International: Analysis and Inspiration of the Centralized Biogas Plant Development Model in Denmark

Preface

Biogas technology is an effective means of treating and utilizing agricultural waste. European countries have always valued the development of biogas technology and industry, leading the world in biogas plant development models.

Denmark is a strong country in European animal husbandry and started early in using biogas technology to treat livestock manure. In the 1970s, farm-scale biogas plants emerged due to the energy crisis, but problems such as low gas production, unstable plant operation, and lack of professional maintenance led to many plants being abandoned. In the early 1980s, based on the experiences and lessons from farm-scale biogas plant development, Danish agricultural economics researchers proposed shifting the biogas construction model towards the Centralized Anaerobic Digestion (CAD) model. CAD, first proposed by Denmark, centrally processes dispersed livestock manure through anaerobic digestion. Its distinct characteristics have had a profound impact on the country's biogas industry development, attracting attention and interest from other nations. The UK began studying the feasibility of introducing CAD in the early 1990s; France built six centralized biogas plants between 2000 and 2005 to process livestock manure and food processing waste; Japan has also introduced and built centralized biogas plants and conducted economic analyses of them.

China is a major country in animal husbandry. Rural biogas plants have always been closely linked to livestock farming, with construction sites often located at farms. They primarily use livestock manure as feedstock to produce biogas for the farm's own consumption or for external centralized gas supply. Because biogas plants are often bundled with farms, the feedstock source is relatively单一 (simple). They are significantly affected by adjustments in the livestock industry and market changes. Plants are frequently abandoned due to unstable feedstock supply, poor specialized management, and low economic benefits. Currently, the construction of rural biogas plants in China is developing towards larger-scale and more sophisticated projects, with increasing attention paid to feedstocks like straw and kitchen waste. The Ministry of Agriculture has proposed breaking the "two bundles": the bundling of biogas plants with farms or farming communities, and the bundling of fermentation feedstock with livestock manure. This places new demands on optimizing the construction and operation models of rural biogas plants in China. The "13th Five-Year Plan" period is a critical time for the transformation and upgrading of China's rural biogas plant construction. Understanding the development history and current status of the centralized biogas model in Denmark, and analyzing its characteristics and problems, can provide relevant inspiration for the development of biogas plants in China.

The Danish Centralized Biogas Plant Model

1 Development History of Centralized Biogas Plants in Denmark

In 1984, the County of North Jutland in Denmark implemented the "Village Energy Project." The first centralized biogas plant in Denmark was built in the village of Vester Hjermitslev to process livestock manure and demonstrate the village's energy self-sufficiency. From 1985 to 1987, the project continued to support the construction of two more centralized biogas plants (see Figure 1). The initial development of centralized biogas plants was slow, but these earliest operational projects explored and demonstrated the economic feasibility and viability of the model, laying the foundation for subsequent biogas plant development.

Fossil fuel prices began to fall in 1985. Denmark rejected the development of nuclear power, focusing instead on developing renewable energy sources like wind and biomass. Combined heat and power (CHP) generation using natural gas was promoted by the government. Concurrently, water pollution caused by ammonia nitrogen infiltration from farmland raised concerns. The Danish Environmental Protection Agency introduced regulations requiring extended storage times for livestock manure and restricting the over-application of manure on fields. This left farmers with no place to dispose of surplus manure waste. Given this situation, the Danish government decided to promote centralized biogas plants. In 1985, it initiated a project for the comprehensive benefit development and demonstration of large-scale biogas plants based on livestock manure. This was upgraded in 1988 to the "Biogas Action Programme," which provided subsidies of up to 40% and low-interest loans for owners investing in biogas plant construction, and implemented monitoring and management for all completed plants. The period of the Biogas Action Programme was the fastest development period for centralized biogas plants: In its first four years of implementation, it supported the construction of five centralized biogas plants. After the programme expired in 1991, the government re-launched it several times between 1992 and 2001, driving construction forward at an average pace of 1-2 plants per year and supporting the addition of 11 new centralized biogas plants (see Figure 1). The significant improvement in the economic viability of centralized biogas plants drove the rapid increase in their numbers. Plant scale expansion, stable operation, and the diversification of feedstock sources led to a yearly decrease in production costs per cubic meter of biogas. Figure 2 shows that biogas production costs fell from 21 Danish Kroner (DKK) in 1984 (1 DKK ≈ 0.96 RMB, 2016) to 2 DKK in 1995, approaching the natural gas procurement price for district heating companies (3.8 DKK/m³) in 1991. On the other hand, the average profit margin for the centralized biogas plant industry increased year by year from 1992 to 2000 (see Figure 2).

