Control methods for remediation of ash-related problems in fluidised-bed combustors

Investigations into control methodologies for mitigating ash-related problems such as particle agglomeration and bed defluidisation during fluidised-bed combustion of low-rank coals are conducted using a laboratory scale spouted bed combustor. Two control methods are investigated viz., the use of alternative bed materials and pretreatment of coal. Bauxite and calcined sillimanite are used as alternative bed materials in the spouted bed combustor while burning a South Australian low-rank coal. Samples of the same coal subjected to Al pretreatment, water washing and acid washing are also tested. Experimental results indicated that both methods are effective to different extents in reducing particle agglomeration and bed defluidisation. Tests with calcined sillimanite and bauxite as bed materials showed no agglomeration for longer periods than with sand runs at the same bed temperatures. Al pretreatment and water-washing were also found to be effective and resulted in extended combustion operation. Al enrichment in ash coating on bed particles has been identified as the key mechanism for prevention of agglomeration and defluidisation by these control methodologies. For water-washing, the principal reason behind agglomeration and defluidisation control is the reduction in sodium levels.     

Effect of coal blending on particle agglomeration and defluidisation during spouted-bed combustion of low-rank coals

The possibility of blending coals to alleviate particle agglomeration and bed defluidisation during fluidised-bed combustion (FBC) of several low-rank coals was exploited. A laboratory scale spouted bed combustor was employed to fire coal blends from two lignites with a sub-bituminous coal at ratios of 50:50 and 90:10, at temperatures ranging 800°C. Experiments showed significant improvements in FBC operation with the coal blends compared to the raw lignites. No particle agglomeration and bed defluidisation were evident after 15 h of operation with the blends at 800°C. Chemical analyses indicated that the formation of low temperature eutectics was suppressed by calcium aluminosilicate phases from the sub-bituminous coal, rendering the surface of ash-coated particles dry and less sticky. This was identified as the key mechanism for the control of particle agglomeration and bed defluidisation in FBC, which led to extended combustion operation with the coal blends.     

Effect of Ca- and Mg-bearing minerals on particle agglomeration defluidisation during fluidised-bed combustion of a South Australian lignite

Behaviour of calcium and magnesium during fluidised-bed combustion (FBC) of a South Australian lignite was investigated using a laboratory scale spouted bed combustion system. Combustion experiments were aimed at investigating the effectiveness of Ca- and Mg-bearing minerals (as alternative bed materials) in controlling particle agglomeration and bed defluidisation during FBC combustion of low-rank coals. Additional experiments performed with a Ca-treated coal investigated the role of Ca in agglomeration and defluidisation process. Experimental results indicated that both Ca/Mg-bearing minerals and Ca-treated coal were effective to different extents in reducing bed defluidisation. Tests with calcite (as the bed material) and Ca-treated coal runs (with sand as the bed material) showed trouble free operation for 8–10 h before bed defluidisation incurred. Tests with magnesite (as the bed material) showed no agglomeration and defluidisation tendencies for longer operating periods (∼12 h at 800°C). Mg-bearing compounds have been found to be effective in controlling defluidisation and allowed extended combustion operations. On the other hand, high levels of Ca either in coal or in bed material have been found to delay and decrease the severity of agglomerates formed.     

Multilateral approaches for investigation of particle stickiness of coal ash at low temperature fouling conditions

Particle stickiness is a key parameter for increasing ash deposition in gasification process. We conducted multilateral investigations to evaluate particle stickiness of coal ash at low temperature fouling conditions through Watt and Fereday’s viscosity model, dilatometry (DIL) and laser flash apparatus (LFA) technique. Seventeen coals were employed for ash deposition experiments under gasification condition through drop tube furnace (DTF). The low viscosity not only led to increasing ash deposition behavior, but also increasing the particle size of deposited ash. From DIL analysis, the ash sintering behavior increased with increasing temperature due to increase of particle stickiness. The high amount of Fe₂O₃, CaO and MgO components resulted in low sintering temperature and high reduction of physical length. Through LFA analysis, the thermal conductivity increased with increasing temperature, because of increasing particle stickiness. In addition, its value was correlated with the propensity of common fouling indices.     

