Gasifier temperature

The operating conditions in the gasifier (temperature and pressure) and the reaction kinetics (residence time, concentration of the constituents, and rate constants) determine the extent of conversion or approach to equiUbrium. 

The advanced pressurized flmd-bed gasifiers currently ready for demonstration have internal diameters of around 3.6 m (12 ft) and are 20 to 28 m (66 to 92 ft) in height. Production of more than 100,000 mVh of gas (at gasifier temperature and pressure) is achieved by the advanced gasifiers, equivalent to 33,260 kg/h of bituminous coal (air blown). 

The Shell process uses partial oxygen gasification. Because insufficient oxygen exists for complete combustion (20-30% of the oxygen required for complete combustion is used), only a fraction of carbon in the coal is oxidized completely to C02. The heat released from this combustion provides most of the energy needed for endothermic coal gasification reactions and raises the gasifier temperature. Some steam is usually added to prevent excessive... 

Therefore, gasifier temperature and pressure can be controlled to maximize the concentration of desired product, CH4 or H2 and CO. However, in order to determine the optimum operating conditions other factors such as the gasification... 

The conversion of fuel nitrogen (o ammonia has been calculated for a number of steady-state periods with different fuels and different operational parameters of the gasifier (temperature, pressure, load, etc.). The calculations suffer from two difficulties, one being the uncertainty of the fuel nitrogen analysis, and the other the uncertainty concerned with the measurement of the biomass fuel flow. 

Figure 4.15. Effect of O/C ratio on gasifier temperature and carbon conversion, (a) Effect of O/C ratio on GE gasifier parameters, (b) Effect of O/C ratio on cold gas efficiency.

Several safety features have been included in the control system. The exit gas thermocouple is connected to a variable temperature controller to maintain gasifier temperatures below safe operating limits. A pressure switch is connected to the outlet point of the gasifier which will shut the combustion fan down if the pressure exceeds 120 cm water column. Various pieces of equipment are also interlocked to protect the system. 

In practice the gas composition can often be approximated by the actual equilibrium composition. The deviation can be corrected for by assuming a somewhat lower gasifier temperature than the real one. Another correction method defines an apparent equilibrium parameter determined experimentally ... 

The pyrolysis gasifier temperature (greater than 800° C for most coals) is controlled by the char fueled circulating bed combustor temperature (greater than 900° C) and the solids circulation rate, and the gasifier bed temperature may be limited to lower than optimum values by ash agglomeration limitations in the circulating bed combustor. 

The coal/water slurry is pumped into the gasifier burner together with oxygen. The gasification takes place rapidly at temperatures in excess of 2,300°F, under which conditions the coal is converted primarily to H2, CO, and CO2 no liquid hydrocarbons are found in the gas. The water in the coal slurry not only serves to convey the coal to the gasifier, but also moderates the gasifier temperature so that excessively high temperatures are not achieved. 

The gasifier temperature is controlled by manually adjusting the oxygen flow rate. 

Since the value of x, i.e. quantity of H2 burned with 0ex02, cannot be determined only from the results of ultimate analysis and gas analysis, we assume reasonable value of x based on the rate of combustion of H2 and CO. When gasifying temperature is too low to maintain the combustion rate of CO, only H2 seems to be virtually burned. In this case, x become 20ex and equation (58) is valid. 

It is seen that reactions (1, 2, 3, and 7) are exothermic, whereas the remainder are endothermic. In practice the reactor or gasifier temperature is controlled by the amount of steam that is added. When oxygen rather than air is used, a larger amount of steam may be added, which results in a richer product gas. With regard to coal gasification processes, at present the principal interest is in the production of pipeline quality gas as a means of replacing natural gas as supplied to domestic consumers. 

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