The Danish Parliament passed the **Danish Energy Reform Act** in 1999, planning to change the method of incentivizing renewable energy development after 2003. The feed-in tariff for biogas-generated electricity was to be adjusted from a fixed price to a floating price that varied with market conditions. Due to concerns about the uncertainty of future energy prices, biogas plant owners stopped investing in new construction. After the last plant was built in 1998, the construction of centralized biogas plants stalled, and the Biogas Action Programme was subsequently terminated. By this point, Denmark had built a total of 20 centralized biogas plants, mainly located in the western part of the country, where livestock farming density is higher. Despite the stagnation in construction, most existing centralized biogas plants continued to operate well.

Based on the draft EU Renewable Energy Directive (Directive 2009/28/EC) initiated in 2007, Denmark needed to increase its renewable energy share from 17% in 2005 to 30% by 2020. Concurrently, driven by considerations regarding climate change, carbon emission reduction, and the sustainable development of animal husbandry, the government's energy policy towards biogas warmed. The 2008 **VE-Lov** (Executive Order on the Promotion of Renewable Energy) stipulated annually increasing feed-in tariffs and market premium subsidies for biogas electricity. The 2009 **Green Growth Agreement** stated that by 2020, 50% of Denmark's livestock manure must be treated using anaerobic digestion technology. It offered 20% government subsidies and 60% municipal loans for biogas plant construction, and gave biogas suppliers equal opportunities as natural gas suppliers. The **Danish Energy Strategy 2050**, formulated in 2010, further mentioned that Denmark would vigorously develop renewable energy, aiming to be completely independent of fossil fuels by 2050. The emergence of favorable energy policies led to a revival of interest in centralized biogas plants. However, due to the impact of the global financial crisis, only two new centralized biogas plants were added in Denmark between 2009 and 2012. Compared to farm-scale plants and sewage treatment plant biogas plants, the number of centralized plants remained relatively small, but their biogas production accounted for 45% of the total output, making them a major component of the Danish biogas industry (see Table 1). As Denmark continues to advance its "Green State" initiative, achieving the livestock manure treatment targets of the Green Growth Agreement would require building approximately 20 more centralized biogas plants. It is expected that centralized biogas plants will see greater development in the future.

Figure 1. Changes in the cumulative number of centralized biogas plants in Denmark (1984-2000)

Figure 2. Changes in biogas production costs for centralized biogas plants in Denmark (1984-1995)

2 Centralized Biogas Plant Model and Characteristics

A centralized biogas plant typically serves a small town, supplying gas and heat to around 2,000 residents (farmers). The basic model (see Figure 3) involves a biogas digester of approximately 2,000 m³, using mesophilic or thermophilic fermentation (large plants currently often use thermophilic fermentation at 52-55°C). Fermentation feedstocks primarily include livestock manure and surplus sludge from sewage treatment plants. Plant organization and operation often adopt a cooperative model. The plant owner organizes, or farmers themselves transport, manure from dispersed livestock farms to the centralized plant using trucks, typically within a collection radius of about 10 km. Denmark places great emphasis on environmental hygiene. Manure feedstock must remain in the thermophilic digester for a Minimum Guaranteed Retention Time (MGRT) of 6-10 hours (see Table 2) and undergo treatment for at least 7 days (Hydraulic Retention Time, HRT). The feedstock or digestate must also pass through a sanitation tank for pasteurization, with an MGRT of 5.5 hours to 1 hour at 55-65°C (see Table 2). The digested residue and liquid (digestate) are separated and stored in tanks, then transported by tanker trucks to fields for application during the growing season. The produced biogas is primarily used for CHP generation: electricity is fed into the grid, and the heat supplies community heating.

