DK201500463A1 - Chemical recuperation of hydrocarbonic fuel using concentrated solar power - Google Patents

Abstract

In a process for reforming natural gas to a mixture of hydrogen and methane comprising hydrodesulfurization of the natural gas feed, pre-reforming and reforming of the hydrodesulfurized feed, separation of the hydrogen-rich fuel gas from the hydrogen-lean gas, recycling the hydrogen-lean gas to the pre-reforming and reforming units, and feeding the hydrogen-rich fuel gas to a power plant, the endother mic reaction, which produces the mixture of hydrogen and methane, is promoted by concentrated solar power via a heating medium, preferably a molten salt or a mixture of molten salts. Thereby it becomes possible to improve the overall process economy in connection with construction and operation of power plants fuelled by natural gas in regions with high natural gas prices.

Description

Title: Chemical recuperation of hydrocarbonic fuel using concentrated solar power

The present invention in general relates to chemical recuperation of hydrocarbonic fuels, especially natural gas, using concentrated solar power. More specifically, the invention relates to a process, in which ordinary prereforming and reforming catalysts are utilized to reform natural gas to a mixture of hydrogen and methane. The product gas, which has a higher energy content (heating value) than the natural gas, can subsequently be used in a power plant to generate electricity.

The energy augmentation can be up to 10% depending on the reformer temperature.

The product gas is produced in an endothermic reaction, in which the reaction is provided by concentrated solar power via a heating medium, such as a molten salt or a mixture of molten salts.

The present process is especially relevant in connection with the construction and operation of power plants fueled by natural gas, where the process has a major impact on the overall process economy.

The production of hydrogen gas by reacting a carbon-hydrogen containing species with water is described in US 7,537,750 B2. The water is provided as a water reforming in-flow at a reforming temperature to produce a primary reacted gas flow containing hydrogen gas. The carbon-hydrogen containing species is heated together with water to the reforming temperature with solar energy.

The heating can be performed by heating a molten metal to at least the reforming temperature with solar energy and using the molten metal to heat the carbon-hydrogen containing species and water to at least the reforming temperature. The water reforming inflow is pre-heated by heat exchange from the primary reacted gas flow, the latter being reacted with water to produce additional hydrogen gas in a secondary reacted gas flow.

US 2013/0181169 A1 describes a method and a system for reforming hydrocarbon gas, which includes stripping from the hydrocarbon gas at least most of gaseous impurities of a type and/or quantity which would normally interfere with efficient catalytic reforming in order to provide stripped hydrocarbon gas including carbon dioxide. The stripped hydrocarbon gas is optionally compressed and then reacted in a solar radiation receiving reactor having a catalyst that is heated by concentrated solar radiation impinging thereon. Thereby an output gas mixture comprising hydrogen gas and carbon monoxide is provided.

A process for the production of hydrogen in a reactor system comprising a steam reforming reaction zone is disclosed in WO 2013/137720 A1. The steam reforming reaction zone comprises a reforming catalyst and a membrane separation zone comprising a hydrogen-selective membrane. The process involves a reaction system of so-called open architecture, wherein the reforming zone and the membrane separation zone operate independently of each other. The heat for the re forming reaction is provided through heat exchange from liquid molten salts, preferably heated by solar energy.

In US 2012/0274078 Al, a hybrid concentrated solar combined cycle (CSCC) power plant based on solar reforming technology, a method of generating electricity using the system and a solar reformer for use in the system are provided. The system enables integration of a concentrated solar power plant (CSP) and a combined cycle gas turbine (CCGT) power plant, resulting in a hybrid system that can correct known issues related to large-scale concentrated solar power generation. The solar reformer provides for the storage of solar energy in a reformate fuel during the reforming reaction and subsequent release through a gas turbine combustion reaction. Fuels directed into the gas turbine power plant can be alternated between hydrocarbon fluid and the reformate fuel dependent upon available solar thermal energy.

EP 2 607 303 Al discloses a method of storing energy in the form of electric current or heat in a reactant gas mixture containing methane by partial conversion of methane under addition of water vapor to hydrogen, carbon dioxide and carbon monoxide by an endothermic partial reforming process, as a result of which a product gas mixture arises.

