Patent Application
20140170046
There is an increasing level of awareness concerning the emission of certain volatile organic compounds (VOCs), combustion byproducts such as carbon monoxide and NOX, and dioxin/furans from industrial plants such as cement manufacturing facilities. With this heightened level of awareness, more stringent environmental regulations are being adopted to ensure low emissions from these industrial plants. In some cases, the level of emissions currently experienced may not be adequately reduced using existing technologies in order to meet new environmental regulations. In other cases, existing technologies for emissions controls in other applications are prohibitively costly in industrial applications. Consequently, there is an interest in developing new systems for controlling these high levels of emissions to meet newly proposed regulations, and that is an object of the present invention.
Very few cement and lime kilns have installed specialized controls for emissions of organic compounds. Cement kilns in the United States have attempted the use of Regenerative Thermal Oxidizers (RTOs), such as those devices described in U.S. Pat. No. 5,352,115 and U.S. Pat. No. 5,562,442. These devices subject the process gases to intense oxidizing conditions produced by applying a direct heat source and introducing air to the system in order to incinerate the organic compounds in the gas stream. The exhaust gases are heated to a temperature in excess of 800° C., and much of this heat is recovered through the system and used for preheating the gas stream prior to the combustion region. The devices require a clean fuel for heating, as soot and ash introduced from the fuel can reduce the thermal efficiency of the unit. These devices are of limited applicability in use with cement and minerals processing systems for several reasons—they are small in size relative to the process gas stream that must usually be treated (requiring multiple, parallel units), the intense oxidizing conditions produced in the unit can oxidize SO2 present in the gas stream to SO3 (which necessitates the use of a wet limestone scrubber or other SO2 control device prior to the RTO), and the use of additional fuel firing for operation will increase emissions of carbon monoxide and carbon dioxide. Combined, these disadvantages make RTOs very difficult to install in industrial plants such as cement kilns because of the large space required for the RTO and supporting equipment, the high cost of installation, and the high cost of operation.
An alternative approach that may be attempted is the use of catalytic means of the destruction of organic compounds and carbon monoxide. Catalyst applications in cement kiln systems have generally been targeted towards the removal of nitrogen oxides through “Selective Catalytic Reduction”, but the art of using catalysts in similar applications for the removal of organic constituents may also be practiced, as in U.S. Pat. No. 6,156,277. In this method, the exhaust gas from a cement kiln is directly passed through a reduction catalyst for the destruction of the emissions from the kiln system. The temperatures required for the chemical reactions for the destruction of NOx and other emissions products is generally between 250° C. and 450° C. in practice, although wider temperature ranges are available with specific designed catalysts. In a typical cement kiln process, the exhaust gas from a preheater/precalciner system is typically in such a range. While this temperature range is conducive to a high activity for the catalyst, these systems often see issues associated with the loading of particulate matter in the exhaust gas stream. Typical dust loadings in these gas streams may be in the range of 20 to 50 grams of particulate per cubic meter of exhaust gas, although dust contents exceeding 150 grams of particulate per cubic meter of exhaust gas can be seen. The particulate matter is typically comprised of the fine dust fraction of feed material introduced to the kiln system which is not completely captured within the kiln or preheater system. This fine dust is comprised of varying amounts of and compositions containing calcium, aluminum, silica, and iron, as well as sodium, potassium, chlorides, sulfur and minor constituents such as phosphorous, arsenic, thallium, and zinc. Depending on the catalyst structure in use, any of these compounds can cause degradation of the catalytic effect of the system through de-activation of the surface, poisoning of the catalyst, erosion of the catalytic surface, or the blocking of the catalyst surface from contact with the gaseous constituents. In addition, the dust content of the gas stream requires larger openings in the catalytic structures in use in these systems, requiring larger catalyst structures to obtain the same catalytic surface as is found in other industrial applications. In systems where SCR is practiced, soot blowers for dedusting and periodic cleaning of the catalyst surface are required. The loss in efficiency associated with dust loading therefore leads to higher costs for these systems for design and operation than in comparable industries with low dust loads.
