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Show heat-extraction rate by the embedded tubes. The maximum heat flux to the tubes was found to be about 83 X 103 Btu/h-tt2 at the firing rate of 88 X 103 Btu/h and excess air of 7%. The heat-extraction rate by the' grate decreased with the firing rate increase, as expected, because the combustion zone moved upward with increasing firing rate. The effect of the combustion regime on heat transfer was so remarkable that the overall heat-extraction rate was reduced by half with doubling the firing rate. The heat-extraction rate can be increased by adding more rows of the embedded tubes. The tests also demonstrated that the overall heat transfer to the tubes embedded in the porous matrix was 30% to 40% higher than to the same tubes without the porous matrix at similar operating conditions. The maximum heat-transfer coefficient achieved during these tests is estimated as high as 45 Btu/h-tt2-oF. CONCLUDING REMARKS Because only a limited number of tests were performed during the evaluation stage, the data collected should be viewed only as concept-proving and trend-showing rather than conclusive. The results and conclusions based on the preliminary tests on the bench-scale surface combustor-heater are summarized as follows: • • For the firing rates tested (from 57 X 103 to 110 X 103 Btu/h), combustion was stable at ve'g' high I~vels of combustion intensity with just one row of tubes (up to 0.44 X 10 Btu/h-W::). The location of the combustion zone was dependent on the firing rate and excess air. High combustion efficiency was demonstrated at low excess air levels (5% to 7%) and various firing rates while maintaining low pollutant emissions, which were as follows: NOx emissions were as low as 10 to 20 ppm (at 0% 02) depending on excess air CO emissions varied considerably during testing. On the average, though, the CO emissions (at 0% 02) were 100 to 150 ppm. At some conditions, CO emissions below 50 ppm were achieved. • Significant heat-transfer enhancement to the load in the porous matrix was demonstrated. The heat extraction was, on the average better than 50%. The maximum heat flux to the tube surfaces was 83 X 103 Btu/h-ft'2. However, heat transfer is strongly affected by the combustion regime. 20 INSTITUTE o F GAS TEe H N 0 LOG Y |