Heat-and-power in industrial fermentation processes

Large-scale fermentation using genetically engineered micro-organisms has frequently been proposed as a future alternative means of obtaining bulk organic chemicals from plant and organic waste materials. Sometimes it is implied that in some way this would conserve fossil fuels, or become more attractive if petrochemical feedstock costs rose dramatically. The paper examines the energy requirements of large-scale fermentation processes, and focuses on the high power requirement. First of all it is shown that the ratio of power to heat required for fermentation and downstream processing (distillation, drying, etc.) is usually around 0·7. The high power requirement is due to the need to cool and aerate the fermenter. The primary energy required is thus high. Two means by which this can be minimised are examined--combined heat and power, and heat pumping from refrigeration equipment. An analysis of the comparative costs of the two approaches shows that with current United Kingdom energy prices energy cost savings between 15 and 30% are possible using such integrated energy systems. The combined heat and power scheme uses less fossil fuel than the alternative, but its higher capital cost makes it a longer term investment. Overall fossil fuel consumption for chemicals manufacture is shown to be similar to that of the normal synthetic routes.