now a hot topic, the updating of technologies and models is changing with each passing day, and the project parameters change rapidly. Therefore, more investigations should be conducted to enhance the reliability and applicability of the research results.