In an entrained flow slagging coal gasifier, pulverized coal is entrained above the ash fusion temperature (1300~1600℃). The major components in the product gas are $H_2$ and CO and the mineral matter in the coal is melted and extracted as molten slag. The reactivity of coal and properties of ash and molten slags were measured. The effects of $O_2$/C, $H_2O$/C, and reaction temperatures on carbon conversion, the product gas compositions, cold gas efficiency, and slag composition in a drop tube reactor (DTR) and an entrained flow gasifier have been determined.
The used char was prepared at 80℃/min and the reactivity of char-$O_2$ reaction and $CO_2$ adsorption under isothermal conditions was determined in a thermo gravimetric analyzer (TGA). Another char was prepared in a DTR at 1880℃/s. For most coals tested, pyrolysis in TGA exhibits weight loss is smaller at temperature below 400℃, but increase significantly at temperature between 400-600℃ due to evolution of volatile matter. The weight loss of coal in a DTR produced 1.5 times of volatile matter of the parent coal. As the pyrolysis temperature is increased, the pore structure in char can be correlated with the volatile matter evolution and pore surface area. The temperature at the maximum char surface area increases with the rank of coal. The reactivity of char oxidation initially increases to a maximum value at around 20-30% conversion and decreases monotonically thereafter. The maximum reactivity of char in a DTR with oxygen at 550℃ is higher than that of char prepared in the lower heating rates. As the reaction temperature is increased, the formation time of ash layer becomes shorter. Although the reactivity of coal decreases with the coal rank, lower active sites in the char react with oxygen in a temperature programed reaction (TPR). The reactivity at lower temperatures (550-550℃) is different that at a higher temperature reaction (800℃). Adsorption of $CO_2$ in char has good correlation with the reactivities at the higher temperature.
Typical ingradients in the ash can be divided into two groups, namely the acid components ($Al_2O_3$, $SiO_2$) and the basic components (CaO, $Fe_2O_3$, MgO). The temperature drop of slag melting is caused by the basic oxide flux and CaO is selected as the best additive.
The characteristics of ash melting and slag viscosity with an addition of CaO as a flux have been determined to maintain the slag tapping condition at the given temperature in an entrained flow coal gasifier. As flux addition increases, ash fusion temperature decreases to a minimum value around 30-40% (CaO content, ash basis) and then increases. The minimum range of ash fusion temperature exhibits an eutectic effects between the base and acid components.
The viscosity was measured using high temperature rotational viscometer (Haake-1700) under nitrogen atmosphere. Low content of silica slag (Alaska coal) has a low viscosity but it has a critical point lower than 250 poise and solidification occurs below 1300℃. Whereas, with high content silica slag coal, the slag viscosity decreases with the addition of flux since CaO as a oxide donor which prevent silicate formation from crystallization. The decrease of slag viscosity with an addition of CaO results in lower the critical temperature $(T_{cv})$. As CaO addition is increased from 15 to 40%, $T_{cv}$ decreases form 1375℃ to 1325℃. At the gasification temperature of 1400℃, the range of flux addition were 40% (Drayton), 25% (Datong) and 20% (Roto) as the CaO concentration in the fluxed ash to maintain a viscosity of 250 poise. Comparision between the prediction model (Urbain) and the measured values, the agrrement is good for high silica slag (Drayton, Datong) than the low silica slag (Alaska) with the standard deviation range of 18~62% with the addition of flux.
In the present study, a modified DTR has been devised to find the optimum operating conditions in an entrained flow coal gasifier to determine the effects of reaction temperature, ratios of oxygen/coal, steam/coal, and residence time on the coal gasification performance at the temperature range of 1000-1550℃ compositions of $H_2$ and CO in the product gas increase and $CO_2$ decreases and $H_2$/CO mole ratio somewhat decreases with increasing reaction temperature since the char-$CO_2$ reaction becomes more dominant than the char-$H_2O$ reaction above $1000\,^\circ\!C$. The gas production rate $(CO + H_2 + CH_4)$ reaches a maximum value around the ash fusion temperature due to the reduction of reactivity caused by pore plugging from ash fusion. From the comparision between the experimental data and the values from the equilibrium model, rather poor agreement is observed at the oxygen/coal ratio below 0.2 whereas, the agreement improves with increasing oxygen content. In CO formation from Drayton and Datong coals, the experimental data lies below those of the equilibrium model. In the present study, the optimum value of $O_2$/coal ratio is found to be in the range of 0.6-0.9 at 1500℃ and atmospheric pressure. In the product gas yield along the reactor length with different coals, the production of $CO_2$ initially increases, and then decreases as the char is converted to CO, followed by $H_2+CO$ formation. The oxygen consuming rate or carbon conversion exhibits different pattern with the reactivity of coal. A high reactivity coal (Alaska) rapidly reacts with oxygen, and the product gas composition is almost uniform above 0.4 m from the bottom of the reactor whereas, Drayton coal requires longer residence time.
In the entrained flow gasifier, coal slurry (62.5%, w/w solids) was fed into the reaction vessel with oxygen. In the gasifier, the highest temperature was detected near the top of the reactor and gas temperature decrease along the reactor height by heat loss and the endothermic reactions. The obtained gas composition in the entrained bed reactor is similiar to that in the DTR experiment. However, carbon conversion decreases with increasing coal feed rate. The $H_2S$ content in the product gas decreases in the range of 1500~2500 ppm with increasing oxygen feed rate with 0.77% sulfur content of Datong coal. Based on the gas compositions, the water-gas shift reaction is almost in equilibrium at the exit of gasifier. The product gas has caloific value of 1900 - 2100kcal/N㎥ and the cold gas efficiency is 64% at the oxygen/coal ratio of 0.9 at 1530℃. From the slag analysis, it can be seen the sensible decrement of alkaline ($Na_2O, $K_2O$, MgO) and volatilization of $SO_3$. On the other hand, slag deposit shows increment of Fe form and the Fe compound in slag deposit at the wall is transformed from iron oxide to metallic iron by unreacted char at the reducing gas environment. The formation of metallic iron is solidified at the wall or slag tap and viscosity is varied with the composition. With the satisfactory slag tapping, the bottom wall temperature should be maintained at 50~100℃ above the ash fusion temperature.
