Method for Enhancing Combustion Reactions in High Heat Transfer Environments

Abstract

The present invention relates to a method of combusting a fuel gas with a stoichiometric or near stoichiometric amount of molecular oxygen in the presence of a controlled amount of a diluent to enhance the extent of combustion reactions in high heat transfer environment. The energy released is utilized to heat a fluid by direct contact with the flame. The diluent can be different from the fluid to be heated with respect to composition, temperature or pressure. The diluent can be same as or derived from the fluid to be heated.

Claims

1. A combustion method, comprising: a) providing a fuel, an oxidant, a diluent that is not a fuel, and a fluid to be heated; b) reacting the fuel with the oxidant in the presence of a diluent to form a first heated fluid; c) forming a heated fluid by directly contacting the fluid to be heated with the first heated fluid; wherein the heated fluid contains lower concentration of carbon monoxide, hydrogen, and/or molecular oxygen than obtainable by directly contacting the third fluid with an otherwise identical flame without dilution.

2. A direct contact heating method, comprising: a) providing a fuel, an oxidant, a diluent that does not contain fuel; b) reacting the fuel with the oxidant in the presence of a sufficient amount of the diluent to form a first heated fluid; c) forming a heated fluid by directly contacting the fluid to be heated in a controlled manner, such as through physical or aerodynamic staging, with the first heated fluid; wherein the heated fluid contains lower concentration of carbon monoxide, hydrogen, and/or molecular oxygen than obtainable by directly contacting the fluid to be heated with an otherwise identical flame without dilution.

3. The method of claim 1, wherein the diluent is a portion of the fluid to be heated or derived from the fluid to be heated.

4. The method of claim 2, wherein the diluent is a portion of the fluid to be heated or derived from the fluid to be heated.

5. The method of claim 1, wherein the diluent is different from the fluid to be heated with respect to one or more of temperature, pressure, and composition.

6. The method of claim 2, wherein the diluent is different from the fluid to be heated with respect to one or more of temperature, pressure, and composition.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0024] The objects and advantages of the invention will be better understood from the following detailed description of the preferred embodiments thereof in connection with the accompanying figures wherein like numbers denote same features throughout and wherein:

[0025] FIG. 1 is a plot of flame characteristics at equilibrium as a function of diluent concentration;

[0026] FIG. 2 is a plot of the total heat released as a function of diluent concentration;

[0027] FIG. 3 is a schematic of a direct contact heating process configuration;

[0028] FIG. 4 is a schematic of an alternate process configuration;

[0029] FIG. 5 is a plot of temperature and residual oxygen in heated stream as a function of diluent amount;

[0030] FIG. 6 is a plot of kinetic modeling results;

[0031] FIG. 7 is a plot of net heat released as a function of diluent amount; and

[0032] FIG. 8 is a plot of net heat released as a function of fluid initial temperature.

DETAILED DESCRIPTION OF THE INVENTION

[0033] For illustrative purposes the problem that the present invention addresses will be described with reference to a direct contact heat exchange system that heats a fluid stream by combusting a fuel gas containing methane and an oxidant containing molecular oxygen. For many applications operators seek to avoid adding anything other than pure combustion products (CO.sub.2 and H.sub.2O) to the fluid stream. Therefore, the oxidant fed is likely to be pure oxygen, and stoichiometric amounts of fuel and oxygen are utilized to provide the required heat.

[0034] Combustion of fuel such as methane with a stoichiometric amount of oxygen can result in a flame having an adiabatic flame temperature in excess of 5500 F. This flame, a heated gas mixture at such high temperature under chemical equilibrium conditions contains considerable amounts of dissociated products CO and H.sub.2. Thus, the heat available for direct contact heating from this heated gas mixture is only a fraction of the fuel calorific value. In accordance with the present invention, when such a heated gas mixture is formed by combustion reactions occurring in the presence of a diluent that is not a fuel then, the chemical equilibrium shifts towards more complete combustion products, in other words at least some of the dissociated CO and H.sub.2 form CO.sub.2 and H.sub.2O, respectively.

