Comprehensive analysis of rural heating by methanol heating stove: Economy, emissions, and energy consumption

Abstract

Coal-fired heating, mainly used in North China, has caused serious environmental problems. Schemes to replace coal have been established, but they are not suitable for all scenarios. This paper proposes a heating scheme that uses a methanol heating stove (MHS) to meet the demand for clean, low-cost rural heating. The economy, emissions, and energy consumption of the MHS were analyzed. The scheme was compared with five common rural heating methods. The effects of the methanol price, power grid transformation, and gas pipeline construction on the annual cost of each heating scheme were investigated using a theoretical model. The MHS emitted 66 % less CO₂ and 95 % less SO₂ and NO_x and consumed 54 % less standard coal than bulk coal heating stoves. The low electricity supply required to vaporize methanol contributed to the cost and emission reduction. Methanol price fluctuations warrant government subsidies to incentivize the implementation of the scheme. For increased adoption, we recommend that the MHS be used in areas with low population densities and methanol prices. This approach can therefore be an ideal substitute for “coal-to-electricity” and “coal-to-gas” methods in areas that lack access to power or gas supplies and can replace air-source heat pumps in severe cold zones.

Introduction

North China is cool and dry for most of the year, and winter is especially cold due to its geographical location. The region has several rural provinces where coal is the main source of heat in winter. However, coal is the primary contributor of air pollutants (including greenhouse gases), which accumulate in the atmosphere in the winter and form haze (Xu et al., 2014; Guorui et al., 2017). Consequently, North China is covered in a blanket of haze during this season, which negatively affects human health (Siyi et al., 2018). Rural areas are the major consumers of bulk coal for heating. The annual consumption of coal for heating in China is approximately 400 Mtce, of which 200 Mtce is bulk coal (National Development and Reform Commission et al., 2017). Therefore, clean heating techniques can help protect the environment in this region. By 2016, the total heating area in China had reached 20.6 billion m², to which rural buildings contributed 31 %. Bulk-coal heating stoves for rural heating are inefficient and emit large amounts of air pollutants (Liqun et al., 2019; Jinda et al., 2019). Recent PM_2.5 emissions data in typical rural areas in North China suggest that the average annual concentration of PM_2.5 in rural areas is 3.1 times the national air quality standard (Kankan and Jie, 2020).

To alleviate air pollution, including haze, the country issued a series of policies to guide and promote clean transformation of the heating mode. In September 2013, the State Council issued the Action Plan of Prevention and Control of Air Pollution. In March 2017, the Ministry of Ecology and Environmental Protection issued the Beijing–Tianjin–Hebei and Surrounding Areas 2017 Air Pollution Prevention and Control Work Plan. In December 2017, 10 ministries and commissions issued the Clean Heating Plan for Winter in the Northern Region (2017–2021), which promoted the development and implementation of various forms of clean energy heating and alleviated the air pollution caused by heating in the winter to some extent (han and Wenying, 2019). During 2017–2019, the Chinese government selected 43 cities to initiate a clean heating pilot project and provided them with financial support (Ministry of Finance of the People’s Republic of China, 2019). Centralized heating, such as regional boiler rooms and cogeneration, is generally adopted in cities. Therefore, the energy source of urban heating systems can easily be transformed from traditional to clean energy under the incentive of national policies (Yuanzheng et al., 2019). However, decentralized heating systems are often adopted in rural areas because buildings are sparsely distributed. The transition of conventional heating to clean heating is more difficult in rural areas than in cities because of the diversified energy and decentralized heating forms in the former. Consequently, centrally treating pollutants is also difficult (Xi et al., 2019). Recent data (2014–2018) provide a positive outlook, demonstrating that rural coal consumption decreased by approximately 19.3 %, whereas natural gas consumption increased by 5.5 billion m³. However, biomasses such as straw are still burned by direct combustion, which is inefficient as it produces large quantities of air pollutants (Building Energy Conservation Research Center of Tsinghua University, 2020).

