Combustion characteristics of semicokes derived from pyrolysis of low rank bituminous coal

Various semicokes were obtained from medium-low temperature pyrolysis of Dongrong long flame coal.The proximate analysis,calorific value and Hardgrove grindability index(HGI) of semicokes were determined,and the ignition temperature,burnout temperature,ignition index,burnout index,burnout ratio,combustion characteristic index of semicokes were measured and analyzed using thermogravimetry analysis(TGA).The effects of pyrolysis temperature,heating rate,and pyrolysis time on yield,composition and calorific value of long flame coal derived semicokes were investigated,especially the influence of pyrolysis temperature on combustion characteristics and grindability of the semicokes was studied combined with X-ray diffraction(XRD) analysis of semicokes.The results show that the volatile content,ash content and calorific value of semicokes pyrolyzed at all process parameters studied meet the technical specifications of the pulverized coal-fired furnaces(PCFF) referring to China Standards GB/T 7562-1998.The pyrolysis temperature is the most influential factor among pyrolysis process parameters.As pyrolysis temperature increases,the yield,ignition index,combustion reactivity and burnout index of semicokes show a decreasing tend,but the ash content increases.In the range of 400 and 450 °C,the grindability of semicokes is rational,especially the grindability of semicokes pyrolyzed at 450 °C is suitable.Except for the decrease of volatile content and increase of ash content,the decrease of combustion performance of semicokes pyrolyzed at higher temperature should be attributed to the improvement of the degree of structural ordering and the increase of aromaticity and average crystallite size of char.It is concluded that the semicokes pyrolyzed at the temperature of 450 °C is the proper fuel for PCFF.

Prediction of remnant volatile matter in the semicokes from coal partial gasification

The semicokes from different coals were prepared under various temperatures by partial gasification method in a fixed bed. According to the analysis of the Vaaf in the coals and semicokes, a new method is presented to predict the content of remnant volatile matter in the semicokes from coal partial gasification. The two fuel character indexes FV and FC were introduced to symbolize the second volatilized temperature and the fully volatilized temperature respectively. Then according to the proximate and ultimate analysis data of raw coal, the content of Vdaf in the semicokes prepared under various temperatures can be predicted.

Modeling study of combustion process of oil shale semicoke in a circulating fluidized bed boiler

In this paper,the combustion process of oil shale semicoke was investigated by modeling,where intraparticle mass transfer resistance was especially considered.The ash formation and attrition characteristics of oil shale semicoke were also investigated by a laboratory test procedure.The burnout time and residence time of different particle sizes were predicted,and the maximum particle size of which oil shale semicoke could be burned out in a 3 MW circulating fluidized bed boiler was obtained.

Influence of nozzle height to width ratio on ignition and NOx emission characteristics of semicoke/bituminous coal blends in a 300 kW pulverized coal-fired furnace

To improve the ignition behavior and to reduce the high NOx emissions of blended pulverized fuels(PF)of semicoke(SC),large-scale experiments were conducted in a 300 kW fired furnace at various nozzle settings,i.e.,ratios(denoted by hf/b)of the height of the rectangular burner nozzle to its width of 1.65,2.32,and 3.22.The combustion tests indicate that the flame stability,ignition performance,and fuel burnout ratio were significantly improved at a nozzle setting of hf/b=2.32.The smaller hf/b delayed ignition and caused the flame to concentrate excessively on the axis of the furnace,while the larger hf/b easily caused the deflection of the pulverized coal flame,and a high-temperature flame zone emerged close to the furnace wall.NOx emissions at the outlet of the primary zone decreased from 447 to 354 mg/m3(O2=6%),and the ignition distance decreased from 420 to 246 mm when the hf/b varied from 1.65 to 3.22.Furthermore,the ratio(denoted by SR/SC)of the strong reduction zone area to the combustion reaction zone area was defined experimentally by the CO concentration to evaluate the reduction zone.The SR/SC rose monotonously,but its restraining effects on NOx formation decreased as hf/b increased.The results suggested that in a test furnace,regulating the nozzle hf/b conditions sharply reduces NOx emissions and improves the combustion efficiency of SC blends possessing an appropriate jet rigidity.

Effective diffusivity of oxygen in the ash layer of Huadian oil shale semicoke

Diffusion of oxygen in the ash layer usually dominated the combustion of oil shale semicoke particles due to the high ash content.Thus,effective diffusivity of oxygen in the ash layer was a crucial parameter worthy of careful investigation.In this paper,the effective diffusivity of oxygen in the ash layer of Huadian oil shale semicoke was measured directly using an improved Wicke-Kallenbach diffusion apparatus.The experimental results showed that higher temperature would lead to a higher effective diffusivity and a thicker ash layer had the negative effect.Especially,the effective diffusivity along the direction perpendicular to bedding planes was much lower than that along the direction parallel to bedding planes.In addition,an effective diffusivity model was developed,which could be used to describe the mass transfer of oxygen in the ash layer of oil shale semicoke.