[0035] This is illustrated in FIG. 1 that plots results of chemical equilibrium calculations. Adiabatic flame temperature is plotted on the left-side y-axis against mole % of diluent in a pseudo mixture of diluent and oxygen as x-axis. The molar ratio of dissociated products [CO+H.sub.2] to complete combustion products [CO.sub.2+H.sub.2O] is plotted on the right-side y-axis against mole % of diluent in a pseudo mixture of diluent and oxygen as x-axis. The calculations assumed adiabatic combustion of methane (fuel) with stoichiometric amount of oxygen (oxidant) at 350 psia with combustion reactions occurring in the presence of various amounts of CO.sub.2 added as a diluent to the combustion reaction mixture. Without limiting how the diluent is added to the combustion region, the effective diluent concentration (x-axis values) allows one skilled in the art to estimate the amount of diluent required to practice the current invention. The effective diluent concentration in this exemplary embodiment assumes pure CO.sub.2 is fed to the combustion region such that the combustion reaction mixture gets diluted by the amount of CO.sub.2 required to achieve the desired flame characteristics. In the absence of diluent or when the effective diluent concentration is low the chemical equilibrium calculation results suggest the flame, a heated gas mixture contains significant amounts of dissociated species CO and H.sub.2. Under such conditions unreacted oxidant (residual oxygen) remains in the combustion mixture since it has not been consumed by the H.sub.2 and CO. Contacting a large quantity of a fluid to be heated with the flame could quench the combustion process too soon, limiting conversion of dissociated species, thus resulting in higher amounts of CO, H.sub.2, and/or O.sub.2 in combined, heated fluid. This effect is illustrated in FIG. 2 using the same methane-oxygen flame as FIG. 1. Therefore, it is important that the residence time and temperature of reactive species in the flame be managed in a controlled manner to decrease the degree of dissociation and residual amounts of unreacted oxygen, fuel, and dissociated species in the heated gas mixture and increase the heat released. The present invention identifies a promising solution that delays mixing of fluid to be heated with a flame until the combustion reaction is complete; and reducing flame/reaction temperature to maximize conversion of dissociated species into complete combustion products.

[0036] Turning to FIG. 3, and as illustrated in this exemplary embodiment, a process configuration is shown that heats a fluid 10 flowing through a conduit 100 or equivalent conveyance. Conduit 100 is made of any material capable of carrying the fluid stream at temperatures and pressures suitable for downstream operations. A combustion device 200 is positioned to be in fluid communication with fluid flowing through conduit 100. Oxidant and fuel are combusted in the combustion device in the presence of a diluent 20 to form a first heated gas mixture. The first heated gas mixture containing heat released from combustion reactions is utilized to heat the fluid stream 10.

[0037] It should be noted that FIG. 3 depicts introduction of diluent 20 into the combustion device, which may have an enclosure forming a combustion chamber within which the fuel and the oxidant combust before their combustion products contact the fluid stream 10 flowing through the conduit 100. As shown in FIG. 3, two pipes at different sizes can be mounted to the same flanges on which a burner is installed. The inner pipe, a shroud which is smaller, will enclose the burner flame and it will have ports drilled closer to the burner face. Diluent introduced into the annular space between the smaller and larger pipes, for example from the ports on the flanges that are in circular pattern surrounding the burner will be entrained into the inner tube thus mixing with the combustion reaction mixture. It also provides cooling to the inner tube wall. The amount of diluent added to the combustion mixture depends on several factors, including the size of the inner tube and the opening area of the ports on the inner tube. The length of the inner tube is another critical parameter in this design since it is important that the combustion is fully completed at the end of this inner tube before mixing with fluid to be heated takes place.

