Bioenergy Co-Products

Production of bioenergy typically results in production of non-energy co-products – products for which there is an economic use, and by-products – those for which there does not yet exist an economic use (discussed below).  All of the conversion pathways can result in co-product creation, but primary co-products are discussed here: distillers grains, bagasse, glycerol, and biochar.

The primary co-products of corn grain ethanol are cereal by-products of the distillation process, notably dried distillers grains (DDGs) and wet distillers grains (WDGs).  About 60% of the U.S. fuel ethanol is produced by dry-mill plants, from which DDGs and WDGs are the co-product.  (Wet mill plants co-produce wet or dried corn gluten feed, corn gluten meal and corn germ as the primary co-product.)  Both DDGs and WDGs can be used for animal feed, mostly cattle, but transportation costs are less for DDGs than distillers grains that have not been dried.  Most DDGs and WDGs are used in the dairy and feedlot industries.

Bagasse (figure 2.7) is the fibrous material left after sugarcane or energy cane has been crushed and the juice extracted.  It can be used directly as a feedstock (i.e., burned directly in biopower combustion), but it can also be used in cellulosic biofuel production.  Bagasse is sometimes used as a primary fuel for heat and electricity use in sugar mills.

Glycerol, also known as glycerin, is the primary co-product of biodiesel production.  It is a liquid co-product of the transesterification of oils and fats.  Glycerol is used in a range of products including pharmaceuticals, cosmetics, and in the food industry as a sweetener, emulsifier, and solvent.  Approximately 950,000 tons of glycerol are produced in the U.S. annually.  However, because of the growth of biodiesel production, there is currently more glycerol produced than market demand.  As a result, a substantial amount of glycerol is considered surplus.  Current R&D efforts in the U.S. and elsewhere are determining the suitability of glycerol as a fuel for combustion in biopower and heat production, for example.

A primary co-product of gasification and pyrolysis is biochar, or black carbon.  Biochar is a form of charcoal, and most commonly is used as a fertilizer or soil amendment.  The amount of biochar produced varies according to temperature of thermal conversion process, with cooler temperatures resulting in more biochar.  There is currently a great deal of scientific interest in the potential use of biochar to sequester carbon in soils.