Biogasification potential of desugarized molasses from sugarbeet processing plants.

Abstract

Sugar production from sugarbeet (Beta vulgaris) generates a co-product stream called raffinate, desugarized molasses, or concentrated separated by-product (CSB). About 0.25 metric tonnes of raffinate is generated for every metric tonne of sugar produced. In this paper, the potential of biogasifying the raffinate to produce methane fuel is investigated. Several aliquots of CSB raffinate (0.1 – 0.2 kg), obtained from the American Crystal Sugar Company, MN, were biogasified in batch mode in a bench-scale anaerobic downflow stationary fixed film reactor (DSFFR) at a thermophilic temperature of 55 OC. It was found that about 0.118 m3 of methane at STP was produced per kg raffinate. This volume of methane was recovered in the biogas within a 2 day period indicating very good degradability. For every kg of raffinate fed into the anaerobic digester about 94 g COD of organic matter will be discharged with the digester effluent along with 27 g of microbial biomass (or sludge). Every metric tonne of raffinate can generate 4,300 MJ (3.7 MMBtu/US ton) of thermal energy from the combustion of the methane or about 300 kWhe of electricity can be generated (assuming 25% efficiency of converting thermal energy to electricity). Additional key words: Anaerobic digestion; biogas; raffinate; CSB; sugar beet 90 Journal of Sugar Beet Research Vol. 47 Nos. 3 & 4 N 40% of refined sugar consumed in the USA is produced from sugarbeet (Beta vulgaris L.). Beet sugar processing generates significant quantities of both solid (tailings, spent pulp) and liquid (molasses, wastewater) by-products and organic wastes. For example, American Crystal Sugar Company, East Grand Forks (EGF) plant processes 8,310 (metric) tonnes of sugar beets daily to produce 1320 tonnes of sugar. The plant also produces 3500 m3 of wastewater, 432 tonnes of tailings, 1273 tonnes of pressed spent pulp and 332 tonnes of desugarized molasses (Polematidis et al., 2008). The molasses stream from the crystallization process is usually further desugarized in chromatographic separators to recover about 70% of the sucrose. The desugarized molasses by-product stream contains water, residual sugars, raffinose, betaine, amino acids, nitrogen compounds, organic acids and inorganic salts. In some plants the betaine is recovered as another value added product. The desugarized molasses stream is concentrated (also called concentrated separated by-product, CSB raffinate) and sold as animal feed for about US$20/tonne. Sugar production from sugarbeet is an energy intensive operation due to the need for drying and evaporation, and a variety of fossil fuels including coal and natural gas are used. In this paper, we investigate the potential of converting CSB raffinate stream into biogas fuel. Biogas produced in this manner would be a renewable biofuel and could potentially displace some of the fossil fuel used in the plant. Biogasification (or anaerobic digestion) is a biochemical process in which the organic compounds (like carbohydrates, proteins and fats) are mineralized to biogas under anaerobic conditions by the concerted and syntrophic action of a mixed culture of naturally occurring microorganisms. Biogas is a mixture of methane (50 – 70%) and carbon dioxide (30-50%) and can be used as fuel in boilers or for electricity generation with very little clean up. Biogas can also be converted to renewable natural gas by removing carbon dioxide and other trace contaminants and injected into natural gas pipelines for transportation. The process of anaerobic digestion has advantages over aerobic waste treatment processes since it not only reduces the polluting potential (oxygen demand) of the waste but also produces a biofuel. The process does not reduce the nutrient (nitrogen and phosphorus) levels. Even though there have been some studies pertaining to anaerobic digestion of molasses from sugarcane refineries (Romli et al., 1994, 1995), it is not implemented in practice as molasses in itself has a high value and by converting to biogas not much value is added to the feedstock. However, the situation may be different for the desugarized molasses stream as it is a low value product. Moreover, since a major Aug. Dec. 2010 Biogasification Potential 91 fraction of the sugars have been removed, its methane production potential could have also decreased and may not produce significant methane to make its biogasification economically viable. This paper presents results from laboratory scale investigations on the biochemical methane potential of desugarized molasses from a sugarbeet processing plant. The anaerobic digestion process is analyzed in terms of extent of organic matter converted to biogas, undegraded residual organic matter and methane production rate. Based on the results a simple analysis was carried out to determine energy potential of the raffinate.