Gasification

Steam gasification of Coal

Normally coal is gasified by the use of oxygen-blown entrained flow gasifiers. However, steam gasification will provide higher H2/CO ratios and lower CO2 emissions.
 
The importance of the raw gases H2/CO ratio becomes obvious from the overall conversion reactions. Methanation transfers the coal’s energy content into methane and produces CO2. It generates a gas, which contains nearly 50% CO2 and CH4 (dry base) and additional amounts of excess steam.
Additional CO2 comes from the heating of the gasifier by means of coal combustion:
 
Steam-Gasification         2 C + 2 H2O -> CH4 + CO2 with steam gasification
Combustion (for indirect     C + O2 -> CO2
heating of the gasifier):
 
Using oxygen-blown gasifiers instead of steam-blown gasifiers increases the CO2 content of the raw SNG by a factor of 1.5
 
Partial oxidation + CO-Shift 4 C + 2 O2 + 2 H2O -> CH4 + 3 CO2 with oxygen gasification
 
and reduces the process efficiency accordingly. Thus the most CO2-efficient upgrading of coal into Substitute Natural Gas requires a steam/allothermal gasification process.
Furthermore the maximum plant size for the methanation of coal is in any case limited by the capacities of existing natural gas pipelines. Medium-scale SNG plants with fluidized bed gasifiers are more feasible than large-scale oxygen blown gasifiers.

Basic Design Large-scale Heatpipe-Reformer

Gasification of coal is usually based on oxygen-blown entrained flow gasifiers. As shown earlier, the upgrading of coal into Substitute Natural Gas (SNG) preferably requires a steam gasification process, which offers a synthesis gas composition perfectly suited for methanation. A technology that provides a synthesis gas with a particularly high H2/CO-ratio is the Biomass Heatpipe-Reformer. The Heatpipe-Reformer-Technology was developed within the FP5 Project “Biomass Heatpipe-Reformer” (EU-Project ENK5-CT-2000-00311) and enables the production of a high-quality syngas, which is perfectly suitable for a methanation installed downstream, due to

• an ideal H2/CO ratio of 3:1
• a gasification pressure > 5 bar.

Concepts for Heatpipereformer

Figure 3: Small-scale and Large Scale concept of the Heatpipe-Reformer

First biomass fuelled 500 kW Heatpipe-Reformer plants are already under construction and in demonstration and are available for first coal gasification tests within this project. However the main innovation of this project will be the application of the Heatpipe-Reformer concept for large scale plants. Due to the fact that the coal ash will significantly increase the erosion impact on the heat pipe surfaces the large-scale design proposes to establish the heat exchange by means of heat pipes, which are integrated in an external bed material heater/cooler. Reduced particle velocities in the external bed material coolers reduce the erosion. Thus the integration of super heater surfaces in external bed material coolers of CFB boilers is already state of the art. Increased temperatures and under-stoichiometric conditions necessitate additional investigations on the particle impact to the Heatpipe surfaces.