Lian Zhang

CCSEM analysis of ash from combustion of coal added with limestone

Abstract 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 1.0xa0m and kept at 1573xa0K in all the runs, while the bottom-side has a length of 0.5xa0m and kept at 1173xa0K. SO 2 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.

In situ desulfurization during combustion of high-sulfur coals added with sulfur capture sorbents☆

Abstract Two Chinese coals, added with two types of sulfur capture sorbents, were combusted in a drop tube furnace to investigate effect of reaction temperature on sulfur removal during coal combustion. Limestone was used as sorbent and mixed with coal physically for sulfur removal. In addition, another sorbent, calcium acetate, synthesized from natural limestone, was also used for in situ removal of sulfur; it was impregnated into raw coals before combustion. The first series of experiments were carried out in the furnace having downside temperature of 1173xa0K (the upper side of furnace was at 1573xa0K). The results proved that calcium acetate captured more sulfur than limestone. In order to understand the effect of reaction temperature on in situ sulfur removal of sorbents, the second series of experiments were carried out at the uniform furnace temperature ranged from 1373 to 1673xa0K. Moreover, the sulfur removal capability of ashes, taken from combustion of coal with sorbents in drop tube furnace, was studied at 1173xa0K using thermogravity. The calcium distribution in ashes was analyzed using a novel calcium-based compounds CCSEM category. The results indicated that at certain temperature, higher sulfur removal efficiency was obtained for calcium acetate than that for natural limestone, which is mainly due to the fine dispersion of calcium in impregnated coal so that a good contact was obtained between calcium and sulfur-containing coal particles; increasing the temperature lowered the sulfur removal capabilities of sorbents since the sorbents were captured by inherent aluminosilicate; the sulfur content in raw coal affects the utilization of sorbents significantly in coal combustion. In addition, ashes, rich in calcium, can adsorb SO 2 at 1173xa0K; the sulfur removal efficiency of fly ash is at least the same as that of natural limestone.

Investigation of a direct melting dehydrated sewage sludge pilot plant

A direct melting dehydrated sewage sludge furnace was tested to obtain the basic operating parameters. Observation of particles (fly ash and deposited particulates) inside the pilot plant was performed to determine the particle properties. The results showed that dehydrated sewage sludge could be directly melted at a temperature of 1400?C with stable combustion. With regard to the fly ash, the majority of the particle size distribution was observed to be in the range 0.1?1 ?m, and amongst heavy metals, the highest concentration in the ash was for zinc. For the deposited particle fraction, most were observed to be in the size range 2?20 ?m and the main inherent chemical compound was associated with metal phosphate. Moreover, at different temperatures inside each chamber, the compositions and chemical compounds were found to change. To estimate the amount of deposited particles, the deposition flux was calculated by various deposition mechanisms, and it was found that diffusion was the dominant deposition mechanism, followed by thermophoresis and gravity, respectively.