[0038] The amount of injection, and its location, could be controlled by the size and number of penetrations in the shroud. Although the shroud design may look similar to that suggested by U.S. Pat. No. 7,770,646, the purpose of the shroud is different. The penetrations near the flame would be designed to ensure mixing with the flame species in the actual flame, as opposed to avoiding the flame to just provide cooling for the shroud (as in U.S. Pat. No. 7,770,646). The driving force for diluent flow through the perforations could be either higher pressure of the diluent fluid, or by entrainment from the flame (similar to a venturi). Through knowledge of the mixing characteristics of the burner and careful design of the perforated shroud control of mixing the diluent fluid can be controlled to maximize heat release.

[0039] The shroud material should be chosen to avoid corrosion or other mechanical failures. Using an oxyfuel flame that could potentially attach to the shroud may limit the material choices significantly. One material that could be, for example, utilized is sintered silicon carbide, which is advertised to have very high thermal conductivity, high resistance to thermal shock stresses, and high corrosion resistance in oxidizing, reducing or other corrosive atmospheres.

[0040] In another exemplary embodiment the diluent 20 can be derived from fluid stream 10. The combustion device may consist of just inner pipe, shroud with ports drilled on this pipe at closer to the burner end thus allowing proper amount of diluent addition to combustion reaction mixture. In yet another exemplary embodiment of the present invention an entrainment limiting device can be employed. In this embodiment the shroud may or may not have ports drilled to facilitate diluent addition to the combustion reaction mixture. A recess placed around the burner limits the amount of fluid stream 10 added as a diluent to the combustion mixture reaction mixture. Through an understanding of the entrainment/mixing pattern of the burner it is possible to control the amount of diluent that is drawn in through the exit of the recess. For example, if the recess is very short, then the jet behaves very similar to a free jet in a fluid stream with rapid mixing of the fluid into the flame products. In the other extreme if the recess is very long then no fluid is drawn into the flame. Basic burner characteristics, such as fuel and oxidant nozzle sizes, can also be used to control the degree of mixing.

[0041] This way controlled mixing of a sufficient quantity of a diluent with the combustion reaction mixture can be provided. The degree of mixing is defined based on the burner design and the recess design, and can be calculated using jet entrainment correlations, computational fluid dynamics (CFD) or other tools available to those skilled in the art. For instance, in the exemplary embodiments described above the controlled mixing of diluent with the combusting reaction mixture of fuel and oxidant results in reduced residual oxygen, fuel, and/or dissociated combustion products than if the diluent had not been added. In contrast, uncontrolled mixing of a large quantity of fluid to be heated with an otherwise identical flame without dilution can rapidly quench the flame, not allowing sufficient time for dissociated radicals to react and form CO.sub.2 and H.sub.2O. The controlled mixing avoids the quenched radicals inability to recombine.

[0042] FIG. 5 is a simplified schematic of yet another exemplary embodiment. This process configuration employs plurality of stages. As shown, a small, controlled, amount of the fluid to be heated is entrained into the burning fuel-oxygen mixture. The resulting first heated fluid will contain less products of incomplete combustion. At the exit of the shroud the remaining fluid to be heated mixes quickly with the first heated fluid, resulting in the desired heated fluid. Since the CO and H.sub.2 are avoided in the first stage, that is, the equilibrium temperature and concentration of the first heated mixture favors CO.sub.2 and H.sub.2O, the overall combustion efficiency is improved. Since the mixing, and therefore quench time between the first heated fluid and the fluid to be heated can impact the net heat release, it may be advantageous to mix in multiple stages as shown in FIG. 4. First, a small amount of diluent is mixed entrained into the shroud to mix with the burning fuel and oxidant to form the first heated mixture. A controlled amount of the fluid to be heated is introduced into a first stage. Mixing of this portion of the fluid to be heated with the first heated mixture results in an intermediate heated fluid with a high enough temperature to enable the combustion reactions to be complete and reach an equilibrium concentration, or near equilibrium, that contains little or no CO or H.sub.2. The remaining fluid to be heated is mixed with the intermediate heated fluid to generate the final desired heated fluid. The method shown in FIG. 4 is particularly useful when the introduction of the diluent could cause problems with flame stability or pollutant formation.

