Analysis of Ignition Characteristics and Influencing Factors of Combustible Fly Ash in Boiler Start-Up Stage Flue Gas

Abstract

In order to realize the full-time denitration of a boiler, high-temperature flue gas needs to be introduced when SCR is conducted during the boiler start-up stage, and there is a fire risk due to the presence of combustible fly ash at this time. Therefore, research on reburning and the explosion risk of tail flue gas encountering high-temperature flue gas during start-up and shutdown was carried out. A small testbed was designed to record the temperature of the flue gas and the composition of the flue gas before and after the test, and the ignition characteristics of combustible fly ash in the flue gas were systematically studied. The ignition temperature of combustible fly ash in various conditions was obtained, the ignition characteristics of combustible fly ash in the airflow were analyzed, and the effects of combustible gas, high-temperature flue gas temperature, and fly ash composition on ignition were also analyzed. The results show that the flue gas temperature in the test section was about 400 °C, the low-temperature flue gas temperature increased from 650 °C to 813 °C, and the combustible fly ash did not ignite regardless of whether alcohol was added as a combustible gas component. When the volatile content of combustible fly ash was 10~26.7%, the ignition temperature was 660~760 °C. The lower the volatile content of combustible fly ash was, the higher the ignition point was. When alcohol was added as a combustible component of gas, the ignition point decreased by about 50 °C. The critical ignition temperature of combustible fly ash in this test was lower than that under actual power plant operation conditions.

1. Introduction

In order to realize the full-time denitrification of a boiler, external fuel combustion is needed during start-up, shutdown, and low-load operation to produce high-temperature flue gas mixing, which improves the inlet temperature of SCR (Selective Catalytic Reduction) and makes the denitrification system run normally. The reason for using a denitrification system is that boilers generate a certain amount of nitrogen oxides (NOx) during operation, which poses significant harm to the environment and human health. Boiler denitrification is the process of converting the nitrogen oxides in flue gas into nitrogen gas and water vapor, which can effectively reduce the harmful emissions from flue gas released into the environment. In the process of burning fossil fuels in coal-fired power plant boilers, alkanes will destroy carbon–hydrogen and carbon–carbon bonds and form combustible components, such as short-chain alkanes, olefins, and hydrogen. Aromatic hydrocarbons may undergo dehydrogenation and condensation reactions, resulting in coke and high-temperature flue gas. At the same time, the flue at the tail of the boiler will contain combustible flue gas and fly ash left by the insufficient combustion of raw coal, which are mainly composed of CO, hydrocarbons, and carbon particles [1,2]. When hydrocarbons and unburned carbon particles in boiler tailpipe encounter high-temperature flue gas [3], it may cause violent combustion and even an explosion. Therefore, it is necessary to study the risk of flue gas re-ignition and explosions when coal-fired boilers encounter high-temperature flue gas during start-up and shutdown.

Many scholars have studied the minimum ignition temperature and ignition characteristics of dust. The minimum ignition temperature of a dust cloud refers to the minimum temperature at which the temperature of the dust cloud suddenly changes, that is, the minimum temperature at which the dust cloud ignites when it is heated in a mixture with air [4]. It is an important parameter for evaluating the sensitivity of industrial dust explosions, and it is also an important basis for implementing fire- and explosion-proof safety designs and explosion-proof electrical designs and selecting the correct type [5]. The research results of Jiang Haipeng [6] showed that the minimum ignition temperature of coal dust clouds with volatile contents ranging from 7.09% to 37.45% was between 580 and 730 °C. Song Yinlian [7] studied the ignition temperature of four coals with different volatile contents and found that the ignition characteristics from best to worst were lignite, blended coal, raw coal, and washed coal, with ignition temperatures of 367.6 °C, 368.2 °C, 372.3 °C, and 435.8 °C, respectively. Zhao Dan [8] tested the minimum ignition temperature of coal dust clouds for 10 coal samples with particle sizes less than 75 μm (volatile contents ranging from 6.10% to 42.70%) and found that the minimum ignition temperature ranged from 412 to 851 °C. Wang [9] tested the minimum ignition temperature of three different coal samples with volatile contents of 11.19%, 32.45%, and 42.26%, respectively, and found that the corresponding minimum ignition temperatures of the coal dust clouds were 880 °C, 580 °C, and 520 °C.

