Aromatics from biomass

Benzene is an important compound in producer gas from medium-temperature gasification. Depending on the operating conditions, producer gas can contain 0.5-1 vol.% benzene. However, 1 vol.% benzene means 10% of the energy contained in the gas, which is a significant amount. Depending on the gasification feedstock, the benzene concentration can increase up to 2.5 vol.%. Besides benzene, also toluene is produced, but the yield of toluene is one order of magnitude lower than that of benzene. Xylenes are also produced in low concentrations.

The capture of BTX (benzene, toluene and xylenes) is a promising option for the co-production of energy/biofuels and green chemicals. Given the high added value of BTX, their recovery is an interesting alternative to reduce the production cost of biofuels such as SNG.

There are a number of projects and initiatives for the production of aromatics from biomass and biomass derivatives, e.g. (bio)chemical and enzymatic conversion of carbohydrates, fermentation of carbohydrates, or (catalytic) pyrolysis of lignin.

There are two routes of production of benzene upon pyrolysis, namely the lignin route and the cellulose route. In all cases, benzene is a secondary pyrolysis product, that is, it is produced by reactions between primary pyrolysis products. The benzene formation from lignin comes from deoxygenation of phenol (dominant at 700-800°C) or acetylene cyclic polymerization (dominant at 850-1000°C). From cellulose, benzene is formed by the reactions between the secondary pyrolysis gases (propylene, butane, dimethyl furan, acetylene). Olefins and BTX are non-equilibrium intermediates and will react further given a prolonged residence time. Benzene can react with another benzene molecule or with acetylene to form polyaromatic cyclic hydrocarbons (PAHs). Polymerization reactions can be prevented by lowering the partial pressures of reagents (benzene, acetylene). From previous tests at ECN, lignin seemed to be the main source of benzene.