Patent Application
20150059625
Combustion in stoker, circulating fluidized bed, rotary kiln, pre-heaters and other equipment utilize solid fuel to make steam, electricity, or heat for manufacturing products such as cement, lime, refractory, or other similar materials.
Stoker boilers are designed to combust fuels on a grate which may be moving or stationary. Combustion air travels up through small spaces in the grates providing the primary combustion air to the fuel, typically coal of a size from 0.25-1.5 inches. However, other fuels such as biomass (paper, wood) refuse derived fuel (RDF), bagasse (sugar cane residue), and shredded tires have been successfully used as fuels in these boilers. The majority of stoker boilers in use in the United States consist of spreader stokers with traveling (moving) grates that have a significant portion of the combustion air provided by overfire air ports in order to increase turbulence, provide for better gas-oxygen mixing, and reduce the formation of carbon monoxide (CO), and nitrogen oxides (NOx) by having a more uniform temperature distribution in the combustion volume. In these systems, the grate moves continuously from the back toward the front of the boiler where the ash is dropped into an ash sluice at the bottom of the units. The fuel is typically fed by means of rotors that attempt to provide a uniform layer of fuel on the back side of the grate as it moves toward the front. The estimates of the number of these boilers in operation in the United States is in the thousands, with some estimates as high as 10,000 of various sizes.
Fluidized-bed combustion (FBC) is a combustion technology used in power plants. Fluidized beds suspend solid fuels in upward-blowing jets of air during the combustion process. The result is a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides more effective chemical reactions and heat transfer. FBC technology was adapted to burn petroleum coke and coal mining waste for power generation in the early 1980s in the United States. At that time, U.S. regulations first provided special incentives to the use of renewable fuels and waste fuels. FBC technology spread to other parts of the globe to address specific fuel quality problems. The technology has proved well suited to burning fuels that are difficult to ignite, like petroleum coke and anthracite, low quality fuels like high ash coals and coal mine wastes, and fuels with highly variable heat content, including biomass and mixtures of fuels.
The technology burns fuel at temperatures of 1,400 to 1,700° F. (760 to 930° C.), a range where nitrogen oxide formation is lower than in traditional pulverized coal units. But, increasingly strict U.S. regulations have led to the use of ammonia DeNOx systems even on FBCs.
Fluidized-bed combustion evolved from efforts in Germany to control emissions from roasting sulfate ores without the need for external emission controls (such as scrubbers-flue gas desulfurization). The mixing action of the fluidized bed brings the flue gases into contact with a sulfur-absorbing chemical, such as limestone or dolomite. More than 95% of the sulfur pollutants in the fuel can be captured inside the boiler by the sorbent. The sorbent also captures some heavy metals, though not as effectively as do the much cooler wet scrubbers on conventional units.
Because of the method of combustion, the level of emissions from spreader stokers in the past has always been an environmental concern, primarily for oxides of sulfur (SOx), but now also for other pollutants such as hydrochloric acid vapor (HCl) and heavy metals such as mercury (Hg). These pollutants are also a concern for fluidized bed boilers.
Non-recyclable paper and other feed stock streams are a source of material that because of their composition (wrapping paper that is laminated or contains foreign materials such as foil-coatings, photographic film, microwave containers, hardcover books, frozen food boxes, thermal fax paper, carbon paper, blueprints, aluminum foil boxes and binders, etc.) cannot be economically recycled back into usable paper. This material must currently be disposed of in a landfill, which is both expensive and requires long-term monitoring. Also, public concern over landfills makes the future expansion of the existing landfill sites difficult, and the permitting of new sites almost impossible. Non-recyclable paper and other materials have a high heat content which is suitable for use as a fuel in solid fuel boilers, and this material is also readily available from the packaging and labeling industries, as well as from municipal refuse.
The terms chlorine, fluorine, sulfur, and mercury, herein, are used to describe all of the compounds that contain chlorine, fluorine, sulfur, and mercury which may exist in a combustor during a combustion process. The present invention is premised on the realization that a compressed fuel source, such as a pellet or briquette, formed with compounds that dissociate under combustion temperatures to form ions and/or molecules which react with pollutants, such as hydrochloric acid (HCl), sulfur oxides (SOx), fluorine, and mercury, are effective to reduce the pollutants emitted during combustion. By selecting the appropriate compound that decomposes easily at combustion temperatures to form ions and/or molecules which react with the chlorine, sulfur, fluorine or mercury, forming a stable solid compound at combustion temperatures, allows one to remove these pollutants in-situ in the ash, avoiding or reducing the need for post-combustion treatment of the combustion gases.
Compounds that decompose within combustion temperature and form stable chloride species were most effective at removing chlorine. For example, barium hydroxide, which decomposes below or around 1400° F. and forms a very stable chloride, provides excellent efficiency, as well as calcium carbonate and strontium hydroxide. Compounds that decompose at higher temperatures, such as calcium oxide and magnesium oxide, reduce pollutants emissions but may be less efficient.
