The present invention relates to an apparatus for heating steam formed from cooling water in a heat exchanger for hot gas, comprising a primary heat-exchanger vessel having a compartment for cooling water, an inlet for the gas to be cooled, an outlet for cooled gas, an outlet for heated steam and a collecting space for maintaining generated steam. In the compartment for cooling water at least one primary evaporator tube is positioned through which, when in use, the hot gas flows. Due to heat exchange between cooling water and the hot gas via the evaporator tube walls the water evaporates and steam is formed. The steam flows upwards to the collecting space for maintaining generated steam. This steam is further heated in a secondary tube-shell heat exchanger vessel, also referred to as the xe2x80x98super heater modulexe2x80x99, positioned in the compartment for cooling water. In such a super heater module the generated steam is heated against the gas, which has been partially reduced in temperature in the primary evaporator tube.
Such an apparatus is described in EP-A-257719. The apparatus disclosed in this publication consists of a submerged superheater module, consisting of a shell-tube heat exchanger, wherein the partially cooled gas is fed to the shell side of the superheater module and the steam to the tube side of the superheater module. The two flows are contacted in the superheater in a co-current mode of operation.
Applicants found that when the apparatus according to EP-A-257719 is used to cool gas comprising contaminants such as carbon, ash and/or sulphur, which is for example the case for synthesis gas produced by gasification of a gaseous or liquid hydrocarbonaceous feedstock, leakage can occur. It is believed that fouling of the apparatus at the gas side causes leakage. Although the apparatus was cleaned regularly the leakage problems persisted. Fouling, especially when the synthesis gas is produced by gasification of a liquid hydrocarbon, in particular heavy oil residues, will also result in that the heat exchange capacity of the apparatus will gradually decrease with run time. As a result, the temperature of the process gas leaving the heat exchanger will increase gradually with runtime. If the temperature of the process gas leaving the primary heat exchanger apparatus exceeds a certain temperature, typically 400-450xc2x0 C., the temperature of the tubes that transmit the process gas downstream of the primary heat exchanger will be so high that they may be damaged. Therefore, the apparatus has to be shut down in order to clean the tubes. The runtime of an apparatus after which the tubes have to be cleaned is referred to as xe2x80x98cycle timexe2x80x99.
It is an object of the present invention to provide for an apparatus for heating steam in a heat exchanger for cooling a hot gas wherein the cycle time is maximised and/or the leakage problems are avoided. The hot gas is especially a hot process gas comprising compounds, which cause fouling of the heat exchange surfaces of the apparatus. Such compounds are especially soot and, optionally, sulphur. Reference herein to soot is to carbon and ash. This object has been met by an apparatus for heating steam formed from cooling water in a heat exchanger for hot gas, comprising a primary heat-exchanger vessel having a compartment for cooling water, an inlet for the gas to be cooled, an outlet for cooled gas, an outlet for heated steam and a collecting space for maintaining generated steam;
at least one primary evaporator tube positioned in the compartment for cooling water and fluidly connected to the inlet for the gas to be cooled,
at least one steam tube for withdrawal of generated steam from the collecting space for maintaining generated steam via a steam outlet of said collecting space,
at least one secondary tube-shell heat exchanger vessel, xe2x80x98super heater modulexe2x80x99, positioned in the compartment for cooling water, wherein the generated steam is further heated against partially cooled gas from the primary evaporator tube,
wherein the primary evaporator tube is fluidly connected to the tube side of the super heater module and the steam tube for withdrawal of generated steam is fluidly connected to the shell side of the super heater module; and
wherein means for adding water to the generated steam entering the super heater module are present.