The gas is then cooled in a heat recovery steam generator (HRSG) to produce a temperature at which it can be cleaned. The HRSG consists of two boilers. The first one generates high-pressure steam and is included inside the same pressurised vessel as the reaction chamber. The second one generates medium pressure steam. Both have a special design in order to reduce fouling (Fig. 2). The gas used for quenching is taken from the cold gas flow that leaves the HRSG. Since the amount of fuel is fixed by the synthesis gas that the turbine demands, operators can control gasification reactions by adjusting the flows of oxygen and steam (actually oxygen/fuel and steam/fuel ratios). Quench gas flow is also modified but this parameter is used to control temperature distributions and fouling in the HRSG and it is not considered here.