China’s biomass gasification power generation technology

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Abstract: China has a large amount of available solid biomass, mainly consisting of agricultural waste and wood waste. Biomass is scattered in distribution, making its collection and transportation difficult. Under China’s current conditions, it is hard to employ large-scale combustion technologies; therefore, small- to medium-scale biomass gasification power generation technologies (200–5000 kW) hold unique advantages in China. Due to the tight electricity supply in China, along with significant waste of biomass materials and their low cost, the cost of generating electricity through biomass gasification is around 0.2–0.3 yuan per Kw·h, which is close to or even lower than that of conventional power generation. The investment required per unit of capacity is only about 3,500–4,000 yuan per Kw, which is 60%–70% of the cost associated with coal-based power generation; therefore, it has the potential to compete in the market. China currently possesses the capability to build MW-scale biomass gasification power generation projects. However, the technology still has some issues; the most prominent ones are secondary pollution of water and limited applicability to various types of biomass. Moreover, the system’s power generation efficiency is low, with a thermal efficiency of only around 15%. Therefore, it is necessary to strengthen efforts in three areas at present: first, to research and improve tar cracking technology in order to completely reduce secondary pollution of water; second, to advance this technology in order to increase the overall thermal efficiency ； Third, establish demonstration projects in areas where conditions permit, carrying out commercial demonstrations tailored to the characteristics of different types of waste in order to fully prove the reliability and cost-effectiveness of this technology, thus creating the conditions for the widespread adoption of biomass gasification technology. r% C* T9 + C4 \7 J7 t6 U! ~$ k* B 1.% P" r% \% u- a4 |9 d( h Introduction) j/ {! u( H; Biomass is an important renewable energy source, which is widely distributed and available in large quantities. However, due to its low energy density and dispersion, it is difficult to process it on a large scale, which is precisely why biomass utilization remains low in most developing countries. Biomass gasification power generation technology (BGPG) enables high utilization rates on a smaller scale and provides a high-quality form of energy; it is particularly suitable for rural areas and developing regions. Therefore, it is an important technology for utilizing biomass and represents an important direction for development. Due to its vast territory and abundant biomass resources, China faces relatively tight power supply conditions; therefore, biomass gasification for power generation offers favorable conditions for development. By vigorously promoting this technology in China, its advantages and economic efficiency can be fully realized. - l9 m6 E1 K# {8 H 2. The current status of biomass gasification power generation technology in China 2.1 Overview of development 8 f* I4 _' F3 ?# R. ?6 K China began researching and applying biomass gasification power generation technology at an early stage; back in the 1960s, a 60KW system for generating power using rice husks was developed. Currently, the two main types of shell power generation in use are 160KW and 200KW. In recent years, research has been conducted on 1MW biomass gasification power generation systems, with the aim of developing medium-scale biomass gasification power generation technologies suitable for China’s national conditions. A 1MW biomass gasification power generation system was completed and put into operation last year, and its process schematic is shown in Figure 1. This system is improved in many aspects compared to the 200KW gasification power generation system, but due to investment issues, the problems related to wastewater have not been fully resolved. 2.2 Key Technologies 4 ^* G6 z' y" M- }5 N* W: r9 e 2.2.1 Gasification Furnaces% Q* u- n& G- Q& ?/ g6 m# h ; R7 P4 A: b' Y China has conducted research on various gasification methods and has developed multiple types of gasification furnaces; the ones currently in use include top-absorption type, bottom-absorption type, open-type, and fluidized-bed type furnaces. In principle, various gasification furnaces can be used for gasification-based power generation, but currently, three types have been developed and are in operational use: the open-bottom suction type, the bottom suction type, and the circulating fluidized bed (see Table 1). The power generation capacity can range from a few kilowatts to several thousand kilowatts, which provides the foundation for further development of gasification-based power generation technology. 2 V* a2 z$ J% k , W$ n" t8 v% M( ?, | 2.2.2 Gas engines Gas engines include natural gas, methane, and biomass gas with low calorific value. The development of low-calorific-value gas engines in our country is poor; those with a small power output (100 KW) are essentially modified from diesel engines, and there are no standardized products available. Research has also been conducted on high-power (500KW) units, 2 d2 U. i6 w+ \; x( w. x Table 1. Gasifier types in biomass gasification power generation in China: 5 G C7 |! o: w8 f L. Gasifier types: layered downward draft type, downward draft type, circulating fluidized bed. Fuel types: bark, wooden logs; rice husks, wooden logs; rice husks, wood chips. Capacity: 2.0–30.0 kWe, 60–200 kWe, 400–4000 kWe. Gas calorific value: 4100–5300 KJ/m3, 3800–4600 KJ/m3, 4600–6300 KJ/m3. Gasification temperature: 1100℃*, 700–800℃, 650–850℃. Cold air efficiency: 70%, 50%, 65–75%. However, due to issues with exhaust temperature and control technologies, no mature products have been developed yet. The available standard products are 160KW and 200KW. Due to the low power of individual units, medium-scale gasification power generation systems must consist of multiple gas engines operating in parallel, which to some extent limits the further increase in the power output of the gasification system. $ q, ~4 c6 B7 I6 p& z. g9 \4 Y( }" y 2.2.3 Gas purification + ^; z9 W1 F* @ ?