Combined low thermal alkali addition and mechanical pre-treatment to improve biogas yield from wheat straw

Abstract

One of the main obstacles for a more widespread use of wheat straw for renewable energy production is the absence of a low-cost technology which promotes its biodegradation. The aim of this study was to determine whether methane yield from wheat straw could be improved by using alkali at low concentration and ambient temperature if the fiber fraction had previously undergone mechanical pretreatment. After combining mechanical and alkali pretreatment, a linear correlation between alkali concentration and hydrolysis of the fiber content, mainly hemicelluloses, was found. A positive effect by combining briquetting of wheat straw with sodium hydroxide addition was found with the hemicellulose content decreasing by up to 66%. In general, alkali improved the ultimate methane yield and the methane yield obtained after 30 days. Briquetting reduced the optimal alkali concentration needed to reach the maximal improvement in methane yield compared to merely shredding. The results proved that it is not necessary to work with the high alkali concentrations traditionally used to improve methane production if the straw is mechanically pretreated prior to alkali addition. Methane yield obtained after 30 days as well as net energy yield rose by around 40% when briquetting was combined with 2% sodium hydroxide. Therefore, a low concentration of alkali, especially sodium hydroxide, applied at ambient temperature can improve the energy and the economic balance of pretreatment.