Several studies have been published on clean heating with a focus on rural buildings. Wu (Shu, 2020) reviewed the rural energy policies in China from 1949 to 2018 and evaluated the implemented policy tools, including subsidies and electrification. Zhang et al. (Yali et al., 2020) analyzed the pollutant monitoring data of 35 pilot cities and found significant decreases in the concentrations of pollutants, including SO₂ and NO_x, during the heating period in winter. Wang et al. (Zhen et al., 2019) statistically analyzed the heating energy consumption patterns of 136 villages in Hebi City and demonstrated the influence of the heating cost on the heating scheme choice. This finding indicated that cleaning heating schemes are more affordable for households with higher incomes. Zhang et al. (Qunli et al., 2017) examined the technical and economic feasibility of low-temperature air-source heat pumps (ASHPs) for heating in cold regions. Further, Zhou et al. (Zhongren et al., 2008) studied the development of rural household energy structures and recommended a strategy for sustainable energy development in rural areas.

Liu et al. (Hongxun and L.Mauzerall, 2020) compared the economy of several common types of clean-heating equipment with and without subsidies in the rural areas of Beijing, Tianjin, and Hebei. They found that the costs of ASHPs, direct heating electric heaters, or natural gas wall-hung boilers (NGBs) incurred by farmers would significantly increase without the provision of subsidies. To make clean heating schemes more feasible in rural areas, challenges such as unreasonable energy structures, low heating-equipment efficiency, and insufficient energy supply must be solved (Mengjie et al., 2018). For instance, the supply of natural gas or electricity to remote villages without pilot policy support requires the installation of new gas pipelines and power grids (Ren and Zhujun, 2017; Boqiang and Yunming, 2020). Moreover, rural power grids may have obsolete infrastructures, insufficient supply loads, and excessively long supply lines. Therefore, natural gas or electricity for heating in such areas cannot be implemented without government subsidies (Jinze et al., 2020). In the northwest and northeast of China where the heating demand is greater and the environment is more fragile, coal combustion with high pollution emissions remains the primary heating method, as the clean heating schemes adopted by the pilot cities are not as suitable or effective for these regions (Wenzhe et al., 2020). It is evident that the conventional clean heating solutions are not suitable for rural areas in China considering pollutant emissions. One of the best solutions to this issue promises to be the use of economic and feasible methanol heating in the aforementioned areas.

The reliance of China on oil and natural gas imports has been increasing annually, accounting for 72.7 % and 42.2 % of the total consumption in 2019, respectively (Petroleum, 2020). The country is rich in coal resources, which are the primary source of energy, and its energy structure will continue to rely on coal in the near future (National Energy Administration, 2015). However, in the context of environmental protection, current regulations prohibit the direct burning of coal for heating, rendering many coal reserves unusable. An advantage of the MHS is that methanol fuel can be derived through clean coal-conversion technology, avoiding direct combustion. Further, the country is the largest producer as well as consumer of methanol. It also uses indirect chemical synthesis processes to convert low-grade coal into methanol, which has been an effective approach for clean and efficient coal conversion in the country (Chijen and B.Jacksonab, 2012). Yao et al. (Yuan et al., 2018) used lifecycle assessment methods to compare the energy consumption and environmental impacts of using different fossil fuels to produce methanol. Li et al. (Changhang et al., 2018) evaluated the environmental effects of coal coking and gasification as typical coal-based methanol production technologies. Li et al. (Jingying et al., 2018) used the lifecycle method to compare the economy of three methanol production routes: coke oven gas, coal gas, and natural gas.

