Patent Application
20070105060
The present invention relates to burners for fluid fuels and in particular burners for volatile organic fuels such as natural gas or a fuel rich in natural gas. More particularly the present invention relates to heaters having improved radiant efficiency and which burn cleaner.
There is a significant field of art relating to non-flame catalytic combustion. In this type of process a fuel to be burned passes over a supported catalyst at an elevated temperature and the fuel is consumed. The most common of this type of catalytic burner is the catalytic converter in cars. The support is typically in the shape of an open honeycomb and the exhaust gasses pass through the honeycomb where combustion gasses are consumed or converted to more environmentally acceptable products. The converter technology does not use flame impingement of the present technology.
German Patent DE 195 32 152 B4 discloses the use of a coating of metal oxide based catalysts such as FeO, Fe2O3, and Fe3O4 on fireproof (refractory) materials placed on the walls of a residential fireplace and/or chimney. The catalyst help burn the “off gas” from the solid fuel burning in fire. The reference does not teach or suggest a flame impingement process as required by the present invention. Further the reference teaches iron oxide catalysts which are not included in the scope of the present invention. U.S. Pat. No. 3,565,830 issued Feb. 23, 1971 to Keith et al., assigned to Englehard Minerals and Chemicals Corporation discloses a catalytic converter comprising a honeycomb support having a porosity of at least 0.1 to 0.3 cc/g upon which is deposited a platinum group metal. The converter differs from the heater of the present invention in that it does not require a direct flame impingement.
U.S. Pat. No. 6,431,856 issued Aug. 13, 2002, to Maenishi et al. teaches a refractory supporting one or more combustion catalysts in a combustion chamber. However, the refractory is shaped in such a manner that the fuel gas or combustions products thereof pass through the refractory. The present invention requires a refractory or lining through which neither the fuel nor the combustion products pass.
The present invention seeks to provide a simple method to improve the heating and particularly the radiant heating of a heater. More particularly the present invention seeks to provide a heater, and particularly a high temperature heater, having a higher wall temperature in or adjacent a flame impingement zone and a higher emisvity (radiation) under equivalent fuel and combustion air input.
The present invention provides in a fluid fueled heater, comprising a mechanical burner, a flame impingement zone adjacent said burner, and a refractory or lining in and adjacent to the flame impingement zone which refractory or lining does not transmit fuel to the burner the improvement of substantially coating the refractory or lining with a metal oxide catalyst or metal oxide catalyst precursor other than iron, iron oxides and mixtures thereof (e.g. the catalyst or precursor can not be Fe, FeO, Fe2O3, Fe3O4 or mixtures thereof) to promote the burning of one or more of the fuel, and combustion products from the fuel.
The present invention further provides a process for preparing a refractory or lining having a metal oxide or metal oxide precursor component other than iron oxides, and mixtures thereof, which catalyses the burning of a fluid fuel or combustion products from a fluid fuel when subject to flame impingement selected from the group consisting of:
a) direct application of a coating composition comprising said metal oxide or metal oxide precursor to the refractory or lining surface; and
b) incorporating said metal oxide or metal oxide precursor into the refractory or lining composition during manufacture of said refractory.
The present invention further provides a method to increase burner stability by substantially coating the refractory or lining in the flame impingement zone with a metal oxide catalyst or precursor for a metal oxide catalyst other than iron, iron oxides and mixtures thereof, to promote the burning of one or more of the fuel and the combustion products.
As used in this specification “substantially coating” means coating the refractory or lining in the flame impingement zone.
The heaters of the present invention may be used in a number of applications typically industrial applications where process streams are heated while passing through metallic tubes in an oven, furnace or radiant heater including the hot box of a steam cracker. Generally, the present invention is useful in high temperature burner applications having a flame impingement zone. Additionally the present invention provides improved flame/burner stability.
The refractory material may be any type of refractory materials that are commonly used in the construction of a furnace refractory wall. Examples of such refractory materials include dolomites, silicon carbide, aluminates (Al2O3), aluminum silicates, chromites, silica, alumina, zirconia (ZrO2), and mixtures thereof, preferably silica, alumina (Al2O3), aluminum silicates, zirconia, (ZrO2), and mixtures thereof. Such a refractory may optionally be non-porous in nature, even though the mentioned refractory materials are typically porous. Typically the refractory will be porous and have a porosity of not less than 0.1 cc/g. Typically the porosity may be from 0.1 to 0.5 cc/g, preferably from 0.1 to 0.3 cc/g. The intended coating is to cover a two dimensional refractory wall surface, rather than to impregnate a significant depth of refractory material with the intended combustion catalysts. More specifically, it is irrelevant how deep the coated catalysts may penetrate into the refractory wall. Generally the coating may have a thickness from 0.0001 to 5 mm thick, typically 0.0001 to 1 mm thick. The refractory could take any convenient form such as bricks, plates and slabs.
The present invention also contemplates a lining rather than or in addition to the refractory. The lining needs to be suitable for the temperature at which the burner is operated from 600° C. to 1300° C., typically from 850° C. to 1300° C., preferably from 900° C. to 1300° C. The lining may be a suitable metal applied to or over the refractory. The metal could be cast or wrought iron or more typically steel such as stainless or high temperature steel. For lining on the blades of natural gas turbines, the metal is typically nickel based alloys. The metal surface may be attached to or encompass (e.g. folded over) a refractory substrate. The metal would protect the refractory from ablative losses.
