Axial concentration profiles and N2O flue gas in a pilot scale bubbling fluidised bed coal combustor

Atmospheric Bubbling Fluidised Bed Coal Combustion (ABFBCC) of a bituminous coal and anthracite with particle diameters in the range 500–4000 μm was investigated in a pilot-plant facility (circular section with 0.25 m internal diameter and 3 m height). The experiments were conducted at steady-state conditions using three excess air levels (10%, 25% and 50%) and bed temperatures in the 750–900 °C range. Combustion air was staged, with primary air accounting for 100%, 80% and 60% of total combustion air.For both types of coal, virtually no N₂O was found in significant amounts inside the bed. However, just above the bed-freeboard interface, the N₂O concentration increased monotonically along the freeboard and towards the exit flue.The N₂O concentrations in the reactor ranged between 0–90 ppm during bituminous coal combustion and 0–30 ppm for anthracite. For both coals, the lowest values occurred at the higher bed temperature (900 °C) with low excess air (10%) and high air staging (60% primary air), whereas the highest occurred at the lower bed temperature (750 °C for bituminous, 825 °C for anthracite) with high excess air (50%) and single stage combustion.Most of the observed results could be qualitatively interpreted in terms of a set of homogeneous and heterogeneous reactions, where catalytic surfaces (such as char, sand and coal ash) can play an important role in the formation and destruction of N₂O and its precursors (such as HCN, NH₃ and HCNO) by free radicals (O, H, OH) and reducing species (H₂, CO, HCs).     

Axial and radial CO concentration profiles in an atmospheric bubbling FB combustor

Atmospheric Bubbling Fluidised Bed Combustion (ABFBC) of a bituminous coal and anthracite with particle diameters in the range 500-4000 μm was investigated in a pilot-plant facility (circular section with 0.25 m internal diameter and 3 m height). The experiments were conducted at steady-state conditions using three excess air levels (10, 25 and 50%) and bed temperatures in the 750-900 °C range. Combustion air was staged, with primary air accounting for 100, 80 and 60% of total combustion air. The effect of limestone addition was also tested.Large CO concentrations were observed inside the bed, up to 8 and 6% (v/v) in the cases of anthracite and bituminous coals, respectively. These concentrations decreased sharply as the gases emerged from the bed, and the CO flue gas concentration observed was in general less than 2000 and 4000 ppm, respectively. The CO flue gas concentration increased with air staging and with limestone addition, but decreased with either excess air or temperature increase. The observed results confirm the influence of sand particles (and probably of SO₂) in the ‘quenching’ of the oxygenated free radicals (HO and HO₂) reactions responsible for the CO oxidation inside the bed.     

Emission of nitrogen oxides in a vortexing fluidized bed combustor

This study investigated experimentally the effects of fuel types and operating conditions on NO emission in a 0.45 m I.D. pilot scale vortexing fluidized bed combustor. Rice husk, soybean, high and low sulfur subbituminous coals, and bituminous coal were used as fuels. Silica sand was employed as the bed material. The effects of various operating conditions, such as bed temperature, excess air ratio, and stoichiometric air flow rate on NO emission were investigated. The dependence of the conversion of fuel-N on O/N and H/N weight ratios of the fuel was explored to understand the effect of fuel composition on NO emissions. The results show that the H/N ratio is a better indicator than the O/N ratio to represent the conversion of fuel-N to NO. Soybean was mixed with other fuels to study its characteristics for reducing NO emission. Taguchi method was applied to analyze the priority of operating conditions for dominating NO emission. It is found that the excess air is the most important factor to dominate NO emission.     

