Yang Guofeng 1, Yu Youfang 2Sheng Kui Chuan1
(1. School of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou 310058, China; 2. College of Applied Engineering, Zhejiang Business College, Hangzhou 310053, China)
Abstract: In order to investigate the effect of feed rate on the composition and pollutant emission of flue gas during biomass pellet combustion, a 5G-20 / 85-0.23 biomass pellet burner Stop 13s, into the 2s stop 15s, into the 2s stop 17s, into the 2s stop 19s, into the 2s stop 21s (respectively 2/13, 2/15, 2/17, 2/19, 2/21 said), the corresponding average Flue gas CO, O at feed rates of 6.9, 6.2, 5.5, 5.0 and 4.5 kg / h 2And NO xChanges in content, examine the CO and NO xThe results showed that when the feed rate decreased from 6.9 kg / h (2/13) to 5.0 kg / h (2/19), the CO content in the flue gas Gradually decreased to a minimum of 66 mg / Nm at 5.0 kg / h 3NO xThe change in content is similar to that for CO, reaching a minimum of 8.8 mg / Nm at a feed rate of 5.0 kg / h (2/19) 3; The content of CO in flue gas fluctuated most with operating time at a feed rate of 6.9 kg / h (2/13); the CO content fluctuated periodically at different feed rates, and the fluctuation period was in good agreement with the feeding period , While NO xIn general, the average feed rate of 5.0kg / h, that is, feed into the 2s stop 19s can significantly reduce pollutant emissions.
Biomass pellet molding fuel is made of sawdust, straw and other agricultural and forestry waste as raw materials, the use of machinery and equipment compressed into cylindrical pellets and solid texture of the molded, can be used as industrial boilers, cooking stoves, heating stove and agricultural heating fuel, With the advantages of small size, large energy density, easy transportation and storage, etc. With the implementation of renewable energy development and utilization and energy conservation and emission reduction policies in China, the production and sale of biomass pellet fuel has entered the commercial operation stage, and large-scale use Particulate fuel is in the ascendant and in some areas there is even a shortage of particulate fuel.
In recent years, biomass pellet fuel burners have been widely used in small hot water boilers, agricultural greenhouses, heating stoves for heating facilities and domestic heating stoves, etc. However, most of the current burner condition parameter settings exist More unreasonable place, resulting in the use of flue gas emissions, restricting the promotion of its application. [1]In biomass pellet fuel suitability studies, it was found that burner particulate emissions typically ranged from 98 mg / Nm 3So flue gas Greenman blackness is grade 2. Zhang Xuemin et al [2]Studying the emission characteristics of the burners in different feeding modes found that the CO content in the flue gas is usually 800mg / Nm 3About, NO xThe content of 134mg / Nm 3Therefore, it is necessary to study the relationship between the burner operating parameters and the pollutant emissions in order to clean the particulate fuel, because most of the existing particulate burners have high soot concentration and CO emission during use Combustion and discharge standards to provide a scientific basis.
The feed rate is one of the important working parameters of the particle burner, which is directly related to the burner heat load and pollutant discharge. After the burner is optimized and processed, the structural parameters such as the combustion chamber and the amount of air intake have been Therefore, the reasonable regulation of the feed rate becomes the key factor.If the feed rate is too large, the oxygen supply will be insufficient and the CO, NO xOther pollutants such as high emissions, polluting the environment [3]Feed rate is too small not only reach the required heat load, but also because of excessive air intake, resulting in combustible gas residence time in the furnace is too short, causing incomplete combustion, will also increase pollutant emissions[4].
DIAS and so on [5]In the study of the relationship between feed rate and CO content, it was found that the CO content in flue gas gradually decreased with the increase of feed rate from 5 kg / h to 6 kg / h under the condition of constant excess air ratio, In addition, some scholars have found that the air introduced into the furnace is not fully utilized in the initial stage of the feed rate increase, making full use of the feed rate as the feed rate continues to increase , The CO content began to decrease. When the feed rate exceeded a certain critical value, oxygen combustion occurred in the furnace and CO content increased again [6-7]In summary, the existing studies mainly focus on the impact of feed rate on CO emissions, while the feed rate of NO xHowever, the impact of emissions is seldom reported, and the fluctuation characteristics of pollutant content in flue gas with burner operating time are not clear.Therefore, in this paper, a small-scale hot water boiler supporting biomass particle burner as the object, Feed rate of CO and NO in flue gas xSuch as the impact of content, examine the CO and NO xThe fluctuation characteristics and variation of the emission with the operation time of the burner provide a theoretical basis for selecting the feed rate rationally and reducing the emission of biomass particles during the combustion process.