The catalyst may be selected from the group oxides of perovskites, hexaaluminate, spinels, PtO, PdO, MgO, Ce2O3, CoO, Cr2O3, CuO, MnO, NiO, and their precursors and mixtures thereof. The catalyst does not include iron, iron oxides, and mixtures thereof or their precursors. Preferably the catalyst is selected from the group consisting of PtO, PdO, MgO, Ce2O3, CoO, Cr2O3, CuO, MnO, NiO and mixtures thereof. Most preferably the catalyst is selected from the group consisting of PtO, PdO, Ce2O3, CoO, Cr2O3, CuO, MnO, NiO and mixtures thereof.
The catalyst may be prepared as an emulsion, solution or a paste (“mud”) and applied to the refractory or lining in any convenient manner such as painting, spraying or roller. The coating could be applied to the refractory prior to incorporation into the heater or after incorporation into the heater for example during routine maintenance or during a turn around. The catalyst may be, incorporated into the refractory or the liner during manufacture. Although, as noted above, the affect is primarily a surface affect and the catalyst need not be incorporated into the interior of the refractory or liner.
The fuel should be fluid. Preferably the fuel is a gaseous hydrocarbon such as natural gas or a natural gas rich (e.g. greater than 30% (volume) of natural gas) fuel. However, the fuel could also be liquid such as naphtha stream, a gas oil or a vacuum gas oil, and the like.
As noted above the refractor or liner is preferably exposed to the burner flame or at least a portion of the burner flame in the flame impingement zone. There are several improvements which may be achieved by the present invention. There is a more complete combustion of the fuel and in particular a more complete combustion of the initial combustion products from the fuel. There is also a reduction in carbon monoxide and NOx in the exhaust gases
In the experiments a combustion testing apparatus (CTA) was used.
Table 1 lists two sets of parameters for both fuel rich flame (FRF) and normal flame (NF) conditions.
Two commercially available combustion catalysts, made of non-noble metals and intended for use at temperature up to 1200° C., were obtained from catalyst providers. Each catalyst was provided in an undisclosed slurry form and brush-coated on the surface of a firebrick. The coated catalysts were then left to dry under ambient conditions for 24 hours before the coated bricks were heat-treated in a muffle oven to calcine the catalyst. These two coated bricks, plus an un-coated firebrick as reference, were tested using the CTA and under the test conditions specified in Table 1. The test results are given in Table 2.
Five nitrates salts (Mg(NO3)2, Ce(NO3), Co(NO3)2, Cu(NO3)2, and Mn(NO3)2) were dissolved in 200 ml of de-ionized water each to prepare five nitrate solutions of a nominal 5 weight % in concentration. A spray bottle was used to contain one of these solutions at a time and to spray coat the nitrate solution on a fresh refractory brick surface. Each brick surface, albeit not controlled exactly, was sprayed with one solution for 30 times. The wet bricks were then left in a laboratory fume hood at ambient conditions to dry for about 15 hours. After the drying, each of these five coated bricks was put in a muffle oven at room temperature, which was heated at about 10° C./min from ambient temperature to 600° C. and held at 600° C. for 30 minutes before cooling down. After a complete cooling to ambient temperature, these bricks were tested under the same testing conditions as used for Example 2. The heat treatment procedure was developed based on a set of separate experiments using a thermal balance, which confirmed that this heat treatment procedure will convert the nitrates into their corresponding oxides. More precisely, these oxides are primarily MgO, Ce2O3, CoO, CuO and MnO, respectively.
Using again the CTA, these five coated bricks were tested under both NF and FRF conditions as given in Example 2. However, it is worth mentioning that due to some modifications made to the CTA (the spacing between the burner and the firebrick) before this batch of tests, these results should not be compared with the results given in Example 2. Nevertheless, the results in Table 3 were obtained under the identical conditions and therefore can be compared.
These results confirm that there are noticeable improvements in the measured temperatures for all five oxides under the FRF condition. For example, the maximum increase in brick surface temperature is about 30° C. with CuO while the maximum rise in chamber temperature was observed from MgO for about 50° C., which shows also a maximum increase in exhaust temperature for about 32° C. In contrast, these observed improvements become less significant under NF test condition. For example, the highest increase in surface temperature is about 12° C. from the MgO coated brick, whilst decreases in surface temperature were also observed with the CuO and MnO coated bricks. However, from both test conditions, the emissivity with the coated bricks are seen to improve, possibly due to the changed surface compositions.
In order to evaluate the potential benefits from using the above mentioned catalysts as refractory coating, an ethylene furnace simulator (SPYRO from TECHNIP PYROTEC) was used to simulate the radiant box operation of a SRT butane cracker which is heated by natural gas at a NOVA Chemicals' production plant. Using the real operating parameters (about 4,000 kg/hr butane feed), two cases were considered for simulation: a base case and a modified case which assumes about 14° F. temperature rise on refractory wall. Considering the same butane conversion in both cases, the simulation results (Table 4) show clearly that with about 14.4° F. (8° C.) temperature rise on refractory temperature, the overall efficiency in radiant box increases by 0.98%. As a result, a slightly less firing is required, suggesting a possible saving in fuel gas. Furthermore, with the refractory wall temperature increasing, a slight increase in coil outlet temperature could also be realized. Considering together that the maximum tube skin temperature is lowered by 7.5° F. (about 4° C.) than in the base case, these results do suggest that the radiant box become more homogenous in terms of temperature.