An advanced ash fusion study on the melting behaviour of coal,oil shale and blends under gasification conditions using picture analysis andgraphing method

This study investigates the potential of solid fuel blending as an effective approach to manipulate ash melting behaviour to alleviate ash-related problems during gasification, thus improving design, operabil-ity and safety. The ash fusion characteristics of Qinghai bituminous coal together with Fushun, Xinghua and Laoheishan oil shales (and their respective blends) were quantified using a novel picture analysis and graphing method, which incorporates conventional ash fusion study, dilatometry and sintering strength test, in a CO/CO2 atmosphere. This image-based characterisation method was used to monitor and quan-tify the complete melting behaviour of ash samples from room temperature to 1520 ℃. The impacts of blending on compositional changes during heating were determined experimentally via X-ray diffraction and validated computationally using FactSage. Results showed that the melting point of Qinghai coal ash to be the lowest at 1116 ℃, but would increase up to 1208 ℃, 1161 ℃ and 1160 ℃ with the addition of 30％–50％ of Laoheishan, Fushun, and Xinghua oil shales, respectively. The formation of high-melting anorthite and mullite structures inhibits the formation of low-melting hercynite. However, the sintering point of Qinghai coal ash was seen to decrease from 1005 ℃ to 855 ℃, 834 ℃, and 819 ℃ in the same blends due to the formation of low-melting aluminosilicate. Results also showed that blending directly influences the sintering strength during the various stages of melting. The key finding from this study is that it is possible to mitigate against the severe ash slagging and fouling issue arising from high calcium and iron coals by co-gasification with a high silica-alumina oil shale. Moreover, blending coals with oil shales can also modify the ash melting behaviour of fuels to create the optimal ash chemistry that meets the design specification of the gasifier, without adversely affecting thermal performance.     

Agglomeration in fluidised bed gasification of biomass

Three promising biomass fuels for southern Mediterranean regions were tested for their agglomeration tendency in an atmospheric lab-scale fluidised bed (FB) gasifier using quartz and olivine as bed materials. The defluidisation temperatures of the energy crops Giant Reed (Arundo donax L.) and Sweet Sorghum bagasse were respectively approx. 790 °C and 810 °C, in both bed materials, while the agro industrial residue olive bagasse caused defluidisation of the quartz bed at 830 °C and olivine bed at > 850 °C. Agglomerates from these tests were analysed with SEM/EDS. Coatings and necks between bed particles were formed due to ash derived potassium silicate melt. For the first two fuels cluster-type agglomerates around remains of char particles were observed. Thermodynamic equilibrium simulations of each chemical system were performed to cross examine the predicted ash melting temperatures and chemistry with experimental findings. Predictions of potassium liquid compounds, like K₂O·SiO_2(l) were verified by EDS analyses on the particle coatings. FB gasification of olive bagasse resisted defluidisation up to higher temperatures because of its lower potassium and higher calcium content, especially in the case of olivine bed. The latter experimental finding coincided with thermodynamic predictions.     

Reactivities of Blair Athol and Nang Tong coals in relation to their behavior in PFBC boiler

The combustion reactivities of Blair Athol (BA) and Nang Tong (NT) coals were measured by thermogravimetry to understand their different behaviors in the PFBC boiler. The reactivity of BA was much the same as that of NT coal but their chars showed different characteristics. BA char of higher surface area (25 m²/g) showed slightly higher reactivity than that of NT char of smaller surface area (7 m²/g). BA coal showed heterogeneous ignition even at its particle size of as large as +355 μm while NT coal showed homogeneous ignition at the average particle size over 75 μm heterogenous one occurring with finer particle size (−75 μm). Higher calorific value of BA volatile matter and higher reactivity of its char than those of NT coal causes of its heterogeneous ignition with an intense DTA peak, which may lead to local heating at its combustion and to yield reactive CaO from limestones causing of bed materials agglomeration in the PFBC.     

AGGLOMERATION BEHAVIOR IN A BUBBLING FLUIDIZED BED AT HIGH TEMPERATURE

Minimum fluidization velocity and agglomeration behavior were investigated at high temperature in an 80?×?30?mm two-dimensional quartz fluidized bed and in an 82?mm i.d. circular fluidized bed. Bed materials tested were two sizes of glass beads as well as three sizes of fluidized bed combustor (FBC) ash. The minimum fluidization velocity decreased with increasing bed temperature, whereas the minimum sintering fluidization velocity increased with the bed temperature. The sintering of glass beads belongs to visco plastic sintering, the first type. FBC ash agglomerate has higher amounts of SiO₂, Al₂O₃, Na₂O, K₂O, and SiO₂ than in the original ash, indicating that low melting eutectics were formed and that the liquid phase in a silicate system was formed. The agglomeration of FBC ash belongs to the second type, an excessive quantity of liquid being formed by melting or chemical reaction.     