# i Gas purification is the weakest link in biomass gasification in China. Currently, water washing is used, which not only reduces system efficiency but also generates tar-containing wastewater; therefore, it is a key topic for future research. From the perspective of current technology, tar cracking to reduce tar and water treatment for wastewater recycling are two key issues in gas purification. 2.3 The economic viability of biomass gasification power generation in China: The investment in biomass gasification power generation mainly includes three components: the gasification and purification systems, the power generation equipment, and the civil engineering works. Taking a 1000KW rice husk gasification power generation system as an example, its investment composition is shown in Table 2. As can be seen from Table 2, since hull gasification does not require specialized collection and transportation equipment nor storage facilities, the capital investment required is only 3,500 yuan per kilowatt, **which is lower than that of small coal-fired power plants (about 6,000 yuan per kilowatt). Furthermore, as the capacity decreases, the proportion of gasification equipment, supporting equipment, and wastewater treatment equipment increases. As a result, the unit cost of BGPG rises as capacity decreases; when the power is less than 60 KW, the unit cost is higher than that of small coal-fired power plants (see Figure 2). Table 2. Investment budget for 1MW gasification power generation (in 10,000 yuan)
(1) Gasification furnace: 5MWth, including a separator.
(2) Water washing unit: 2500 Nm3, including a venturi tube.
(3) Gas turbines: 5 units of 200kW each.
(4) Power distribution equipment: distribution panels and cables.
(5) Water treatment system: used for treating wastewater.
(6) Supporting equipment: fans, water pumps, motors, pipes, etc.
(7) Project installation.
(8) Infrastructure: a 200m2 factory building and foundations.
(9) Control system.
Total cost: approximately 0.27 yuan/kWh for 1000KW rice husk power generation (see Table 3). This cost is close to that of coal-fired power plants, but it is far lower than the cost of generating electricity using diesel engines. At smaller power generation scales, the proportion of labor and maintenance costs **increases**, which causes the cost of power generation to rise as the power output decreases. When the power is less than 100 KW, the cost of power generation approaches that of large diesel generators, resulting in a loss of competitive advantage. 2.3.3 Environmental protection investments: Since the problem of tar generated during biomass gasification has not yet been resolved, in order to avoid secondary pollution, it is necessary to invest funds in installing wastewater treatment equipment. The amount of investment required for wastewater treatment varies depending on the system used; generally speaking, the lower the power capacity, the higher the proportion of investment needed. This is also the main reason why low-power BGPG systems are difficult to be widely adopted. Moreover, since sewage treatment requires a large amount of space, many users refrain from treating their sewage in order to save costs and reduce operating expenses, which leads to environmental problems. ! V2 c+ Z6 l+ I1 t% n0 X6 b+ F% C 3. Key issues affecting the application of biomass gasification technology 2 }- w) S6 a e/ g/ ^, T4 T0 \4 j 3.1 Biomass collection and preprocessing Due to its low energy density and scattered distribution, the collection and transportation of biomass constitute a major part of the costs associated with it. Preprocessing of biomass requires additional equipment and investment, and both of these factors increase the operating costs of biomass gasification for power generation. Figure 3 shows the relationship between biomass cost and power generation cost. As can be seen from the figure, when the price of biomass exceeds 200 yuan per hour, the power generation cost of BGPG is higher than 0.45 yuan per kilowatt-hour, at which point BGPG loses its economic attractiveness. Figure 4 shows the current market prices of different types of waste, reflecting the impact of various collection and transportation conditions on the cost of biomass. As can be seen from the figure, under current conditions, the cost of collecting biomass waste over a distance of 50 kilometers is higher than 200 yuan per ton, making it uncompetitive. This indicates that current BGPG technology can only be applied in situations where biomass waste is concentrated, without the need for collection, transportation, or preprocessing. 