The present invention utilizes a non-hazardous secondary material as a fuel source for spreader stokers and fluidized bed boilers along with a novel and unique method of capturing pollutants such as HCl, SOx, HF, and heavy metals during combustion by means of in-situ sorbent capture.
A sorbent as used herein reacts with potential air pollutants precursor chemicals which are found in the pellet itself, or are formed as a result of the combustion process. The sorbent reacts by various mechanisms such as adsorption, absorption, and chemical reaction in the combustion process with these potential air pollution compounds to form stable solid compounds which can then be removed with the ash preventing the emissions of these air pollutants to the atmosphere.
In one embodiment, the process of the present invention involves pelletizing the non-recyclable paper and other biomass materials which involves shredding the raw materials into small pieces which can then be forced through a die under pressure. This forms a coherent rod which is cut to form pellets. A flexible sorbent doping process is proposed which can be used to add solid sorbent material within the pellets and/or on the exterior of the pellets, and can be applied as either a dry sorbent material, or as a slurry of sorbent which can be used as a raw material or sprayed on to the surface of the pellet, or by allowing the pellets to be placed in a bath of sorbent solution, or by means of any combinations of these methods. The sorbents associated with the pellets when burned react with HCl, SOx, HF and heavy metal vapors which may be present in the raw material and produced during fuel combustion to form products that remain with the ash.
The compressed fuel, which is referred to hereinafter simply as a pellet, is formed by combining one or more fuel sources and a chemical additive or sorbent designed to capture certain pollutants emitted during the combustion process, such as HCl, HF, SOx, heavy metals and the like. The fuel source can be a wide variety of different fuel sources, basically anything that can be combusted. In particular, the present invention is suitable for use with pelletized feedstocks and, in particular, pelletized recycled material.
Such pelletized feed stocks are designed to meet specifications for use in the specific boilers. In order to meet these specifications, the materials are tested for the specific properties required for: 1) combustion, 2) ability to make the fuel, 3) emissions requirements, and 4) comparability to traditional or currently utilized fuels. Feed stocks are qualified by testing the material for ultimate and proximate analysis, MACT metals, chlorine, fluorine, and mercury, The goal of this testing is to insure that the fuel will meet the comparison criteria to the existing solid fuels it will replace or be blended with.
The materials garnered and processed for pellet production include flexible packaging materials (plastic film waste containing primarily polypropylene and polyethylene and smaller quantities of polyester, nylon, and/or polystyrene), nonwovens (polypropylene and polyethylene products normally used in the industrial packaging, food packaging, diaper, baby wipe, and medical industries) and pressure sensitive labels/tapes (printing industry materials such as: labels and tapes are paper-based materials with varied layers of poly and adhesive coatings).
The feedstock products also contain cellulose, plastics, adhesives, laminates, wood, and various other wastes with sufficient heating value to be used as a fuel, and chemical properties that will allow it to meet air emissions regulations when combusted in a traditional solid fuel boiler (non-CISWI boiler, i.e., not a waste incinerator) under the EPA alternative fuels program. The feedstock can originate from industrial, commercial, and residential sources.
The industrial materials are received at the facility in various forms: baled, un-baled, loose material, compactor trucks, palletized, and in Gaylord boxes. The material is stored in separate areas by type in order to be mixed in the correct proportions to feed the various fuel making processes. The second step in the production process is pre-mixing and then pre-shredding according to heating value specifications (and assessed through laboratory testing). The third step is metals removal through the use of magnets. From that point the pre-shredded materials are additionally mixed to achieve the desired heating value specification (generally 8,500 to 14,000 BTU per pound) necessary for the engineered fuel custom specification for the specific boiler/application. Upon post mixing, the materials are again re-shredded and then processed to make pellets.
Other pelletized or briquetted fuel sources can also be used in the present invention, such as coal briquettes. Additives or sorbents for acid gas emission control are preferably mixed with the raw materials prior to densification. The engineered fuel is then compressed and shaped in order to produce pellets of uniform shape and consistency allowing for transport and having the characteristic of a particular ¼″ to 1 ½″ diameter by up to 4″ in length and shaped to allow use in existing coal-fired stoker boilers, circulating fluidized bed (CFB) boilers, or other applicable solid fuel systems.
Introduction of chemical additives into the engineered fuel pellets will limit the emission of acid gases (such as HCl, SOx, and HF), and heavy metal emissions (e.g., mercury) by reacting with the acid gases and other emissions forming compounds such as heavy metal vapors which may be formed during combustion process in the boiler. The resulting metal salts and other compounds such as heavy metals are retained in the bottom ash and fly ash of the boiler thus reducing the emissions through the boiler stack to the atmosphere. While the primary goal of the additive is to reduce HCl, the additives will also reduce the sulfur oxides (SOx), HF, and heavy metal emissions from the combusted pellets as well as from any fuel that the pellets are blended with. Additives to reduce mercury emissions, lower NOx emissions, reduce slagging or fouling in the boiler, improve the performance of electrostatic precipitators and other possible uses can be incorporated into the fuel pellets through this invention.