In addition to the traditional methanol production methods using fossil fuels as raw materials, renewable methods (e.g., biomass fermentation and CO₂ hydrogenation), which can absorb agricultural waste, greenhouse gases, and renewable electricity, can be used to produce methanol (Hobson et al., 2018). Shih et al. (Fong et al., 2018) proposed “liquid sunshine technology,” which combines solar energy with CO₂ and H₂O to produce methanol. They evaluated the significance of methanol as a substitute for fossil fuels to achieve the goals of environmental conservation, economic growth, and energy security. The use of renewable energy sources, such as solar and biomass to produce “green” methanol fuel, is key to achieving the goal of carbon neutrality by 2060 as it can greatly aid in forming a closed carbon loop, thereby creating an ecologically balanced cycle between fuel and nature. In July 2019, the National Energy Corporation issued a “Notice on Solving Related Issues in the Process of Promoting ‘Coal to Gas’ and ‘Coal to Electricity’ and Other Clean Energy Heating,” emphasizing the need to adapt clean heating schemes to local conditions (National Energy Administration, 2021). Inner Mongolia, Shanxi, Xinjiang, and other northern provinces are the main producers of methanol in China, where the heating demand is higher than in other regions in the country. As a liquid energy carrier with the simplest structure and the highest hydrogen-to-carbon ratio, methanol is being used as a clean alternative fuel for boilers and stoves in more than 10 provinces in the country, including Hebei and Shanxi. Since 2016, more than 1000 such boilers have been in operation in China, consuming over 2 million tons of methanol (Center for global energy strategy studies of Peking University and Methanol Institute, 2018).

Feng (Xiangfa, 2006) reviewed the development of alcohol-based fuels in China, compared them with other alternative fuels, and recommended steps to promote methanol as a fuel. Ran et al. (Jingyu et al., 2014) optimized the structure of the ejector of a fully premixed methanol burner, which enabled a wider range of load adjustments. In October 2018, the Jinzhong Municipal Government of Shanxi Province issued the “Implementation Plan for the Clean Heating ‘Replace Coal with Methanol’ Project in Jinzhong City in Winter (Interim)” (Jinzhong Municipal People’s Government, 2018). In January 2019, it released the “Implementation Plan for Jinzhong Supporting Methanol Boiler and Related Industries To Be Larger and Stronger,” which mandated the construction of a methanol boiler product assembly and manufacturing center, a methanol fuel storage and distribution center, and an engineering technology research and development center (Jinzhong Municipal People’s Government, 2019). In June 2019, four supporting documents were issued to promote the clean heating project “Replace Coal with Methanol,” which covers the “Replace Coal with Methanol” project acceptance, safety supervision, and industrial insurance (Jinzhong Municipal People’s Government Website, 2021). Since 2018, the Jinzhong Municipal Government has been piloting a project on clean heating with a methanol boiler in rural areas, and 25,550 MHSs were installed by the end of 2019.

Recent data offer evidence that the MHS is being adopted as a viable clean heating technology by rural areas with no access to central heating and gas pipelines, or with obsolete power infrastructures and insufficient grid capacities. As part of further research, the use of methanol fuel as a boiler fuel and the applicability of MHSs in rural clean heating and household heating must be studied. Moreover, the energy consumption, economy, and pollutant emission characteristics of MHSs must be investigated before they are applied on a large scale.

In this study, the differences in the standard coal consumption, annual cost, and emissions between MHS and traditional heating schemes (bulk coal heating stove (BCHS), NGB, direct electric wall-hung boiler (DEB), thermal storage electric wall-hung boiler (TSEB), and ASHP) were investigated. The effects of the methanol price, grid expansion, and gas pipeline construction on the economy of each heating scheme were evaluated. The application areas were identified and suggestions were made for improving MHSs based on the evaluation results. This study will facilitate the optimal selection of household heating schemes and promote the sustainable development of methanol heating.

The rest of the paper is structured as follows. Section 2 introduces the MHS principle. Subsequently, Section 3 describes the thermal load model of a typical house in a rural area of China and the evaluation indicator models of each heating scheme in terms of energy consumption, economy, and emissions. Based on these factors, Section 4 presents a case study with detailed parametric analysis to show the merits of methanol heating. Then, Section 5 discusses the technical characteristics and development prospects of methanol heating. Finally, Section 6 summarizes the main conclusions of the study, as well as its limitations and the scope for future research.