Patent Application
20060029534
The present invention relates to a method for reducing ammonia emissions in combustion exhaust gases and other waste streams using an oxidation catalyst. More specifically, the invention relates to the reduction of unreacted ammonia downstream of a Selective Catalytic Reduction (“SCR”) system simultaneously with the reduction of other gaseous pollutants. The invention utilizes a catalyst based on noble metals, such as Pt, Pd or Ru, whereby ammonia is oxidized to produce one or more reaction products having a higher oxidation state of nitrogen, preferably elemental nitrogen and secondarily other species such as nitrous oxide, nitric oxide or nitrogen dioxide. The efficiency of the process in reducing the unreacted ammonia can be increased by optimizing the catalyst operating conditions such as the temperature and space velocity.
The SCR system is a known method used to control nitrogen oxide emissions from stationary combustion sources (NO and NO2, collectively referred to herein as “NOX”). SCR systems normally involve the addition of a chemical agent to the exhaust stream that reacts with nitrogen oxides on the catalyst surface. The main product of the reaction is elemental nitrogen (N2). However, a small amount of by-products such as nitrous oxide (N2O) will also be formed.
The most commonly used chemical agents for SCR systems include ammonia NH3 and urea (NH2)2CO. Both agents have similar mechanisms of NOX conversion to molecular nitrogen. The SCR process using ammonia injection takes place according to the following chemical equations:
4NH3+4NO+02.4N2+6H2O and
4NH3+2NO2+02.3N2+6H2O
Ideally, the products of the SCR process should include only elemental nitrogen and moisture (H2O). In practice, however, the process efficiency is less than 100% due, for example, to incomplete diffusion of the emissions to available catalytic reactive sites or an incomplete reaction with the catalyst due to limited reaction time, imperfect mixing during the SCR step, etc. These non-ideal environments invariably cause at least some un-reacted ammonia to be released at the outlet of the SCR system. The un-reacted ammonia, commonly referred to as “ammonia slip,” if released to the atmosphere, can cause various harmful effects due to its toxicity to human health and its tendency to react with other atmospheric constituents resulting in the formation of fine particles that reduce atmospheric visibility and cause adverse human health effects. Thus, it is desirable to minimize or eliminate ammonia slip at the outlet of any SCR system.
The present invention provides a new method for reducing ammonia emissions in combustion exhaust gases or other waste streams by utilizing an additional catalyst strategically positioned downstream of the SCR system catalyst that is capable of converting the ammonia slip to less harmful substances, ideally elemental nitrogen. The cost of exemplary systems using the invention can also be reduced by using certain conventional process components and hardware developed for atmospheric pollution control of stationary combustion sources.
In certain aspects, the invention includes a process for treating the exhaust gases generated by a stationary combustion source equipped with a selective catalytic reduction (“SCR”) system by subjecting the unreacted gaseous ammonia downstream from the SCR system to an oxidation catalyst in order to produce reaction products having an oxidation state higher than ammonia, such as elemental nitrogen, nitrous oxide, nitric oxide or nitrogen dioxide.
The invention can also be used in applications for treating waste streams generated in non-combustion applications. In both combustion and non-combustion embodiments, the emissions of ammonia and other X—NHi compounds can be significantly reduced (where X represents part of the compound containing C, H, O, or N atoms and i=0, 1, 2, 3, 4) through direct contact with the oxidation catalyst. That is, the X—NHi compounds in the waste stream react with oxygen on the oxidation catalyst, resulting in reaction products having nitrogen in oxidation states higher than the X—NHi compounds. Typically, the ammonia and other X—NHi compounds originate from nitrogen-containing agents present in the waste stream upstream of the oxidation catalyst such as anhydrous ammonia, aqueous ammonia, urea, cyanuric acid, amines, syngas, process gas, biomass or nitrogen-containing fuels.