As an improvement over these “high dust SCR” applications, systems have been proposed which include a step for removal of the dust present in these industrial applications prior to the catalyst structure. These systems comprise an additional cleaning step utilizing a dust filter or a precipitator prior to the catalytic structure, such as is described in US Application 2010/0307388. In this arrangement, the gases coming from a cement kiln system are first passed through a dust precipitation system to remove particulate matter. The gases are then passed through the reduction catalyst for destruction of the targeted pollutants. After treatment in the reduction catalyst, the hot gases may then be used in other devices found in the industrial facility, such as grinding mills, and vented through a stack. This “low dust SCR” arrangement offers several advantages over the “high dust SCR” arrangement, including a longer lifetime for catalyst structures before replacement, the usage of smaller openings between catalyst plates or honeycombs which allows for a smaller and less costly catalytic structure, and lower operating costs associated with less replacement of catalyst. This arrangement does come with several disadvantages. The requirement for a dust collection device such as a filter or precipitator is an added piece of process equipment that comes with installation and long term operating and maintenance expenses. The additional pressure drop through the precipitator or filter, in addition to the catalyst structure, will increase the power requirements on any fans utilized for drafting gas through the overall system. In addition, the layout requirements of the cement kiln or minerals processing facility will often make it difficult or impossible to fit both the filter or precipitator and a catalytic structure within the confines of available areas for installation.
In view of the prior art issues, the objects of the present invention include improving the control of various undesirable emissions from cement and minerals, and obtaining a high efficiency of catalytic activity such as is found in a “low dust SCR” applications while having a high inlet dust loading similar to that encountered in “high dust SCR” applications, while utilizing fewer pieces of equipment and a lower pressure drop than a “low dust SCR” system.
The above and other objects are achieved by utilizing a device fitted with a filter element or filter elements which are pretreated with the catalyst or composed of the catalytic materials dispersed through the filter elements.
According to the invention, there is a method for the reduction of organic compounds and other emissions from an industrial plant having a cement or mineral kiln or calciner system that has a high level of emissions. The invention treats the exhaust gas stream from the cement or minerals processing plant on a filter medium in order to remove entrained particulate, and destroys the targeted pollutant within the structure of the filter medium. Particulate captured on the surface of the filter medium is periodically removed from the surface of the medium to prevent blockage of the porous filter medium and to avoid undesireable increases in energy consumption at the processing plant. Such removal can be achieved by a number of methods, including subjecting the filter medium to sonic or ultrasonic vibration or the mechanical removal of particulate matter with a solid object. Pre-treatment of the exhaust gas stream can be used to enhance the pollutant destruction capabilities of the filtration device, or to prevent oxidation of entrained pollutants to less desirable compounds. This invention is not limited to cement or lime plants. It can be used in any industrial processing plant where the emission of organic compounds, total organic carbons or volatile organic carbons, carbon monoxide, nitrogen oxides, or dioxin/furans require a very high degree of treatment for attainment of regulatory requirements, such as, for example, in plants that use long dry cement kilns, short cement kilns with precalciners, and lime kilns.
Although the invention is particularly directed to the reduction in emissions of organic compound emissions, the present invention also applies to the removal of other products of incomplete or partial combustion such as carbon monoxide, condensable VOC's, nitrogen oxides, and dioxin/furans that contaminate manufacturing processes. Many of the organic compounds that this invention is directed towards fall under numerous overlapping categories of compounds, such as Total Organic Carbon (TOC), Total Hydrocarbons (THC), and Volatile Organic Compounds (VOC), and this invention is broadly aimed to the various compounds which are classified under these general categories. Also, while emphasis is placed on a cement manufacturing process, the present invention is applicable to other minerals and kiln manufacturing processes, such as lime manufacturing processes and other industrial processes where very high starting emission levels of these contaminate compounds can not be sufficiently controlled using existing methods, or where existing methods of control are cost prohibitive.
Emissions of organic compounds from industrial process may originate from a variety of different sources within a system. In minerals processing systems such as cement kilns, these sources may include incomplete combustion of fuels fired within the system, decomposition or partial combustion of organic species within feed components, contamination of gas streams with organic materials such as from oiled-compressors, introduction of organic components in process water used for cooling, and from introduction of ambient air which may contain organic components in small quantities. Also, the oxygen concentrations at the exhaust of many industrial processes such as cement kilns are kept low in order to improve efficiencies within the systems, but such low oxygen concentrations inhibit full combustion of these various organic compounds prior to release from the system. These minor components may contribute to localized conditions such as smog, and are therefore seeing increased levels of regulation.