A new rheometer for direct measurement of the flow properties of coal ash at high temperatures

Development of more effective technologies of utilising low rank coals for power generation has been driven by a demand for higher efficiency, low capital costs and minimal environmental impacts. Fluidised bed systems are regarded as one of the more promising alternative technologies for power generation to overcome the disadvantage of the existing pulverised coal burning power generation plants for low rank coals. However, ash deposition and bed agglomeration are potential problems in fluidised bed processing of coals with high alkali and sulphur contents. In order to gain a better understanding of the mechanism of agglomeration in fluidised beds, a good knowledge of the rheological behaviour of coal ash deposits at high temperatures and under the processing conditions is necessary. Rheological characterisation of materials at high temperatures is difficult due to lack of standard instruments and reliable measurement techniques. We have recently developed a rheometer that has the capability of measuring the rheological properties of coal ash slag over a wide range of temperatures from 600 to 1300°C and under different processing atmospheres. In this paper the features of this unique instrument are described and the experimental technique developed for flow property measurement is outlined. Some typical measured rheological properties of coal ashes from different Australian low-rank coals are presented and discussed to illustrate the potential applicability of the rheometer for high-temperature rheological characterisation. Furthermore, by means of the experimental results obtained it is demonstrated that the alkali content of the coal ash plays a significant role in controlling the rheological characteristics of the ash deposit, which in turn has an important implication on agglomeration in fluidised bed combustion processes.     

灰熔聚流化床粉煤气化技术

灰熔聚流化床粉煤气化技术是８０年代以来开发研究的。该技术是在流化床用氧化剂形成局部高温的办法来提高碳转化率，因而拓宽了煤种适应性，使灰渣在高温下团聚成又大又重的粒子而减少了排灰碳损失。     

Studies on the combustion behaviour of blends of Indian coals by TGA and Drop Tube Furnace

Combustion behaviour of blends of two Indian coals of same rank with wide variation in mineral matter content were studied using Thermogravimetric Analyzer (TGA) and Drop Tube Furnace (DTF). The characteristic TGA parameters determined from the burning profiles showed both additive and non-additive behaviour. The burnout temperature and peak temperature showed a linearly decreasing trend with the increasing proportion of the high ash coal. Deviation from the linear trend was observed in the case of the reactivity parameter. The high ash coal showed better TGA reactivity than the low ash coal. The higher TGA reactivity could arise from the combined effect of mineral matter and the nature and distribution of the macerals, particularly those of the inertinite group.The burnout in DTF showed a nonlinear effect. The burnout behaviour of the coals and their blend observed in DTF was not similar to that reflected in TGA. Coal blends with less than 50% of high ash coal showed better burnout than the individual coals.     

Agglomeration of coal ash in fluidized beds

The defluidization behaviour of ash derived from Indian coal by combustion in a fluidized bed has been studied. Sintering temperatures for ash in several ranges of particle size were measured with a dilatometer. In agreement with the earlier work on other coals it was found that above the sintering temperature pairs of complementary, limiting values of fluidization velocity and bed temperatures exist which mark the onset of defluidization when the ash particles are heated in a fluidized bed. A linear relation was observed between bad temperature and limiting defluidization velocity. The constants in the corresponding equations were calculated for two size ranges of particles.     

ALKALI-INDUCED AGGLOMERATION OF SOLID PARTICLES IN COAL COMBUSTORS AND GASIFIERS

The effect of alkali adsorption on the agglomeration of particles of bauxite, kaolinite, emathlite, lime and two types of coal ash is studied. An agglomeration (adhesion) temperature is defined which characterizes the adhesion propensity of particles. An experimental technique is developed to measure this agglomeration point in situ and without sample quenching or removal. The effect of alkali adsorption on the agglomeration characteristics of the substrates is determined. The agglomeration temperature for all substrates decreases a:, alkali content increases. At low loadings the alkali adsorption enhances particle agglomeration by forming new compounds of lower melting points. At high concentrations, the agglomeration point of all substrates approaches the melting point of condensing alkali. Under these conditions, adhesion and agglomeration are caused by a layer of molten alkali which covers the exterior of the particles.     

A study of agglomeration of coal-ash in fluidized beds

Experimental data on agglomeration of coal-ash particles in a fluidized bed have been presented. It has been observed that above the “initial sintering temperature”, the ash particles are defluidized at velocities above their minimum fluidization velocity. The expression for minimum fluidization velocity has been modified by including a force-term due to the phenomenon of sintering in addition to those due to drag, gravity and kinetic energy.     

DSC法测定煤的比热

本文采用差示扫描量热仪（ＤＳＣ）对１４种含有不同挥发分的中国煤，在２０～１７０℃范围内测定了煤的比热，并校正为干燥无灰基煤的比热。结果表明，煤的比热随温度的变化和挥发分含量的不同而变化，依此建立了煤的比热和温度及挥发分含量的数学关联式。用此模型可以估计中国煤在２０～１７０℃范围内比热的值，其误差小于１０％。     

Fouling propensities of blended coals in pulverized coal-fired power station boilers

This paper presents the experimental results on the fouling propensity of five pairs of blended coals (19 coals and blends) tested in the Australian Coal Industry Research Laboratory (ACIRL) furnace. The results showed that the coal D has the highest fouling propensity among tested coals and blends. A parameter, growth rate (mm/h), is used to numerically rank the fouling propensity based on photos of fouling deposits taken over a period of test time. The growth rate correlates the fouling propensity better than the build up rate or the fouling coefficient. Five empirical fouling indices are examined against the fouling propensities of the above 19 coals and blends, and another 10 coals and blends previously tested in the Energy and Environmental Research Cooperation (EER) furnace. The linear correlation between the flue gas exit temperature/initial deformation temperature (FGET/IDT(ox.)), a measure of the overall heat transfer in the furnace, and the Na₂O, g/GJ, is proved to be a good tool for predicting the fouling propensity of coals. There is also a relationship between the FGET and the growth rate.     