Comparisons of pulverized coal combustion in air and in mixtures of O2/CO2

Pulverized coal combustion in air and the mixtures of O₂/CO₂ has been experimentally investigated in a 20 kW down-fired combustor (190 mm id×3 m). Detailed comparisons of gas temperature profiles, gas composition profiles, char burnouts, conversions of coal–N to NO_x and coal–S to SO₂ and CO emissions have been made between coal combustion in air and coal combustion in various O₂/CO₂ mixtures. The effectiveness of air/oxidant staging on reducing NO_x emissions has also been investigated for coal combustion in air and O₂/CO₂ mixtures. The results show that simply replacing the N₂ in the combustion air with CO₂ will result in a significant decrease of combustion gas temperatures. However, coal combustion in 30% O₂/70% CO₂ can produce matching gas temperature profiles to those of coal combustion in air while having a lower coal–N to NO_x conversion, a better char burnout and a lower CO emission. The results also confirm that air/oxidant staging is very effective in reducing NO_x emissions for coal combustion in both air and a 30% O₂/70% CO₂ mixture. SO₂ emissions are proved to be almost independent of the combustion media investigated.     

High temperature desulphurization by fine limestone during staged fluidized‐bed combustion

This paper reviews the SO₂ emission from a 0.3 m² stainless‐steel fluidized‐bed combustor. Fine coal was premixed with fine limestone and fed pneumatically under the bed. The SO₂ emission was found to depend largely on air staging ratio and bed temperature, which agrees with previous observations. The SO₂ emission observed in sorbent‐free tests (reported earlier by Khan and Cibbs, 1995) was found to be proportional to the sulphur content of the fuel when limestone was added, the sulphur capture at a fixed Ca/S molar ratio was dependent on oxygen stoichiometry and bed temperature. Finely sized limestone enhanced the effectivity of the sorbent at low bed temperature and air staging ratio. During staged combustion, the combustion efficiency depended largely on primary air to coal ratio. Around 90% combustion efficiency was observed at 1 m/s fluidizing velocity which was reduced when fluidizing velocity was increased to 1.5 and 2 m/s. This reduction is due to increased elutriation of finer coal particles from the combustor.     

O2/CO2气氛下煤燃烧SO2/NO析出特性

在水平管式炉上研究了O₂浓度、CO₂浓度、温度及石灰石添加等各参数对O₂/CO₂气氛下徐州烟煤和龙岩无烟煤燃烧过程中SO₂/NO排放特性的影响。结果发现,O₂/CO₂气氛下,烟煤和无烟煤燃烧SO₂/NO的析出规律与空气气氛下不同,同等O₂浓度下析出量比空气气氛下小。O₂/CO₂气氛下,随着O₂浓度的提高,烟煤和无烟煤SO₂/NO排放量均增大;随着CO₂浓度的升高, SO₂/NO排放量均减小。O₂/CO₂气氛下,石灰石添加对SO₂排放的抑制作用低于空气气氛下;石灰石添加对NO的排放有一定减排作用。对煤灰的元素分析显示O₂/CO₂燃烧对SO₂的抑制主要是由于煤灰的自固硫能力增强,而对NO的减排作用则是促进燃料N向其他含N气体的转换。     

Conversions of fuel-N,volatile-N,and char-N to NO and N2O during combustion of a single coal particle in O2/N2 and O2/H2O at low temperature

Oxy-steam combustion is a promising next-generation combustion technology. Conversions of fuel-N, volatile-N, and char-N to NO and N₂O during combustion of a single coal particle in O₂/N₂ and O₂/H₂O were studied in a tube reactor at low temperature. In O₂/N₂, NO reaches the maximum value in the devolatilization stage and N₂O reaches the maximum value in the char combustion stage. In O₂/H₂O, both NO and N₂O reach the maximum values in the char combustion stage. The total conversion ratios of fuel-N to NO and N₂O in O₂/N₂ are obviously higher than those in O₂/H₂O, due to the reduction of H₂O on NO and N₂O. Temperature changes the trade-off between NO and N₂O. In O₂/N₂ and O₂/H₂O, the conversion ratios of fuel-N, volatile-N, and char-N to NO increase with increasing temperature, and those to N₂O show the opposite trends. The conversion ratios of fuel-N, volatile-N, and char-N to NO reach the maximum values at < O₂ > = 30 vol% in O₂/N₂. In O₂/H₂O, the conversion ratios of fuel-N and char-N to NO reach the maximum values at < O₂ > = 30 vol%, and the conversion ratio of volatile-N to NO shows a slightly increasing trend with increasing oxygen concentration. The conversion ratios of fuel-N, volatile-N, and char-N to N₂O decrease with increasing oxygen concentration in both atmospheres. A higher coal rank has higher conversion ratios of fuel-N to NO and N₂O. Anthracite coal exhibits the highest conversion ratios of fuel-N, volatile-N, and char-N to NO and N₂O in both atmospheres. This work is to develop efficient ways to understand and control NO and N₂O emissions for a clean and sustainable atmosphere.     

