Method for combined generation of synthesis gas and power

Abstract: A method for the combined production of synthesis gas and electrical power including at least a step for the production of synthesis gas by combusting a second fuel stream with combustion air to flue gas and passing the flue gas in heat conducting relationship with steam reforming reactions of a hydrocarbon feedstock in a heat exchange reactor, and a step for the production of electrical power by combusting a first fuel stream with combustion air in a gas turbine combustion chamber to a flue gas and expanding the flue gas in a gas turbine; wherein the flue gas from one of the steps is used as combustion air in the other step.

Description: The present invention is directed to combined production of synthesis gas and electrical power by utilising air from a gas turbine firstly in a heat exchange reformer and thereafter in a secondary combustion unit. The invention provides in particular a method of generating power at a high efficiency by using compressed air from a gas turbine air compressor as combustion air firstly in a heat exchange steam reformer (HER) and subsequently in a secondary combustion for raising flue gas temperature from the HER before the gas is expanded in a gas turbine expander. HER is a conventionally available hydrocarbon reforming technology. The advantage of HER reformers is a high degree of convective heat transfer compared to heat transfer by radiation like in a reforming furnace. This is obtained by a significant combustion air excess, which limits the flue gas temperature. Conventionally, the HER is fed with preheated desulphurized hydrocarbon stream mixed with steam. The reforming process is performed in presence of a reforming catalyst arranged in the HER. The steam reforming process proceeds by below reactions: C n H m + n H 2 O → n CO + (n+m/2) H 2 CO + H 2 O → CO 2 + H 2 The concept of the HER implies that the endothermic reactions are conducted in such a manner that the effluent from the reformer catalyst bed by cooling provides part of the necessary heat for the reforming process. The remainder of the required reformer duty is supplied by burning fuel in the HER combustion chamber and producing a hot flue gas stream, which is passed at the outside of the catalyst bed walls in heat conducting relationship with the above steam reforming reactions in the bed at near atmospheric or elevated pressure. The combustion is carried out with excess of air for the attainment of a proper flue gas temperature to obtain predominantly convective heat transfer to the catalyst through the catalyst bed walls. Synthesis gas produced in the HER is applicable for several syntheses e.g. methanol or for the production of hydrogen or carbon monoxide by means of downstream process sections. In the conventional layout, the HER is supplied with combustion air in excess of a combustion air compressor. The necessary power for such and other miscellaneous equipment (i.e. compressors, blowers, pumps etc.) in a HER based plant will usually be supplied by power import or onsite generation of power typically by steam turbines or gas turbines. A gas turbine requires, analogous to the HER, significant excess of combustion air in order to control the flue gas temperature. EP patent application No. 0569796 discloses a method for operating a process furnace useful in endothermic reaction processes such as steam reforming. This method comprises heating a radiant chamber housing a number of heat transfer tubes, the heat being provided by radiant burners which combust fuel with hot turbine exhaust as all or part of the combustion air. The flue gas produced then travels through a series of convection coils in a split channel design, where heat transfer takes place, before it is finally discharged. It is mentioned that the method allows greater control over the operating temperature and enthalpy content of the gases in the flue channels, and that the sensible heat of turbine exhaust can be used to enhance the efficiency of the burners and to reduce the fuel consumption. Based on the fact that both an HER reformer and a gas turbine require control of flue gas temperature, we found that improved process efficiency is achieved when oxygen depleted exhaust from HER is used as combustion air in the gas turbine. The entire calorific energy of exhaust gas from the first process down to ambient temperature is transformed into combustion air preheating duty in the subsequent combustion process without loss of entropy. Accordingly, the present invention is a method for the combined production of synthesis gas and electrical power including at least a step for the production of synthesis gas by combusting a second fuel stream with combustion air to flue gas and passing the flue gas in heat conducting relationship with steam reforming reactions of a hydrocarbon feedstock in a heat exchange reactor, and a step for the production of electrical power by combusting a first fuel stream with combustion air in a gas turbine combustion chamber to a flue gas and expanding the flue gas in a gas turbine, characterised in that the flue gas from the steam reforming step is combusted with the first fuel stream in the electrical power producing step. By the inventive method, power is produced at high efficiency in a gas turbine in combination with a heat exchange reformer. The fuel amount required for the co-production of power and synthesis gas is converted with an efficiency of about 80-90%, which is not achievable by a separate power generation. As another advantage, sequential use of combustion air requires only one common flue gas waste heat recovery section instead of two separate recovery sections, whereby the overall flow of flue gas is considerably reduced. By the method of the invention, combustion air for use in the HER is passed to a gas turbine compressor, where it is compressed and sent to the HER combustion chamber for use in burning of a liquid or gaseous fuel stream, supplying an appropriate flue gas stream as heating medium in the reformer; the oxygen containing