[0043] In accordance with the present invention a diluent that is not a fuel can be utilized to modify the flame reaction mixture. Although excess oxidant could be used as the diluent, the resulting increased O.sub.2 in the product could be detrimental to many applications. The diluent can be introduced separately or mixed with oxidant or mixed with fuel or mixed with both oxidant and fuel. The amount of diluent added to the flame is controlled to modify the residence time and temperature of reactive species in a manner that promotes complete combustion. The resulting flame is allowed to interact with at least a portion of the fluid to be heated, forming a high temperature gas. The high temperature gas then mixes with the remaining portion of fluid to form the heated fluid.

[0044] In an exemplary embodiment the diluent and the fluid to be heated are different fluids. This could be advantageous in avoiding pollution formation. For example, the diluent can be a non-nitrogen containing fluid to avoid formation of NOx in the first zone if the fluid to be heated contains nitrogen. Yet in another embodiment the diluent may be derived from the fluid to be heated. And most importantly the introduction of fluid to be heated into the combustion products must be sufficiently slow to accommodate complete reaction between CO, H.sub.2, and O.sub.2 prior to thermal quenching this reaction.

[0045] The present invention identifies a promising solution that delays mixing of fluid to be heated with a flame until the combustion reaction is complete; combusting a fuel with molecular oxygen containing gas in the presence of a diluent to form a flame, a heated gas mixture containing higher amounts of heat released than possible when combusting in the absence of a diluent. Additional benefits include less severe process conditions since the flame/heated gas mixture temperature can be considerably lower when the diluent is used compared to that when the diluent is absent. The invention is further explained through the following examples based on various embodiments of the invention, which are not to be construed as limiting the present invention.

Example

[0046] The process configuration depicted in FIG. 3 was modeled to heat a CO.sub.2 stream. The CO.sub.2 stream mixes rapidly in a controlled manner with the products of flame formed by combustion of a fuel gas such as methane with an oxidant containing molecular oxygen. For this particular example the burner was assumed to be a simple coannular type (i.e. no swirl) designed to combust 1000 scfh methane and 2000 scfh of oxygen, generating a turbulent diffusion flame. Heat released by the combustion heated a stream of 10,000 lb/hr CO.sub.2 at 200 psig by direct contact. The mixing of CO.sub.2 can be controlled, in part through the selection of the burner geometry, including the configuration of any recess. Since no devices or methods, such as bluff bodies or swirl were assumed to increase flame zone mixing, the mixing in the flame zone was estimated using standard entrainment correlations for reacting turbulent diffusion flames known in the art. This estimate of mixing rate was coupled with a kinetic model where the flame and the post flame region were described by a series of perfectly stirred reactors (PSR). The amount of gas entrained into the flame at each location was added to the PSR for that location. Using this modeling technique, direct contact heating scenarios with varying amounts of CO.sub.2 mixing with the O.sub.2/methane combustion reaction mixture followed by remainder of the CO.sub.2 mixing with the flame products to form the heated CO.sub.2 stream were modeled. The 10,000 lb/h CO.sub.2 stream was divided into two portions. The first portion was added to the perfectly stirred reactors (PSR) describing the flame region. This first portion served as a diluent in whose presence the combustion reactions proceeded. The product of the flame region, heated gas mixture provided the thermal energy for heating the second portion (remainder of the CO.sub.2). Both the heated gas mixture (flame) and the second portion of CO.sub.2 served as feed to the PSR describing the post flame region that produced the heated CO.sub.2 stream. The oxygen conversion was estimated assuming residence time to be 200 milliseconds. The modeling results are plotted in FIG. 5 with the amount of diluent added to the combustion reaction mixture as x-axis, final temperature of heated CO.sub.2 stream as left-side y-axis and the ratio of residual oxygen in the heated CO.sub.2 stream to that fed to the burner as right-side y-axis. To establish a base line, the common prior art practice that restricts mixing between the CO.sub.2 and the O.sub.2/methane in the flame zone was also modeled. This could be accomplished through such means as separating the flame completely from the CO.sub.2. As can be seen from FIG. 5 this condition (0 lb/hr CO.sub.2 into flame) results in low final CO.sub.2 temperature and high residual O.sub.2, both indicators of incomplete combustion. FIG. 5 also shows that when the diluent, CO.sub.2 is present with the O.sub.2/methane reaction mixture in the flame zone, then there exists an optimal diluent amount that maximizes performance for the particular process configuration modeled.

