Downdraft gasifier is efficient approach and low tar
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Efficiency and Low Tar Production in Downdraft Gasifiers
Introduction to Downdraft Gasifiers
Downdraft gasifiers are a popular choice for biomass gasification due to their ability to produce gas with a viable heating value and low tar content. This makes them suitable for various applications, including internal combustion engines and chemical processing. The design and operational strategies of downdraft gasifiers play a crucial role in enhancing their efficiency and reducing tar production.
Design Innovations for Low Tar Production
Internal Recirculation and Separate Combustor
One effective design involves the use of internal recirculation of pyrolysis gas and a separate combustor in the partial oxidation zone. This setup mixes pyrolysis gases with gasifying air, burning the mixture in a combustor to maintain a very low tar content in the producer gas. This method has achieved tar contents as low as 10.6 mg/Nm³, significantly lower than traditional downdraft gasifiers, with a cold gas efficiency of approximately 82.7% on average.
Swirl Flow and Extended Reduction Zone
Another innovative approach involves changing the fluid dynamic behavior of the mixture in the combustion zone by using wall nozzles to create a swirl flow. This modification, combined with an extended reduction zone, enhances the efficiency of tar thermal cracking and the Boudouard reactions, resulting in tar contents below 10 mg/Nm³ without significantly affecting the heating value of the producer gas.
Modular Design with Thermal Integration
A modular downdraft gasifier with a thermal integration unit has also shown promise. This design returns hot fuel gas exiting the gasifier to the drying and pyrolysis zones, improving gasification reactions and reducing tar content. The gas produced from peanut shell waste using this method contained quality combustible gases suitable for heat and power applications.
Performance and Efficiency
Cold Gas Efficiency and Heating Value
The cold gas efficiency (CGE) of downdraft gasifiers can vary based on design and operational parameters. For instance, a study on an Imbert downdraft gasifier indicated that decreasing the throat diameter could reduce tar content but also decrease CGE by approximately 7%. Conversely, a double air stage downdraft approach achieved a high temperature in the oxidation zone for better tar cracking, resulting in a maximal lower heating value (LHV) of 5.25 MJ/Nm³ and an overall polygeneration efficiency of 95.84%.
Impact of Air Flow Rate
The air flow rate is a critical factor influencing the quality of the producer gas. Optimal air flow rates can enhance the conversion of carbon and hydrogen into combustible gases like CO, CO₂, CH₄, and H₂. For example, an air flow rate of 3.06 m³/hr resulted in significant conversions, producing gas with a high heating value and low tar content.
Conclusion
Downdraft gasifiers, through various innovative designs and operational strategies, have demonstrated high efficiency and low tar production. Techniques such as internal recirculation, swirl flow creation, and modular designs with thermal integration significantly enhance the performance of these gasifiers. These advancements make downdraft gasifiers a viable and efficient option for biomass gasification, suitable for a wide range of applications.
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