Technology for Producing Biogas from Light Feedstocks

The raw materials are unloaded from trucks into slurry tanks or onto a conveyor belt that serves as a storage area for solid raw materials. A screw conveyor feeds the raw materials to the preparation area.

The raw material enters the biomass feed hopper, which grinds, mixes, and keeps the mixture loose. A magnetic separator is used to remove metal objects from the mixture. The cleaned mixture is fed into the hydrodynamic preparation mixer. Here, the raw material mixture is blended with “dagestat”—the returned fermented substrate from the digester. The mixer homogenizes the mixture and removes any impurities while maintaining a constant temperature. The result is a homogeneous substrate for fermentation at the desired temperature.

The process takes place in a large industrial reactor—a digester. Fermentation is accelerated by efficient mixing using a high-speed vertical mixer, which ensures 98% homogeneity of the substrate. To activate the process, additional enzymes may be added: Fe, Co, Mo, Ni, Mn, Cu, Zn. Maintaining the temperature regime at ??°C and effective foam suppression are important factors. The result is the production of raw biogas, which is fed through a gas pipeline into the anaerobic digestion reactor.

In the digester reactor, biogas rises into the dome membrane, which regulates the pressure, and is discharged through a pipeline via a gas blower for purification. The digestion of residual organic matter from the digestate into biogas occurs due to the presence of side mixers on the reactor walls. The digestate from the digester is fed to the decantation and purification stages.

The digestate is separated by a separator into a liquid fraction—the supernatant—and a solid fraction of dewatered sludge—the cake. The SR 25% cake is collected in a hopper and shipped to the warehouse as a solid organic biofertilizer. The decant, purified by flotation and settling, becomes a liquid organic biofertilizer. The purification sludge—the photo-sludge—is the most active biomass and is returned to the anaerobic reactor.

Unpurified biogas contains up to 2,000 ppm of hydrogen sulfide. As a corrosive gas, it causes corrosion of boiler and cogeneration equipment. Preliminary purification of biogas is possible in a digester reactor using a sulfur-reducing support mesh colonized by sulfur-oxidizing bacteria of the genus Thiobacillus. This solution removes 50% of the hydrogen sulfide from the biogas. Subsequent purification is carried out using a chemical wet scrubber-desulfurizer through adsorption with a 4% sodium hydroxide solution. If further purification is necessary, carbon filters are used. The purified biogas has 60% moisture content and a residual hydrogen sulfide level of 50–100 ppm.

The purified biogas is fed by fans to cogeneration units, where it is converted into electrical and thermal energy. Part of the thermal energy is used to maintain a temperature of 37°C in the anaerobic reactors and for other plant needs. The electricity generated is the final product and is fed into the local power grid via a transformer substation. Excess biogas is utilized in an emergency flare.