Search

In this paper, experiments were carried out to obtain the actual operation characteristics of external heat exchangers (EHEs) in the first 600 MW supercritical CFB boiler. The furnace temperatures and some operational parameters in EHEs were measured and the heat transfer coefficients of heating surfaces in EHEs were calculated. The results showed that by controlling the solid flow rates to the EHEs, the furnace temperature were adjusted for good performance at various boiler loads and the bed temperature were maintained stable at full load. The heat transfer coefficients in EHEs were mostly over 200 W/(m 2 ∙ K), which contributed to the flexible temperature regulation. The tube panel wall temperatures were found to be a saddle-shaped distribution, which was high in the bed center but low near the wall. Packet renewal theory was used to explain such uneven temperature distribution. These results confirm the importance of EHEs to the furnace temperature control of the supercritical CFB boilers, but the uneven tube wall temperatures need to be well solved in the future. [ABSTRACT FROM AUTHOR]

The NO x emission of circulating fluidized bed (CFB) boilers in China is more and more stringently controlled due to the updated national emission regulation (NER). Based upon the theory of CFB combustion and previous research results related to NO x formation mechanism, a technical route was proposed by means of re-specifying the fluidization state in CFB boilers, to achieve the ultra-low NO x emission without installing the deNO x equipment additionally. The main method is to increase the solid circulating rate and decrease the bed inventory as well as the mean size of bed material, which would influence the bed temperature and oxidizing/reducing atmosphere distribution, and thereby benefit for the reduction of NO x emission. Besides, the reasonable control of bed temperature and air supply should be considered. This technique has been demonstrated to be effective in three CFB units with different capacities (150 t·h − 1 , 260 t·h − 1 and 560 t·h − 1 ) and burning different types of coal. No obvious decrease of combustion efficiency was found in the field tests. The technical route provides a feasible approach to further control the NO x emission for CFB boilers. [ABSTRACT FROM AUTHOR]

Abstract It is difficult to maintain enough high circulating ash rate in circulating fluidized bed (CFB) boilers if they burn the Indonesian coals due to their low ash content. Hence, the ash formation data for these coals is of significance to assessing mass balance in CFB boilers. Two kinds of Indonesian coal that were burned in the 550 MW supercritical CFB boilers in Korea, Kideco and KCH, were tested with static combustion and cold sieving method to study their ash formation characteristics. Results showed that the average primary ash particle size of these two coals is quite fine overall and nearly no gangue exists. Besides, their attrition rate constant K af is much higher than that of some other bituminous or lean coals, and with the increase of particle size, the K af decays exponentially for Kideco, while it initially increases and then decreases for KCH. In addition, the physical and chemical properties of these two Indonesian coals' ash obviously change in particle size, including density, pore structure, chemical composition, etc. The larger ash particles are composed of some hard materials, such as quartz (SiO 2) and mullite (Al 6 Si 2 O 13). While some other materials with lower hardness value, like hematite (Fe 2 O 3) and gehlenite (Ca 2 Al 2 SiO 7), become major mineral components of smaller and softer ash particles. These results indicate that the difference of chemical compositions results in the discrepancy of attrition capability for different size ash particles. Graphical abstract The ash attrition rate of low-rank coals including these two Indonesian coals is much higher than that of some other bituminous or lean coals, and larger ash particles are more difficult to be worn. Unlabelled Image Highlights • Two kinds of Indonesian coal were tested to obtain their ash formation data. • The ash properties obviously change in particle size. • The difference of chemical compositions may result in the discrepancy of attrition rate for different size ash particles. [ABSTRACT FROM AUTHOR]

Abstract Circulating fluidized bed (CFB) boilers face sever challenges in China due to the new ultra-low emission rules, more specifically, the NOx emission must be lower than 50 mg/m3. In this paper, NOx evolution inside CFB furnaces and the gas-solid flow characteristics were reviewed. It is found that NOx emission can be reduced through re-specification of fluidization state, namely, improving the bed quality. A low NOx emission (de-NOx) technology applied to CFB boilers was thus proposed. The keys of the technology are decreasing the average size of bed materials, limestone included, and increasing the solid circulating rate by improving the performance of circulating loop including improving cyclone efficiency, controlling slag discharge, etc. Some industrial practices showed that when the average size of bed inventories is markedly reduced, and when using ultrafine limestone, NO x emission can be significantly decreased without undermining high efficiency desulfurization. This paper provides a new solution to low-cost pollution control of CFB boilers. Highlights • NOx emission can be reduced through re-specification of fluidization state, namely, improving the bed quality. • Using ultrafine limestone can relieve the contradiction between high efficiency desulfurization and de-NOx combustion. • A low NOx emission technology applied to CFB boilers was proposed. • Some industrial practices showed that the ultra-low NO x emission can be achieved by applying this technology. [ABSTRACT FROM AUTHOR]

