The Sofia council decided to invest in renewable energy to make use of the many tons of biowaste (vegetative material), 20,000 tons per annum, from the waste removal collections that take place daily in Sofia and surrounding area.
To make this possible it was decided to go for wet fermentation where the substrate or raw material ferments and the biogas is collected. After three weeks the bacteria which promote/digest during fermentation to produce Biogas that can be collected. The solid waste however was to be composted.
This type of fermentation where the biomass has to be separated from household waste and wrapping materials before going into the fermentation process is a labour intensive procedure. All small plastics or wrappers that remain in the bio waste could cause damage to pumps or other transport pipes used in the fermentation plant.
A model of the well-functioning plant used in Hamburg Germany.
Dreyer & Bosse supplied two CHP, a 255Kw and 600Kw as well as a Wolf heating unit that runs on oil to enable startup of the fermenter and to compensate for added heat when the biogas level is down. (see photo) This heating boiler was installed to guarantee heating supplies for the fermenter in winter and for local housing under contract with the Sofia council for their heating supply.
With this type of fermentation process Dreyer & Bosse paid special attention to making sure the harmful substances were removed from the gas therefore improving the quality of the bio gas. This has been acheived by using the tried and tested Dreyer & Bosse gas coolers as well as providing advice on what is needed to monitor the gas quality .
The Dreyer & Bosse CHP will eventually supply electricity and heat for private homes and industry in Sofia and therefore reduce the dependency on fossil fuels that cause global warming.