Pressure gasification

The exergetic efficiency of the two alternatives is shown in relation to the gasification parameters (pressure, temperature). A downstream waste heat boiler has great exergetic advantages particularly at low and medium gasification pressures. 

The minimum steam mass flow to ensure a stable fluidisation results from the bed diameter, the gasification pressure and the minimum fluidisation velocity, which depends mainly on the particle size, A large diameter, a high pressure and a large particle size leads to an elevated mass flow. The estimated steam mass flow for fluidisation always has to be equal or larger than the required steam mass flow for the gasification process. 

The gasification pressure should be as small as possible. Hence, the gasification pressure is determined by the required inlet pressure of the syngas at the gas engine and the pressure losses caused by heat exchangers and piping. 

The most important operating parameters are coal concentration in the CWS, gasification temperature, gasification pressure, residence time, and oxygen/carbon ratios. These effects are discussed below. 

Due to the dry feeding of the Shell gasifier, the 02 consumption is reduced, and the efficiency is increased by -2% compared with the CWS feeding system. However, the maximum gasification pressure is limited to about 5 MPa due to the dry-feeding system, as opposed to 8 MPa for some CWS feeding systems. 

It uses 15%-25% less oxygen consumption compared with slurry feed gasification processes. The trend for dried pulverized coal gasification is to replace N2 with C02 and increase the gasification pressure, which can further reduce the 02 consumption.37... 

The low gasification pressure requires extra syngas compression for the methanol synthesis process. The bulk density of the released dust is much higher than that of CWS gasification. 

The gasification pressure in partial oxidation processes has been gradually increased to a range of 60-90 atm, which helps save energy for compression. Texaco s commercial gasifiers for ammonia plants operate at 8,5-9 MPa [371. 

In general, the cost and performance of membrane integrated plants benefit from high gasification pressures, which increase the Hj partial pressure... 

It can be seen from Table 9.13 that the designed energy consumption of synthetic ammonia production is changed with the difference in the raw materials, gasification pressure and plant scale. 

For fluid-bed and entrained-flow processes, the gasification pressure should be raised from atmospheric to 20 to 60 bar to reach higher single-unit capacities (up to 500 MW) and lower capital investment. 

In addition to the coal properties, the reasonable and unified boundary conditions need to be defined in order to evaluate and compare processes. The first section of Table 5.11 presents process-related variables. The gasification pressure was set to 30 bar, a typical value for integrated gasification combined cycle (IGCC) power plants as well as some chemical syntheses (e.g., Fischer-Tropsch) [60,65]. In the case of slagging entrained-flow gasification, the gasification temperature was set at >100 K above the ash fluid temperature for the reasons discussed in Section 4.5.2. A typical value is between -1-100 and -1-150 K [61]. The thermal capacity of the modeled reactors was set to a thermal input of 500 MW on the basis of the LHV as a suitable size for a state-of-the-art reactor. 

Gasification pressure 30 bar Suitable for IGCC/several syntheses [60]... 

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