Some scholars have explored the factors that affect the ignition temperature of coal dust. Chen Jinjian [10] used coal dust with a volatile content of 25.1% to study the factors that affect the ignition temperature and found that as the concentration increased, the ignition temperature of the coal dust decreased first and then slightly increased. The ignition-sensitive mass concentration was 1.364 kg/m³, and the minimum ignition temperature was 621 °C. Dejian Wu et al. [11] investigated the minimum ignition temperature of coal dust in an O₂/CO₂ atmosphere with an O₂ mole fraction ranging from 20% to 50% using three types of coal powder in the BAM furnace experiment. Yuqiao et al. [12] studied the ignition temperatures of Loy yang brown coal and Datong bituminous coal in a wire mesh reactor and found that the thermal conductivity of the gas surrounding the particles had a significant impact on the observed particle ignition temperature. Hao Liu et al. [13] studied the effects of particle size, oil shale content, dust concentration, dispersion pressure, and inert dust on the minimum ignition temperature (MIT) of coal and oil shale mixed dust clouds (COSMD) in a dust cloud MIT testing system. They studied the potential mechanisms of these factors for the COSMD MIT and their interactions. Deng Jun [14] studied the minimum ignition temperature of three kinds of pulverized coal with volatile contents of 31.16%, 26.52%, and 18.58%, respectively, and found that under the same pulverized coal concentration, the minimum ignition temperature decreased with a decrease in the particle size. The minimum ignition temperatures were 570 °C, 597 °C, and 660 °C when the particle size was 25 μm, respectively. Mishra and Azam [15] obtained the minimum ignition temperature of coal dust cloud at different concentrations using coal samples with a volatile content of 17.73%. They found that the optimum concentration of particle sizes of 38~74 μm and 74~212 μm was 2564 g/m³, and the lowest ignition temperatures were 460 °C and 470 °C, respectively. The optimum concentration of coal samples with particle size less than 38 μm was 3419 g/m³, and the lowest ignition temperature was 415 °C. Jafar [16] studied the effect of particle size and concentration on the minimum ignition temperature of coal dust. These results showed that the characteristics of coal dust concentration and particle size affected the minimum ignition temperature. When the coal dust concentration was lower than 1000 g/m³, the minimum ignition temperature decreased with an increase in the dust concentration. Cao Weiguo [17] evaluated the explosion characteristics of pulverized coal with five different particle sizes. The experimental results showed that when the particle size of pulverized coal was reduced to 25~48 μm, the lowest ignition temperature was 793~803 K. Liu Tianqi [18] selected eight coal samples and studied the factors affecting the minimum ignition temperature of coal dust clouds. He found that the lower the degree of coal metamorphism was, the easier it was for coal dust clouds in closed high-temperature spaces to catch fire, and there was a greater potential danger of coal dust cloud explosion. Wang Qiuhong [19] tested the minimum ignition temperature of four kinds of pulverized coal with different volatile contents and found that, with an increase in the coal dust cloud concentration, the minimum ignition temperature of the four kinds of pulverized coal showed a trend of first decreasing and then increasing. Under the optimum concentration of 1500 g/m³, the lowest ignition temperature of coal dust cloud with 10.89% volatile matter was about 620 °C.

At present, most of the research focuses on the explosion characteristics of boiler pulverizing system and other dust environments, but there are few research papers on the reburning and explosion risk of tail flue gas encountering high-temperature flue gas during start-up and shutdown. Therefore, a small testbed was built, and a hot blast stove was used to simulate the working conditions of a boiler start-up. Fly ash with different volatile contents was used to systematically study the ignition characteristics and influencing factors of fly ash in flue gas and to analyze the secondary ignition risk of combustible fly ash during SCR in the industrial boiler start-up stage.

2. Experimental System

A small-scale hot air furnace and flue gas testing section were used as the research object to simulate the start-up conditions of a boiler. The outlet temperature of the hot air furnace, ignition temperature of the testing section, and the composition of flue gas before and after the test were measured. The experimental system is shown in Figure 1, and it included a fan, electric heater, hot air furnace, testing section, micro feeder, and injection pump. The front end of the testing section was connected to the hot air furnace, and the back end was connected to a spray filter device, which cooled and filtered the flue gas. The hot air furnace used liquefied petroleum gas as fuel and produced high-temperature flue gas at 600–900 °C by adjusting the cooling air flow. The electric heater produced low-temperature flue gas at 150 °C, and the gas medium was air. The two gases mixed at the connection between the hot air furnace and the testing section to form flue gas at 300–400 °C, which was used to simulate the SCR inlet flue gas environment. When investigating the ignition characteristics and influencing factors of combustible fly ash, a micro feeder was used to feed combustible fly ash with a concentration of 25 g/m³ into the low-temperature flue gas, and a trace amount of alcohol was injected at a rate of 10 mL/min to simulate volatile organic compounds.