8 g) K! Y+ F" W' A# r 6 t4 O Y+ u4 G! D% j 3.2 Secondary pollution of wastewater As mentioned earlier, the high costs and space requirements associated with wastewater treatment have made wastewater issues a major obstacle to the widespread adoption of BGPG at present. Especially for gasification power generation systems with a capacity of less than 400 KW, installing wastewater treatment equipment will **increase investment and operating costs, leading to many users who require 200 KW giving up on using gasification power generation technology. ; X, N* T$ f* w. D, ? h 4. Solutions to the problems faced in developing BGPG in China" `! N" V2 G; q2 h 4.1 Technical research and improvement 4.1.1 Tar cracking technology and wastewater treatment processes h! ]" M- s- g% G8 }; Q7 o Tar cracking is a method to completely eliminate secondary pollution. Only by minimizing the amount of tar can wastewater generation be avoided. Of course, it is difficult to ensure complete absence of tar regardless of the process used; therefore, it is necessary to use a certain amount of water for cooling and cleaning. As a result, research on the treatment and recycling of wastewater is also essential. Only by addressing the issue of secondary pollution can biomass gasification power generation technology compete on an equal footing with other technologies. I# n. Z+ E" z; ^ T& b% ?7 Z2 c( ~$ W) w( r 4.1.2 Improvements in the power generation cycle and enhancement of system efficiency Due to limitations in gasification efficiency and gas turbine efficiency, the efficiency of a simple gasification-gas turbine power generation cycle can hardly exceed 20%; as a result, the biomass consumption per unit of electricity generated is generally greater than 1.1 kilograms (dry weight)/kWh. From the analysis of power generation costs, it is evident that raw material costs constitute the largest portion of these costs. If the amount of biomass available cannot be reduced, it will be difficult to make use of the biomass resources that need to be collected and preprocessed. Therefore, in the long term, improving the overall efficiency of the system is a prerequisite for the widespread adoption of BGPG. : v1 X4 R$ B. q7 S) p3 c- e. V6 X From a purely technical perspective, biomass IGCC can effectively improve the overall efficiency of BGPG; however, due to the limitations in tar treatment technologies and gas turbine technology, it remains challenging to develop biomass IGCC in China. Therefore, how to utilize existing mature technologies to develop systems that are economically viable and offer significant improvements in efficiency is a key challenge in the current development of BGPG. Figure 5 shows a combined cycle concept based on a more mature gasification-gas turbine system, which has three features: (1) low technical complexity, as no advanced gas purification technologies are required ； (2) The system’s power generation efficiency improves significantly, reaching around 28%, which is on par with that of small coal-fired power plants ； (3) Due to the maturity of the technology, the equipment used are traditional, standard products, resulting in a low investment per unit – around 4,000 to 5,000 yuan per kilowatt. Therefore, taking both technical and economic factors into account, this system is a suitable choice for China’s conditions, especially for capacities ranging from 4 to 10 megawatts. It is one of the directions for future research and development in our country. 4.2 Accelerating the application and demonstration of BGPG in China 5 Compared to developed countries, China has a favorable market environment for biomass gasification power generation. However, cost analysis shows that even if the issue of secondary pollution is addressed, the large-scale collection and transportation of biomass still increase the cost of power generation, rendering it uncompetitive from an economic perspective. Therefore, the main users of biomass gasification for power generation at present are likely to be enterprises or regions that have large amounts of biomass waste and no issues with collection and transportation. To fully demonstrate the technical and economic advantages of BGPG, it is essential to carry out commercial demonstrations in these enterprises, so that BGPG can be gradually accepted by them. On this basis, the technical performance of BGPG is improved and enhanced, and the possibility of applying BGPG to the large-scale processing of agricultural straw or forest waste is explored. Considering the current characteristics of enterprises in China, rice mills and timber processing plants are the ones most likely to use BGPG. China produces nearly 200 million tons of rice and over 10 million cubic meters of wood-based panels each year; as a result, there are hundreds of large-scale rice milling plants and wood-based panel factories. Therefore, even though BGPG is currently targeted only at these two types of enterprises, it still possesses great market potential. 4.3 Ensuring power purchase and encouraging the use of BGPG: In China, the capacity of individual gas engines is relatively low (200 kilowatts), which does not meet the requirements for connecting such units to the power grid. However, as a renewable energy source, biomass plays an important role in reducing pollution and protecting the environment; therefore, policies aimed at encouraging and supporting its use should be implemented. In addition to allowing smaller units (100 kilowatts) to be connected to the grid, a minimum purchase price should also be established to ensure the continuation of biomass-based power production. At present, the relevant ministries have begun to formulate policies in this area; the key lies in how to implement them effectively. / {4 K3 ~& @; u 5. Conclusion 8 `# ]+ a# m3 R’ e5 T- h/ Y+ i) R Gasification for power generation is an effective way to make use of biomass energy in a decentralized manner, and it is quite suitable for China’s current economic level and development status. China has a solid technical foundation for biomass; as long as secondary pollution is addressed, it is capable of competing with other conventional power generation technologies. To develop and accelerate the adoption of biomass gasification technology, three actions should be taken at present: first, research on tar treatment technologies to completely eliminate secondary pollution ； Second, improve gasification power generation technologies and systems to enhance overall efficiency and further reduce power generation costs ； Third, establish guarantee policies to encourage the application of biomass gasification power generation technology.