(374as) Effects of Impurities in Syngas On Ethanol Production Via Fermentation

Biomass is an attractive energy feedstock because it is an abundant, domestic, renewable resource that can be converted to liquid transportation fuels. Currently, most of the research in the US is centered on breaking down feedstocks to sugars and fermenting sugars to ethanol. Although this process is comparatively simple, it only converts part of the carbon to the end-products such as ethanol. An alternative to this process is gasification of biomass to produce syngas, followed by fermentation of syngas to ethanol by a microbial catalyst--more carbon in the biomass can be converted to end products such as ethanol. For this gasification-fermentation process, most research is still at the laboratory scale and uses ?clean? syngas instead of biomass-generated syngas (denoted biomass syngas). For rapid commercialization of this new process, the biomass syngas must be used. However, there is a great difference in the compositions between the ?clean? syngas and biomass syngas. The ?clean? syngas is only made of CO, CO₂, and H₂. For the biomass syngas, there are other potential gas impurities such as methane (CH₄), acetylene (C₂H₂) , ethylene (C₂H₄), ethane (C₂H₆), benzene (C₆H₆), hydrogen sulfide (H₂S), sulfur dioxide (SO₂), ammonia (NH₃), nitrogen (N₂), carbonyl sulfide (COS), oxygen (O₂), water (H₂O), and mono-nitrogen oxides (NO_x) as well as tars and ashes. Some of these minor gases may interfere with the fermentation process. In previous work, the effects of biomass syngas from gasified switchgrass on cell concentration and acid / ethanol distribution were investigated in comparison with ?clean? bottled gases of similar compositions for CO, CO₂, and H₂. Clostridium was used with the following key findings: (a) the cells stopped growing after switching to biomass-generated syngas; (b) the cells could recover from dormancy after switching back to clean syngas; and (c) the cells stopped consuming H₂ . In addition, biomass syngas affected the acetic acid / ethanol product distribution. Tars were the most likely culprit for cell dormancy and product redistribution. After prolonged exposure, the biocatalyst adapted to the tars. The addition of a 0.025 um filter in the gas clean-up system can minimize this effect to negligible levels. In our subsequent investigation, the effects of impurities from syngas were dealt with individually?this is the focus of this presentation. It was found that NO would inhibit cell growth and hydrogenase activity at concentrations above 40ppm. From our recent ammonia studies (both bottle batch and continuous gas feeding bioreactor), it was concluded that ammonia would have an inhibitory effects on cell growth and hydrogenase activity at concentrations above 250mM in media owing to the effects from increases in osmolarity. It has been categorized through modeling that both NO and ammonia ion are non-competitive inhibitors for hydrogenase activity. Issues associated with scale-up and the need for syngas processing will also be addressed.

Since the syngas composition from various biomass and coal blends will generate different gas compositions, an understanding of the positive or adverse effects of impurities from syngas on ethanol production will provide critical information regarding the need for efficient gas cleaning processes for commercialization.