Figure 3. Centralized Biogas Plant Model

"Compared to farm-scale biogas plants, centralized biogas plants have distinct advantages and characteristics: 1) They are not dependent on a single farmer or farm, ensuring stable feedstock supply and enabling stable year-round gas production. 2) They mostly use thermophilic fermentation with additional pasteurization, offering low sanitation costs, effectively killing pathogens and weed seeds present in the manure, avoiding pathogen transmission risks, ensuring the quality and safety of the digestate, and helping improve the environmental quality of livestock farming. 3) They use mixed feedstock fermentation, resulting in higher biogas yields and producing large quantities of high-quality digestate that can meet the needs of large-scale crop farmers. 4) They can accept industrial organic waste from domestic and foreign sources, further enhancing the plant's economic viability. 5) Their close integration with district heating creates a distributed energy development model that effectively utilizes renewable energy like biogas, reducing energy costs and carbon emissions. 6) They often adopt a cooperative management model, allowing for coordinated plant operation, specialized equipment maintenance, and efficient storage and transport of manure and digestate, significantly reducing farmers' costs for waste treatment.

3 Development Bottlenecks and Problems of the Danish Centralized Biogas Plant Model

After nearly a decade of rapid development, the Danish centralized biogas plant model gradually encountered development bottlenecks in the mid-to-late 1990s: 1) **Feedstock shortages limited development.** As the number of centralized plants increased, so did their demand for feedstocks like manure and organic waste. The limited supply of organic waste could not meet the needs of all plants, leading to competition among plant owners for high-quality feedstocks. This competition drove up feedstock prices, increased operating costs, reduced economic benefits for plants, and dampened owners' confidence in new investments. 2) **Declining feedstock quality slowed development.** With the expansion of Danish farms, farm-scale biogas plants also grew. Some farmers gained the capacity to store and process more of their own manure. They began to retain high-quality, high-organic-content feedstocks for their own use, supplying centralized plants with lower-quality, low-organic-content feedstocks. This poorer feedstock further undermined the economic viability of centralized plants, suppressing construction impulses. 3) **The government's energy policy shift weakened development.** The 1999 Danish Energy Reform Act and the amendment to the Electricity Subsidy Act changed the feed-in tariff for biogas electricity from a fixed price to a floating price, adding uncertainty to the profitability prospects of biogas plants. In 2001, the Liberal Party won the Danish election. The new government focused on energy supply efficiency and economy, canceling support for biogas construction and research projects, including the Biogas Action Programme, which dealt a blow to the development of the biogas industry.

Inspiration from the Danish Centralized Biogas Plant Model for China

Denmark is a major livestock farming country in Europe. Its biogas plants have gradually evolved from being initially a tool to solve energy problems to becoming a means of comprehensively managing agricultural and environmental issues. China's biogas development path is similar to Denmark's. In the current context of biogas plant transformation and upgrading, and the promotion of circular agriculture, the Danish centralized biogas plant development model offers valuable lessons for China.

1 Diversified and Sufficient Feedstock Sources Ensure Stable and Efficient Plant Operation

Stable feedstock supply was key to the success of Danish centralized biogas plants. By cooperating with multiple farms, these plants avoided dependence on a single farm, mitigating the risk of plant shutdown due to feedstock shortages caused by poor management at one farm. Furthermore, centralized plants mostly use mixed-feedstock fermentation. Besides livestock manure, they also process industrial organic wastes (e.g., from food processing, slaughterhouses, pharmaceuticals) and organic household waste. Compared to single-feedstock fermentation, co-digestion can improve microbial fermentation efficiency and enhance the buffering capacity and organic loading rate of the digestion system. Feedstock diversity further ensures stable plant operation and benefits the quality of the biogas, digestate, and residue produced.

