Optimal moisture for rapid compaction of coal logs for freight pipelines

The cost of producing coal logs and thus the economic competitiveness of coal log pipelines is directly related to the compaction time necessary to make the coal logs. Previous research has demonstrated the ability to make laboratory scale coal logs with a 5-s compaction time. It was also observed that for rapidly compacted coal logs, coal log circulation performance (resistance to abrasion in a commercial pipeline) is maximized, if the logs are compacted from a coal mixture at optimal moisture content. For the bituminous Mettiki coal that has been studied, this optimal moisture content is about 9%, if the compaction time is 5 s. The practical application of this is that if stockpiled coal is wetter than optimum, it would require drying of the coal to compact it at the optimum moisture content. Obviously, this would effect the cost of coal log fabrication. The authors hypothesize that an alternative to drying coal exists. If the coal logs are to be made at mix moisture contents that are above the optimum value, then it is hypothesized that coal log quality can be maintained if the compaction time is increased. In fact, we believe that if compaction time is increased such that the bulk density and moisture content of the compacted coal logs are the same, then the circulation performance of the coal logs will be the same. In this paper, the results of experiments designed to test this hypothesis are presented. The experimental data show that there is a range of mix moisture contents over which rapidly compacted quality coal logs can be formed. This range could be determined experimentally for any coal. For the Mettiki coal studied here, the range is 6% to 9%. For mix moisture contents above the optimum (high end of the range or 9% for the Mettiki coal), logs can be made with optimal dry bulk densities and log moisture contents if the compaction time is sufficiently slow. Logs with the same dry bulk density will have the same circulation performance. These results indicate that the hypothesis is valid.