Effect of circulating ash from CFB boilers on NO and N₂O emission

Abstract

NO and N₂O emissions from circulating fluidized bed (CFB) boilers are determined by their formation and destruction rates in the furnace. The effect of circulating ash from a CFB boiler on NO and N₂O emissions were investigated in a laboratory-scale fluidized bed reactor. The results show that the residue char in circulating ash and the CO generated from the char play an important role in NO reduction and N₂O formation; however, active components of circulating ash such as CaO, Fe₂O₃ accelerate the decomposition of N₂O. Experiment was also conducted on a 75 t/h CFB boiler fueled with the mixture of anthracite and biomass. The lower residue carbon content of circulating ash in this experiment is lower; therefore, the reacting rate of NO deoxidize is limited. This result verified the conclusion of laboratory research.

Keywords

CFB boiler / circulating ash / NO reduction / N₂O thermal decomposition / biomass

Introduction

As a clean coal combustion technology, CFB boiler develops very quickly especially in China [1]. The results of measurements on many coal fired CFB boiler plants show that NO concentration is usually below 2×10^-4 in volume which is remarkably lower than that on PC boilers [1,2]. Because of the lower combustion temperature in CFB boilers, N₂O concentration in flue gas is usually higher than that in PC boilers. Much attention has been paid to the discharge of N₂O because the increase of N₂O concentration in the atmosphere will lead to worse greenhouse effect and will destroy the ozonosphere [3-6].

It is believed that the nitrogen oxide in the flue gas of a CFB boiler mainly comes from the fuel. According to the fluidization characteristic in the furnace of a CFB boiler, the furnace can be divided into dense-phase zone in the lower furnace and lean-phase zone in the upper furnace. The coal is vaporized in the dense-phase zone where most NO is formed. The residue char from the coal are fluidized and carried into the lean-phase zone. There is residue char in circulating ash from every part of the CFB boiler including the lean-phase zone and the cyclone separator. The percent conversion of nitrogen in the fuel to NO is mainly affected by the content of residue char, CO concentration and the content of minerals such as FeO_x and CaO in circulating ash [5,7,8]. N₂O is mainly formed by the fuel nitrogen in the coal through reactions such as NO and residue char [5,8,9]. Besides combustion temperature and excess air ratio, the dense circulating ash is also considered to be an important influencing factor for nitrogen oxide emission [1].

A laboratory-size CFB reactor is used in this research. The concentration variation of NO and N₂O before and after the CFB reactor are measured to study the effect of circulating ash from CFB boilers on nitrogen oxide formation and emission. The experiment is also conducted on a 75 t/h CFB boiler fueled with the mixture of anthracite and biomass.

Laboratory experimental

CFB circulating ash

The circulating ash is mainly made up of coal ash, calcined lime and residue char. The circulating ash used in the experiment is taken from a coal fired CFB boiler plant in China. Quartz sand which is believed to be alert for nitrogen oxides thermal decomposition was used as bed materials in the reactor too. The chemical compositions of the bed materials were measured by a fluorescent X-ray spectrometer while the characteristics of bed material surface were analyzed with a mercury injection apparatus AutoPoreⅣ, which shows that the specific area of quartz sand is mainly made up of the outer surface, and there are little micropores or slots on its surface. Meanwhile, there are many micropores and slots on the surface of circulating ashes; as a result, the specific area of the circulating ash is much bigger. If gas-solid reaction takes place on the surface of circulating ash, the big specific area would be favorable for the adsorption and diffusion of gas molecules.

The measurement shows that the porosity of circulating ash is 0.468 while that of quartz material is 0.470.

The content of residue char in circulating ash was measured with an electric muffle furnace through ignition test. The results are shown in Table 1.

Table 1 shows that the quartz sand is mainly made up of SiO₂, that the content of Fe₂O₃ and CaO which are believed to be active catalysts for the heterogeneous reaction of N are negligible in quartz sand while the content of Fe₂O₃ and CaO on the surface of the circulating ash [7,10] is 6.1% and 2.5%.

Experiment rig

The laboratory-size CFB reactor used in the research, as shown in Fig. 1 is mainly made up of a flue gas generator, a CFB reactor and a FTIR to measure NO and N₂O concentration on line.