In China, biogas plants are mostly built adjacent to farms and rely entirely on a single farm's feedstock, which cannot guarantee long-term, stable, and efficient operation. Taking pig farms as an example, factors like changes in the live pig market, industrial layout adjustments, and farm relocation significantly impact biogas plants: market fluctuations lead to volatility in pig farming, affecting stable feedstock supply; industrial layout adjustments and farm relocation can directly lead to the abandonment of the bundled biogas plant. Additionally, the primary feedstock for rural biogas plants in China is livestock manure from farms; feedstocks like industrial waste, municipal sludge, and organic waste are rarely used. Bundling biogas plants with farms limits the application of co-digestion, wasting valuable biomass resources and hindering the economic viability of biogas plants. Southwest China has 9,262 township-level divisions, with dispersed urban and village layouts and uneven terrain. Although large-scale pig farming is developing rapidly in this region, the number of smallholder farms is still substantial and their scale is growing, posing a significant risk of non-point source pollution. Considering this, the Danish centralized biogas plant model could be referenced. Building large-scale biogas plants in the peri-urban areas of this region or between villages could collect and process manure from smallholder farms and organic waste from towns. The biogas produced could be supplied centrally to urban residents, and the digestate could be returned to farmland in a timely and coordinated manner. This would be beneficial for comprehensively managing the urban and rural environment and promoting regional circular agriculture development.

2 A Model of Regional Energy and Environmental Integration

Denmark has achieved remarkable success in CHP and district heating. It began exploring the establishment of CHP plants and district heating networks, promoting them nationwide, in the early 20th century. Early fuels mainly included natural gas, coal, oil, straw, wood pellets, and household waste. Using CHP for district heating improves fuel utilization efficiency and maximizes the use of local fuels. As biogas production costs decreased (see Figure 2) and public environmental awareness increased, more and more CHP plants started using clean energy sources like biogas, especially in areas poor in natural gas resources. Currently, the vast majority of biogas in Denmark is used for CHP. Centralized biogas plants produce biogas on a scale that can stably fuel CHP plants, using this as their primary revenue stream and integrating closely with the district heating network. District heating has strong public welfare attributes; the government exempts biogas heating from taxes, maintaining heating prices at $50-100 per MWh.

Chinese biogas plants are generally small in scale; plants with digester volumes above 1,000 m³ account for only about 5% of the total. Energy supply often only meets the plant's own operational needs, resulting in poor comprehensive benefits. Northeast China commonly uses coal-fired boilers for heating, which has a significant environmental impact. In fact, this region is China's granary, with large amounts of straw not effectively utilized; large-scale livestock farming is developing rapidly, generating substantial manure requiring treatment. Northeast China is at a similar latitude to Denmark and has a cold climate. Given the opportunity for transformation and upgrading of China's biogas plants, this region could learn from the Danish centralized biogas plant model. Building large-scale biogas plants using mesophilic/thermophilic processes between several villages and towns could centrally process organic waste from the region, maximizing resource utilization. Retrofitting coal-fired boilers and supporting the construction of CHP plants and gas/heating pipeline networks could provide public welfare-based centralized gas and heating supply to residents in the regional villages and towns, with government subsidies or tax incentives. The waste heat from CHP could supply the biogas plant's own normal operation. The digestate could be stored and managed by the centralized plant owner and sold to large-scale farmers for field application at appropriate times. Compared to current biogas plants in Northeast China, the centralized model could better leverage the advantages of comprehensive energy and environmental benefits.