The energy is stored into the reactant gas mixture. The reforming process is based on a reaction temperature in accordance with a chemical equation, where methane is reacted with water to give three moles of hydrogen and CO, a Sabatier-inverse reaction, where methane is reacted with two moles of water to give four moles of hydrogen and CO; and/or a shift-reaction, where the CO is reacted with the water to give one mole of hydrogen and carbon dioxide. The reactant gas mixture is natural gas and/or a fermentation gas. A proportion of the hydrogen created by the endothermic partial reforming process is controlled by the reaction temperature of the partial reforming process and by a pressure at which the partial reforming process occurs. The reforming process is carried out by catalysts at a reaction temperature of 250-350°C. The methane contained in the reactant gas mixture is converted by a partial reformation of 2-5%. The gas mixture resulting after the partial reforming corresponds to guidelines for natural gas quality in accordance with DVGW-guidelines (technical rule, worksheet G260, gas composition, May 2008). The stored energy originates from a regenerative energy source that is a wind turbine. The heat is removed from the product gas to preheat the reactant gas mixture and to evaporate the added water. The hydrogen and carbon dioxide contained in the resultant product gas mixture are reacted before consumption of the product gas mixture by a partial exothermic methanation in methane and water vapor. A common reactor vessel is used for both the partial reformation process and the methanation. A device for storing energy in the form of electric current or heat in a reactant gas mixture containing methane by partial conversion of methane under addition of water vapor to hydrogen is also disclosed.

WO 2014/107561 A1 describes a process for converting carbon dioxide to hydrocarbon fuels using solar energy harnessed with a solar thermal power system to create thermal energy and electricity, using the thermal energy to heat a fuel feed stream to a temperature of 650-800°C. The heated fuel feed stream comprising carbon dioxide and water is fed to a syngas production cell comprising a solid oxide electro- lyte, wherein the carbon dioxide and water are converted to carbon monoxide and hydrogen to produce a syngas stream. This syngas stream is then fed to a catalytic reactor, in which the syngas stream is converted to a hydrocarbon fuel stream.

The use of an external carbon-free source like solar energy to provide heat for highly endothermic conversion processes like steam methane reforming (SMR) has been studied and reported at the 17th World Hydrogen Energy Conference, Brisbane, Australia, 15 to 19 June 2008. The option of using molten nitrates as solar heat carriers and storage media to continuously provide SMR process heat is presented in the report. A molten nitrate mixture of NaNCR and KNO3 has interesting features as a large scale storage medium and has been positively long tested as solar heat carrier and heat storage medium up to 565°C. Thus, a constant-rate heat supply for an energy demanding process like SMR, avoiding daily start-up and shut-down operations, can be achieved despite the intermittent solar power availability.

Finally, in DE 39 33 284 C2, solar energy is converted continuously into high-temperature heat of a fluid which is stored and transformed to electrical energy. The reaction partner of a methane reforming process is heated to produce a gas which is stored with a high energy density and is directed to a fuel cell for electrical energy generation. Preferably solar energy received by a radiation collector is transferred to a gas, e.g. air or carbon dioxide which passes through a pipe to a reformer with cracking tubes and back via a cooler and a compressor. The cracking tubes receive methane and water from a heat exchanger in which the gas produced, which is high in H2, is cooled. A shift converter raises the hydrogen share, and a pressure vessel eluates further carbon dioxide shares. The hydrogen is passed to a fuel cell together with air or oxygen. This minimises the volume required for storage and achieves a better efficiency.

The present invention relates to a process for reforming natural gas to a mixture of hydrogen and methane, said process comprising the following steps: a) hydrodesulfurization of the natural gas feed in a hydrodesulfurization (HDS) unit, b) pre-reforming and reforming of the hydrodesulfurized feed in a pre-reforming unit followed by a low temperature reforming unit, c) separating the hydrogen-rich fuel gas from the hydrogen-lean gas, d) recycling the hydrogen-lean gas to the pre-reforming and reforming units, and e) feeding the hydrogen-rich fuel gas to a power plant, wherein the endothermic reaction, which produces the mixture of hydrogen and methane, is promoted by concentrated solar power via a heating medium.

The heating medium is preferably a molten salt or a mixture of molten salts.

