Experimental Investigate on Pre-combustion Characteristic and Flame Shape of an Adjustable Axial Swirl Burner for Pulverized Coal Combustion


 The 14 MW pilot–scale pulverized coal experiment system was built, pre-combustion characteristic and flame shape of adjustable axial swirl burner was investigation, and influence of swirl number also was discussed. The temperature and species concentration distribution in pre-combustion chamber showed that a high temperature oxygen-free and high CO concentration zone appeared around central pulverized coal, which enhanced burnout and inhibiting or reduction NOx. The pre-combustion characteristic belonged to high temperature preheating combustion technology. The captured flame image showed that flame shape beyond pre-combustion chamber outlet belonged to turbulent diffusion flame. Then flame shape scales were obtained. For different swirl numbers, ignition, wall temperature, mean maximum temperature, oxygen-free boundary and CO concentration was discussed, the results indicated that S=1.67 had advantaged for burnout and reduction NOx emission on the basis of stable ignition and avoiding high temperature corrosion. Mean flame length and diameter and divergence angle also were analyzed, the results showed that S=1.67 case had moderate flame length and best diameter and divergence. It was favorable for flame stability and high temperature region in furnace part, and increasing burnout and deceasing NOx emission. Therefor the optimal selection was S=1.67 case under experimental condition.


Introduction
Coal still is the main primary energy (Chang et al. 2016;Guan 2017) in some area of world over a period of time.
For pulverized coal (PC) combustion, NOx emission in pollution control is considered continuously. A great quantity of low nitrogen combustion technologies have been researched and applied, such as air staging and fuel staging (Yang et al. 2014 and2015), fuel rich/lean combustion (Li et al. 2014), flue gas recirculation (Kim et al. 2007), preheating-combustion (Ouyang et al. 2013 (Ouyang et al. 2014;. And adjustable swirl burner also is considered and researched as a method to satisfy variable conditions Zhang et al. 2018). Thus, the investigation of preheating-combustion processing for adjustable swirl burner is necessary.
For PC preheating-combustion technology, the preheating mothed included individual preheating device (circulating fluidized bed or preheating chamber), increasing air temperature (air preheater) and precombustion equipment (pre-combustion chamber burner). Ouyang et al. (2013; experimentally discussed the effect of limestone on pollution emissions of lignite, bituminous, anthracite, semi-coke by circulating fluidized bed preheating. And preheating and combustion characteristic of individual anthracite also was studied. Bituminous and ultra-low volatile coal based fuel was researched on NOx emission and preheating combustion characteristic by . The influence of air distribution and burner type on NOx emission of two kinds bituminous and low volatile was investigated in down preheating chamber and down tube furnace system by . And Li et al. (2012) researched the effect of air temperature on combustion characteristic and NOx emission in experimental furnace. The above results indicated that preheating-combustion technology was suitable for different PC types. For pre-combustion equipment, Jiang et al. (2014) numerically discussed the combustion characteristic of middle volatile coal in industrial boiler, which double cone pre-combustion chamber swirl burner was adopted. And the precombustion characteristic of coal water slurry in chamber also was studied by Mo et al. (2018). But, the experimental data in different shape chamber was deletion. To fully understand the function of pre-combustion chamber, the experimental investigation on pre-combustion chamber needed to be developed.  Ti et al. (2016). The above results showed that optimized burner outlet structure was selected with different conditions, evidenced that adjustable swirl burner was a great method. Chen et al. (2011) researched and optimized the influence of a conical rings concentrator on the gas/particle flow characteristics in central fuel rich swirl burner outlet region, and reasonable levels, distances between adjacent rings and cover ratios were obtained in specific condition. Li et al. (2014) studied the influence of fuel rich/lean on combustion characteristic and NOx emission by speeding rate of inner and outer primary air duct. The research showed that reasonable fuel rich/lean needed to be adjusted. The investigation of literature also included the effect of swirl number, which was achieved by adjustable axial swirler . The influence of tangential swirl vane angles were discussed on flow and combustion characteristic and NOx emission for central fuel rich swirl burner (Ti et al. 2017;Li et al. 2017).
Effectiveness between swirl intensity and air staging was studied on burnout characteristic and NOx emission by Sung et al. (2015). The above literatures about swirl intensity indicated that reasonable swirl intensity or swirl number needed to be selected with different conditions. Although the adjustable structure was not used in some investigations of three parts, the all results showed that adjustable swirl burner had advantage in boiler variable conditions.
In this paper, an adjustable axial swirl burner was selected to research pre-combustion characteristic in precombustion chamber and flame shape beyond precombustion chamber outlet. And the influence of swirl intensity also was discussed, which was achieved by position of axial swirler. The investigation of different PC types and operating loads will be considered in future.