The flue gas from the CFB boiler is simulated by the mixture of nitrogen, compressed air, CO₂ and stream. The standard gases of NO and N₂O are added too. The flux of N₂, compressed air, CO₂ and stand gases are controlled by mass flowmeters. Meanwhile, the steam is supplied by a specially designed generator mainly made up of a mini-bump and an electric heater.

The inner diameter of the reactor is 25 mm with a distributor in the middle. The reacting gas can flow through the reactor upward passing the distributor while the bed material can be kept on it. The reacting temperature can be controlled by the adjustable globar heating element and monitored by a K type thermojunction. There is a charging pipe on top of the reactor. The weighted circulating ash can be kept in the charging pipe and added into the heated reactor when the charging valve is opened. The exhaust gas from the reactor is cooled, dried, filtered and then delivered into the FTIR. The FTIR is equipped with a #A10720 VEN 0.2l/4 m gas cell. The FTIR can measure the concentration of NO, CO and N₂O simultaneously on line with a measurement accuracy of ±5%.

Experimental operation

The percentage compositions of simulated flue gas are shown in Table 2.

The total flux of the reacting gas is 0.2 m³/h and the reacting temperature around the distributor is 850°C. During experimental operation, 28 g of quartz sand is put into the reactor first and then 2 g of circulating ash. NO, CO and N₂O concentration in exhaust gas are measured by the FTIR on line. The measurement lasted for 10 min with a frequency of 10 s.

The steam and some of the CO₂ are removed from exhaust gas before measurement, as a result, the measured NO, CO and N₂O concentration is higher than that in exhaust gas. Researches show that the NO concentration is almost the same during passing through the reactor when there is quartz sand only [8, 10], so NO concentration is constant and can be used as the reference for concentration reduction. The effect of H₂O and CO₂ removal on NO and N₂O measurement would be excluded by the concentration reduction.

Results of laboratory experiment

The reduced NO, CO and N₂O concentration in exhaust gas changes after circulating ash is added into the reactor. The measurement result is shown in Fig. 2. NO and N₂O concentration in reacting gas are 10^-4 and the reacting temperature is 850°C.

In Fig. 2, the FTIR starts to work when circulating ash is added. Because of the influence of the volume of pipes and FTIR gas cell, the exhaust gas concentration is delayed. The exhaust gas component is kept almost the same except for some vibration within the first 100 s. During this 100 s, NO concentration is kept at 10^-4. N₂O decomposes when passing through the heated reactor and its concentration in exhaust gas is about 0.6×10^-4. No CO is detected in exhaust gas by the FTIR.

One hundred seconds after circulating ash addition, the gas component in exhaust gas changes. NO concentration decreases first and then increases. It is the lowest at about 220 s after the addition of circulating ash. However, it increases to about 10^-4 at about 500 s, which means that the influence of circulating ash on NO concentration was removed by then.

N₂O concentration increases at first to around 0.7×10^-4 within about 180 s after circulating ash addition and then decreases gradually to about 0.54×10^-4, which means that the addition of circulating ash accelerates the thermal decomposition of N₂O.

CO concentration in exhaust gas is measured too. The results show that CO is detected between 120 s and 300 s after adding circulating ash. Maximum CO concentration detected during the experiment is 0.14×10^-4.

It is found that all the reaction of residue char with H₂O, CO₂ and O₂ in flue gas would generate CO:

(1){\mathrm{H}}_{\mathrm{2}}\mathrm{O}+\mathrm{X}(\mathrm{s})\stackrel{}{\to }{\mathrm{H}}_{\mathrm{2}}+\mathrm{C}\mathrm{O}

(2)\mathrm{C}{\mathrm{O}}_{\mathrm{2}}+\mathrm{C}(\mathrm{s})\stackrel{}{\to }\mathrm{2}\mathrm{C}\mathrm{O}

(3){\mathrm{O}}_{\mathrm{2}}+\mathrm{2}\mathrm{C}(\mathrm{s})\stackrel{}{\to }\mathrm{2}\mathrm{C}\mathrm{O}

The content of residue char in circulating ash is only 2.18%. At reacting temperature, the reaction between CO and O₂ is easy, and as a result, some of the CO is oxidized to CO₂. Therefore, CO concentration detected in exhaust gas is small.

