OCR Text |
Show the calculated energy flows and temperatures at each zone. Column 1 of Figure 8b shows the axial distribution of chemical energy released into the zone. The nonzero values indicate the presence of the luminous flame within the zone. The sum of the zone contributions equals the firing rate, the product of mass feed rate of fuel, and its higher heating value. The ratio is in proportion to the flame volumes occupying the zones. Column 2 shows the net radiation loss from the zone to other zones. In Case I, gas-to-gas radiative transfer is not considered so all entries are zero. Column 3 shows the radiative loss from the gas stream to the waterwalls, Qra ; column 4 shows the radiative interchange from the gas stream to the refractory walls, Qref in Equation (1). Note that the transfer to the refractory is much greater than to the waterwall, roughly in proportion to the refractory/waterwall surface area ratio (10:1). Columns 5 and 6 are the analagous flows due to convective transfer. Because convective transfer goes as the gas-to-wall temperature difference, the heat flux (per unit area) to the cold waterwall is much higher than to the hot refractory wall. The heat flows are thus very similar even with the relatively small waterwall area. Note that the total convective transfer only approaches the radiant transfer at the low temperatures near the firebox exit. Column 7 shows the sensible energy carried out of the zone, mhj. The first row ("zone 0") shows the energy carried in with the preheated air, generally between 5 and 10 percent of the firing rate. As a check on the energy balance, the sum of the sensible energy leaving the final zone and the total convective and radiative losses from the gas stream should equal the sum of the combustion air sensible energy and the total chemical energy. Columns 8 and 9 contain the total heat loss through the refractory and waterwall, Qref and Q ^ . The values include the contributions of both convection and radiation. The radiative loss to the refractory, Qref, is ra d equal to Qref, the net radiation from the gas stream to the refractory, minus r a d • Qr£J , the sum of the reradiated and reflected components which are transmitted to the waterwall. The total waterwall heat loss, summed over the furnace length, is approximately three times the loss through the refractory. The final two columns show the calculated bulk gas temperatures and refractory wall temperatures. The difference in each zone has been fixed at 200°F for the baseline case. The results of varying the inputs are summarized in Table 2. Four key temperatures, from Zones 2, 4, 10, and 16 are listed. Most of the inputs were varied by 20 percent of their absolute values in the direction of increasing 5.5.15 |