Experiment system
14MW pilot-scale pulverized coal experiment system 1, 2, 4-blower; 3-Roots fan; 5-storage facilities; 6-feeding device; 7mass flowmeter; 8-valve; 9-ignition device；10-thermocouple; 11-air compressor; 12-Signal exchanger;13-desktop computer; 14-sampling device; 15-filter unit; 16-flue gas analyzer 17-high speed camera;18notebook computer;19-protective wall; 20-adjustable axial swirl burner. 1-primary air duct; 2-isolate duct; 3-inner secondary air duct; 4-outer secondary air duct；5-tertiary air duct; 6-axial movable swirl vanes; 7combustion stabilizing device; 8-expanding cone shape pre-chamber; 9location of measuring points.  Fig. 1 showed the schematic diagram of experiment system. The PC in storage facilities was fed to mix with primary air by feeding devices, the speed was controlled by frequency of feeding devices. During each case, three times of 10 min feeding speed calibration were carried out, and error was maintained at ±8%. A Roots fan and three blowers respectively provided primary air, inner and outer secondary air, tertiary air. The flow rate of mass flowmeter was adjusted by valve. The diesel oil was used to help PC ignition and stable combustion, then was stopped. The control system was responsible for start and stop of the whole experiment system. The 1.5m-length stainless steel K-type thermocouple was adopted in temperature measurement. It could achieve temperature online display by signal converter. The measurement range of the thermocouple was 0-1300 o C with an error of 0.5%. When fluctuation range of temperature was ±10 o C, it was recorded as the measurement value.
Flue gas was sampled and quickly cooled by a watercooled sucking probe, and samples through unit and were filtered then analyzed online by a MRU VARIO PLUS flue gas analyzer to obtain the species concentrations. The water-cooled sucking probe were consisted by a centrallylocated sampling pipe and double-deck stainless steel tube with cooling water. Flue gas species concentration had an accuracy consisting of ±2% for measurement values. In this processing, 60 group data were measured at each The scale was arranged at the same section with the burner axis, then 8337 flame pictures was obtained to analysis flame shape.

Swirl burner and experimental conditions
The schematic diagram of the burner was shown in Fig.   2, which had flame stabilizing device, isolated area of PA and ISA, ISA adjustable axial swirler, expanding cone shape pre-combustion chamber et al.. Fig. 3 showed the axial swirl vane on adjustable swirler. Axial distance of burner outlet section and the radial distance of central axis was defined as X and R respectively. Four sections X=268, 536, 670, and 804 mm were selected. Measurement points of each section were respectively 10, 60, 110, 160, 210, 260 mm from the wall surface of pre-chamber and the last  In this combustion experiment, Shenfu long flame coal in China was used, and its basic characteristics were shown in Table1 where di was the inner diameter of swirl duct, do was the outer diameter of swirl duct, and θ was the special swirl vane angle. But the original swirl number formula needed

Results and discussion
Pre-combustion characteristic and flame shape of adjustable axial swirl burner were researched by above experiment section. Influence of swirl number also was considered to select reasonable case under the condition  At X=536-804 mm area, a significant low temperature zone could be seen around central axis, indicated that PC underwent pyrolysis processing.    Finally, the original image was converted to monochrome image with selected limit. According to pixel value (0 or 1) distribution of monochrome image, flame pixel length ( ) and diameter ( ) were obtained and real divergence angle (α) was calculated. The flame real length ( ) and diameter ( ) were translated using following formulas:

Flame shape
where and were reference objection real length and height, and were reference objection pixel length and height. To decrease errors of flame shape, 1000 images in 8337 images of each case were processed to obtain mean value. The specific data will be shown in Section 3.3.
Flame shape pixel scale was depended on limit ratio in image processing. For length example, Fig. 10 showed influence and selection of limit ratio on flame pixel length.
The length decreased alone limit ratio increasing with different S. Relative horizontal region was selected as arrange used of limit ratio, such as 20-60%. But S=1.14 case appeared two regions due to unstable flame wake, Fig. 10 influence and selection of limit ratio later region was used to select limit ratio. Finally 50%limit ratio was adopted in all image processing.

Influence of swirl number
Influence of swirl number was discussed to select optimizing S by temperature and species concentration and flame shape scale. Ignition, central and wall region and specific flame shape scale were analyzed.
The maximum temperature of X=256mm cross-section was considered as ignition temperature to discuss influence of S. Heat recirculation of isolate duct area and stable flame device ignited PC, the heat recirculation processing was reflected by ignition temperature. Ignition temperature profiles in Fig. 11 indicated that heat recirculation processing for S=1.14 case was stronger than others. But temperature more than 1000 O C was able to For central region, the mean maximum temperature and CO concentration and NOZ boundary was shown in Fig.   11 and Fig. 12. The profiles indicated that mean maximum CO concentration and NOZ boundary for S=1.67 case were higher than others, and mean maximum temperature also was moderate. There were beneficial for increasing burnout and inhibiting or reducing NOx comparing with others in pre-combustion processing.
The temperature of measuring point from wall 10mm was defined as wall temperature to reflect high temperature corrosion of pre-combustion chamber. Fig. 13

Conclusions
Pre-combustion characteristic and flame shape and influence of S were experimental investigation for an adjustable axial swirl burner, the conclusions were as follow: 1. For the adjustable axial swirl burner, a high temperature, oxygen-free and high CO concentration zone was formed in pre-combustion chamber. It was enhanced to increase pulverized coal burnout and inhibit or reduction NOx in pre-combustion processing.
2. For the adjustable axial swirl burner, typical turbulence diffusion flame appeared beyond precombustion chamber outlet. Pre-combustion chamber had the ability of stable flame.
3. For the adjustable axial swirl burner, influence of S was discussed and analyzed on ignition, temperature of central and wall region, atmosphere and flame shape scale.

Declarations
Data Availability: No data were used to support this study. The burn of the experiment is available from the authors.

Conflicts of Interest:
The authors declare that there are no conflicts of interest regarding the publication of this paper.