1 materials and methods
1.1 Test Materials
Biomass pellet fuel was taken from Zhejiang Hongyong Fuel Processing Plant of Jinhua City, Zhejiang Province, and the main raw materials were wood processing residues of Chinese fir, pine, etc. The average particle diameter of pellet fuel was 9.0mm and the density was 1200kg / m 3; Its industrial analysis components, elemental composition and calorific value are shown in Table 1. Of which: The industrial analysis components were determined according to the "Solid Biomass Fuel Industrial Analysis Method" (GB / T 28731-2012); the heat value was determined according to the "Biomass Solid Molding Fuel Test (NY / T 1881.1-2010); the C, H and N contents were determined by elemental analysis (EA1112, Carlo Erba, Italy) and the mass fraction of O was subtracted from C, H, N, S and the drying base Ash content obtained by calculation Particle fuel in the indoor dry and cool place sealed with plastic bags stored until the test access.
1.2 test platform and equipment
1.2.1 biomass particle burner test platform
Based on the burners used in 5G-20 / 85-0.23 biomass hot water boiler made by TianNan New Energy Technology Co., Ltd in Lanxi City, Zhejiang Province, our group designed and built a biomass particle burner test platform , Mainly by the furnace, feeding system, control and testing instruments and other components, the structure diagram shown in Figure 1. Furnace body by the inside and outside, followed by the furnace insulation and insulation layer, the furnace from the bottom to the top of the order Ash collection chamber → grate (an air inlet) → combustion chamber → secondary air inlet → cooling water pipes; using axial parallel twin-screw feed mechanism; air volume regulation and testing instruments are: 130FLJ2WYD4-2 centrifugal fan Shanghai Xinxing Electromechanical Group Co., Ltd.), SLDLUGB-DN40 intelligent integrated vortex flowmeter (Nanjing Shun Tat Instrument Co., Ltd.), laboratory manual air butterfly valve regulator device (pipe diameter 110mm, length 1200mm).
1.2.2 burner work flow
Add a certain amount of particles to the silo, start the power and run according to the set program, followed by: feeding (20s) → ignition (4min) → ignition is successful, normal operation (gap feed) → stable combustion → stop Continue to burn 15min after the end feeding mechanism feeding intermittent feed (periodic feed) feed, the upper screw by a few seconds pause for a few seconds for a cycle, the lower screw is continuous operation. The use of twin-screw mechanism and the gap into The main purpose of the material is to effectively prevent the combustion chamber from tempering and burning back the particulate fuel in the silo[8].
1.3 test method
1.3.1 Determination of flue gas composition
Smoke in the CO, NO, NO 2, NO xAnd O 2Detection of content: Refer to "Technical Requirements and Test Methods for Continuous Monitoring System of Flue Gas from Stationary Sources" (HJ / T 76-2007) and Testo350 Flue Gas Analyzer (Testo, Germany) .The NOx content of this detector is NO With NO 2And according to the detector requirements, after the burner is running stable (furnace temperature almost unchanged), the flue gas probe is fixed at a distance of 50cm above the furnace body, and the cross-section of the chimney was 30 ~ 45 ° angle measurement. Before the calibration flue gas analyzer, the detection time 5min, sampling frequency 10s / time, automatically save the average, maximum and minimum values of each flue gas composition within 10s.
1.3.2 feed rate setting
1.3.3 once into the wind and the second intake air flow measurement
The first and second inlet air are respectively provided by two centrifugal fans of the same type. A butterfly valve for adjusting the air volume is installed at the front of the air inlet of the fan. A vortex flowmeter is installed at the front end of the air outlet of the fan at a diameter of 15 times (60 cm) for measuring the gas The secondary air ratio is expressed as the secondary air volume / total air volume, and the total air volume is the sum of the primary air volume and the secondary air volume.
