The effect of oxygen on formation of syngas contaminants during the thermochemical conversion of biomass

The effect of oxygen on the formation of syngas contaminants during the thermochemical conversion of carbonaceous feedstocks has been quantified using an integrated biorefinery plant operated at a biomass input of about 4.5 metric tons/day. This plant combines solids steam reforming and gases steam reforming processes for the conversion of biomass to syngas. It was found that the presence of low concentrations of oxygen (in air) during the thermochemical conversion process had a significant effect on the formation of contaminants in the syngas. For example, particulate organic carbon compounds (organic particulate contaminants) increased from 3.3 to 122 mg/m3 when the oxygen input was increased from 225 ppm to 4.1 vol.% during the thermochemical conversion of wood to syngas. It is proposed that the primary free radical (∗) species H∗, OH∗, O∗, CH3 ∗ and OOH∗, formed from the presence of O2 in this high-temperature process, react with the myriad of organic compounds in the syngas at varying rates, depending upon their structure and reactivity. These processes represent the primary chemical mechanisms for the formation of high molecular weight hydrocarbons, polynuclear aromatic hydrocarbons, oxygenated hydrocarbons and polymeric materials, commonly referred to as organic particulate contaminants. The potential importance of these free-radical oxidation processes was supported by measuring the concentrations of selected oxygenated hydrocarbons in the syngas over a range of 225 ppm to 4.1 vol.% of O2 in the thermochemical process. The concentrations of oxygenated polycyclic aromatic hydrocarbons (hydroxy-naphthalene, dihydroxy-naphthalene, dihydro-indene-2-one, and benzo-pyranone) increased by 732, 244, 83 and 195 times, respectively, when the oxygen concentration was increased from 225 ppm to 2.5 vol.%. These increases were due to the free-radical oxidation of the highly reactive PAHs during the thermochemical processes. The importance of these oxidation processes was further confirmed by studying the decrease of easily oxidized olefins. For example, the concentrations of 1,3-butadiene, acetylene, propene and ethene decreased by 9.3, 5.2, 4.5 and 3.4 times, respectively, when oxygen in the plant was increased from 1.6 to 2.5 vol.%. It is concluded that the formation of organic particulate contaminants during the thermochemical conversion of carbonaceous feedstocks can be minimized by maintaining the concentration of oxygen below 500 ppm.