Ladda ner examensarbetet - Profu

SUMMARYAt NSR in Helsingborg, Sweden, organic household waste is digested and converted into biogasand bio-fertiliser. The incoming waste contains a small fraction of non-sorted waste suchas plastics, metal and paper. These materials, especially plastics, can cause operational problemsin the digester and pollution of the bio-fertiliser. In order to separate these particles fromthe digestion substrate, the waste requires pre-treatment. For two years, a screw press hasbeen applied for pre-treating the waste at NSR. In the pre-treatment process, food waste isgrounded and mixed with water to form a slurry. The slurry is separated into a dry fraction(reject) and a liquid fraction in the press. The liquid fraction is the digestion substrate and issent to the digester while the reject is sent to combustion. Though, the separation in the screwpress is not complete and thus organic, easily degradable matter ends up in the reject.In order to evaluate the efficiency in the screw press and to estimate the loss of easily degradablematter (and thus loss of methane), an assessment of the mass- and energy balances wascarried out. The composition of the in- and outgoing fractions was analysed with the purposeof determining the distribution of organic material in the two outgoing fractions. The methanepotential in the liquid fraction was compared with the methane potential in the slurry so as toestimate the loss of methane.The results of the mass balances showed that 63 % of the organic material that enters thescrew press ends up in the liquid fraction and 37 % ends up in the reject. One ton of waste thatenters the pre-treatment facility will eventually result in 1.5 tons of liquid and 0.2 tons of reject.Analysis of the composition was carried out in the slurry, the liquid fraction and the reject.These analyses showed that the liquid fraction contains a higher concentration of easilydegradable matter such as fat and protein than the reject. In the incoming material, as well asin the reject, visual plastic objects and large fibres were present, while no visual large particlesoccurred in the liquid fraction.Analyses of the methane yield in the three streams showed that the liquid fraction has thehighest methane potential. However, a considerable amount of methane can be produced fromthe reject and thus, a large amount of potential methane is lost in the present situation. Approximately65 % of the methane that potentially can be produced in the slurry can be producedfrom the liquid fraction.Methane is a gas with a high energy value and can be converted to vehicle fuel. The energy inthe reject is converted into heat and electricity when combusted in a combined heat and powerplant. Just over 12 GWh of methane could be produced from digestion of the slurry. The correspondingenergy production from the liquid fraction is 8 GWh. From combustion of the reject,almost 2 GWh of electricity can be produced and close to 4 GWh of heat. The totalamount of energy that can be produced by applying the screw press is therefore larger whenthe screw press is applied. However, the energy in the gas is considered as a higher form ofenergy and the selectivity in the screw press should therefore be increased in order to achievea higher methane production from the liquid fraction. Though, pre-treatment in the screwpress gives benefits in terms of increased operational stability and a possibility to use the digestionsludge as a bio-fertiliser.