Ash deposition characteristics of Moolarben coal and its blends during coal combustion

We report a systematic and comprehensive laboratory investigation of the ash deposition behavior of Moolarben (MO) coal, which has recently begun to be imported into Korea. Ash deposition experiments were conducted in a drop tube reactor, and a water-cooled ash deposit probe was inserted into the reactor to affix the ash. The tests were conducted using five types of single coals (two bituminous and three sub-bituminous, including MO coal) and blended coals (bituminous coal blended with sub-bituminous coal). Two indices represent ash deposition behavior: capture efficiency and energy-based growth rate. A thermomechanical analysis evaluated the melting behavior of the resulting ash deposits. The MO coal had the least ash deposition of the single coals due to its high melting temperature, indicated by high ash silica content. Indonesian sub-bituminous coals formed larger ash deposits and were sticky at low temperatures due to relatively high alkali content. However, blends with MO coal had greater ash deposition than blends with other bituminous coals. This non-additive behavior of MO coal blends is likely due to interactions between ash particles. Coals with higher silica content more effectively retain alkali species, resulting in lower melting temperatures and larger ash deposits. Therefore, we recommend that when blending in a boiler, silica-rich coals (SiO₂>80%, SiO₂/Al₂O₃> 5) should be blended with relatively low-alkali coals (Na₂O+K₂O<3%), and the blending ratio of the silica-rich coals indicates less than 10%, which can safely operate the boiler.     

Combustion in bubbling fluidised bed with bed material of limestone to reduce the biomass ash agglomeration and sintering

A bed material of limestone was used in order to reduce/eliminate the tendency for bed material agglomeration and sintering that normally occurs in plants that operate with the traditional silica bed material. Combustion tests were carried out in a bubbling fluidised bed (BFB) combustion pilot plant (1 MWth). Mass balances of the inorganic elements and ash characterisation with respect to bed agglomeration, fouling and emissions were performed in the BFB combustion pilot plant.Limestone bed material with particle sizes between 0.25 and 2 mm, corresponding to a mean fluidisation velocity of 1.2 m/s and at a mean bed temperature of 775 °C, were chosen. It has been successfully proven that the limestone bed material eliminates the bed agglomeration. The calcium particles, which escape from the limestone bed material and are adhered on heat exchangers, reduce the sintering of ash deposits on the tubes.     

The effect of halloysite additive on operation of boilers firing agricultural biomass

The paper presents results of investigations on using halloysite as an additive in biomass-fired boilers. It has been shown that in the case of a few different agricultural biomasses the halloysite addition increased the ash sintering temperature to the values noted for coals. This is an effect of bonding sodium and potassium in the form of chlorides and other compounds. In practical terms the halloysite additive may reduce slagging and fouling of boiler heating surfaces as well as deteriorate the agglomeration processes in fluidized beds. Moreover, addition of halloysite reduces the amount of KCl and NaCl present in ash (and therefore in ash deposits) thus decreasing the rate of high temperature corrosion.     

Study on the ash fusion temperatures of coal and sewage sludge mixtures

The coal, sewage sludge, water and chemical additives are milled to produce coal-sludge slurry as a substitute for coal-water slurry in entrained-flow gasification, co-gasification of coal and sewages sludge can be achieved. The ash fusion temperature is an important factor on the entrained-flow gasifier operation. In this study, the ash fusion temperatures (DT, ST, HT and FT) of three kinds of coals (A, B and C), two kinds of sewage sludges (W1 and W2) and series of coal-sewage blends were determined, and the mineral composition during the ash melting process was analyzed by X-ray diffraction (XRD). The results showed that the ash fusion temperatures of most coal-sewage blends are lower than those of the coals and sewage sludges. The ashes have different mineral composition at different temperature during the heating process. It was found that the mineral composition of AW1 blend ash is located in the low-temperature eutectic region of the ternary phase diagram of SiO₂-Al₂O₃-CaO. The minerals found in BW1 blend ash are almost the same as those in B coal ash. Kyanite is detected in CW1 blend ash, which results in the ash fusion temperatures of CW1 blend ash higher than those of C coal. We found that sodium mineral matters are formed because of NaOH added to W2, which can reduce the ash fusion temperature of coal-sewage blends.