Cold gas efficiencies

The first stage assures high carbon conversion and optimum slag removal. The second stage reduces the raw product gas temperature to about 1000°C. This latter step helps to improve cold gas efficiency relative to other slurry fed processes and to lower waste heat recovery costs (15—18). 

The Shell syngas contains about 80-83% of the energy in the coal feed (the cold gas efficiency). This high efficiency is due to the high carbon conversion (>96%) during gasification (Tables 3.4 through 3.6).5... 

The mass and heat balances of the process are satisfactory when coal is gasified to a 53% carbon conversion in the gasifier and the remaining 47% of carbon is sent to the regenerator. From the heat values of the produced fuel gas and the input coal, the cold-gas efficiency (91 vol% H2 with 9 vol% CH4 298 K, 0.1 MPa) was calculated to be higher than 0.77. 

Once the heating value of the biomass and of the producer gas are known, the cold gas efficiency of the plant can be determined ... 

One central step considered in all flowsheet options, is an entrained flow gasifier operating at a flame temperature up to 1500°C. The hot tar-free syngas generated in the gasifier flame should be quenched with crushed char powder (not shown in the flowsheet) to increase the cold gas efficiency by chemical quenching [16]. 

To maximise the cold gas efficiency of the CASST process, heat supply by combustion of volatiles and heat demand by gasification of charcoal should be balanced. A preliminary system assessment study showed that charcoal yields of 0.38 kg/kg dry fe are required to balance heat demand and supply (3). 

For some experiments the gasifier cold-gas efficiency was determined, which varied between 65 and 82%. However, these figures are not very accurate. 

With one or two exceptions, an increase in biomass moisture content to the gasifier results in an increase in tfo. This is despite a fell in electrical output, which is outweighed by increased hot water production. The increase in hot water production conies fiom the product gas water heater, where a fall in gasifier cold gas efficiency gives rise to more sensible heat in the product gas. 

Although many processes are reported for biomass gasification, BCL (Battelle-Columbus Laboratory) Process is applied for this analysis [5]. According to the report, the system is fast-fluidized bed and indirectly heated. The temperature is 863 C and the operating pressure is 0.101 MPa. As for product gas characteristics, the yield of (kmol/ton-dry feed) is 45.8, and HHV (MJ/Nm3 raw gas) is 15.19. Cold gas efficiency is 80.1%. 

Table 4 gives an overview of the main experimental results obtained using the DWSA test rig, regarding process conditions, LCV gas composition, heating value, carbon conversion and cold gas efficiencies. 

Beside the problems of gas cleaning and waste water treatment the upscaling of fixed-bed gasifiers is limited to app. 1 MWe. But even in this relatively small power range electrical efficiencies between 18 and 28%, depending on the efficiency of the IC engine and the cold gas efficiency of the gasifier, are possible. The ratio electrical power to heat output (a) reaches 0,45 up to 0,65. 

Figure 4.9. Predicted effect of steam/02/coal ratio on peak cold gas efficiency and equilibrium temperature (adapted from Smoot and Smith15).

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