The invention is further illustrated by the drawings, where Fig. 1 shows a typical plant for running the process, and Fig. 2 is a plot illustrating the payback time (in years) as a function of the NG price (in $/Nm3) for various power plants. The invention is described in more detail in the following examples. The invention, however, is not limited to these examples.

Example 1

Referring to Fig. 1, a natural gas stream (1) containing 90% methane and 5% inerts at 40 barg is mixed with hydrogen containing recycle gas (15) and pre-heated to 350°C using superheated steam (2). A small amount of steam (3) is added to the pre-heated gas. The resulting mixed gas (4) is then introduced to the desulfurization reactors (hydrogenation and hydrogen sulfide removal) R1.

The sulfur-free gas (5) is mixed with more superheated steam (19) at 484°C, and the resulting mixture (5) at 428°C is then passed through the pre-reformer reactor R2 wherein the higher hydrocarbons, such as ethane and propane, together with a fraction of methane are converted to hydrogen and carbon oxides. The effluent gas (6) from R2 at 400°C is mixed with the recycle gas (14) and introduced to the steam reformer reactor R3. The inlet gas (7) enters the reformer at 196°C. The reformer is a tubular reactor with a molten salt (20) flowing in its shell side at 650°C. The molten salt is circulated between the solar tower and the reactor (20, 21). The heat of the molten salt provides the heat necessary to the endothermic steam reforming reaction. The tubes are loaded with a steam reforming catalyst, and the gas passes through them and exits the reactor at 640°C.

The reformed gas (8) is cooled to 40°C in the heat exchanger trains of E2, E3 and E4. Boiler feed water (16) is preheated in E6 and fed to the boiler E3. The saturated steam from the boiler E3 is split into two streams: one fraction as export stream (17) and another fraction (18) to produce superheated steam in the super-heater E2.

The cooled product is flashed into the separator vessel VI wherein water (10) is separated from the gas product (11).

A hydrogen-rich stream (12) is recovered from the gas product as the final product in the gas-gas separation unit Ml. A possible separation unit can be a membrane unit or a pressure swing adsorption (PSA) unit. In this example, a membrane unit is utilized, producing the hydrogen-lean gas (13), which is pressurized in the compressor Cl and then recycled back to the reaction loop.

The results appear from the tables below.

Table 1

Composition of streams in percent

Table 2a

Stream specifications

Table 2b

Stream specifications (continued)

Table 2c

Stream specifications (continued)

Example 2

Natural gas (NG) is a very attractive energy source, but the exploitation thereof is rather dependent on the actual price of NG. The main purpose of the process according to the invention is to increase the heating value of NG by utilizing solar heat via a molten salt. In the process, NG is partially reformed (endothermic reaction) to a hydrogen-containing gas with higher heating value. The added energy comes from the solar energy.

Fig. 2 is a plot illustrating the payback time (in years) as a function of the NG price (in $/Nm3) for various power plants. It appears from the figure that the NG price is very different in different regions around the world, USA and the Middle East being cheapest, while Europe is medium priced and Japan being quite expensive.

Further it appears from the figure that the higher the NG price, the better is the overall economy of the process. With sufficiently cheap NG available there is no real point in going through a complex/expensive process like the process of the invention to increase the heating value by a few percent (in the present process, the energy augmentation can be up to 10% depending on the reformer temperature) . Therefore, the process of the invention is especially desirable in countries or regions (Japan in this example) where NG has to be imported at a high price.

Claims (3)

1. A process for reforming natural gas to a mixture of hydrogen and methane, said process comprising the following steps : a) hydrodesulfurization of the natural gas feed in a hydrodesulfurization (HDS) unit, b) pre-reforming and reforming of the hydrodesulfurized feed in a pre-reforming unit followed by a low temperature reforming unit, c) separating the hydrogen-rich fuel gas from the hydrogen-lean gas, d) recycling the hydrogen-lean gas to the pre-reforming and reforming units, and e) feeding the hydrogen-rich fuel gas to a power plant, wherein the endothermic reaction, which produces the mixture of hydrogen and methane, is promoted by concentrated solar power via a heating medium.

2. Process according to claim 1, wherein the heating medium is a molten salt or a mixture of molten salts.

3. Use of the process according to claim 1 or 2 to improve the overall process economy in connection with construction and operation of power plants fuelled by natural gas in regions with high natural gas prices.