The NO in reacting gas is deoxidized by reducing agents such as residue char, CO and residue nitrogen in the circulating ash. Some researches show that circulating ash has a catalytic effect on the reaction between NO and CO [8].

(4)\mathrm{2}\mathrm{N}\mathrm{O}+\mathrm{2}\mathrm{C}(\mathrm{s})\stackrel{}{\to }{\mathrm{N}}_{\mathrm{2}}+\mathrm{2}\mathrm{C}\mathrm{O}

(5)\mathrm{2}\mathrm{N}\mathrm{O}+\mathrm{2}\mathrm{C}\mathrm{O}\stackrel{\mathrm{c}\mathrm{i}\mathrm{r}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{s}\mathrm{h}}{\to }{\mathrm{N}}_{\mathrm{2}}+\mathrm{2}\mathrm{C}{\mathrm{O}}_{\mathrm{2}}

(6)\mathrm{N}\mathrm{O}+(-CN)\stackrel{}{\to }\mathrm{C}\mathrm{O}+{\mathrm{N}}_{\mathrm{2}}

When residue char and residue nitrogen are all consumed by the O₂ in reacting gas, reactions (4) and (5) are over. As a result, the NO concentration in exhaust gas recovers to that before adding circulating ash.

N₂O concentration first increases and then decreases, which shows that the reaction rate of N₂O formation increases after adding circulating ash. N₂O is produced by deoxidization of NO with residue char or residue nitrogen [8, 9-11]

(7)(-\mathrm{C}\mathrm{N})+\mathrm{N}\mathrm{O}\stackrel{}{\to }{\mathrm{N}}_{\mathrm{2}}\mathrm{O}+(-\mathrm{C})

NO concentration in exhaust gas is low, which shows that the reaction rate of N₂O formation is very low. When the residue char and residue nitrogen are all consumed by O₂ in the reacting gas, the reaction rate of N₂O formation decreases while the reaction rate of N₂O thermal decomposition is accelerated by the circulating ash. Metal oxides, such as Fe₂O₃ and CaO in the circulating ash, are believed to be active catalysts for N₂O thermal decomposition [11,12]; therefore, N₂O concentration in exhaust gas is lower than that before adding circulating ash.

(8)\mathrm{2}{\mathrm{N}}_{}\mathrm{O}\stackrel{\mathrm{c}\mathrm{i}\mathrm{r}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{s}\mathrm{h}}{\to }{\mathrm{N}}_{\mathrm{2}}+{\mathrm{O}}_{\mathrm{2}}

The experiment shows that circulating ash has some influence on both NO and N₂O concentration. The concentration of circulating ash in the CFB boiler is higher than that in the laboratory-size CFB reactor. Since the O₂ concentration in dense-phase is usually lower, the CO concentration is much higher than that in the reaction zone. The measurement result on operating coal fueled CFB boilers is as high as 10^-3 [2]. With the CO in reacting gas increasing, the reaction rate (5) is accelerated. As a result, the NO concentration decreases along the furnace. The NO concentration in the flue gas of CFB boilers is usually lower than 2×10^-4, lower than that in the lower part of the furnace. The experiments in this research show that the CO concentration in the furnace is important for NO deoxidization in the CFB boiler furnace.

The residue char and residue nitrogen in circulating ash would deoxidize the NO to N₂O. The reaction rate of N₂O formation is relatively higher than that of N₂O thermal decomposition, so N₂O concentration increases with the increase in furnace height. The active components in circulating ash have a strong influence on N₂O concentration in flue gas too. As the reaction rate of N₂O thermal decomposition increases with the reaction temperature, the N₂O concentration in flue gas decreases with the increasing temperature in the furnace [5,13].

It is concluded from the research that operation parameters, such as residue char content in circulating ash, temperature, O₂ concentration, have a great influence on NO and N₂O concentration in flue gas. The changed operation parameters would decrease nitrogen oxide emission.

