Patent Application
20090011290
1. Field of the Invention
This invention relates to heat and thermal chemical processes, in particular, recuperation systems of gas-fired devices. More particularly, this invention relates to thermochemical recuperators and the use of thermochemical recuperators as fuel reformers for providing reformed fuel to a combustion process.
2. Description of Related Art
Natural gas is the most abundant energy source available after coal. Because it is inexpensive and burns very cleanly relative to other energy sources, particularly with respect to coal, more ways are continually being investigated for using natural gas as a fuel. In addition, because natural gas is in abundant supply, using more natural gas as an energy source provides a means for reducing dependence on imported foreign oils.
Over the past several years, fuel cells, which typically use hydrogen (H2) as a fuel, have been receiving a substantial amount of attention due to their almost emission-free operation. The primary exhaust from a fuel cell using hydrogen, as with other systems in which hydrogen is used as a fuel, is water. It will, thus, be apparent that substantial environmental benefits may be realized from the use of hydrogen as a fuel in applications other than fuel cells, such as combustion processes in industrial furnaces and the like. However, one problem associated with the use of hydrogen in such applications is the requirement for ready availability of the hydrogen in a form suitable for use therein. Thus, one issue which needs to be addressed is the production of H2 in a manner which satisfies the availability requirements.
Several reforming technologies to produce H2 are known, including autothermal reforming, partial oxidation reforming, plasma reforming, and steam reforming. Reforming of natural gas or other hydrocarbons produces H2-enriched products which, in addition to H2, may also include CO, CO2, and carbon. At the present time, about 90% of the hydrogen produced around the world is from reforming natural gas, as a result of which demand for natural gas is increasing considerably. Recently, efforts to develop various kinds of fuel reformers to reform liquid or gaseous fuels to produce H2-enriched fuels have increased substantially. Most of these reformers use steam reforming technology, which requires heat and steam.
In a typical combustion process, a significant amount of energy is wasted. Thus, if this energy can be used to reform a lower quality fuel to produce a higher quality fuel, combustion efficiency will increase significantly.
It is, thus, one object of this invention to provide a method and apparatus for increasing the efficiency of conventional combustion processes.
It is another object of this invention to provide a method and apparatus for decreasing the fuel consumption of conventional combustion processes.
It is another object of this invention to provide a method and apparatus for thermochemical recuperation reforming using exhaust gas from a combustion process as a thermochemical fuel conversion reactant.
These and other objects of this invention are addressed by a system for fuel reforming comprising at least one wall enclosing a combustion chamber having a primary fuel inlet, a primary oxidant inlet, and an exhaust gas outlet. Disposed downstream of the combustion chamber is a first stage heat exchange vessel having a first stage exhaust gas inlet in fluid communication with the exhaust gas outlet of the combustion chamber and having a first stage exhaust gas outlet. A reformer fuel conduit having a reformer fuel inlet in fluid communication with a reformer fuel supply and having a reformed fuel outlet in fluid communication with the combustion chamber is disposed within the first stage heat exchange vessel. To enable use of the exhaust gas as a reactant in the reforming process, the first stage exhaust gas outlet is in fluid communication with the reformer fuel inlet. In this manner, the exhaust gas from the combustion chamber is used to increase the enthalpy of the fuel and the combustion oxidant, e.g. air, and to increase the chemical energy of the fuel. The increased enthalpies and chemical energy of the fuel provide higher heat input to the combustion chamber, increase efficiency of the combustion process and decrease the fuel consumption by the combustion process.
In accordance with the method of this invention, the exhaust gas is used for chemical conversion (reforming) of a fuel (referred to herein as a “reformer fuel”) to a state of higher chemical energy and for combustion air preheat. In order to reform the reformer fuel, it is mixed at a certain ratio with the exhaust gas, after which the mixture is heated in a reformer to reform the fuel. As a result of the reforming process, the reformed fuel that is produced contains hydrogen and carbon monoxide which may be supplied to burners for combustion. The thermal reforming process is accompanied by absorption of a considerable amount of heat (endothermic process), thus recovering much more heat in comparison with conventional thermal-only recuperation that leads to a potential for a substantial increase in furnace productivity and improved product quality, a substantial increase in system thermal efficiency, reduction in specific consumption of fuel, and considerable reduction in pollutant emissions. The amount of the exhaust gas for reforming may be slightly higher, equal to, or less than theoretical values to provide the highest thermal process efficiency. A reformed fuel cooling device may be used to reduce the temperature of the fuel at the combustion chamber fuel inlet and/or partially remove moisture from the fuel.
These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein:
As used herein, the term “combustion process” refers to the burning of a fuel in a combustion chamber, such as an industrial furnace, and explicitly excludes the combustion of a fuel in internal combustion engines, turbines, and the like.