In this paper, the design principle of Fluidization State Specification (FSS) is reviewed and its applications in CFB boiler are introduced. In the early 2000s, FSS design principle was proposed to determine the fluidization state in the upper furnace as the basis of engineering design. The principle was initially adopted by engineers to select a proper superficial gas velocity to prevent the severe erosion on water walls. It was used in many CFB furnace designs including the world's largest 600 MW supercritical one in China. Later, FSS design principle was extended to reduce the pressure head of the draft fan to save energy consumption. Industrial application showed that by changing the amount of bed inventory, CFB boilers could operate at a re-constructed fast bed state, not only saving ~30% power of the draft fans, but also further reducing the erosion on water walls and improving combustion efficiency. Recently, FSS design principle was further improved and applied to achieve ultra-low NOx and SO 2 emission. A third coordinate with improved bed quality, i.e., size distribution of bed material was proposed to add on the original two-dimensional fluidization state diagram defined by the superficial gas velocity and the solid circulation rate. Industrial practices showed that under the guideline of the improved FSS design principle, it was possible for CFB boilers to reach over 99% desulfurization efficiency with limestone injected into furnace, and less than 50 mg/Nm 3 (at 6% O 2 ) NOx emission at the furnace exit. FSS design principle promotes the booming of ultra-low emission CFB boilers in China. [ABSTRACT FROM AUTHOR]

A set of comprehensive on-site experiments was conducted to assess the performance of the louver-type primary air (PA) concentrators on anthracite ignition and combustion in a 600 MW supercritical arch-fired boiler (AFB) equipped with slot burners. The temperature profiles along the PA stream from fuel-rich and fuel-lean streams were measured under various conditions. The separation efficiency of the PA concentrators of two selected burners was also measured and its effect on the ignition of the PA streams was studied. Experimental results revealed that separation performance of some louver concentrators was even opposite to design setting. The ignitability of PA stream, however, was more profoundly depended on the configuration of burner, the position of the nozzle and the SA distribution than the separation performance of the louver concentrators. On the other hand, confirmed by numerical simulation, the unsatisfied separation performance of the louver-type concentrators could result in overall poor ignition of PA, causing over-shooting of the flame penetration length, and high loss of ignition and difficulties for combustion organization. Some suggestions are given to the retrofit and new design of combustion system. [ABSTRACT FROM AUTHOR]

Co -firing coal slime in circulating fluidized bed (CFB) boilers is an effective method for utilizing coal slime, as industrial solid waste, with high water content, high viscosity, high ash content and low calorific value. In order to study the morphology and drying characteristics of coal slime dough injected into the furnace, this paper studied in detail the influence of compression degree and water content on the tensile force of coal slime dough. The results show that the degree of compression is positively correlated with the tensile force of coal slime dough, while, the water content is negatively correlated. A one-dimensional quadratic mathematical relationship between the tensile force of coal slime dough and the water content is established. At different feeding locations, i.e., at the top or middle of the boiler furnace, the breaking length of the coal slime dough was predicted. With top feeding, the length is 0.17 m, and that value for the middle feeding is about 0.15 m. Using the coupling model of the movement and drying of coal slime dough inside the furnace of the CFB boiler, the temperature change of the slime dough was calculated. The results showed that the surface temperature of coal slime dough rose rapidly during the drying process, then decreased slowly before it dived into the dense bed. When the furnace temperature is 900 °C and furnace height is about 38 m, the temperature of the slime surface can reach 200 °C at the highest. • Complete the morphology and drying characteristics of coal slime dough ejected into the furnace. • Propose a one-dimensional quadratic mathematical relationship between the tensile force of coal slime dough and the water content. • Study detailed the influence of compression degree and water content on the tensile force of coal slime dough. • Calculate the surface temperature of coal slime dough change of the slime dough. [ABSTRACT FROM AUTHOR]

