Gasification can be utilized to convert to coal, biomass and waste materials into fuel/synthesis gas. To improve the gasification technology, development of gasifier, which is a main technology of gasification, and improvement of rate of gasification should be achieved. Therefore in this study, development of gasifier and improvement of gasification reaction could be achieved by an internally circulating fluidized bed reactor and a mixed catalyst $[Ni(NO_3)_2+K_2SO_4]$, respectively.
Among the coal gasification processes, the fluidized bed process with their inherent advantages of high heat transfer and easy handling of solids seems to be a natural choice for the gasification process. However, the intrinsic problems in fluidized beds are known to be high carbon losses due to coal shattering, and subsequent elutriation of fines, and low conversion of reactant gases due to gas bypassing. To solve these problems, a draft tube was inserted in a fluidized bed to divide the fluidized bed into two reaction zones. Two reaction zones can be attained for coal combustion with air feeding in the draft tube and coal gasification with steam feeding in the annulus zone; or vice versa. By fluidizing solids in the draft tube at a velocity about 7-10 times of $U_{mf}$ and the annulus at 0.7-1.5 $U_{mf}$, it is possible to induce a gross circulation of bed material up the draft and down to the annulus region. Solid circulation within the reactor provides energy transfer from the combustion zone into the gasification zone and reduces the elutriation rate of fine coal particles from the reactor significantly. By installing a gas separator over the draft tube, high calorific value gas in the annulus zone can be obtained. To increase calorific value of the product gas in the annulus zone, gas bypassing from the draft tube to annulus region should be minimized since this gas bypassing would cause both dilution and burning of the gasification product gases and consequent degradation in the product gas quality. To reduce gas bypassing from the draft to annulus region, a draft tube having orifices at the bottom part was devised based on the findings in previously studies. Based on our previous findings on catalytic activity for coal gasification, $Ni(NO_3)_2, K_2SO_4, K_2CO_3$ and $[K_2SO_4 + Ni(NO_3)_2]$, were used to improve the reaction rate of steam gasification and to control the composition of the product gas.
On the other hand, to understand the overall coal gasification reaction, individual reaction steps ($O_2$-char combustion, $H_2O$-char gasification, pyrolysis) should be identified. Therefore, in this study, the kinetic data of coal combustion and steam gasification of Australian's subbituminous coal in a thermobalance reactor and pyrolysis in a fluidized-bed reactor have been determined to account the individual reaction. Based on the obtained kinetic and pyrolysis data, a coal gasification model in a fluidized bed reactor has been proposed.
In the thermobalance reactor, the activation energies of combustion and steam gasification are 27 kcal/mol and 40 kcal/mol in chemical reaction control regime, respectively. The reaction rate equations can be expressed with the apparent activation energies and rate constant of the individual reaction steps as
Combustion: $\frac{dX_1}{dt} = k_1 exp (-\frac{E}{RT})P_{o_2} (1-X_1)^{2/3}$
where
$k_1 = 75785 [1/s \cdot atm]$ and $E_1/R$ = 13523 [K], for chemical reaction control,
$k_1 = 0.44 [1/s \cdot atm]$ and $E_1/R$ = 3342.4 [K], for pore diffusion control,
$k_1 = 0.046 [1/s \cdot atm]$ and $E_1/R$ = 1166 [K], for gas film diffusion control
Steam gasification : $\frac{dX_2}{dt} = 6474.7 exp (- \frac{19544}{T}) P_{H_2O} (1-X_2)^{2/3}$.