The initial sample 0 of combustible fly ash was analyzed for its composition, and the content of each component after the treatment is shown in

Table 1. Analysis of the combustible fly ash composition.

Components	Ash	Moisture	Volatile Matter	Fixed Carbon
Unit	%	%	%	%
Sample 0	16.45	14.27	26.7	42.58
Sample 1	18.43	0	20	61.57
Sample 2	19.32	0	15	65.68
Sample 3	21.9	0	10	68.1

. Samples 1, 2, and 3, with volatile contents of 20%, 15%, and 10%, respectively, were prepared from the initial sample 0 by drying at 350 °C for 35 min, 60 min, and 75 min in a muffle furnace, respectively. The fineness of all samples was R90 ≈ 20%.

When high-temperature flue gas produced in a hot blast stove is mixed with low-temperature flue gas produced in an electric heater to form 300~400 °C flue gas, the possibility of the ignition of combustible fly ash is low. In order to fully obtain the combustion characteristics of flammable fly ash and further obtain the critical ignition conditions, the temperature of high-temperature flue gas was gradually raised from 600 °C to 950 °C, the temperature of low-temperature flue gas was controlled at 130~160 °C, and the temperature of mixed flue gas was 300~400 °C. Then, the flue gas at the outlet of hot blast stove and the flue gas at the front and back ends of the test section were collected using a testo 350 flue gas analyzer, and the flue gas components before and after adding flammable fly ash were analyzed. Finally, the flammable fly ash with different volatile content was tested to observe the ignition situation.

3. Test Results and Analysis

3.1. Ignition Characteristics of Combustible Fly Ash in the Gas Stream

Using liquefied petroleum gas as fuel, a hot blast stove can produce high-temperature flue gas at 600~1000 °C. The main components are shown in

Table 2. High-temperature flue gas components at the outlet of hot blast furnace.

Components	O2	CO	CO2	NO	NO2
Unit	%	ppm	%	ppm	ppm
Numerical	14.1	25	4.8	32	2.5

, with O₂ accounting for 14.1%, CO₂ accounting for 4.8%, and the NO and CO components accounting for only 32 ppm and 25 ppm, respectively, which shows that the combustion was sufficient. The flame situation of the hot blast stove fire port is shown in Figure 2, in which a blue flame emitted from the side fire port also indicates that the combustion was sufficient.

Combustible fly ash with a volatile content of 20% was transformed into flue gas. At this time, alcohol was not added as a combustible gas component, which is characterized by CO. As shown in

Table 3. Ignition of the combustible fly ash at different temperatures.

High-Temperature Flue Gas Temperature	Low-Temperature Flue Gas Temperature	Post-Mixing Temperature	CO Concentration at the Front of the Test Section	CO Concentration at the End of the Test Section	Whether It Is on Fire
(°C)	(°C)	(°C)	(ppm)	(ppm)
740	163	559	75	96	No
845	154	620	115	158	No
900	156	669	282	415	No
942	145	697	239	278	Yes

, when the temperature of the high-temperature flue gas and mixed test section increased continuously, the volatile matter precipitated from fly ash in the test section increased, and the difference in the CO concentration between the front and back ends of the test section became larger and larger. When the mixed temperature reached 669 °C, the CO concentration at the back end and front end of the test section was far greater than the difference in the CO concentration between the front and back ends at other temperatures, indicating that the volatilization level is the highest at this time, and the temperature should reach near the combustion threshold. Finally, when the temperature of the test section reached 697 °C, it ignited, and the difference in the CO concentration at the front and back ends was reduced to a certain amount because of the combustion of volatile matter.

The relationship between the flue gas temperature and CO concentration at the front and back ends of the test section is shown in Figure 3. It can be seen that with an increase in the flue gas temperature, the CO concentration increased continuously, indicating that the volatile matter in combustible fly ash continuously precipitates at a high temperature. By comparison, it was found that the CO concentration detected at the back end of the test section was obviously higher than that at the front end, which indicates that volatile matter is continuously precipitated from the front end to the back end of the test section. When the temperature reached about 675 °C, the concentration of CO decreased obviously, which indicates that CO began to oxidize and continued to exotherm. When the temperature reached 700 °C, the ignition of combustible fly ash was observed.