Experiment on CFB boiler

Comparing the combustion parameters of coal and biomass, it can be found that the volatile matter content in biomass is higher than that in coal, and the combustion of biomass is much easier than coal. The biomass can enhance the coal combustion ignition and burnt off characteristics; as a result, the residue char content in the circulating ash of the CFB boiler fueled with biomass or the mixture of coal and biomass is lower than that fueled with coal only.

The experiment is also conducted in a 75 t/h CFB boiler. In the experiment, the boiler runs under the condition of 80% to 90% full load. Under different test conditions, the operation parameters, such as temperature, pressure and air flow rate, are recorded; meanwhile, the flue gas composition before the electrostatic precipitator is measured with a portable gas analyzer G-40. Samples of fly ash, bottom ash and circulating ash are collected and analyzed in the laboratory.

During the experiment, the boiler is fueled with the mixture of anthracite coal and pelletized biomass (corn straw) with different mixing ratios. The anthracite coal is marked as C and the two kinds of biomass are marked as biomass A and biomass B. The property of the anthracite coal, biomass-A and biomass-B are listed in Table 3.

The nitrogen content is 0.94% in the anthracite, 1.46% in biomass A and 1.54% in biomass B. Unlike the low nitrogen content biomass such as wood [14], the NO_x formation characteristics from co-combustion is more complex and affected by operating conditions.

The NO_x emissions are measured with different mixing ratios of biomass mixture. The test conditions are listed in Table 4. The mixing ratios are biomass percentage composition by weight. The coal and the biomass are mixed at the coal yard and then loaded into the fuel silo in front of the furnace by belt conveyor. The equivalence ratio is controlled from 0.83 to 0.87 during the test. The bed temperature is controlled from 900 to 920 °C by changing the primary air ratio.

The field experiment shows that the biomass mixed in the coal promotes the coal combustion reaction, resulting in higher combustion efficiency and an increase in the boiler thermal efficiency. The residue carbon content (ρ) in bottom ash, fly ash as well as circulating ash decreases with the increase of the percentage of pelletized biomass (η) in the mixture ranging from 0 to 25%. The residue char content in circulating ash is shown in Fig. 3.

NO_x emissions are represented by NO because NO₂ concentrations in all tests are found to be zero. Meanwhile, N₂O emission from biomass is reported to be negligible [14]. N₂O can be rapidly destroyed in the flame by radicals and molecules through reaction (8) [14,15].

NO concentrations in flue gas are measured by a portable gas analyzer G-40. The simple point is located in the horizon flue gas duct before the electrostatic precipitator. Figure 4 shows the NO concentrations under different mixing ratios of 0%, 10%, 15%, 20% and 25% respectively. If the bed temperature difference is neglected, it seems that NO emission increases with the increase in mixing ratio.

The NO concentration increase can be explained by the higher nitrogen content in the biomass. The nitrogen content increases with the biomass mixing ratio. Winter et al. [14] have investigated the relation of nitrogen conversion to NO and fuel-nitrogen content and found that the higher the nitrogen content of the fuel, the lower the proportion its conversion to NO is. The nitrogen conversion to NO for different biomass mixing ratios is calculated and the result shows that the nitrogen conversion to NO increased with the mixing ratio. The NO concentration increase is caused not only by higher nitrogen content in the biomass, but also by the increasing nitrogen conversion to NO.

As shown in Fig. 3, the burnt off characteristics are enhanced by co-combustion of coal and biomass, which leads to a lower carbon content in the bed material, especially in circulating ash. The study shows that the concentration of CO in the flue decreases too [16,17]. The NO reduction in the upper combustor region with co-combustion by reactions (4), (5) and (6) decreases, and more NO remains in the flue gas. The experimental results by the CFB boiler test conform to the conclusions of laboratory research.

Conclusions

There is much circulating ash in the furnace and cyclone separator of CFB boilers. This research shows that circulating ash has an important influence on NO and N₂O emission. Because of the reaction between NO and circulating ash, the residue char can dioxide the NO and make its concentration decrease with the increase in furnace height while the N₂O concentration increases with the increase in furnace height. Metal oxides, such as CaO and Fe₂O₃, have a catalytic effect on N₂O thermal decomposition, so the N₂O concentration in flue gas decreases with the increase in temperature. The result is also verified in a CFB boiler fueled with the mixture of anthracite and biomass.

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