• Waste management in Japan
• Circular economy in Japan
• Waste management in Asia
• Disaster waste management

Introduction

An energy circulation system, household biogas plant (HBP) (Fig. 1) (see the May 2012 issue [in Japanese] for more details), is widely used in many parts of the world, especially in developing countries. The system generates biogas for household use through microbial fermentation in a small-scale tank using organic waste such as food scraps, livestock manure and human manure as raw materials. Recently, Germany and other developed countries also began to promote small-scale biogas plants. China particularly has a long history of HBPs, and the highest number of plants in the world, which is reported to have reached 40 million in 2010. Japan also has several Chinese-style biogas plants which residents administer.

In China, wider use of biogas plants has brought three benefits to rural areas: improved livelihoods, improved environment and stable supply of recyclable energy. Therefore, the promotion of such plants has been regarded as an effective policy measure for the development of rural areas and resource circulation. This type of biogas plant in China is not simply a downsized system. Unlike the biogas plants in Japan, the US, and European countries, the Chinese-style plants use a simple and low-efficiency system that seeks to achieve cost reduction. Although it is a low-efficiency system without any heating device or stirring device, it has remarkable characteristics such as on-site generation of energy for cooking and efficient usage of energy through direct utilization of generated biogas as fuel. In Japan, however, these characteristics of Chinese HBPs are not yet well recognized. That may be because most research studies on this system are conducted in China, and because many technical documents are only available in Chinese. This article will explain the Chinese government's policy measures regarding the HBPs in rural areas, and the status of their development and distribution.

History of household biogas plants in China and relevant national policies

The history of HBPs in China started about 90 years ago. The first practical application of a biogas plant was reported in the 1920s. The plant for a six-person household provided biogas for cooking and lighting purposes. In 1935, the first biogas-related company was established. In 1958, the Chinese government began to promote the installation of biogas plants mainly in rural areas. In the early 1970s, construction of HBPs that used human and animal waste as a fuel source became widespread across the country. However, the plants that were built during this time had many problems such as immaturity of the technology, low quality construction materials, and lack of appropriate management. As a result, the number of such plants gradually decreased. In the 1980s, biogas-related industries in China began to construct a new generation of biogas plants based on the analysis of their past experiences and failures. In the 2000s, with rapidly growing economy, China started to face the challenge in ensuring a stable supply of energy, and the use of renewable energy was given special attention. Meanwhile, economic inequality between urban and rural areas and the outflow of rural population to urban areas became social problems. Consequently, development of rural areas became a major concern. In 2006, China's Renewable Energy Law came into effect, and the government announced a renewable energy development plan over the medium to long term. The plan set a goal of boosting the number of rural households installing biogas plants to 40 million by 2010; this goal was recently accomplished as mentioned above.

Thanks to the introduction of these biogas plants, rural households benefited from a more stable supply of fuel to replace firewood, improved sanitation through appropriate management of human and animal waste, and the use of resulting sludge as agricultural fertilizer. These changes improved their livelihood and environmental condition. Such rapid dissemination of the plants was made possible mainly through the Chinese government's financial support. The law "Rural Biogas Construction State Debt Program Management Method," which was enacted in 2003, stipulated that the central government subsidizes 800 RMB to 1,200 RMB, depending on the economic level of the region, to a biogas digester (with 8 m³ capacity) project run by an individual. Considering that the current average annual income in rural China is about 6,000 RMB/person, the subsidy makes up about 20% of it. In conformity with the central government's policy, local governments have established more detailed implementation plans: training biogas plant engineers, providing plant construction and maintenance services through these trained engineers, and providing preferential tax treatment to biogas-related businesses. In this way, China has strongly pushed its national policy on the promotion of HBPs.

The number and distribution of household biogas plants

Since the 2000s, the number of rural HBP installation in China has increased rapidly. Until the 1990s, the total number of plants had remained at approximately 5 million, but in the 2000s, more than 1 million plants were built each year. The total number reached 35,070,000 at the end of 2009. When we examine the number more closely, Hebei Province marks the northern limit for the distribution range of the administrative districts (provinces and autonomous regions) having the large number of plants (more than 1 million) in the district. In northeastern and northwestern China with severe cold climate in winter, the number of plants for each administrative district is clearly small in relative terms, approximately 0.8 million at most for an administrative district. Since a household digester tank basically runs without using heat, the problem of inefficiency of the reaction in the cold regions has been pointed out. A low-cost heating system for a tank was developed, but the number of plants with such systems has not increased much.

In comparison, in the warmer regions, the operating efficiency of digester tanks is considered higher. Nevertheless, the number of plants is not necessarily large in these warmer regions, and their distribution is rather concentrated in inlands. In fact, the number of plants is clearly much smaller in Shanghai City, Jiangsu Province, and Zhejiang Province on the coast where a farmer's income is more than twice the national average. There are three areas, each having more than 3 million plants: Henan Province, Sichuan Province and Guangxi Zhuang Autonomous Region. Total amount of 12.4 billion m³ of biogas is generated annually from the HBPs across China. The annual biogas production in each of the above three areas amounts to 1.2-1.7 billion m³. Generally, the annual production of biogas per household ranges from 242 to 720 m³; the warmer regions south of Shanghai clearly show higher values compared to other regions. The calorific value of 1 m³ of biogas is 6,000 kcal. This means that the total energy from the biogas production across China amounts to 744 x 10⁹ kcal, equivalent to the energy produced from 106 million tons of standard coal.

Challenges for household biogas plants

The following challenges have been identified for household biogas technologies: (1) acceleration of hydrolysis of recalcitrant substances, such as cellulose and lignin in agricultural residues; (2) improvement in reaction speed and reduction of start-up time in the low-temperature areas; (3) stable production of biogas; (4) diversification in biogas usage methods; (5) appropriate plant management; and (6) utilization of digestion liquid as liquid fertilizer. Moreover, inadequate input of raw materials in households often leads to the problem of insufficient production of biogas. Therefore, utilization of remaining biomass, such as agricultural residue, will become more important for increasing biogas production. There remain other scientific and technical challenges to overcome, including improvement in stability of digester tanks.