Combustible fly ash was sampled from the front end and the back end of the test section, and composition analysis was carried out. The moisture and volatile content of the combustible fly ash sampled from the back end of the test section is shown in Figure 4. Because the combustible fly ash sampled from the back end of the test section stayed in the high-temperature environment of the test section for a period of time, the moisture and volatile were continuously consumed, so the moisture and volatile content levels of the combustible fly ash at the back end of the test section were obviously lower than those at the front end.

3.2. Influence of Combustible Gases in Flue Gas on Combustible Fly Ash Ignition

In order to explore the influence of the combustible gas components in flue gas on the ignition of combustible fly ash, trace alcohol was injected into the flue gas, and the gas components formed after alcohol volatilization were used to simulate the combustible components in actual power plants. The volatile content of combustible fly ash used in the test was 26.7%. The relationship between the temperature and CO in the test section after mixing high- and low-temperature flue gas and the corresponding ignition points are shown in Figure 5 and Figure 6. When only flammable fly ash was added, the CO concentration in the test section increased with an increase in the flue gas temperature. When the temperature approached 650 °C, the CO concentration reached the highest level, about 1000 ppm; then, it began to decrease obviously, which indicated that the temperature had caused the flammable components to undergo an oxidation reaction at this time, and the reaction was accompanied by heat release that made the temperature in the test section continue to rise rapidly. Finally, a spark of flammable fly ash ignition was observed at 663 °C. When alcohol was added as the combustible gas component, the ignition point dropped to 615 °C, which is 48 °C lower than that when only combustible fly ash was added. This is because the combustible gas component produced by alcohol made the CO concentration in the test section rise rapidly. The CO concentration in the working condition of adding combustible gas from 475 °C was higher than that when only combustible fly ash was added, so the concentration threshold of oxidation reaction was reached ahead of time, and the ignition point dropped obviously after adding the combustible gas component.

Once the ignition point is reached, the flammable fly ash particles form a clear flame track, which can be recorded using a high-speed camera.

Table 4. Parameters of combustible fly ash before and after ignition.

	Volatile Content of Combustible Fly Ash (%)	Alcohol Flow (mL/min)	Ignition Temperature (°C)
only combustible fly ash	26.7	0	663
combustible fly ash and alcohol	26.7	10	615

shows the fly ash volatile content, alcohol flow rate, and ignition temperature of the combustible fly ash used in the test. Figure 7 shows the situation before and after ignition observed at the observation port of the test section when only combustible fly ash was added. When ignition occurred, the color of the observation port was brighter than that when it was not ignited, and an obvious flame trajectory could be seen. When alcohol was added and ignition occurred, the flame formed in the test section was obviously brighter than that when only fly ash was added, which indicates that combustible gas components also participate in ignition combustion. The situation before and after ignition when combustible fly ash and alcohol were added together is shown in Figure 8.

3.3. Effect of High-Temperature Side Flue Gas on Combustible Fly Ash Ignition

In the actual operation of a power plant, in order to increase the SCR inlet temperature, the mixed high-temperature flue gas temperature can be adjusted to a certain extent and generally kept within 300~400 °C, so that it can adapt to the working temperature of the SCR catalyst. In order to study the influence of high-temperature flue gas temperature on the ignition of combustible fly ash, the flue gas temperature on the high-temperature side was changed when the temperature was unchanged after mixing it in the test section. The experimental results are shown in

Table 5. Effect of high-temperature flue gas on combustible fly ash ignition (only combustible fly ash).

High-Temperature Flue Gas Temperature	Low-Temperature Flue Gas Temperature	Post-Mixing Temperature	CO Concentration	Whether It Is on Fire
(°C)	(°C)	(°C)	(ppm)
649	105	422	80	No
709	105	440	79	No
754	117	368	14	No
813	141	435	37	No

. First, the flue gas temperature at the outlet of the hot blast stove was raised from 650 °C to 813 °C. In this process, the mixing temperature was controlled at 370~440 °C, and only combustible fly ash was added. It was found that the combustible fly ash did not ignite under this working condition. Compared with Table 3, it was found that the temperature after mixing was lower, and the CO concentration was much lower than 1000 ppm, which was very different from the previous ignition temperature of combustible fly ash and the concentration threshold of the CO oxidation reaction. Although a CO oxidation reaction occurred after an increase in the temperature, the combustible fly ash did not ignite.

