SYNTHESIS GAS COOLING
Gases leave the gasifier reactor at high temperatures varying from 550°C for some dry-ash moving-bed gasifiers to 1600°C for dry-coal feed entrained-flow slagging gasifiers.
With the exception of natural gas feeds, the synthesis gas from a gasification reactor is contaminated with various components that must be removed before the syngas is suitable for its final use, whether as chemical feedstock or as fuel. These contaminants, which may be particulates, sulfur or chlorine compounds, tars, or others, must be removed, and all such removal processes, even the so-called hot-gas clean-up processes, operate at temperatures considerably lower than that of the gasifier itself. Thus there is always a necessity to cool the syngas. In most cases it will also be desirable to make good use of the sensible heat in the gas, for example, by raising steam. On the other hand, differences in the contaminants, which vary from feed to feed, as well as characteristics of the different gasification processes themselves, lead to a considerable variety of solutions to the syngas cooling task.
The highest temperature gasification processes are the entrained-flow slagging processes. On cooling the gas any entrained-flow ash particles will inevitably pass through the critical temperature range, where the ash becomes sticky. Every gas-cooling concept has to take this into account and quench the gas as quickly as possible to a temperature at which the ash becomes dry, typically about 900°C. There are a number of different methods for achieving this, which are described in more detail in what follows.
Fluid-bed coal gasifiers have typically outlet temperatures of 900-1000°C. Apart from some problems with tar, the hot gases can be used for the generation of a reasonable quality steam in a syngas cooler. Even after passing cyclones, the gas does contain some fly ash, and hence, in the design of the syngas cooler, the danger of potential erosion problems should be taken into account. When limestone is added to the feed to bind the sulfur, unconverted CaO may react back with C02 in the gas to CaC03 at temperatures below 950°C, which may enhance fouling in the syngas cooler.
Most biomass gasifiers operate around 900°C and the same syngas cooling issues apply as for other fluid-bed gasifiers. Moreover, as biomass ashes are rich in alkali carbonates, these may condense out or desublimate and cause additional fouling between 600 and 900°C. At lower temperatures the condensation point of volatiles in the gas is the main cause of fouling, as the condensate is ideal for catching ash particles.
Although in the case of moving-bed gasifiers the temperatures are low (300-550°C) the cooling of the gas is often complicated by the presence of tars in the gas that may foul heat exchangers when the temperatures drop below the condensation point of the heaviest tar components. In practice, this means that only low — or medium — pressure saturated steam can be produced in these syngas coolers.
In the Lurgi dry-ash and the BGL slagging gasifier the transition from slagging to nonslagging regime occurs within the coal bed and does not cause problems as the bed is continuously moving in a downward direction, which keeps the reactor wall free from slag deposits.