The interior doping process involves adding the proper amount of chemical additive in with the shredded paper and material and thoroughly mixing the mixture to insure a uniform distribution of the chemical additive with the pellet. The exterior doping process involves spraying wet or dry, micronized chemical additive directly onto the material before and/or after it is injected into the die for pelletizing. During this process, the chemical additive acts as a lubricant insuring a uniform pellet size and long life for the die.
The chemical additive, which acts as a sorbent, can be one of, or a mixture of several of the compounds formed from the elements in the alkali metal, and/or alkaline earth metal group of the periodic table, as well as other metals such as copper. Any non-toxic metal which will react with chlorine, sulfur, fluorine, and heavy metals can be used. However, commercial considerations provide practical limitation. The chemical additive's counter ion or cation will be any counter ion that includes only oxygen, carbon and hydrogen. Thus, this would include carbonates, bicarbonates, oxides, hydroxides, certain organic counter ions, such as acetate as well as other carboxylate compounds.
There are a number of other materials that can be used as sorbents, such as trona (an ore of sodium bicarbonate and sodium carbonate), as well as all of the alkali metal and alkaline earth oxides, hydroxides, and carbonates and bicarbonates either in pure form or as ores (e.g., limestone). Strontium carbonate due to its low toxicity is particularly suitable. Also, the metal oxide, copper oxide, is potentially a candidate material. Generally, sufficient sorbent is added to react with all chlorine ion as well as sulfur oxides generated during burning.
The chemical additive, as injected, should be one which decomposes at or below the combustion temperature to form a reactive compound which subsequently reacts at combustion temperatures to form a stable solid compound containing chlorine, sulfur, fluorine, heavy metals, or combination of these compounds which would then be removed with the ash. Preferably, the chemical additive decomposes at about 1000° F.-1800° F. and forms a chloride that is stable at 1400° F., preferably higher, such as 1500° F., 1700° F., 2000° F. or higher. Generally, the combustion temperatures will be around 1750° F. to 2000° F. Thus, a compound that remains stable within the temperature range of the combustion zone may not be available to react with formed pollutants.
Further, if the formed compound, such as the fluoride or chloride, decomposes at combustion temperature, in other words is not stable, at least some of the pollutant, such as the chloride, will be re-emitted in the flue gas and there will be a reduction of pollutant capture. However, the combustion zone is very dynamic and temperatures vary widely. So as shown in the test lab a wide variety of alkali and alkaline earth compounds function and remove chlorine.
To determine the amount of sorbent to be combined with the pellets one should test the feed material for chlorine, sulfur, fluorine and heavy metals (the polluting compounds) and have sufficient, and preferably excess sorbent to stochiometrically combine with all of the polluting compounds present in the pellets. Generally 1-15% by weight is effective.
The formed pellets are combusted in a solid fuel combustor or boiler. As the fuel heats up, the chemical additive sorbent will decompose along with the chlorine, sulfur, fluorine, and heavy metal compounds, allowing the metal ions to react with chlorine, sulfur, fluorine and heavy metal compounds to form corresponding solid salts which will then be trapped and removed with the ash. This will remove the noxious components from the fuel, allowing a wide variety of materials to be used and reducing the need to dispose of non-recyclable materials in landfills,
To test the efficacy of various compounds at removing chlorine, paper pellets were formed with a known chlorine content of about 1657 ppm on a dry basis. Various concentrations of compounds formed from Group I and Group II metals were added at various concentrations and the pellets combusted.
First, the chlorine removal efficiencies and the decomposition temperature of different additives were studied, as shown in
In order to eliminate the influence caused by different additive amounts, the utilization efficiency (U) (removal efficiency/stoichiometric additive-to-chlorine ratio) was used. And the relationship between U and the decomposition temperature of additives is recalculated and shown in
As to the relationship between the chlorine removal efficiencies and the decomposition temperature of different metal chloride, as the metal chloride is very stable and hard to figure out its thermal decomposition temperature, the chlorine removal efficiencies in relation to the melting points and boiling points were studied respectively. Also two kinds of efficiency were calculated as shown in
This data demonstrates that all tested components remove chlorine. More effective compounds such as Ba(OH)2 and Sr(OH)2 had lower decomposition temperatures and formed chlorides which were stable at higher temperatures.
This has been a description of the present invention along with the preferred method of practicing the present invention. However, the invention itself should only be defined by the appended claims, WHEREIN WE CLAIM:
| Number | Date | Country | |
|---|---|---|---|
| 61873602 | Sep 2013 | US |
