| Pollution caused by coal combustion is one of the key issues in global air pollution. Nitrogen oxide(NOx) is one of the main pollutants from coal combustion, 67% of NOx came from coal combustion. The impact of air pollutants emissions from thermal power on ecological environment has become more serious. However, China has various types of coal-fired boilers and diverse sources of coals. Also, changes in parameters during actual operations will greatly affect NOx emissions. Hence, effectively reducing NOx emissions from boilers, especially from those fired with low rank coal is significant for the reduction of NOx emissions from Chinese coal-fired power plants. The Entrained Flow Reactor with Multiple Reaction Segment system(EFR-MRS) has been designed and built by author of the paper, which can simulate the actual combustion conditions in the coal-fired utility boilers. A detailed study on NOx reduction from staging combustion of Chinese low rank coal has been conducted in this paper. In addition, experimental research on air-staged combustion of two high fuel-N bituminous coals has been carried out in this facility. An experimental investigation to reduce the NOx emissions during burning lignite has been carried out in a 600 MW tangential pulverized coal firing rectangle furnace, which verified the value of the research results in this paper.The numerical simulation of EFR-MRS system and the optimization of burners structure were analyzed and researched by the CFD software, error analysis of this system was also analyzed. The facility partially simulates the vertical one-dimensional combustion conditions in the coal-fired utility boilers and allows studying NOx emissions behavior by staging combustion. Error analysis shows that the total experimental system error can be controlled within ± 4.5%.The impact of reduction zone stoichiometric ratio(λ1), residence time(τ1), and coal quality on NOx emissions in the case of air-staged combustion has been investigated. An comprehensive NOx reduction index(Sz) was proposed to denote the relation between the maximam NOx reduction rate and coal quality. The study employs EFR-MRS and focuses specially on low rank coal(four lignites and one sub-bituminous coal) and two high volatile bituminous coal in China. The results show that in air-staged combustion, the level of NOx emitted drops as λ1 decreases, reaching a high reduction rate. The longer the τ1 is, the higher the reduction rate will be. Coals with higher volatile content have a lower conversion rate of fuel nitrogen to NOx, benefiting the reduction of NOx emissions. The higher the Sz is, the higher the NOx reduction rate is. In particular, satisfactory NOx reduction for the low rank coal is achieved at the residence time between 1.0s and 1.5s, depending on coal. For high volatile bituminous coal, the gain in performance for residence time greater than 1.0s is generally minimal. Moreover, air-staged combustion does not significantly reduce the burnout rate of the high volatile coal.In this study, experimental research on air-staged combustion were carried out in order to test the NOx emissions with two high fuel-N bituminous coals. The results show the NOx emissions increased with increasing fuel-N under un-staged combustion, and the fuel NOx conversion decreased with increasing fuel-N under air-staged combustion. While minimum NOx emissions could be achieved when the residence time and air stoichiometric ratio of reduction combustion zone are 0.8s~1.0s and 0.75, respectively. In addition, Russia bituminous coal showed similar better burnout rate levels to Datong bituminous coal.This paper specially focuses on NOx reduction from high volatile pulverized coals by fuel-staged combustion, especially from low rank coal in China with strong slagging tendency, high ash, high moisture, and low calorific value. The results are very important to further enrich the database of coal reburning. The influences of the main process parameters, including reburn fuel fraction(Rff), stoichiometric ratio(λ1 and λ2), residence time in reduction zone(τ2), and fuel properties, on NOx emissions were studied experimentally using an EFR-MRS. The present experiments verified the comprehensive NOx reduction index(Sz) proposed by the authors for air-staged combustion is still applicable to correlate the maximum NOx reduction rate and coal characteristics in fuel-staged combustion. The experiments also verified that the NOx reduction rate increases with the increase of Rff, with the increase of τ2, and with the decrease of λ2. The results also showed that these parameters have a critical range value, which is 15%~20% for Rff, less than 0.8 for λ2, and less than 0.8s for τ2, respectively. NOx emissions drop with the decrease of the λ1 to 0.8. For high volatile coals, the effect of fuel-staged combustion on the reduction of NOx emissions increases as the volatile content or Fuel-N in the coal increases. The NOx reduction rate increases while the comprehensive NOx reduction index goes up. Moreover, fuel-staged combustion did not significantly reduce the burnout rate of the high volatile coal.An experimental investigation to reduce the NOx emission during burning lignite has been carried out in a 600 MW tangential pulverized coal firing rectangle furnace, with a technique combining horizontal bias combustion(HBC) burners in the primary zone and various pulverizer off-stream combination and varied separated over fire air(SOFA) combination. The experiments show that NOx emission is decreased by improving the burnout air rate or reducing the export of oxygen, while appropriate readjustment of the secondary air and separated over fire air injected have a positive effect on NOx emissions, the unit of NOx emission can even reduced to 209 mg/m3. Through these methods, the slagging of the unit has also been a great relief. |
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