O2/CO2气氛下煤燃烧SO2/NO析出特性

在水平管式炉上研究了O₂浓度、CO₂浓度、温度及石灰石添加等各参数对O₂/CO₂气氛下徐州烟煤和龙岩无烟煤燃烧过程中SO₂/NO排放特性的影响。结果发现,O₂/CO₂气氛下,烟煤和无烟煤燃烧SO₂/NO的析出规律与空气气氛下不同,同等O₂浓度下析出量比空气气氛下小。O₂/CO₂气氛下,随着O₂浓度的提高,烟煤和无烟煤SO₂/NO排放量均增大;随着CO₂浓度的升高, SO₂/NO排放量均减小。O₂/CO₂气氛下,石灰石添加对SO₂排放的抑制作用低于空气气氛下;石灰石添加对NO的排放有一定减排作用。对煤灰的元素分析显示O₂/CO₂燃烧对SO₂的抑制主要是由于煤灰的自固硫能力增强,而对NO的减排作用则是促进燃料N向其他含N气体的转换。     

CCSEM analysis of ash from combustion of coal added with limestone

Combustion of three Chinese coals, mixed with limestone physically, was carried out in drop tube furnace. The drop tube furnace consisted of two parts, the top side has a length of about 1.0 m and kept at 1573 K in all the runs, while the bottom-side has a length of 0.5 m and kept at 1173 K. SO₂ removal efficiency of about 80 and 73% were obtained in the combustion of Yanzhou with high and low sulfur, respectively. In contrast, for Datong coal, the De-S efficiency was only about 50% at the molar Ca/S ratio of 2.0; increasing Ca/S ratio to 3.0 had little effect on De-S efficiency. The combustion ashes were analyzed by several techniques including XRD, SEM-EDX and CCSEM (computer-controlled SEM). A novel calcium-based phase definition, based on CCSEM data was developed to investigate the modes of occurrence of added limestone in the ashes. Additionally, the mixture of limestone with kaolinite was injected into the furnace to study their transformation behavior under simulated coal combustion conditions. The governing mechanisms for limestone capturing sulfur and its reaction with the inherited minerals were correspondingly revealed. It was found that under the given coal combustion conditions, the calcium distribution in the ash varied with coal type and residence time. Briefly, more calcium was used for desulfurization or fixed into mineral; as time progressed, the inherited aluminosilicate, small sized excluded particles in the coal matrix, facilitated its reaction with limestone; it also reacted quickly compared to sulfation of limestone in coal combustion. This in turn hampered the efficient utilization of limestone in coal combustion.     

Effects of operational parameters on emission performance and combustion efficiency in small-scale CFBCs

A well-designed CFBC can burn coal with high efficiency and within acceptable levels of gaseous emissions. In this theoretical study effects of operational parameters on combustion efficiency and the pollutants emitted have been estimated using a developed dynamic 2D (two-dimensional) model for CFBCs. Model simulations have been carried out to examine the effect of different operational parameters such as excess air and gas inlet pressure and coal particle size on bed temperature, the overall CO, NO_x and SO₂ emissions and combustion efficiency from a small-scale CFBC. It has been observed that increasing excess air ratio causes fluidized bed temperature decrease and CO emission increase. Coal particle size has more significant effect on CO emissions than the gas inlet pressure at the entrance to fluidized bed. Increasing excess air ratio leads to decreasing SO₂ and NO_x emissions. The gas inlet pressure at the entrance to fluidized bed has a more significant effect on NO_x emission than the coal particle size. Increasing excess air causes decreasing combustion efficiency. The gas inlet pressure has more pronounced effect on combustion efficiency than the coal particle size, particularly at higher excess air ratios. The developed model is also validated in terms of combustion efficiency with experimental literature data obtained from 300 kW laboratory scale test unit. The present theoretical study also confirms that CFB combustion allows clean and efficient combustion of coal.     

