Using char as a catalyst for tar reduction in the production process of DME from biomass
A lot of effort is put into the reduction of using fossil fuels in the energy mix. The transportation sector is a large consumer of mainly liquid , fossil energy. To reduce the share of fossil energy in this sector it is important to increase production of sustainable. liquid fuel. It is important to produce fuel which does not require major changes to infrastructure or engines. Sustainable produced dimethyl ether (OME) from gasified biomass, is a suitable fuel for the transportation sector.
Tars. i.e. hydrocarbons heavier than benzene are a byproduct of the gasification of wood. These tars pose a problem as they are prone to condense. resulting in clogged lines. In addition, it is important to transform the tars into lighter gasses as they contain a significant amount of energy. Biomass char has been put forward as a promising catalyst to reduce the tar content in the gas stream. The advantages of char over other catalysts are low production costs and the constant renewal of the active surface area due to gasification of the char itself
In this work, experiments have been done on the catalytic activity of biomass char on the reduction of the model tar naphthalene (C10H8) and on the consumption of char during these reactions in a downdraft fixed bed reactor. The experiments were done with a simulated producer gas of an oxygen blown circulating fluidized bed gasifier at 850 [0C). The char proved to have good catalytic properties as it promoted a catalytic tar reduction of 16 [%] to 34 [%] with a residence time of 014[s] Char consumption was much less than expected from literature and was measured to be 0.0021 (g.g-1 .min-1] (gram char gasified per gram char present in the reactor per minute time on stream). When it is assumed that all char and tar is converted Into the most important feedstock for DME, H2, they provide 9 [%] and 30 [%], respectively . of the initial H2 feed.
Furthermore, a numerical model was build, using ASPEN. of a DME production facility of 60 [kton] annually with the incorporation of a fixed char bed tar reducing reactor. A sensitivity study of the tar reducing reactor showed a large influence of reactor temperature and char reaction kinetics on reactor size, char consumption, energy consumption, and gas composition. The overall process had a higher heating value efficiency of 72.9 [%] and a 49 [%] carbon efficiency after the major part of the blanket gas of the gasifier. C02. had been removed. Excess heat. in the amount of 30 [MW]. is produced. Due to the low H2 content in the producer gas it is important to recycle CO-rich purge gas to the reformer to convert it to H2 and decrease losses. It proved possible to produce fuel grade DME from the producer gas of an oxygen blown gasifier using a char based tar reformer.