3 Insights on Digestate Application to Farmland

Denmark has advanced experience in utilizing digestate as fertilizer, characterized by direct application to farmland. The Danish government places a high value on ecological environmental protection and hygiene safety: In 1984, an act stipulated the maximum nitrogen application rate per hectare of land, requiring farmers to apply digestate on time and in the correct amounts. In 1989, livestock hygiene regulations and safety regulations were enacted to monitor the content of animal and plant pathogens in digestate and impose strict limits on heavy metals and organic pollutants. All centralized biogas plants are equipped with sanitation tanks with specified minimum guaranteed retention times (see Table 2). On-site laboratories test the physical and chemical properties of the digestate. Tankers transporting digestate must be cleaned and disinfected with 0.2% sodium hydroxide solution to maximize the assurance of digestate quality and application safety. Centralized biogas plants mostly use a cooperative management model, effectively coordinating digestate storage and transport, and can even provide farmers with scientific fertilization plans based on field and crop conditions.

In China, rural biogas plants produce large quantities of digestate, which is often treated using ecological purification or aerobic aeration methods, presenting problems such as large land occupation and high energy consumption. Compared to developed countries like Denmark, the difficulty of directly applying digestate to farmland in China stems from reasons like low per capita arable land and the disconnect between crop and livestock farming. Currently, the management model involving village collectives coordinating and participating in biogas plant operation has shown relatively good results in China. Therefore, regions in China with scarce arable land resources could learn from the centralized biogas plant development model. Equipping plants with large digestate storage tanks, with village collectives and plant owners coordinating the storage, transport, and field application of digestate, could be beneficial. Anaerobic digestion is a key link in circular agriculture and has a certain effect on blocking pathogen transmission. Although China has issued standards and specifications for digestate utilization technology, these have not been elevated to the level of law and are relatively weak in enforcement. Many biogas plants do not prioritize the quality of their biogas, digestate, and residue, which is detrimental to controlling the risk of secondary environmental pollution from digestate. As China's economy develops, consumers have higher requirements for the quality and hygiene of agricultural products. Developing centralized biogas plants facilitates the batch disinfection of digestate, which is conducive to quality control and ensures the safety of its use. Biogas plants could incorporate a pasteurization step in sanitation tanks. Village collectives could supervise plant owners in regularly testing digestate quality. Specialized companies could formulate field application plans, and the government could provide appropriate subsidies for digestate applied to fields according to standards.

Summary and Suggestions

The centralized biogas plant model, pioneered by Denmark, plays a crucial role in the sustainable development of its animal husbandry and energy security. It has received attention from the Danish government and interest from the international community. Emerging in the early 1980s, influenced by factors such as policy support and decreasing biogas production costs, the model was promoted nationwide in Denmark. Construction was fully implemented and experienced over a decade of rapid development, culminating in the completion of 20 plants by 2000. The basic model involves the centralized treatment of manure from several farms. Its advantages include stable feedstock supply, diversified fermentation materials, high-quality digestate, and good economic and comprehensive benefits. However, its development also encountered problems such as competition for high-quality feedstocks, declining feedstock quality, and fluctuations in supportive policies. Currently, the remaining plants continue to operate normally, providing good comprehensive benefits in waste treatment, district heating, and digestate application to farmland. They are a major component of the Danish biogas industry.

China's rural biogas plants primarily focus on treating livestock manure. Although current biogas technology is relatively advanced, plants remain bundled with farms, and fermentation feedstocks bundled with livestock manure. This development model limits the plants' ability to achieve their full potential. To break these "two bundles" and better realize the comprehensive benefits of biogas plants, China could learn from Denmark's biogas industry development experience. It could introduce, demonstrate, and develop the centralized biogas plant model in suitable regions such as the Southwest and Northeast. This would involve centrally processing livestock manure from various farms within a region, along with crop waste and urban household waste. Biogas could be used for centralized gas supply or linked with CHP and district heating. Digestate quality could be managed through collaboration between village collectives and plant owners, with coordinated storage and timely field application. The government would need to provide supporting policies, subsidies, and support. In the current context of building an ecological civilization, constructing a resource-conserving and environment-friendly society, and the transformation and upgrading of biogas plants, developing centralized biogas plants in China could help improve the utilization rate of biomass resources and energy efficiency, enhance agricultural product quality, promote circular agriculture development, prevent and control rural non-point source pollution, and alleviate pressures on agricultural resources and the environment.

Source: *World Agriculture*