Combustion of olive cake and coal in a bubbling fluidized bed with secondary air injection

Combustion performances and emission characteristics of olive cake and coal are investigated in a bubbling fluidized bed. Flue gas concentrations of O₂, CO, SO₂, NO_x, and total hydrocarbons (C_mH_n) were measured during combustion experiments. Operational parameters (excess air ratio (λ), secondary air injection) were changed and variation of pollutant concentrations and combustion efficiency with these operational parameters were studied. The temperature profiles measured along the combustor column was found higher in the freeboard for olive cake than coal due to combustion of hydrocarbons mostly in the freeboard. Combustion efficiencies in the range of 83.6–90.1% were obtained for olive cake with λ of 1.12–2.30. For the setup used in this study, the optimum operating conditions with respect to NO_x and SO₂ emissions were found as 1.2 for λ, and 50 L/min for secondary air flowrate for the combustion of olive cake.     

Synthetic coal chars for the elucidation of NO heterogeneous reduction mechanisms

In the present work, the mechanisms involved in NO–char heterogeneous reduction have been studied using a synthetic coal char (SC char) as carbon source. Another synthetic char (SN char) without nitrogen in its composition has also been employed in these studies. Isothermal reduction tests at different temperatures have been carried out. Two temperature regimes were considered: low temperature (T < 250 °C) where NO chemisorption takes place and high temperature (T > 250 °C) where NO–C reaction occurs. Step response experiments combining consecutive reaction stages with NO and ¹⁵NO were performed in order to determine the role of nitrogen surface complexes, C(N), in the reduction process. The results revealed N₂ and CO₂ to be the main reduction products under the experimental conditions employed in this work. NO chemisorption at lower temperatures results in N₂ emission and surface complexes (mainly oxygenated) formation, while char gasification by NO involves a direct NO attack on the char surface to form surface complexes. As a consequence of desorption of these complexes new sites of reaction are created.     

The formation of N2O during the reduction of NO by NH3

The process of selective non-catalytic reduction of NO, SNCR, is important for limiting emissions of nitrogen oxides from coal-fired power plants. Such a process has been studied for many years, both in the laboratory and under practical conditions. This work was an attempt at elucidating some of the problems associated with the method when used under circulating fluidized bed (CFB) conditions and in particular, the formation of the N₂O by-product. The NO + NH₃ reaction has been studied in the laboratory, over quartz sand in a heated fixed bed flow reactor. In comparison with a combustion environment, the composition of the gas phase was drastically simplified and limited to NO and NH₃, in nitrogen as the carrier gas, with O2 added in some experiments. The product gases were analyzed for NO, N₂O and NH₃. The effects the following parameters were studied: temperature inside the reactor between 850 and 1250 K, height of the sand bed, NH₃/NO molar ratio over the range 0.54–2.0 and the addition of 1 or 2% of O₂ in volume. Baseline tests with an empty reactor were also made. With no sand in the reactor, the results were both qualitatively and quantitatively different. The sand helped to increase the efficiency of NO reduction, particularly at lower temperatures, but N₂O formation also appeared to be strongly enhanced, except at the highest temperatures. Higher molar NH₃/NO ratios favored NO reduction and N₂O production, both with and without sand. The reduction of NO did not appear to require the presence of O₂, but the introduction of 1% or 2% of O₂ gave some benefit. The results confirmed that under practical conditions more attention should be paid to the role of the bed solids in the SNCR process.     

Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle

This paper presents experimental results of a 20 kW vertical combustor equipped with a single pf-burner on pulverised coal combustion in air and O₂/CO₂ mixtures with NO_x recycle. Experimental results on combustion performance and NO_x emissions of seven international bituminous coals in air and in O₂/CO₂ mixtures confirm the previous findings of the authors that the O₂ concentration in the O₂/CO₂ mixture has to be 30% or higher to produce matching temperature profiles to those of coal-air combustion while coal combustion in 30% O₂/70% CO₂ leads to better coal burnout and less NO_x emissions than coal combustion in air. Experimental results with NO_x recycle reveal that the reduction of the recycled NO depends on the combustion media, combustion mode (staging or non-staging) and recycling location. Generally, more NO is reduced with coal combustion in 30% O₂/70% CO₂ than with coal combustion in air. Up to 88 and 92% reductions of the recycled NO can be achieved with coal combustion in air and in 30% O₂/70% CO₂ respectively. More NO is reduced with oxidant staging than without oxidant staging when NO is recycled through the burner. Much more NO is reduced when NO recycled through the burner (from 65 to 92%) than when NO is recycled through the staging tertiary oxidant ports (from 33 to 54%). The concentration of the recycled NO has little influence on the reduction efficiency of the recycled NO with both combustion media—air and 30% O₂/70% CO₂.     

Formation of submicron particulate matter (PM1) during coal combustion and influence of reaction temperature

Combustion of pulverized coals was conducted in a lab-scale drop tube furnace to investigate the formation of submicron particulate matters less than 1.0 μm in diameter (PM₁). Three temperatures, 1473, 1573 and 1723 K were tested. PM₁ was collected by a low-pressure-impactor, which automatically segregates particulates into seven sizes ranging from 0.03 to 0.76 μm. Each size was subjected to several techniques for elemental composition and morphology analysis. The results indicate that there are two major portions formed for PM₁: particulates smaller than 0.1 μm and those larger ones. The former portion, termed as PM_0.1 in this study, was formed by aggregation of elemental vapors generated by either decomposition of organically bound elements in coal pyrolysis or vaporization of inorganic compounds in char combustion. Sulfates and phosphates dominate this portion. On the other hand, the latter portion in PM₁, termed as PM_0.1+, was generated containing two kinds of typical particles: one is in a molten solid shape and rich in Al-silicates and another one in the fractal agglomerate form. Liberation of inherent fine minerals (≤1 μm) and aggregation of nucleates containing vaporized elements are the two major routes for their formation. PM_0.1+ was mainly formed in char combustion. Additionally, both coal type and reaction temperature affected the formation of PM₁ greatly. Combustion of bituminous coals produced more the PM₁ than the anthracite coal did. Increasing temperature produced more the PM₁ as well.     

Impact of air staging along furnace height on NOx emissions from pulverized coal combustion

Experiments were carried out on an electrically heated multi-path air inlet one-dimensional furnace to assess NO_x emission characteristics of an overall air-staged (also termed air staging along furnace height) combustion of bituminous coal. The impact of main parameters of overall air-staged combustion technology, including burnout air position, air stoichiometric ratio, levels of burnout air (the number of burnout air arranged at different heights of the furnace), and the ratios of the burnout air flow rates and pulverized coal fineness of industrial interest, on NO_x emission were simulated to study in the experimental furnace, as well as the impact of air staging on the carbon content of the fly ash produced. These results suggest that air-staged combustion affects a pronounced reduction in NO_x emissions from the combustion of bituminous coal. The more deeply the air is staged, the further the NO_x emission is reduced. Two-level air staging yields a greater reduction in NO_x emission than single-level air staging. For pulverized coal of differing fineness, the best ratio between the burnout air rates in the two-level staging ranges from 0.6 to 0.3. In middle air-staged degree combustion with f_M = 0.75, pulverized coal fineness, R₉₀ (%), has a greater influence on NO_x emission, whereas R₉₀ has little influence on NO_x emission for deep air-staged degree with f_M = 0.61. Air-staged combustion with proper burnout air position has little effect on the burnout. For overall air-staged combustion, proper burnout air position and air-staged rate should be considered together in order to achieve high combustion efficiency.     