The invention in part comprises the use, in conjunction with a kiln exhaust, of a device fitted with a filter element or filter elements which are pretreated with the catalyst or composed of the catalytic materials dispersed through the filter elements. The catalyst utilized in the pretreatment or the construction of the filter element is chosen prior to installation of the filter elements within the device in order to treat those gaseous emissions within the exhaust stream from the industrial process which must be controlled in order to achieve regulatory compliance. While the catalysts are designed for the control of organic compound emissions such as Volatile Organic Compounds (VOC), Total Hydrocarbons (THC), Total Organic Carbon (TOC), and similar classifications of emissions, the catalyst for the pretreatment of the filter elements or the dispersal through the filter elements may also be chosen for the reaction or destruction of other compounds including dioxins, furans, carbon monoxide, or oxides of nitrogen (NOX), or can be used for the oxidation of mercury for further treatment and capture after the device. The catalytic elements used in the treatment or manufacture of the filter elements can contain a mixture of any of vanadium, platinum, palladium, ruthenium, titanium, lanthanum, cerium, yttrium, zirconium, tungsten, manganese, niobium, molybdenum, nickel, iron and copper in compositions designed to remove those emissions which must be reduced in the gas stream.
The filter elements are porous membranes which allow for the passing of exhaust gases through the elements, but of sufficiently small pore size to capture a significant quantity of the dust on the surface of the elements which is exposed to the process exhaust gas containing entrained dust. The surface of the filter element exposed to the process exhaust gas containing entrained dust is referred to as the “dirty side” of the filter element, while the surface of the filter opposite the “dirty side” and through which the dedusted process gases pass to the outlet of the filter is referred to as the “clean side” of the filter elements. The filter element is comprised of the porous substrate as well as the catalytic component of the filter element during the manufacturing process. The filter elements are treated with the catalysts either in whole or in part, with catalysts deposited on both the “clean side” surface and the “dirty side” of the filter element penetrating through a depth to the inside of the filter element, with the maximum penetration of the catalysts being through the entire thickness of the filter element, i.e. from the “clean side” to the “dirty side” of the filter element. In all cases, it is preferred not to have the catalysts applied only to the “dirty side” of the filter element, as this exposes the catalyst to the dust particles which may erode or “poison” the catalyst and reduce its lifetime. The non-catalytic portion of the filter element, which serves as a substrate to the catalyst and as the porous filter for entrained dust in the gas stream, is composed of a material which is designed to retain sufficient filtering properties through the design range at which the filter elements will be exposed for filtration of dust and for catalytic reduction of gaseous pollutants. The non-catalytic composition of the filter element may be comprised of any of porous ceramic, glass fibers, ground quartz, alumino-silicate ceramic fiber, rutile, calcite, corundum, kaolinite, and diatomaceous earth, among others.
The surface of the filter elements which is exposed to the process exhaust gas entering the device is periodically cleaned to prevent excessive accumulations of dust, which would otherwise increase the pressure drop of the device, and thus increase the power consumption of the system in operation. Cleaning of the device may be performed through mechanical cleaning, such as scraping or “rapping” of the filter elements, but is preferentially performed through periodic “pulsing” of gas counter to the flow of the industrial process exhaust gas entering the filter element. Dust which is released from the filter element may be returned to the cement or mineral processing facility, or may be withdrawn and stored for use elsewhere.
The filter element is placed such that the exhaust gases from the cement or minerals industrial process, which contain entrained particulate matter, are passed through its porous filter. The majority of the entrained particulate matter is captured on the surface of the filter element and will not come into contact with the interior of the filter element. Gases passing through the pores of the filter element come in direct contact with the catalytic compounds with which the element has been treated, ensuring contact time between the gas and the catalyst. This reduces the required residence time with the catalyst and allows for the possibility of a smaller installation. By using the filter elements as the catalyst substrate, the steps of separation (of the dust from the gases) and catalytic contact may occur within the same device, also reducing the size and cost of an installation.
By suitable pretreatment or post treatment of the gases around the filter device, additional pollutant controls may be achieved. In one variant of the invention, a sorbent for sulfur emissions may be injected before the device to capture sulfur dioxide emissions prior to the filter elements and the catalyst. In this manner, the sulfur dioxide may be captured prior to the gases contacting the catalyst, preventing the potential formation of sulfur trioxide within the filter elements through catalytic oxidation.
In one variation of the invention, a sorbent for capture of mercury emissions is injected after the filter device in order to capture mercury emissions which have been oxidized in contact with the catalyst.
