Experimental Study On Combustion And NO_x Emission Characteristics Of Char From Pyrolysis Of Low Rank Coal

Low-rank coals (mainly lignite and sub-bituminous coal) account for more than 55% of the coal reserves in China. In the past decades, low-rank coals were mainly directly combusted in power plants and energy utilization efficiency was relatively low in China. These circumstances must be improved to achieve clean and efficient utilization of low-rank coals. Based on the composition and structure of low-rank coals, the grade utilization of low-rank coals was proposed, which is considered to be a clean and efficient solution. The foundation of this solution is effective coal pyrolysis at a low temperature range, with gas, tar, and char produced. Pyrolysis gas and tar are used as high-grade raw materials, while the powdered char (0～10 mm) can be combusted in power plants as it is not suitable for raw materials.Due to better fuel feasibility, circulating fluidized bed (CFB) boilers can be used to achieve the effective utilization of char (0-10 mm). For the purpose of achieving clean and efficient combustion of char, it is necessary to study the combustion and emission characteristics. This paper examined the basic physical and chemical properties of char, and investigated the gemeral combustion and nitric oxides (NOx) emission characteristics of char with online thermogravimetry-mass spectrum (TG-MS) system. Pilot-scale CFB platforms were used to study the fluidization, transportation, combustion and original NOx emission of Shenmu char. Besides, possible measures of deeper controlling NOx emission from char combustion were explored on a new CFB test platform with a post combustion chamber.Char has more developed pore structure. X-ray photoelectron spectroscopy (XPS) was used to analyze the nitrogen morphology of Shenmu coal and Shenmu char. The nitrogen morphology in Shenmu coal and Shenmu char is in the following descending order:N-5, N-6, N-Q and N-X. Combustion of Shenmu char were comprehensively studied with TG-MS system by equivalent characteristic spectrum analysis (ECSA). NH3 and HCN are detected out as the major precursors of NOX while the volumetric of HCN is larger than that of NH3 in combustion of Shenmu char. NO and NO2 emissions are lower for Shenmu char than those of Shenmu coal.Fluidization and transportation characteristics of different types of Shenmu char performed well in pilot-scale CFB test platforms. Uniform temperature field can be formed during the combustion of char. With the increase of combustion temperature, the distribution of apparent particle concentration tends to be uniform in both dense and dilute phase region. The primary/secondary air ratio does not have obvious effece on the distribution of combustion temperature. With the increase of primary air rate, more uniform temperature distribution can be formed. The distribution of combustion temperature is more uniform in condtions of preheated char and coupling pyrolysis and combustion of coal.Unlike Shenmu coal, the disadvantages of Shenmu char combustion include late ignition, poor combustion stability, and poor burnout. The combustion of Shenmu char has a higher dependence on combustion temperature, and it improves when the combustion temperature exceeds 900 ℃. Within traditional CFB combustion temperature range, NOx emission from the combustion of Shenmu char decreases with an increase in combustion temperature, which is in contrast with the combustion of Shenmu coal. With an increase in the excess air ratio and a decrease in the secondary air ratio and the height of the secondary-air port, NOx emission from the combustion of both Shenmu char and Shenmu coal increase. NOX emission decreases using SNCR in the char-fired circulating fluidized bed and ammonia as de-NOx agent is better than urea solution on de-NOx effect.Possible measures of deeper controlling NOX emission from combustion were explored on a new CFB test platform. Combustion characteristics of Shenmu char and the emission characteristics of NOX were studied on the conditions of the secondary air (SA) arrangement, lean oxygen combustion and post combustion, respectively. Experimental results showed that:the approach of modifying the SA arrangement was able to remove the NOX emissions by up to 30%, a certain distance away from the standard 100 mg/m3; the lean oxygen combustion can reduce NOx emissions significantly but resulting in a declined combustion efficiency; post combustion is able to bring the final concentration of NOX emission down to 100 mg/m3 with the lowest value of 51 mg/m3 with meanwhile maintaining the high combustion efficiency of 98.6%. In reducing atmosphere, most fuel-nitrogen is converted to N2. NOX is mainly formed from the reaction of residual N in the char contained in fly ash and O2 from post combustion air in post combustion chamber. Nitrogen in char is gradually released and converted during combustion, not instantaneously.