Effects of operation parameters on NO emission in an oxy-fired CFB combustor

Oxy-fuel Circulating Fluidized Bed (CFB) combustion technology, a very promising technology for CO₂ capture, combines many advantages of oxy-fuel and CFB technologies. Experiments were carried out in a 50 kW_th CFB facility to investigate how operation parameters influence the NO emission in O₂/CO₂ atmospheres. The simulated O₂/CO₂ atmospheres were used without recycling the flue gas. Results show that NO emission in 21% O₂/79% CO₂ atmosphere is lower than that in air atmosphere because of lower temperature and higher char and CO concentrations in the dense bed. Elevating O₂ concentration from 21% to 40% in O₂/CO₂ atmosphere enhances fuel-N conversion to NO. Increasing bed temperature or oxygen/fuel stoichiometric ratio brings higher NO emission in O₂/CO₂ atmosphere, which is consistent with the results in air-fired CFB combustion. As primary stream fraction increases, NO emission increases more rapidly in O₂/CO₂ atmosphere than that in air atmosphere. Stream staging is more efficient for controlling NO emission in oxy-CFB combustion than that in air combustion. Oxygen staging provides an efficient way to reduce NO emission in oxy-CFB combustion without influencing the hydrodynamic characteristic in the riser.     

NO emission on pulverized coal combustion in high temperature air from circulating fluidized bed – An experimental study

High temperature air was adopted by combustion in high excess air ratio in a circulating fluidized bed. Experiments on pulverized coal combustion in high temperature air from the circulating fluidized bed were carried out in a down-fired combustor with the diameter of 220 mm and the height of 3000 mm. The NO emission decreases with increasing the residence time of pulverized coal in the reducing zone, and the NO emission increases with excess air ratio, furnace temperature, coal mean size and oxygen concentration in high temperature air. The results also revealed that the co-existing of air-staging combustion with high temperature air is very effective to reduce nitrogen oxide emission for pulverized coal combustion in the down-fired combustor.     

Preparation and characterisation of calcined Mg/Al hydrotalcites impregnated with alkaline nitrate and their activities in the combustion of particulate matter

The effect of incorporating alkaline nitrates in hydrotalcites for use in the combustion of particulate matter from diesel emissions has been studied. The catalysts were characterised by X-ray diffraction (XRD), N₂ adsorption, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), elemental analysis (EA), atomic absorption spectrophotometry (AAS) temperature programmed reduction (TPR) and Fourier transform infrared spectroscopy (FTIR). Activity measurements were carried out using a thermobalance in air and using a fixed-bed reactor with a NO/O₂ flow. The observed activities decreased in the following order: HTMgAlcCs > HTMgAlcK > HTMgAlcLi > HTMgAlc.     

The influences of coal type on in-bed desulphurization in a PFBC demonstration plant

Desulphurization features of Blair Athol (BA), Witt Bank (WB), Nang Tong (NT) and Drayton (DT) coals were investigated in a 71 MW demonstration plant PFBC boiler operated at bed temperature of around 850 °C, Ca/S molar ratios in feeds of above three, partial pressure of CO₂ at combustion boiler outlet of around 0.9 kg/cm² and boiler load of 50%, to clarify the influences of coal type on the in-bed desulphurization achievement. Bed materials (BM) and fly ashes (FA) were characterized to describe their roles in the desulphurization. SO₂ emission in the stack was found 3, 2, 41 and 27 ppm in the respective combustion of BA, WB, NT and DT. The desulphurization efficiencies were determined by their Ca/S molar ratio of the fine sorbent formed by attrition. The ratios were 1.51, 2.98, 0.79, and 0.67 for BA, WB, NT and DT, respectively. Calcination in the bed increased the attrition rate of the sorbent to allow a better desulphurization in PFBC. However, the very high attrition rate yielded a large amount of unreacted sorbent which was entrained out from the bed, lowering the calcium utilization efficiency.