1.4. Gas composition

The gas composition and its impurities can have an impact on the productivity of the gas fer­mentation process. Greater molar ratio of H2:CO allows greater efficiency in the conversion of the carbon from CO into products such as ethanol, because reducing equivalents are generat­ed from oxidation of H2 (rather than CO). However, CO is also a known inhibitor of hydroge — nase which can affect utilization of H2 during fermentation. In B. methylotrophicum, H2 utilization was inhibited until CO was exhausted [108]. When CO is consumed, acetogens are able to grow using CO2 and H2. Common impurities from biomass gasification or other waste gases are tar, ash, char, ethane, ethylene, acetylene, H2S, NH3 and NO [17, 22, 24, 188].These have been shown to cause cell dormancy, inhibition of hydrogen uptake, low cell growth and shift between acidogenesis and solventogenesis in acetogens [13, 188]. For instance, NH3 from the feed gas readily convert into NH4+ in the culture media and these ions were recently shown to inhibit hydrogenase and cell growth of acetogen "C. ragsdalei” [189]. A number of strategies to mitigate the impact of such impurities have been proposed, for example installing 0.025 mm filters, or the use of gas scrubbers or cyclones, and improvement in gasification efficiency and scavenging for contaminants in the gas stream using agents such as potassium permanganate, sodium hydroxide or sodium hypochlorite [24, 190—192]. H2S does not have a negative effect on acetogens such as C. Ijungdahliiup to 5.2% (v/v) [193].