The present invention substantially reduces the unreacted ammonia slip at the outlet of the SCR system using an oxidation catalyst installed downstream of the SCR system catalyst. For combustion sources that contain at least some oxygen in the exhaust stream, the preferred oxidation catalysts include noble metals that oxidize the ammonia (with the oxidation state “OS” of the nitrogen in the ammonia being −3) to a species of nitrogen having a higher oxidation state, such as elemental nitrogen N2 (OS=0), nitrous oxide N2O(OS=+1), nitric oxide NO(OS=+2), and nitrogen dioxide NO2 (OS=+4). As noted above, it is desirable to maximize the conversion of the ammonia slip to elemental nitrogen and minimize the formation of other nitrogen-containing species. The chemical reactions describing oxidation of the ammonia slip include the following:
4NH3+302→2N2+6H2O
2NH3+202→N2O+3H2O
4NH3+502→4NO+6H2O
4NH3+702→4NO2+6H2O
As those skilled in the art will appreciate, converting ammonia to products with different oxidation states involves different process conditions, depending in part on the specific noble metal catalyst employed in the reaction. The oxidation of ammonia to higher oxidation states (e.g., nitrogen oxides with an OS from +1 to +4) normally require higher temperatures than the temperature required for the oxidation of ammonia to N2. Thus, the temperature for ammonia conversion to N2 should be optimized, i.e., kept high enough to maintain a high rate of ammonia conversion to N2 on the selected catalyst but low enough to prevent conversion of the ammonia to the less desirable nitrogen oxides. Process conditions exist under which most of the ammonia can be selectively converted to nitrogen without formation of significant amounts of nitrogen oxides.
One known application of an exemplary ammonia-to-N2 oxidation system using the invention involves treating the exhaust from gas turbines, whereby an SCR system is used in combination with an ammonia agent.
The emissions control system for a prior art gas turbine system is shown in
It has also been found that the performance of the oxidation catalyst used for ammonia removal, particularly the preferential conversion to elemental nitrogen (rather than to nitrogen oxides), is temperature-dependent. That is, high temperatures tend to favor formation of nitrogen oxides, while lower temperatures tend to decrease the rate of the catalytic reaction. As those skilled in the art will appreciate, intermediate optimum temperatures can be determined for different catalytic systems that favor the formation of elemental nitrogen from the ammonia slip. Even at intermediate temperatures, some formation of N2O and NO may occur. However, the amount can be minimized by selecting appropriate process conditions, namely the reaction temperature, space velocity and specific catalyst composition.
Normally, the operating temperature of the oxidation catalyst should remain close to the operating temperature of the SCR catalyst. For most noble metals, it has been found that the oxidation catalyst temperature should range between about 105° C. and 350° C., with the maximum operating temperature range being about 100° C. to 700° C. It has also been found that acceptable catalyst space velocities range between about 5,000 and 150,000 hr−1.
As noted above, exemplary oxidation catalysts useful in the invention are based on noble metals, preferably platinum and/or palladium, but may include other noble metals (e.g., rhodium or ruthenium). Due to their high reactivity, these noble metals have been found very effective in oxidizing CO, H2 and organic compounds in the exhaust gases from stationary combustion sources. In particular, the catalysts are capable of converting unburned fuels to CO2 and H2O in a wider temperature range with near 100% efficiency. Thus, their placement in the exhaust stream downstream of the SCR can be adjusted to an optimum temperature, thereby maximizing the reaction of ammonia to elemental nitrogen.
Various optimum temperatures and process conditions, such as oxygen concentrations and space velocities, have also been determined in order to maximize the reaction efficiency for specific oxidation catalysts. The oxidation catalysts are screened for maximum activity in ammonia removal. Additionally, the oxidation catalysts can be combined with different additives and in various compositions to maximize the removal of the ammonia. Those skilled in the art will also appreciate that the reaction conditions can be varied to enhance efficiency.
Although the processes for ammonia removal depicted in
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
