Design and performance studies of pilot scale inclined tubular digesters operating on cattle slurry

Abstract

Two pilot scale (1 and 2m3) inclined tubular digesters were designed and constructed adjacent to a large dairy unit. The digesters were instrumented to monitor automatically gas production, temperatures and the electrical energy used for heating. Operation of the digesters was reliable. Aspect ratio, angle of inclination, retention time and type of slurry could be varied independently, and the effect of these parameters on digester performance was investigated in five sets of operating conditions during a 20 month period. These parameters had the following values : Aspect ratio, 10 and 20 ; Retention time, 15 and 20 days ; Temperature 25 and 35 degrees Centigrade ; Feedstock, whole and mechanically separated slurry.Gas yields ranged from 0.282 - 0.318 m3/kg VS added with whole slurry at 35 C. The maximum gas yield was with a digester inclination of 20 to the horizontal, aspect ratio of 10, and retention time 20 days.For a digester inclination of 20°, gas yields ( m3/ kg vs added) from whole slurry were higher at an aspect ratio of 10 than 20 for both 25 and 35°c operating temperatures. At 10 inclination, gas yields were marginally lower at an aspect ratio of 10 for both whole and separated slurry.Stability of the digestion process was greatest at 20° inclination, this being attributed to the more uniform temperature distribution arising from mixing caused by the evolved biogas, and convection currents.Tracer studies which were designed to characterize the flow of feed components through the digester, showed actual retention times to be less than the nominal values at the times they were carried out. With separated slurry, the flow of soluble components was very similar to a completely mixed system.Energy production from the digesters was investigated, digester heating requirement exceeding gross energy production, primarily because of the large heat losses which occurred due to the experimental nature of the plant. It was predicted that this would be improved in farm scale versions.A technical and economic appraisal established that the design could be scaled up to farm scale ( >100m 3 ), but major problems would arise from the need to support a reactor of this size at the optimum inclination of 20°.This was shown in the capital construction cost estimates. Comparisons with the more common continuously stirred tank reactor were made.

Date of Award	Nov 1986
Original language	English
Awarding Institution	Polytechnic of Wales
Supervisor	Dennis Hawkes (Supervisor)