To accomplish high temperature molten ash combustion higher than the melting point of ash while recovering excess thermal energy by drying up sludge appropriately using oxygen gas in combustion.

When oxygen is used in combustion, volume of combustion gas is reduced by a factor of about 4 as compared with air combustion because nitrogen gas is not present and combustion temperature increases in reverse proportion to reduction in the thermal capacity of combustion gas. In case of oxygen combustion, combustion temperature of 1570°C is realized even when water content is 65% (moisture contained in sludge is 1.87 kg/kg-DS). Since nitrogen gas is not present in the combustion gas, volume of combustion gas is small and high combustion temperature can be attained even if the water content is high. When oxygen combustion is applied, high temperature combustion for sustaining molten ash combustion can be ensured without requiring any preheat of oxygen even if dry up of sludge is lessened significantly. Consequently, conventional auxiliary boiler and air preheater are eliminated and since the volume of dry up steam being produced from a waste heat boiler can be reduced significantly, excess thermal energy can be recovered.