The catalytic activities of pure and mixed catalysts of alkali [$K_2CO_3$, $K_2SO_4$, $Ba(NO_3)_2$] and transition metal [$Ni(NO_3)_2$, $FeSO_4$] salts on steam-char gasification at 700 - 850℃ have been measured in a thermobalance reactor.
The effects of catalyst loading (0 - 35 wt%), coal rank (lignite - anthracite), steam partial pressure (25.3 - 91.2 kPa) and composition of mixed catalysts on steam-char gasification have been determined. The conversion data of non-catalytic steam-char gasification is well represented by the shrinking core and the modified volumetric reaction models. The initial reaction rate of non-catalytic steam-char gasification increases with increasing steam partial pressure, but is nearly independent of steam partial pressure in catalytic steam-char gasification. The catalytic activities of pure salts are in the order of $K_2CO_3$ ＞ $Ni(NO_3)_2$ ＞ $K_2SO_4$ \approx $Ba(NO_3)_2$ ＞ $FeSO_4$. The gasification rate with $K_2SO_4$ could be enhanced by addition of other salts and the rank of catalytic activity among the tested mixture salts is found to be $K_2SO_4+Ni(NO_3)_2$ ＞ $K_2SO_4+Ba(NO_3)_2$ \approx $K_2SO_4+FeSO_4$. The reactivity of char increases with increasing catalyst loading of $K_2SO_4+Ni(NO_3)_2$ and gasification temperature. The catalytic effectiveness of $K_2SO_4+Ni(NO_3)_2$ decreases with coal rank. An equimolar mixture of $K_2SO_4$ and $Ni(NO_3)_2$ exhibits the highest catalytic activity and synergistic effect.
To investigate the relationship between catalytic activity and gas compositions of the product gas, steam-char gasification with different catalysts has been carried out in a lab-scale fluidized bed reactor (0.028m I.D × 0.5m high) at a steam partial pressure of 50 kPa. Main components of the product gas in the non-catalytic and catalytic gasifications are $H_2$ and CO. Gas production rate in the catalytic gasification is greater than that in the non-catalytic gasification. The gas production rate increases with increasing bed temperature. The selectivity of gas composition in the product gas with different catalyst types does not exhibit in the catalytic gasification.
To investigate the coal gasification characteristics and to produce the low calorific value gas in a fluidized bed gasifier (0.1 m-I.D × 1.6 m-high) at atmospheric pressure, Australian coal has been gasified with air and steam. In addition, to investigate catalytic effect on Australian coal gasification in the fluidized bed, coal was impregnated with binary mixture of $K_2SO_4+Ni(NO_3)_2$.
The effects of gas velocity (2∼5 $U_f/U_{mf}$), air/coal ratio (1.6∼3.2), steam/coal ratio (0.63∼1.26), and reaction temperature (750∼900℃) on gas composition, gas yield, cold gas efficiency and calorific value of the product gas and carbon conversion have been determined. The contents of product gas ($H_2$, CO, $CO_2$, $CH_4$) increase with increasing gas velocity. With increasing reaction temperature, $H_2$ content increases, CO content exhibits minimum value, but $CO_2$ content exhibits maximum value, and $CH_4$ content is almost constant. Carbon conversion, gas yield, calorific value, and cold gas efficiency of the product gas increase with increasing gas velocity and the reaction temperature. The contents of $H_2$, CO and $CH_4$ decrease, but $CO_2$ content increases with increasing air/coal ratio. With increasing air/coal ratio, carbon conversion, gas yield, and cold gas efficiency of the product gas increase, but calorific value of the product gas decreases. The contents of $H_2$ and $CO_2$ increase, but CO content slightly decreases and $CH_4$ content is nearly constant with increasing steam/coal ratio. Also, gas yield, calorific value, and cold gas efficiency of the product gas increase, but carbon conversion is nearly constant with increasing steam/coal ratio.
The particle entrainment rate increases with increasing gas velocity, but decreases with increasing bed temperature.
Australian coal impregnated with mixed catalyst of $K_2SO_4+Ni(NO_3)_2$ exhibits the highest catalytic activity in char-steam gasification in the thermobalance reactor. With increasing gas velocity, $H_2$ content increases from 12 to 22%, and CO content increases from 31 to 56% in the catalytic gasification compared to those in the non-catalytic gasification. As the reaction temperature increases, $H_2$ content increases from 41 to 59%, CO content increases linearly, but $CO_2$ content decreases. The $CH_4$ content from the catalytic gasification is lower than that in the non-catalytic gasification with increasing gas velocity and the reaction temperature. Carbon conversion and gas yield increase from 19 to 32%, and from 24 to 33%, respectively with increasing gas velocity. Whereas, carbon conversion and gas yield increase from 5 to 8% and from 8 to 34% with reaction temperature, respectively. With increasing gas velocity, calorific value increases and cold gas efficiency increase with increasing gas velocity and reaction temperature. With increasing air/coal ratio, $H_2$ and CO contents increase from 42 to 91%, and from 26 to 63%, respectively. Carbon conversion (14∼45%), gas yield (28∼63%), calorific value (16∼53%), and cold gas efficiency (22∼62%) increase with increasing air/coal ratio. Whereas, $H_2$ content is nearly constant with increasing steam/coal ratio. With increasing steam/coal ratio, carbon conversion (14∼57%), gas yield (5∼46%), calorific value (16∼38%), and cold gas efficiency (7∼44%) increase.
Gasification of Australian coal with air and steam has been carried out in an internally circulating fluidized bed (0.3 m-I.D. × 2.7 m-high) with a draft tube (0.1-m I.D. × 0.7 m-high). The effects of coal feed rate (5∼8 kg/h), gas velocity (6∼12 $U_d/U_{mf}$), and steam feed rate (3.0∼4.5kg/h) on gas composition, gas yield, and calorific value of the product gas and carbon conversion have been determined. The fluidized bed can be divided into two reaction s, namely coal combustion (draft tube) and gasification (annulus) zones by inserting a centrally located draft tube. With the fluidizing velocities of 6∼12 $U_{mf}$ to the draft tube and 0.8∼1.0 $U_{mf}$ to the annulus, gross circulation of bed materials as the heat carrier from combustion to gasification zones can be established. The present coal gasifier can be operated with much lower air/coal ratio compared to conventional fluidized beds. Calorific value, carbon conversion, gas yield, and bed temperature decrease with increasing coal feed rate. Caloric value of the product gas decreases, but carbon conversion increases with increasing gas velocity. Calorific value and carbon conversion decrease with increasing steam feed rate. In the gasification zone the obtained gas compositions are found to be 24%-$H_2$, 25%-CO, 19%-$CO_2$, 4%-$CH_4$ having calorific value of 7.6 MJ/㎥ which is higher than those from conventional fluidized or spout bed gasifiers. By removing the gases from the two reactions zone separately, medium calorific value gas can be obtained in the present internally circulating fluidized bed gasifier.