Similarly, under the condition of adding both combustible fly ash and alcohol, the temperature of the high-temperature flue gas on the hot side increased from 660 °C to 813 °C, and the mixed temperature was maintained at 370–440 °C, as shown in

Table 6. Effect of high-temperature flue gas on combustible fly ash ignition (adding alcohol).

High-Temperature Flue Gas Temperature	Low-Temperature Flue Gas Temperature	Post-Mixing Temperature	CO Concentration	Whether It Is on Fire
(°C)	(°C)	(°C)	(ppm)
660	105	430	330	No
715	102	439	281	No
760	120	370	135	No
813	142	426	350	No

. It was found that the combustible fly ash did not ignite under this condition as well.

3.4. Influence of Combustible Fly Ash Composition on Ignition

In order to study the influence of combustible ash composition on the ignition characteristics, 26.7%, 20%, 15%, and 10% volatile fly ash were delivered to the test section in turn, and the temperatures of the furnace and the test section were continuously increased to find the ignition points of combustible fly ash with different volatile components. The relationship between the volatile content of combustible fly ash and the ignition point is shown in

Table 7. Ignition point of combustible fly ash with different volatiles.

Volatile Matter Content (%)	Only Combustible Fly Ash Ignition Point (°C)	Combustible Fly Ash and Alcohol Ignition Point (°C)
26.7	663	615
20	700	657
15	715	668
10	762	698

. It can be found that the lower the volatile content of combustible fly ash was, the higher the ignition point was. When alcohol was added as the combustible component of gas, the ignition point clearly decreased.

The relationship between the volatile content and ignition point of combustible fly ash is shown in Figure 9. With a decrease in the volatile content, the ignition point gradually increased. When the volatile content was 10%, the ignition point of combustible fly ash was 762 °C. The ignition point of flammable fly ash with the same volatile content when alcohol was added as a combustible gas component was about 50 °C lower than that when only combustible fly ash was added. In reference to Section 3.2, the influence of the combustible gas in flue gas on the ignition of flammable fly ash was explored, and it was determined that the concentration of CO in the working condition of adding alcohol was much higher than that of only combustible fly ash; therefore, the time required for CO to reach the concentration threshold of oxidation reaction was shorter, and the temperature at this time was lower, so the ignition point of flammable fly ash particles was lower. By comparison, it was found that the change trend of the ignition point was almost the same in the two working conditions, and this point could be predicted. According to the results, two formulas were fitted, which is convenient to calculate the ignition point according to the volatile content under similar working conditions:

$$T_1=-563x+810.96$$

(1)

$$T_2=-475x+744.7$$

(2)

where T₁ is the ignition point of only combustible fly ash conditions; T₂ is the ignition point of combustible fly ash and alcohol coexistence; and x is the volatile content of combustible fly ash.

4. Conclusions

A systematic experimental study of the ignition characteristics of combustible fly ash in high-temperature flue gas was conducted using a small test stand to grasp the effects of combustible gas components, the high-temperature side flue gas temperature, and the volatile content on the ignition point of combustible fly ash. The following conclusions were obtained:

(1)

The concentration of CO increased with an increase in the temperature of mixed smoke in the test section, which indicates that the volatile matter in combustible fly ash is continuously precipitated at a high temperature. When the temperature reached a certain value, the combustible fly ash started to ignite and burn. When alcohol was added as a combustible gas component, the flame formed in the test section was obviously brighter, which indicates that the combustible gas component also participates in ignition.

(2)

When the flue gas temperature in the test section was maintained at about 400 °C and the high-temperature flue gas temperature was raised from 650 °C to 813 °C, the combustible fly ash did not catch fire whether alcohol was added as a combustible gas component or not, which can eliminate the risk of secondary combustion and the explosion of combustible fly ash under this condition.

(3)

When the volatile matter content of combustible fly ash was 10~26.7%, the ignition point was 660~760 °C. The lower the volatile matter content of fly ash was, the higher the ignition point was. When alcohol was added as a combustible component of gas, the ignition point decreased by about 50 °C.

(4)

The oxygen concentrations in the high-temperature flue gas and low-temperature flue gas used in this experiment were much higher than those used in an actual power plant, so the critical ignition temperature of combustible fly ash obtained is relatively conservative, that is, it is lower than the critical ignition temperature in an actual power plant, which can provide a certain margin for the design and transformation of power plants in the future.