Abstract
The aim of the study is to determine the energy consumption of the extrusion-cooking process of corn straw under various conditions (screw speed, moisture content), water absorption measurements and water solubility indices as well as biogas efficiency evaluation. The extrusion-cooking of corn straw was carried out using a single screw extruder with L/D = 16:1 at various rotational screw speeds (70, 90, and 110 rpm) and with various initial moisture content of raw material (25 and 40%). Prior to the process, the moisture content of the raw material was measured, and next, it was moistened to 25 and 40% of dry matter. For example, at 70 rpm extruder screw speed, the temperature range was 126–150 °C. Energy consumption of straw pretreatment through extrusion-cooking was assessed in order to evaluate the possibility of using the process in an agricultural biogas plant. Biogas and methane efficiency of substrates after extrusion was tested in a laboratory scale biogas plant and expressed as a volume of cumulative methane production for fresh matter, dry matter, and dry organic matter. Pretreated corn straw moistened to 25% and processed at 110 rpm during the extrusion-cooking processing produced the most advantageous effect for methane and biogas production (51.63%) efficiency as compared to corn straw without pretreatment (49.57%). Rotational speed of the extruder screw influenced biogas and methane production. With both dry matter and dry organic matter, the increase of rotational speed of the extruder screw improved the production of cumulated biogas and methane. Pretreatment of corn straw has a positive effect on the acquisition of cumulated methane (226.3 Nm3 Mg−1 for fresh matter, 243.99 Nm3 Mg−1 for dry matter, and 254.83 Nm3 Mg−1 for dry organic matter). Preliminary analysis of infrared spectra revealed changes in the samples also at the molecular level, thus opening up the possibility of identifying marker bands that account for specific degradation changes.
Highlights
The most common substrates used in biogas plants are corn silage, slurry, and feedstocks in the form of lignocellulosic biomass
Pretreated corn straw moistened to 25% of initial moisture content and processed at the highest rpm during the extrusion-cooking process proved to provide the most efficient conditions for methane and biogas production efficiency
Both in the case of dry matter and dry organic matter, the rotational speed of the extruder screw had an impact on the production of biogas and methane
Summary
The most common substrates used in biogas plants are corn silage, slurry, and feedstocks in the form of lignocellulosic biomass. Corn silage is one of the most widespread substrates of vegetable origin used for co-fermentation in agricultural biogas plants. Waste from livestock production exhibits proper composition for the purpose of methane production. This is, for example, liquid manure, manure or droppings from poultry production. Lignocellulosic biomass contains natural polymers such as cellulose, hemicellulose, and lignin [1,2,3]. Cellulose and hemicellulose as carbohydrates suitable as feedstock are fermentable after hydrolysis for bioenergy production. According to Eastman and Ferguson [7], the hydrolysis of sparingly soluble polymers, such as cellulose, lignins, and decomposable fat, protein, and carbohydrate, is a limiting step in the rate of fermentation
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