This invention relates to thermochemical recuperation systems with exhaust (flue) gas/fuel reforming. The recuperative process is characterized by chemical reaction temperatures in the range of about 500° F. to about 1800° F. and can take place at a low absolute pressure (about 14.7 psig or lower) or high pressure (higher than 14.7 psig). Higher temperature is needed to achieve higher reforming rates of the fuel and higher efficiencies. Exhaust gas/fuel ratio depends upon the type of fuel used in the combustion process. For natural gas or methane (CH4), the exhaust gas/methane optimal mole ratio is from about 1:100 to about 1:3 (⅓×{CO2+2H2O+7.52N2}+CH4) depending upon temperature and exhaust gas composition. The stoichiometric, theoretical exhaust gas mole composition is CO2+2H2O+7.52N2. Reforming processes with lower methane ratios (<1:100) have an essential lack of oxidizers (H2O and CO2) in the reforming mixture; thus, reforming is not efficient. Reforming processes with higher ratios (>1:3) have an excess of essential oxidizers and are favorable for the reforming rate but less favorable with respect to efficiency. For natural gas or methane/steam reforming, the optimal steam/fuel ratio is from about 1:30 to about 2:1 (H2O+CH4).
In accordance with one embodiment of this invention, at least one of the heat exchange vessels employed in the system of this invention is provided with heat transfer means for promoting or enhancing the transfer of heat between the exhaust gases entering the heat exchange vessels and the materials and fluids disposed within the fuel reformer conduits. Such heat transfer enhancements include, but are not limited to, extended heat transfer surfaces such as fins and studs connected with the fuel reformer conduits, vortex generators such as dimples and winglets formed by the inner and/or outer surfaces of the fuel reformer conduits, and fluidized bed and porous matrix techniques.
Mixing of the reformer fuel and the exhaust gas is accomplished by mixer 26 having mixer exhaust gas inlet 27, which is in fluid communication with second stage exhaust gas outlet 22, and having mixer reformer fuel inlet 28, which is in fluid communication with the reformer fuel supply. By virtue of this arrangement, exhaust gas exiting first stage heat exchange vessel 11 through first stage exhaust gas outlet 17 is lower in temperature than the exhaust gas entering first stage heat exchange vessel 11 through first stage exhaust gas inlet 16 and reformer fuel flowing through reformer fuel conduit 18 is reformed.
Disposed within second stage heat exchange vessel 12 in heat exchange relationship with exhaust gas flowing there through is oxidant conduit 23 having oxidant inlet 24 in fluid communication with an oxidant source and having oxidant outlet 25 in fluid communication with combustion chamber 10 by virtue of preheated oxidant line 29. By virtue of this arrangement, exhaust gas flowing through second stage heat exchange vessel 12 heats the oxidant flowing through the oxidant conduit, and the exhaust gas exiting from the second stage heat exchange vessel is lower in temperature than the exhaust gas entering the vessel.
Exhaust gases from combustion processes may be chemically aggressive and cause reformer corrosion. In such cases, it is desirable to clean or chemically treat the exhaust gases prior to being introduced into the reformer. A sorbent bed may be used to prevent or reduce chemically aggressive contaminants in the exhaust gases. Accordingly, in accordance with one embodiment of this invention as shown in
An alternative means for protecting the TCR surfaces against corrosion is to fabricate all surfaces from materials that are chemically inert in the presence of the exhaust gas. These materials include, but are not limited to, a range of steel alloys, ceramics, intermetallics (such as silicon carbide), and composites made by putting a permanent or sacrificial coating on the TCR surfaces.
Others means for protecting the TCR surface against corrosion are shown in
In accordance with one embodiment of this invention, which embodiment is particularly suitable for use with relatively low temperature exhaust gases, i.e. temperatures less than about 800° F., the system comprises an additional two stages of exhaust gas heat exchange as shown in
One of the key requirements of the thermochemical reforming process of this invention is the necessity of effectively utilizing the supply of heat into the chemical reaction zone per unit of time. Because the reaction is endothermic and proceeds only with the addition and effective use of external heat (such as from hot exhaust gas) per unit of time, heat transfer enhancements in accordance with one preferred embodiment of this invention are provided to optimize the transfer of the external heat to the reforming process. As shown in
A method for thermochemical recuperation in accordance with one embodiment of this invention comprises introducing a heated exhaust gas from a combustion chamber into a first stage heat exchange vessel, producing a cooler exhaust gas, followed by introducing the cooler exhaust gas into a second stage heat exchange vessel, producing a further cooled exhaust gas. As the exhaust gas flows through the first heat exchange vessel, the further cooled exhaust gas is mixed with a reformer fuel, such as natural gas, to form a fuel/exhaust gas mixture, which mixture is then introduced into at least one reformer conduit disposed within the first stage heat exchange vessel in heat exchange relationship with the heated exhaust gas, resulting in the production of a reformed fuel. A primary combustion oxidant, i.e. air, oxygen-enriched air or oxygen, is introduced into at least one oxidant conduit disposed in the second stage heat exchange vessel in heat exchange relationship with cooler exhaust gas, producing heated primary combustion oxidant, which is then introduced together with the reformed fuel into the combustion chamber in which the reformed fuel is combusted. In accordance with one embodiment of this invention, the fuel/exhaust gas mixture is preheated prior to being introduced into the at least one reformer conduit. In accordance with one embodiment of this invention, the primary combustion oxidant is preheated prior to being introduced into the at least one oxidant conduit. In accordance with yet another embodiment of this invention, the heated exhausted gas is passed through a sorbent bed for removal of the corrosive contaminants prior to being introduced into the first stage heat exchange vessel.
While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.