In one variation of the invention, a nitrogenated agent such as ammonia, urea, ammonium bisulfate, or flyash may be injected prior to the filter device in order to reduce NOX emissions. For example, injection of ammonia may be placed immediately prior to the inlet of the filter device, or may be injected in excess within the industrial process producing the exhaust gas with the resulting ammonia slip further reacting within the filter device.
Placement of the catalytic filter device into the industrial process is dependent upon the pollutants in the exhaust gas stream that are to be destroyed. The activity of the catalyst and the selectivity of the catalyst for destruction of gaseous emissions are dependent upon the temperature of the gas stream. Organic compounds such as methanol can be destroyed in large percentages even at temperatures as low as 120° C., while organic compounds such as propane may require temperatures as high as 300° C. It is preferential for the destruction or reaction of shorter-chain (less than 7 carbon atoms) hydrocarbons, of single-bonded (i.e. saturated) hydrocarbons, and NOX emissions to place the device as close to the exhaust of the cement or minerals processing system as is possible in order to obtain a gas temperature in the range of 250 to 400° C., and more preferentially 300 to 350° C. If the destruction or reaction of longer-chain hydrocarbons (7 or more carbon atoms), double- or triple-bonded (i.e. unsaturated) hydrocarbons, and/or cyclic or aromatic hydrocarbon compounds are desired, without need for higher temperatures for the treatment of other emissions through catalytic means, then the device may preferentially be used in the temperature range of 80° C. to 250° C., and more preferentially between 150° C. and 200° C.
The catalytic activity of the filter elements may also be enhanced through the treatment of the gas stream with other means. These means would include the use of ozone, peroxide, potassium permanganate, calcium chloride, sodium hydroxide, or other oxidizing species injected upstream of the filter element or within the filter device.
The invention is explained in greater detail below with the aid of drawings.
In the system of the present invention illustrated in
The catalytic filter system 100 is depicted as being positioned between the preheater/precalciner system and the gas conditioning tower, but depending on the configuration of the kiln system and the requirements for gas flows for processing within the system, the catalytic filter system may alternatively be placed at a number of other locations within the system. For example, the catalytic filter system may be placed at location A (between the exit of the preheater system 50 and the hot gas stream 31 removed from the preheater exhaust gas stream 21), at location B (between the hot gas stream 31 removed from preheater exhaust gas stream and the returned gas stream 41 from the solid fuel grinding system), or at location C (after the exhaust of the gas conditioning tower 23).
It may be preferred to utilize sorbents in the process prior to the position of the catalytic filter system in order to capture items that otherwise may oxidize to less preferable components in the catalytic filter systems. Sorbents such as calcium oxide, calcium hydroxide or hydrated lime, trona, activated carbon, or proprietary sorbents such as Minsorb™ or Sorbacal™, may be utilized in this capacity in one or all of locations S1, S2, and S3. The injection of additional reactive agents such as ozone, peroxide, potassium permanganate, calcium chloride, sodium hydroxide, or other oxidizing species, or ammonia, urea, or other nitrogenous compounds for conversion of emission components to those more readily destroyed within the catalytic filter system or to directly improve the destruction of compounds in the catalytic filter system may be utilized in one or all of locations S1, S2, S3, S4 and S5.
The cleaned gases 110 exit the catalytic filter via a duct 111, and can be used elsewhere in the process or vented to atmosphere.
While the filter elements are depicted in
Particulate matter collected on the surface of the filter elements is removed from the surface of the filter and collected in the device 206 and removed through a withdrawal system 207. Pressure monitoring device 208 is used to monitor the difference in pressures attained across the partition between the clean and dirty sides of the filter. The temperature of the gas stream is monitored at 209 The cleaned gases 210 exit the catalytic filter via a duct 211.
Using this invention, the exhaust gases from an industrial plant such as a cement or mineral kiln can be treated to reduce or destroy organic compounds and other pollutants from the exhaust until the total content of organic compounds and other pollutants in the gas stream is below levels that may be considered safe for release to the atmosphere. Treatment of the gas stream may also allow for removal of other pollutants, or additional treatment downstream.
The invention having been thus described it will be obvious that the same may be varied in many ways without departing from the spirit and scope thereof. All such modifications are intended to be included within the scope of the invention which is defined by the following claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2012/046848 | 7/16/2012 | WO | 00 | 1/29/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61574273 | Jul 2011 | US |