1.3.4 particulate fuel combustion theoretical air volume
1.4 Experimental Design
According to preliminary test results, when the average feed rate is 6.2kg / h (into 2s stop 15s), the discharge of pollutants is in a relatively reasonable range, so the feed method is set to enter 2s stop 15s (6.2kg / h), primary air volume, secondary air volume were 47.2m 3/ h, 38.1m 3/ h, that is, the ratio of the secondary air is 0.4 and the excess air coefficient is 2.5. The results show that the CO concentration is 193mg / Nm 3, NO xThe mass concentration is 72mg / Nm 3, Are lower than the relevant national standards: that under the conditions in which the parameters in a relatively reasonable range to keep the other parameters unchanged, only increasing the feed rate, select into the 4s stop 15s (10kg / h) into Materials way of testing, the results show that the CO concentration of up to 3293mg / Nm 3, Far exceeding the national emission standards for pollutants.Therefore, through the air volume control valve at different feed rates, the amount of primary air and secondary air intake air is changed to keep the secondary air ratio 0.4 and the excess air coefficient 2.5 unchanged The gap feed change gradient into the 2s stop 13s, into the 2s stop 15s, into the 2s stop 17s, into the 2s stop 19s, into the 2s stop 21s (2/13, 2/15, 2/17, 2/19, 2 / 21), the corresponding average feed rates are 6.9, 6.2, 5.5, 5.0 and 4.5 kg / h.
2 results and discussion
2.1 CO and O in flue gas 2content
2.1.1 feed rate of CO and O in the flue gas 2Content of CO content in the flue gas directly reflects the volatiles precipitation and combustion conditions, the flue gas in different feed rates of CO and O 2Content shown in Figure 2. It can be seen that, as the feed rate decreases, the CO concentration in the flue gas first decreases and then increases, which is consistent with the results of previous studies [11-13]similar.
The CO content gradually decreased when the feed method was changed between 2/13 and 2/19: indicating that the combustion condition in the furnace was improved and the combustion was more complete with the decrease of the average feed rate, which was mainly attributed to the excess air ratio and The proportion of the secondary air remains the same, the decrease of the feed rate increases the blow in O 2The mixing time with the volatiles allows the two to be more fully mixed [14]When the feed method was changed from 2/17 to 2/19, the CO was 146 mg / Nm 3Directly down to 66mg / Nm 3, A 55% decrease: indicating a significant reduction in the CO content released during this change and a more complete volatilization of the combustion. The CO content increased 2-fold or more at 2/21 feed rates, 205 mg / Nm 3) This is because after the feed rate continues to decrease, the particulate fuel layer above the grate undergoes a 'burn-through' phenomenon, ie, a 'wind leakage' occurs through the primary wind entering below the grate and the residence time of the air entering the hearth decreases , Part of the CO generated by pyrolysis of particles away, too late with the inlet air O 2Mixed combustion and exit from the upper flue gas outlet of the furnace, resulting in a significant increase of CO content in the flue gas.
As the feed rate decreases, the flue gas O 2The content increased first and then decreased, reaching the maximum value (about 16%) when the feed method was 2/19. At different feed rates, O 2Which may be related to the feed mechanism using a screw feed and the combustion chamber of a fixed grate (grate): the particulate fuel is delivered unevenly to the grate by the screw, there are particles in the furnace The phenomenon that the upper part of the grid is close to the side of the material inlet and the other part is lack of particles and produces "air leakage" to different degrees with little air resistance causes the primary air entering the furnace to directly enter the furnace through the gap on the side grid and make the airflow Accelerate, so that the flue gas O 2High content.
