By Hirofumi Okazaki, Kenji Kiyama and Hidehisa Yoshizako of Babcock-Hitachi, and Masayuki Taniguchi of Hitachi, Power and Industrial Systems
Further reduction of nitrogen oxide (NOX) and carbon monoxide (CO) emissions from pulverized coal firing boilers is required. Babcock Hitachi K.K. has developed highly efficient and low NOX combustion technologies using a large-scale test facility and an in-house development Computational Fluid Dynamics (CFD) model.
2. The Test Facility
Table 1 shows an outline of the latest test facility. This facility has a furnace designed on opposed firing system. 12 burners were arranged on three levels, two rows, facing each other to examine combustion phenomena. Five levels of over fire air (OFA) ports are provided to evaluate the effect of residence time on NO X reduction. The test facility has three main advantages as follows;
(1) High accuracy in the measurement of coal flow for each burner
Coal was pulverized at the pulverizer placed beside the test facility and supplied to the furnace. The coal flow for each burner was precisely measured so as to establish conditions estimated in commercial scale boilers.
(2) Evaluation at high combustion temperatures
In general, small-scale test facilities have the tendency of lower heat releases than commercial scale boilers. Therefore we could not increase combustion temperatures in the test facility. The new test facility was installed with a high grade insulator to increase combustion temperatures. The area of insulator was designed using CFD for the same temperature distribution as commercial scale boilers. Figure 1 shows the CFD results. As the result, the maximum temperature in the furnace increased to 1800 degrees Celsius (3270 degrees Fahrenheit). The combustion reaction and NO X formation rate were the same level as actual commercial scale boilers.
(3) Evaluation of flow patterns in the furnace
The latest test facility is sufficiently flexible in burner settings and OFA ports arrangements for establishing reaction times parallel to various commercial scale boilers. The distribution of gas species in the furnace and other characteristics of boiler performance were more accurately evaluated in this facility.
3. Results of NOX and CO emissions
In using this test facility with CFD, the latest combustion technology was developed. This technology was based on the concept of “in-flame NOX reduction technologies” of HT-NR3 burners that enhanced low NOX performance. Two-stage combustion and optimized arrangement of OFA ports achieved low CO and high combustion efficiency.
When in-flame NOX reduction technologies were applied, NOX emissions were greatly reduced through acceleration of NOX decomposition under high temperature fuel rich conditions. Figure 2 shows the mechanism of this phenomena. With these in-flame NOX reduction technologies, NOX can be reduced without increasing unburnt carbon (UBC).
Two-stage combustion allows combustion air supplied from burners and OFA ports placed downstream of the burners. Coal particles form the burners reacted under the fuel rich conditions between the burners and OFA ports and expired under the air rich conditions between OFA ports and furnace exits. If the length between the burners and OFA ports was longer, NO X emissions would be reduced but CO and UBC would be increased due to the length between OFA ports and furnace exits being shorter.
In the latest combustion technology, optimized arrangement of OFA ports made better mixing between the flue gas and the air from OFA ports reducing CO and UBC through shorter length of mixing. Therefore, using this technology, the length the length between the burners and OFA ports could be extended without change to the boiler height.
The concept of in-flame NOx reduction technologies of HT-NR3 burners and an optimized arrangement of OFA ports achieved low NOX and CO emissions and high combustion efficiency.