[0047] The output of the kinetic modeling, shown in FIG. 6, highlights the advantages of the subject invention. As shown in FIG. 6, the amount of oxygen contained in the flame increases along the length of the flame in the absence of diluent addition (i.e. no CO.sub.2 is added to the combustion reaction mixture). This is the result of the flame at or near chemical equilibrium containing high concentrations of dissociated products. This is illustrated in Table 1 for the conditions outlined in this example.

Table 1. Comparison of Equilibrium and Kinetic Results for Example System

[0048]

TABLE-US-00001 TABLE 1 Comparison of equilibrium and kinetic results for example system Optimal CO2 in flame No CO2 in flame Equilibrium Kinetic Equilibrium Kinetic T (F.) 2682 2654 5674 5674 Gas (vol %) H2 0.01% 0.02% 6.16% 6.12% H 0.00% 0.00% 3.09% 3.08% O 0.00% 0.00% 2.79% 2.80% O2 0.05% 0.24% 7.55% 7.64% OH 0.01% 0.02% 9.45% 9.47% H2O 18.35% 18.26% 43.14% 43.12% HO2 0.00% 0.00% 0.02% 0.02% CO 0.11% 0.36% 14.93% 14.87% CO2 81.47% 81.09% 12.86% 12.89%

[0049] As can be seen from the table the no-CO.sub.2 added flame contains a significant amount of oxygen and oxygen containing radicals at equilibrium. When the remaining CO.sub.2 is mixed quickly with this stream this residual oxygen increases due to recombination of the oxygen containing radicals and quenching of the CO/H.sub.2 oxidation reactions. However, under the optimal conditions for this example the amount of O.sub.2 and oxygen containing radicals in the flame is very low. Therefore the quenching of this flame by addition of the remaining CO.sub.2 still results in low residual oxygen.

[0050] Different operating conditions may lead to different optimal mixing rates of the diluent into the flame. This is illustrated in FIGS. 7 and 8 that plot results for heating steam or CO.sub.2 at different pressures and initial temperatures. Some conditions require lower oxygen concentrations while others need higher oxygen concentrations. Rapid quenching of the flame prevents the dissociated species, particularly CO and H.sub.2 present in the flame from reacting with the remaining O.sub.2. Therefore, the net heat released and available for heating the fluid is reduced. The presence of diluent can shift the chemical equilibrium towards more complete combustion products, leading to a higher net heat release. However, at some point the kinetics are slowed down enough due to the presence of the diluent, that the reactions are not complete before the fluid to be heated is introduced. Therefore, the present invention serves to mix enough diluent into the flame zone to maximize the net heat release while still completing the reactions. This method both increases the efficiency of the heating process, as well as reduces the amount of residual species such as CO, H.sub.2, and O.sub.2 in the heated fluid.

[0051] As described above there are several ways to provide the required amount of diluent in whose presence the combustion reactions proceed. For example, burner/combustor design features such as recess geometry, nozzle sizes can be selected to entrain the desired amount of CO.sub.2 into the flame zone. Note, when the CO.sub.2 mixing rate into the flame is higher than the optimum, the resulting flame dilution leads to degradation of the combustion reactions in the flame zone itself. For burner designs employing devices or methods, such as bluff bodies or swirl or defined recess geometry to increase flame zone mixing, computational fluid dynamics (CFD) technique can be used to predict the impact of mixing some of the CO.sub.2 directly into the flame zone.

[0052] Although various embodiments have been shown and described, the present disclosure is not so limited and will be understood to include all such modifications and variations as would be apparent to one skilled in the art.