Science and Engineering

Excessive Pile Size

Tom Richard

Composting comes in many shapes and sizes, from 1 liter vacuum bottles to warehouse sized industrial systems. In all of these systems, the correct pile size balances the heat generated by microbial decomposition with the heat lost through conduction, convection, and radiation, keeping most of the compost between 40°C and 60°C (for more on the mechanisms of heat loss, see the physics page in our background information section).

Passively aerated systems, which depend on diffusion and natural convection for oxygen transport, usually have a large open surface area to encourage air movement, with corresponding convective heat losses. This large surface area also results in conductive and radiant heat loss. Because heat loss in these systems is largely a function of exposed surface area (as well as ambient temperatures), and microbial heat generation largely a function of volume (assuming the environmental conditions are near optimum), for any material and configuration there will be an ideal surface to volume ratio. Larger piles, with a smaller surface to volume ratio, will tend to overheat, while small piles will be too cool. For materials in a typical windrow configuration (where the width of the windrow is about double the height), the ideal height will usually be in the range of 1 to 3 meters. Rapidly degrading, dense mixtures that include grass clippings, food scraps or manure will be at the lower end of this range, while porous, slowly degrading piles of leaves will be at the upper end.

With a forced aeration system, convective heat loss can be increased by increasing the aeration rate. Although this will reduce the average temperature of the pile, one also has to be careful to insure that the temperature extremes are not too great. With very large piles, regions near the air inlet will be excessively cooled and dried, while other regions near the exhaust may be to hot. As with passively aerated systems, the ideal size of a forced aeration pile depends on the characteristics of the material being composted and the geometry of the composting system. For most materials, in systems using ambient air (air once through), the maximum height (or airflow path length) is 2 to 3 meters. Tunnel reactor systems, which can recycle the airflow, typically operate at higher airflow rates with a smaller temperature difference between the inlet and the outlet. Recycling the airflow, or using pre-heated air, thus allows an increase in reactor size while still maintaining a tolerable degree of process control.


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