From coal pyrolysis in a fluidized bed reactor, the effects of reaction temperature and gas velocity on the gas product yield, heating value and solid conversion. The mass fractions of $H_2$ and CO increase but $CH_4$ and $CO_2$ decrease with increasing temperature. However yields (g-gas/g-coal fed) of all the components actually increase with increasing temperature due to the increase of heat transfer. It may indicate that pyrolysis activated by increment of temperature has a great influence on producing $H_2$ and CO in coal and steam-char gasification reactions. With increasing fluidizing velocity, mass fraction of the product gas is not varied appreciably. Heating value of the product gas increases from 920 to 1500 kcal/kg-feed and coal conversion is 12-20% with increasing reaction temperature. Product gas yields from pyrolysis have been correlated with the reaction temperature at an average fluidizing velocity as
$y_{H2}$ = 8.233 × 10{-4} T - 0.073, $y_{CO}$ = 2.168 × 10^{-4} - 0.102
$y_{CO2}$ = 2.999 × 10^{-6} T + 0.039, $y_{CH4}$ = 2.561 × 10^{-5} + 0.011$, T: [K}$
where yi (kg/h) is gas yield/coal feed rate (kg/h) from pyrolysis.
From modeling of coal gasification in the fluidized bed reactor, the modeling of coal gasification based on the correlation of pyrolysis in the literature may give a serious error, especially with coal having higher volatile matter content. Our kinetic data derived from the two-phase theory on coal gasification in a thermobalance reactor and coal pyrolysis in a fluidized bed reactor could be used to predict the product gas compositions of coal gasification.
From coal gasification in the internally circulating fluidized bed reactor, the effects of reaction temperature, $O_2$/coal and steam/coal ratio, coal feed rate, gas velocity and the type of catalysts on the coal gasification performance have been determined.
From the non-catalytic coal gasification, the compositions of the product gas in the annulus region are $H_2$ (36.7-45.1%), CO (19.3-25.5%), $CO_2$ (13.4-15.4%), $CH_4$ (6.6-10.3%). The yields of $H_2$ and CO increase with increasing reaction temperature due to the endothermic gasification reactions of steam-char and steam-$CO_2$. Whereas, the yieof other gadecrease witcreasieaction temperature. Compositions of the product gas in the draft tube have the same trend as of the product gas in the annulus region. The increase of $H_2$ and CO yields may come from volatile matters release from pyrolysis and high steam bypassing from the annulus to draft tube. Calorific value of the product gas from the annulus region is 11-12MJ/㎥ that is much higher than that in the annulus zone of an ICFB with the gap height-type draft tube. In the annulus zone, calorific value decreases with increasing temperature due to the decrease of hydrocarbons and $CH_4$. In the draft zone, calorific value of the product gas is 3.3-4.7 MJ/㎥ which is comparable to conventional fluidized bed or spout bed gasifier. Therefore, it can be claimed that the performance with the orifice-type draft tube is found to be superior compared to the gap height-type draft tube to producing medium calorific value gas in the annulus region due to the reduction of gas bypassing from the draft tube to annulus regions.
For catalytic coal gasification in the present study, four different catalysts $[Ni(NO_3)_2, K_2SO_4, Ni(NO_3)_2+K_2SO_4, K_2CO_3]$ were tested.
The yield of $H_2$ of the catalytic gasification is much higher than that of non-catalytic coal gasification, but, calorific value of the product gas is lower than that of non-catalytic gasification. The stable operations can not be made with $K_2SO_4$ and $K_2CO_3$ at the reaction temperature above 800℃ due to the ash agglomeration. The catalytic coal gasification with the mixed catalyst, $Ni(NO_3)_2+K_2SO_4$, can be readily operated up to 900℃. The performance of coal gasification with the mixed catalyst is superior to that with the single catalyst due to the cooperative effect and the high resistance to catalyst poisoning during the steam gasification reaction.
The catalytic coal gasification performances with the given amount of mixed catalyst $[Ni(NO_3)_2+K_2SO_4]$ in the ICFB with a draft tube and a conventional fluidized bed reactor have been determined. The maximum performance of the catalytic coal gasification can be attained at $850℃ in both reactors, and the catalytic activity in the ICFB is higher than that in conventional fluidized bed reactor.
