Comparison of Fixed Bed Gasifier and Fluidized Bed Gasifier

Energy and environmental issues have become a global focus. With the rapid growth in energy consumption, the extensive use of fossil fuels has led to severe environmental pollution and ecological damage. Additionally, the diminishing reserves of conventional energy sources such as coal, oil, and natural gas make the development of clean renewable energy an urgent necessity for sustainable development.

Biomass is a storable and transportable clean energy source. Biomass gasification technology can convert biomass into gaseous energy and biochar, which can be applied in areas such as power generation, hydrogen production, and heating, with significant development prospects. The biomass gasifier is the core equipment of biomass gasification plant, where biomass undergoes gasification reactions to produce syngas. Biomass gasifiers can be classified into fixed bed gasifiers, fluidized bed gasifiers, entrained flow gasifiers (EF), plasma gasifiers, etc. Among these, fixed bed gasifiers and fluidized bed gasifiers are the most widely used.

Understanding the respective advantages and disadvantages of fixed bed gasifiers and fluidized bed gasifiers is essential. This knowledge aids in selecting more suitable solutions during equipment selection, leading to improved gasification efficiency and economic benefits.

Biomass Fixed Bed Gasifier

The fixed bed is a traditional gasification reactor, typically operating at temperatures around 1000°C. In a fixed bed gasifier, the gasification reaction takes place in a relatively stationary bed layer, where the material remains relatively static in relation to the gas flow. The material in the furnace is generally divided into four stages: drying stage, pyrolysis stage, combustion stage, and reduction stage. The reaction rate inside a fixed bed gasifier is relatively slow.

fixed bed biomass gasifier

Based on the flow direction of the gasifying agent inside the furnace, fixed bed gasifiers can be categorized into four types: updraft gasifiers, downdraft gasifiers, cross-draft gasifiers, and open-top gasifiers.

Fluidized Bed Gasification

Fluidized bed combustion is an advanced combustion technology that has been successfully applied to biomass combustion. However, its application to biomass gasification remains a relatively new topic. Unlike fixed beds, fluidized beds do not have grates. A simple fluidized bed consists of a combustion chamber and a distribution plate, with the gasifying agent introduced into the fluidized bed gasifier through the distribution plate. Depending on the gas-solid flow characteristics, fluidized beds are categorized into bubbling fluidized beds and circulating fluidized beds.

fluidized bed biomass gasifier

In bubbling fluidized bed gasifiers, the gas flow velocity is relatively low, and there is almost no escape of solid particles from the fluidized bed. In contrast, circulating fluidized bed gasifiers have a relatively higher fluidization velocity. The particles carried out of the fluidized bed are collected through a cyclone separator and reintroduced into the furnace for gasification reactions. Fluidized bed gasifiers exhibit excellent mixing characteristics and a higher gas-solid reaction rate.

Comparison of Fixed Bed and Fluidized Bed

Fixed bed gasifiers and fluidized bed gasifiers have their own advantages, disadvantages, and specific application scopes. For instance, fixed bed gasifiers have a simple structure, convenient operation, and flexible operation modes. However, they are suitable only for medium to small-scale production. On the other hand, fluidized bed gasifiers, while suitable for industrial and large-scale applications, have complex equipment, high investment requirements, and depend on a relatively stable market demand for the product gas.

Technical Performance

As of the current situation, the design operational time for both fixed bed and fluidized bed gasifiers is generally less than 5,000 hours. The former has a simple and robust structure, while the latter has a more complex structure and is less mobile once installed. However, fluidized bed gasifiers generally have a smaller footprint, and their capacity is usually larger than that of fixed bed gasifiers.

During startup, fixed bed gasifiers have a slower heating process, requiring a longer time to reach the reaction temperature. In contrast, fluidized bed gasifiers heat up rapidly and can be started and stopped frequently.

During operation, the temperature distribution in the fixed bed is uneven, with solids spending a longer time in the bed, while gas residence time is shorter, resulting in a lower pressure drop. In fluidized beds, the temperature distribution is uniform, and there is good mixing between gas and solids. Both gas and solids have shorter residence times, leading to a higher pressure drop within the bed. The operational load of fixed bed gasifiers can vary between 20% and 110% of the design load, while fluidized bed gasifiers, constrained by the need to maintain fluidization conditions, can only vary between 50% and 120% of the design load due to airflow velocity limitations.

Raw Material Requirements and Products

Fluidized beds have lower material requirements compared to fixed beds. Fixed beds require specific types of raw materials with consistent shapes and sizes, while fluidized beds can accommodate diverse raw materials with varying types, shapes, and particle sizes. The particle size in fixed beds tends to be larger, whereas in fluidized beds, it is smaller.

The primary product of fixed bed gasification is low-calorific-value gas, containing small amounts of tar, oils, benzene, ammonia, and other substances. This gas requires separation and purification processes. In fluidized bed gasification, the gas produced has lower levels of tar and ammonia, with a stable composition and calorific value. The gas from fluidized beds contains more solid particles compared to fixed beds. The temperatures of the outgoing gas and bed in fluidized beds are generally consistent.

Energy Utilization and Conversion

In fixed beds, the uneven temperature distribution within the bed leads to a less effective heat exchange compared to fluidized beds. However, the extended residence time of solids in the bed results in a high carbon conversion efficiency, typically ranging from 90% to 99%. In fluidized beds, the presence of more solid particles in the outgoing gas causes incomplete combustion losses, resulting in a carbon conversion efficiency of around 90%. Both fixed beds and fluidized beds exhibit relatively high thermal efficiency.

Environmental Benefits

Fixed bed gasifiers have low fly ash content in the produced gas, while fluidized bed gasifiers have higher fly ash content. This is because the temperature in a fixed bed can be higher than the ash melting point, causing the ash to melt into a liquid that is then discharged from the bottom of the furnace. In contrast, the temperature in a fluidized bed is lower than the ash melting point, allowing some fly ash to be carried out with the outgoing gas. The environmental impact of fluidized beds is greater than that of fixed beds, necessitating dust removal and purification treatment for the produced gas.

Economic Benefits

In terms of design and manufacturing, the complex structure of fluidized beds results in higher investment compared to fixed beds. In terms of operation, fixed beds have higher raw material requirements, while fluidized beds have lower requirements. Consequently, the operating investment for fixed beds is higher than for fluidized beds. Fixed bed gasifiers have a wide temperature distribution, which may lead to local high temperatures causing ash fusion. This makes them less efficient in terms of capacity, startup time, and difficulty in scaling up for large applications.

Fluidized beds, on the other hand, have the advantages of high gasification intensity and good overall economic performance. Considering both design and operational aspects, fluidized beds offer greater economic benefits compared to fixed beds.

Conclusion

In conclusion, the comparison between fixed bed and fluidized bed gasifiers reveals distinct characteristics, advantages, and drawbacks for each technology. Both technologies play crucial roles in biomass gasification, addressing global concerns about energy consumption, environmental pollution, and the need for sustainable energy sources. The choice between fixed bed and fluidized bed gasifiers depends on specific project requirements, raw material availability, and economic considerations. As technology continues to evolve, further advancements in biomass gasification processes are expected, contributing to a cleaner and more sustainable energy future.

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