A Proposal and Trial of Heat Extraction from a Compost Bed by Water Flowing through the Pipe Buried in the Bed

摘要

As a method of extraction of heat generated in composting process, an operation of heat extraction from a compost bed by water flowing through the pipe buried in the bed had been investigated. The analytical solutions of the water temperature in the pipe and the temperature in the bed were derived from heat conduction and heat balance equations with suitable boundary conditions and several assumptions. These calculated results from the proposed solutions were compared with the results of the experiments which had been made intermittently under the unsteady-state conditions.1) The calculated results of the water temperature at the outlet of the pipe and the temperature profile in the compost bed were in good agreement with the experimental results, so that the proposed mathematical treatment for the heat transfer mechanism would be approximately available for this unsteady-state heat extraction process.2) The water temperature at the outlet of the pipe did not varied significantly with time during the unsteady-state heat extraction process, except for the extremely early stage immediately after supplying water into the pipe. Therefore, it seems that this operation for heat extraction which had been carried out under the unsteady-state condition would be appropriate to obtain approximately constant water temperature at the outlet of the pipe.3) In these experiments, a water temperature at the outlet of the pipe depended upon only two terms because the water temperature at the inlet of the pipe was controlled at a constant value. One of the terms is a driving force of the initial temperature difference between the compost bed and the water in the pipe, and the other is the apparent rate of heat generation in the compost bed. The effect of the former term on the water temperature at the outlet of the pipe was considered to be much greater than that of the latter term.4) To obtain a similar value of the water temperature at the outlet of the pipe to the value given in the unsteady-state operation under the same condition of the flow rate of water in the steady-state operation of heat extraction, the volume of the container for the compost was estimated to be about 10 times larger than that required for the unsteady-state operation, because the apparent rate of heat generation is usually comparatively small. Thus, the steady-state operation seems to be inconvenient in practical use in comparison with the unsteady-state operation.5) Mathematical manipulation in the analytical solutions is so complicated that it requires the simplification of the physical description of the problem by considering the above-mentioned items 1)-4) for the practical application of them.6) It would be also advisable to investigate the maximum amount of heat possibly can to be extracted from the compost bed throughout a series of the intermittently unsteady-state operations, by taking account of both the time interval employed for heat extraction by water and the time required for the recovery of temperature in the compost bed.