Design and selection of biomass gasification processes

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There are various processes for the gasification of biomass. Theoretically, any gasification process can be used to form a biomass gasification power generation system. However, from the perspective of the quality and economic efficiency of gasification-based power generation, biomass gasification power generation requires two basic conditions: stable power generation frequency and continuously adjustable power generation load. Therefore, for the gasification equipment, it is necessary to ensure stable gas quality, adjustable gas output, and continuous operation. Under these premises, the level of energy conversion efficiency in gasification is the key factor affecting the operating costs of gasification power generation systems. Once the gasification method has been selected, from the perspective of system compatibility, the gasification equipment should meet the following requirements: ① The fuel gas should be as clean as possible to reduce the complexity of the after-treatment systems, ensuring that the tar content meets the standards acceptable for internal combustion engines. If a catalytic cracking process is chosen for the subsequent purification system, it is also necessary to ensure that the tar present in the raw gas has properties that facilitate catalytic cracking; ② The calorific value of the gas should be high and stable, in order to increase the output power of the internal combustion engine and thus improve the efficiency of the entire system; ③ The design of the gasification furnace itself as well as the feeding and slag removal systems must take into account the characteristics of the raw materials in order to enable continuous operation; ④ The sensible heat should be utilized fully to improve energy efficiency. From a practical perspective, fixed-bed gasifiers are more suitable for small-scale, intermittent gasification power generation systems. Their greatest advantage is that the feedstock does not require pretreatment, and the equipment has a simple and compact design; moreover, the ash content in the gas produced is low, allowing for simple filtration methods to be used for purification. However, their biggest drawback is that fixed-bed systems are not easy to scale up, making industrial application difficult, and the cost of power generation is generally high. Furthermore, due to issues with feeding and ash discharge, fixed-bed systems are not suitable for design as continuous-operating units, which is detrimental to the continuous operation of gasification power generation systems. Additionally, the quality of the gas produced can fluctuate easily, resulting in unstable power generation quality. All these factors limit the widespread use of fixed-bed gasification technology in gasification power generation systems, and it represents the biggest technical challenge to the industrialization of small-scale biomass gasification power generation systems. Various fluidized-bed gasification technologies, including bubble beds, circulating fluidized beds, and dual-fluidized beds, are quite suitable for gasification-based power generation processes. The fluidized bed operates stably, and its operation can be adjusted continuously. Most importantly, it is easy to scale up, making it suitable for industrial applications in biomass gasification power generation systems. Of course, fluidized beds also have two obvious disadvantages: first, the raw materials need to be pre-treated to meet the requirements of the fluidized bed and the feeding process; second, the gas produced by fluidized-bed gasification contains a high level of fly ash, which makes subsequent gas purification difficult. These two aspects are the main issues that are currently being studied and addressed in the industrial application of biomass fluidized beds. The scale-up design of fluidized bed gasifiers is one of the key technologies that must be addressed when applying large and medium-sized biomass gasification power generation systems. Since air is typically used as the gasification medium in conventional gasification processes, the lower part of a fluidized bed gasifier is usually filled with hot combustion air, while the middle and upper parts contain a gas mixture. The volume of gas in these areas changes significantly; therefore, to ensure that the fluidized bed operates within an appropriate range of fluidization velocity, a variable cross-sectional design is generally adopted, with a smaller cross-section in the lower part and a larger one in the upper part. There are certain limitations to the scaling up of atmospheric-pressure fluidized beds. When the power generation capacity of gasification power systems exceeds 100 MW, the large size of the fluidized-bed gasification equipment and the low efficiency of pressurized gas turbines mean that atmospheric-pressure fluidized beds can no longer meet the requirements of gasification power generation technology. Therefore, high-pressure gasification technology represents an inevitable trend for the large-scale development of this technology. It should be noted that due to the differences in the particle size of biomass, for most crushed biomass, the parameters actually selected in design are much larger than those shown in the table. For example, for crushed straw or ordinary wood chips, the diameter of a 20 MW circulating fluidized bed reaches 3000 mm, rather than the theoretically calculated 2001 mm; from this perspective, high-pressure gasification technology becomes even more important for large-scale biomass gasification power generation systems.