2.1.2 CO content in flue gas fluctuations
CO content changes over time can directly reflect the stability of the combustion conditions in the furnace Figure 3 for different feed rates CO content in flue gas with the running time changes from which we can see: the feed method is 2/13, CO The variation of the content with time is the biggest, which shows that the combustion condition in the furnace fluctuates greatly under this working condition. When the test time reaches 80s, a significant peak appears, which may be that the accumulation of particles in the furnace reaches the maximum at this time. The CO produced by incomplete combustion increased suddenly, then the combustion speed accelerated, the feed rate and combustion rate reached a new equilibrium, and the fluctuation of CO content decreased. The curve of CO content with time went from about peak to peak about 15s, The value and the minimum respectively appear at the end of feed and the end of pause, basically in line with the feed cycle.While in other feed methods, although the CO content in the flue gas fluctuates little, it also changes periodically, And with the corresponding feeding period coincide.Thus, in the intermittent feeding method, the feeding time can not be too long, otherwise it will result in increased particle accumulation, incomplete combustion, CO pollutant emissions High.
2.2 flue gas NO, NO 2And NO xContent
2.2.1 feed rate of flue gas NO, NO 2And NO xEffect of content
Figure 4 for the feed rate of NO, NO 2And NO xContent can be seen from which, NO, NO 2With NO xThe change of content is basically similar.With the decrease of feed rate, the NO in flue gas xThe content showed the trend of decreasing and then increasing: the maximum value appeared when the feed method was 2/13 (78mg / Nm 3), Reaching the minimum at 2/19 (8.8 mg / Nm 3). Correspondingly, the temperature of the lower part of the furnace detected by the thermocouple sensor reached a maximum value of 758 ° C .; when the feeding method was 2/21, NO xThe content increased obviously, correspondingly, the temperature in the lower part of the hearth decreased to 375 ℃, indicating that the increase of temperature in a certain temperature range was beneficial to the inhibition of NO xOf the production and reduce emissions.In the study conditions, the biomass particles did not burn in the air N 2To NO xConversion temperature (1300 ℃ above), basically does not produce thermal NO x[15-16]Due to the low content of N in wood particles, NO xMainly from the oxidation of N in the fuel, so at different feed rates, flue gas NO xContent is low.From Figure 2 and Figure 4 can be seen, with the feed rate decreased, the flue gas NO xThe change of content is similar to the change trend of CO, showing the change after decreasing and then increasing[17-18].
2.2.2 flue gas NO xFluctuation of content
NO in flue gas at different feed rates xContent fluctuations over time as shown in Figure 5. It can be seen from, NO xNo periodic changes are present. When the feed method is 2/19 and 2/17, NO xThe content fluctuates little over time, with a relatively large fluctuation with time when the feeding methods are 2/15 and 2/13, possibly due to the accumulation of particles in the furnace at a certain moment as the feed rate increases, Leading to unstable combustion conditions, promoted NO xThe production and emission concentration changes.As mentioned earlier, NO xThe main source of raw materials is the conversion of N, therefore, the feed rate and the raw material of N to NO xThe conversion is closely related, ie, the higher the feed rate, there will be more raw material of the N element into the furnace [19-20]It can be seen that the feed rate not only affects NO in flue gas xContent, but also affect the volatility.
3 Conclusion
3.1 The feed rate has a significant effect on the CO content in the flue gas with the CO emission being the smallest at a feed rate of 2/19 (average feed rate 5.0 kg / h) with a feed rate of 2/13 (average feed The rate of 6.9 kg / h), the CO emissions maximum, the largest fluctuations. CO content of flue gas cyclical changes in the cycle and feed cycle basically consistent. Batch feed can be guaranteed in the heat load requirements of the conditions Under the test conditions, the choice of feed method 2/19 (into 2s, stop 19s) is more reasonable.
3.2 at different feed rates NO xThe changing trend of content is consistent with that of CO. When the feed method is 2/19 (average feed rate is 5.0kg / h), NO xThe minimum content of 8.8mg / Nm 3, The maximum value of 78mg / Nm appears when the feed method is 2/13 (average feed rate is 6.9kg / h) 3NO at different feed rates xNo periodic changes were present. When the feed method was 2/19 and 2/17, NO xVolatility over time is small.
3.3 CO and NO in flue gas at different feed rates xThe content changes are more consistent. The effect of feed rate on CO emission is greater than that of NO xImpact of emissions.
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