The attrition characteristics of limestone have an important influence on the material balance and the efficiency of desulfurization in circulating fluidized bed boiler. By means of XRD, XRF, SEM and BET analysis, the attrition behaviour and morphology of two kinds of limestone with different particle sizes were studied by investigating the effects of simultaneous reaction of sulfation and calcination, sulfation reaction, oxygen-enriched flue gas atmosphere. The variation trend of attrition rate constant under simultaneous reaction is similar to that under only sulfation condition. Both of these conditions decrease with time and then reach a stable value. The attrition rate of finer limestone is higher than that of coarser limestone in a certain size range. The attrition rate of limestone in air atmosphere is the largest, while, in SO 2 atmosphere it's far less. This is mainly caused by the formation of CaSO 4 layer that reduces the attrition rate. By calculating the conversion of Ca to CaSO 4 , it is found that the conversion increases with time, but the increasing rate decreases gradually. The increase of SO 2 concentration leads to the increase of final conversion. In the oxygen-enriched flue gas atmosphere, the limestone is more attrition-resistant than that in the air. • The attrition characteristics of limestone particles was measured. • SEM analysis was used to observe the surface morphology. • The effect of simultaneous reaction was studied. • The effect of sulfation reaction was observed. • The effect of oxygen-enriched flue gas atmosphere was studied. [ABSTRACT FROM AUTHOR]

Compared to a pulverized coal boiler, a circulating fluidized bed (CFB) boiler may face greater risk if a sudden power failure accident happens due to the massive bed materials in its main loop. In this study, a numerical simulation is conducted to analyze the heat transfer process of the stationary bed materials and estimate the bed temperature based on a real sudden power failure accident that happened in a 170 t/h CFB boiler. The simulation results are consistent well with the measurement data of three measuring points, which indicates that the simulation results are reliable. The simulation results show that the stationary bed materials continuously dissipate heat but the cooling trends in different areas of the bed materials are different. The average bed temperature is estimated to reach 729.2 °C at the end time of the simulation. It is also found that the thermal conductivity of the bed material is the major influencing factor of the bed temperature and heat dissipation process after a sudden power failure, while the effect of the emissivity of the bed material on the bed temperature is slight. • The heat transfer process of a CFB boiler after a sudden power failure is analyzed. • The heat dissipation of stationary bed materials is numerically simulated. • The reliability of the simulation is proved by real accident data. • The emissivity of the bed material affects slightly on the bed temperature. • The thermal conductivity of the bed material is the major influencing factor. [ABSTRACT FROM AUTHOR]

Five types of available high-volatile fuels from different origins were burnt under a variety of operating parameters in bubbling bed to explore the emission characteristics of N 2 O and NO X. The main operating parameters studied were bed temperature, excess air ratio, fuel particle size, and bed material size. It was found that the N 2 O emission decreased as the temperature increased while the NO X emission was incompletely the same. Both the N 2 O and the NO X emissions increased with the increasing excess air ratio. Two series of fuel particle size experiments showed the opposite results, indicating that the influence of particle size on the N 2 O emission depended on fuel types. The N 2 O emission decreased and the NO X emission also increased with the increasing bed material size. After devolatilization, N 2 O and NO X emissions decreased and devolatilization had a greater impact on N 2 O. Moreover, devolatilization had a greater impact on N 2 O, which could obviously reduce N 2 O and NO X emissions. • Fuel can be continuously fed into the combustor at 1 g/min in a stable state. • Five types of high-volatile fuels from different countries with diverse properties were studied. • The experiments were carried out in the bubbling bed condition. • The main operating parameters of N 2 O and NO X were discussed. [ABSTRACT FROM AUTHOR]

The thermal effects of five coal types' pyrolysis processes at heating rates of 5, 10, 15, 20, and 30 °C/min in the temperature range of 30–1000 °C were systemically investigated by thermogravimetric - differential scanning calorimetry. The experimental results show that coal pyrolysis process can be roughly divided into four stages, namely the drying and dehydration stage S1, the degassing stage S2, the pyrolysis stage S3, and the polycondensation stage S4. S1, S2, and S3 are endothermic and S4 is exothermic. For a given coal, the temperature at which the process shifts from the endothermic stages (S1 + S2 + S3) to the exothermic stage (S4) increases with increasing heating rate. The heat absorbed in endothermic stages keeps invariable basically, while heat released in exothermic stage decreases with increasing heating rate. The entire pyrolysis process is exothermic when the heating rates are lower than 10 or 15 °C/min, and endothermic at heating rates higher than 15 or 20 °C/min. The temperature corresponding to the peak in the weight loss curve does not correspond to that in the heat flow curve. Therefore, the characterizations of the coal pyrolysis reaction process by weight loss and heat flow are inconsistent with each other. • Coal pyrolysis process involves both endothermic and exothermic stages. • Turning point from endothermic to exothermic increases with heating rate increases. • Unlike heat released, heat absorbed in endothermic stages keeps basically invariable. • Coal pyrolysis process characterized by weight loss and heat flow does not match. [ABSTRACT FROM AUTHOR]