The objective of this paper is to establish the minimum thickness of heat insulation for an experimental bioreactor, developed at the Institute of Microbiology of the Bulgarian Academy of Sciences, required to ensure heating of the fermentor with its own produced biogas. The experimental bioreactor is a cylinder of height h=935 mm and diameter d=435 mm. It has a water jacket around its shell to heat up the substrate. It is made in stainless steel of thickness 5 mm and of heat transfer coefficient =45 W/m.К. The fermentor is intended to be filled with substrate at 75-80 % of its volume. The bioreactor is designed with heat insulation in mineral wool of heat transfer coefficient =0.035 W/m.К. The biogas output of the bioreactor is 78 dm3 per 24 hours at energy value 6 kWh/m3. Heat loss of the fermentor is calculated using classical heat engineering equations. It is assumed that the substrate heated up in the bioreactor is a high inertia heat system which because of its large mass and not high temperature, will be exposed not to the impacts of the average day and night temperature, but to the monthly average. Results were obtained for the heat transfer coefficient and heat loss of the bioreactor (filled with cattle dung diluted in water) in mesophilic (35 oC) and thermophilic (55 C) mode at different insulation thicknesses. Heat loss of the bioreactor in thermophilic operation mode is 57% higher than in mesophilic mode. The main conclusion from the research is that small bioreactors cannot ensure heating at their own using the produced biogas in wintertime. A highly efficient heat insulation of thickness of about 10 cm is advisable.
R. Georgiev, K. Peychev, I. Simeonov, A. Alexandrov