effluent from the HER is then used for burning of an additional fuel stream producing exhaust gas, which is passed to the gas turbine expander and finally through the common waste heat recovery section. In operating the method of the invention as shown for a specific embodiment in Fig. 1, ambient air 20 is passed to the gas turbine air compressor (GT Compressor) and compressed in one or more stages. Compressed air 22 is passed to HER combustion chamber 90 and mixed and reacted as oxygen source in excess of a liquid or gaseous fuel stream 60 providing a flue gas at a sufficiently high temperature (typically 1300°C) and a high flow rate to supply heat for the heat absorbing endothermic reforming process taking place in the HER reforming section 140. Flue gas 80 being cooled on the flue gas side in the heat absorbing HER reforming section 140; and HER containing residual amounts of oxygen is then passed on to combustion chamber 120 and once more reacted as oxygen source in excess of liquid or gaseous fuel 110 producing flue gas 100. Flue gas 100 is passed to the gas turbine expander and depressurized in the turbine closed to atmospheric pressure. Depressurized flue gas stream 160 is then sent through a common waste heat recovery section (WHS). The gas turbine is producing shaft power, which is optionally used for driving a turbo generator or e.g. a compressor (not shown). In the embodiment shown in Fig. 1, compressed combustion air from the air compressor may be bypassed directly to the secondary combustion chamber. In a comparative example as shown in Fig. 2, ambient air 2 is passed to a gas turbine (GT) compressor and compressed in the gas turbine air compressor in one or more stages. Compressed air is then mixed and reacted as an oxygen source in excess of a liquid or gaseous fuel stream 11 in the gas turbine combustion chamber. Flue gas 8 from the combustion chamber contains substantial residual amounts of oxygen and is subsequently passed to the gas turbine expander (GT Expander), in which the flue gas is depressurized to near atmospheric or elevated pressure, as appropriate for the HER combustion air. Oxygen containing flue gas stream 12 is passed to the HER and once more reacted as oxygen source in excess with liquid or gaseous fuel 6 in HER combustion chamber 9 providing a second flue gas stream at a sufficiently high temperature (typically 1300°C) and a high flow rate to supply heat for the heat requiring endothermic reforming process taking place in catalyst chamber 14 of the HER. Flue gas 10 from the HER being cooled on flue gas side in the heat absorbing HER reforming section 14 is further cooled in a common flue gas waste heat recovery section (WHS). Conditions employed and results obtained in the embodiment shown in Fig. 1 and the comparative example of Fig. 2 are summarized in Table 1 and Table 2, respectively. The lower heating value (LHV) for a fuel is the standard (25°C, 1 atm) heat of combustion with water formed at its hypothetical standard state as a gas at 25°C. Compared to conventional synthesis gas production in an HER without a gas turbine as shown in Fig. 3 and summarized in Table 3, considerable energy saving is obtained by the inventive method. Whereas a conventional HER, as shown in Fig. 3, requires power import of 8.9 MWh/h, excess of power is produced by combined production of power and synthesis gas. By the embodiment of Fig. 1,76 MWh/h power are exported from the plant. The conversion efficiency of additional fuel required in the combined production of synthesis gas and power is, thereby, 94% as apparent from Table 1. Comparatively, the method illustrated in Fig. 2 allows power export of 29.9 MWh/h at a conversion efficiency of additional fuel of 89% (cf. Table 2). Position in Fig. 1 20 40 60 80 100 P[kg/cm 2 g]/ (kPa gauge) 0 20 (1961) 20 (1961) 14.7 (1442) 14.6 (1432) T[°C] 15 25 25 559 1270 Flow [Nm 3 /h] 523687 35249 18339 544086 562896 LHV [kcal/Nm 3 ]/ (kJ/Nm 3 ) 9856 (41238) 9856 (41238) Composition: N 2 76.92 0.48 0.48 74.05 71.59 CO 2 0.03 1.56 1.56 4.14 7.66 O 2 20.69 12.18 4.89 Ar 0.93 0.89 0.86 H 2 O 1.43 0.14 0.14 8.74 15 Hydrocarbon 97.82 97.82 Power Export [MWh/h]= 76 Conversion efficiency of additional fuel [%] = 94 LHV = Lower Heating Value Position in Fig. 2 2 4 6 8 10 P[kg/cm 2 gauge]/ (kPa gauge) 0 20 (1961) 20 (1961) 15 (1471) 0 T[°C] 15 25 25 1000 619 Flow [Nm 3 /h] 538967 31181 19769 551661 573652 LHV [kcal/Nm 3 ]/ (kJ/Nm 3 ) 9856 (41238) 9856 (41238) Composition (vol%): N 2 76.92 0.48 0.48 75.16 72.3 CO 2 0.03 1.56 1.56 2.55 6.65 O 2 20.69 15.47 6.97 Ar 0.93 0.91 0.87 H 2 O 1.43 0.14 0.14 5.91 13.21 Hydrocarbon 97.82 97.82 Power Export [MWh/h] = 29.9 LHV = Lower Heating Value Position in Fig. 3 2 4 6 8 10 P[kg/cm 2 gauge]/ (kPa gauge) 0 20 (1961) 0 T[°C] 15 25 619 Flow [Nm 3 /h] 548604 27380 579060 LHV [kcal/Nm 3 ]/ (kJ/Nm 3 ) 9856 (41238) Composition: N 2 76.92 0.48 72.9 CO 2 0.03 1.56 5.79 O 2 20.69 8.76 Ar 0.93 0.88 H 2 O 1.43 0.14 11.67 Hydrocarbon 97.82 Power Import [MWh/h]= 8.9 LHV = Lower Heating Value

Claims: A method for the combined production of synthesis gas and electrical power including at least a step for the production of synthesis gas by combusting a second fuel stream with combustion air to flue gas and passing the flue gas in heat conducting relationship with steam reforming reactions of a hydrocarbon feedstock in a heat exchange reactor, and a step for the production of electrical power by combusting a first fuel stream with combustion air in a gas turbine combustion chamber to a flue gas and expanding the flue gas in a gas turbine, characterised in that the flue gas from the steam reforming step is combusted with the first fuel stream in the electrical power producing step. The method of claim 1, wherein the combustion air in the steam reforming step is pressurized air from a gas turbine compressor.

Inventors: HOLM-LARSEN HELGE, VOSS BODIL, HOLM-LARSEN, HELGE, VOSS, BODIL
Applicants: TOPSOE HALDOR AS, HALDOR TOPSOE A/S
Application year: 1997
Main classification: C10L3
IPC codes: C01B3, C10K3, C10L3, F02C6, F01K23, F02C7, F02C3