CO₂ Removal from wood gas

• Main Author: Dahiru, R. (Rufai)
• Format: Dissertation
• Language: English
• Published: University of Oulu 2013
• Subjects: Environmental Engineering
• Online Access: http://urn.fi/URN:NBN:fi:oulu-201310301823; http://nbn-resolving.de/urn:nbn:fi:oulu-201310301823

Gasification is considered as one of the most attractive conversion technologies, because the product gas from the process serves as a building block for several industrial applications. However, the use of biomass as a fuel in the gasification process offers a carbon neutral fuel that will alleviate the continuing use of fossil fuels sources. This study was done to evaluate the possible applications of syngas originating from biomass gasification, as a follow up to the earlier biomass gasification research of the HighBio project. The syngas from the gasification process is generally produced in a gasifier. An overview of the different type of gasifiers for biomass gasification that include updraft, downdraft, crossdraft, entrained-flow and plasma gasifiers was presented. The syngas can be utilized in the generation of power, heat, fuels and chemicals. A detailed overview of the promising applications of the syngas in Fischer-Tropsch synthesis, hydrogen production, ammonia synthesis, hydroformylation of olefins, and syngas fermentation was also given. However, for these applications, a high degree of treatment and conditioning of the syngas is required. The treatment is usually carried out to remove undesirable impurities, while the conditioning of the gas is done to get the right H₂ to CO ratio for further applications of the syngas. Raw syngas from gasification processes can contains also impurities such as solid particulates, inorganic and organic impurities, which have to be removed. However, CO₂ is one of the major by-products in a gasification process. The removal of CO₂ is desirable in order to reduce the CO₂ emissions or to meet the downstream process requirement in relation to size and costs. Absorption processes are the most developed techniques in the separation of CO₂ in the industries. However, other techniques such as adsorption, membrane separation, and chemical-looping combustion have recently gained interest. Moreover, two MFI-types zeolite membranes (ZSM5-I and ZSM5-II) were tested in the separation of CO₂ from CO₂/N₂ mixtures in a laboratory scale experiments. Separation factor, gas permeability and CO₂ permeate flux were the parameters used to determine the membrane performance. The highest membrane performances of the feed gas compositions were achieved with low CO₂ gas composition, and at the low temperature separation experiments. However, in order to achieve high recovery and purity of CO₂, the separation experiment of the HighBio CO₂/N₂ feed composition using the ZSM5 zeolite membrane was evaluated at low temperature and constant feed and permeate pressures of 6 bar and 1 bar respectively. Based on the analysis, it is suggested that 6 successive membrane modules should be employed. Further investigations that incorporate other product gas compositions from biomass gasification, as well as the study of other type of inorganic membranes more suitable to CO₂/N₂ separation processes are highly recommended. The economic analysis of the multi-stage ZSM5 zeolite membrane separation could be also another interesting study.