Patent Application
20090139510
Fuel burning appliances require an exhaust system to conduct combustion products including noxious gasses and water vapor to the exterior of a dwelling.
Vent pipes, fittings and adapters, all exhaust systems generally include one or more usually made from a ductile material, such as sheet metal. These components are assembled in place and installed to custom fit the exhaust system to a given space. Vent pipes are usually located between walls, in attics and in crawl spaces where there is little room to work. As a result, the manipulation of the vent pipes is difficult, particularly with regard to connecting vent pipe sections.
One desirable type of combustion product for such appliances is biomass. Biomass fuels are organic and include seed, wood, crops, manure and garbage. Typical biomass fuels which can be burned include corn and wood. Some biomass fuels emit an exhaust when burned that contains corrosive chemicals.
The present technology, roughly described, pertains to a venting system for a biomass burning appliance. The system includes at least one pipe element formed of a steel alloy comprising: a maximum of 0.025 weight percent of Carbon, a maximum of 1.0 weight percent of Manganese; a maximum of 0.030 weight percent of Sulfur; a maximum of 1.00 weight percent of Silicon; a maximum of 0.035 weight percent of Nitrogen; a range of 17.50-19.50 weight percent of Chromium; a maximum of 1.00 weight percent of Nickel; a range of 1.75-2.5 weight percent of Molybdenum; and a range of a combination of Titanium+Columbium having a minimum of percentage of 0.20+4×(C+N) and a maximum of 0.80 percent.
The advantages of the present technology will appear more clearly from the following description in which the preferred embodiment of the technology has been set forth in conjunction with the drawings.
A biofuel venting system is disclosed.
In accordance with the technology, each pipe section component is formed of stainless steel having a composition as follows:
The aforementioned composition is commercially available under the Unified Numbering System (UNS) standard designation of type “S44400” stainless steel, commonly referred to as “444” stainless steel.
A number of stainless steel materials which have corrosive resistant properties are known. Extensive testing under the supervision of the inventors has determined that the aforementioned material exhibits superior properties for biofuel burning appliance venting systems given the corrosive effects of these exhaust and the heat requirements of such appliances.
Biofuel exhaust can cause both chemical corrosion and heat damage. The chemical composition, and in particular, the corrosive elements in a typical biofuel exhaust, were determined before selecting and a number of exhaust system material candidates were analyzed.
To develop the system, testing was first performed to determine the corrosive components of combusting biofuel and in particular corn fuels known commonly as “feed corn”, i“stove corn” and “seed corn”. Combustion of such materials was compared to wood pellet fuels used as a control Feed corn is comprised of whole yellow corn commercially available under the trade name Grainland Select Whole Corn from Country Acres Feed Company, Brentwood Mo. Stove corn is comprised of whole yellow corn commercially available under the trade name Valley View Feeds Stove corn from Balley View/Brubaker Grain, Farmersville Ohio. Seed corn is whole corn that may be treated with one or more of Captan, Metalaxyl, Pirimiphos-methyl, Imidacloprid, 1-[(6-chloro-3-pyridinyl)methyl]-N-Nitro-2-imidazolidinimine, and commercially available under the tradename Gaucho® from Gustafson LLC, Plano Tex.; or treated with Fludioxonil, 4-(2,2-difluoro-q,3 benzodioxol-4-yl)-1H-pyrrole-3-carbonitrile(R)-2[(2,6 dimethylphenyl)-methoxyacetylamino]-propionic acid, methyl ester (mefenoxam) and Chlorpyrifos (Lorbsan), commercially available under the trade name MaximXL® from Sygenta Crop Production Inc, Greensboro N.C.
Elevated levels of potassium chloride and potassium sulfate salts in the air emissions from corn combustion along with elevated levels of acid gases (sulfur dioxide and nitrous oxides) producing acidic emissions were found. Such emissions were responsible for the corrosive environment within the exhaust pipes burning corn biofuels. Tests of corroded pipe confirmed the corroded material on the pipe surfaces as being enriched in sulfur, chlorine and potassium. Analyses of bulk corn fuel confirm high levels of nitrogen, sulfur, chlorine, and potassium as compared to typical wood pellets. This is demonstrated in Table 1 which shows the results of air emission testing on three different fuels.
Table 2 shows an analysis of the components making up the various types of fuels:
An ion analysis of the emissions shown the following amounts of residual materials in a venting system:
Literature reports that normal dent corn, as it is a seed in contrast to a stalk, contains more than 9% protein. Protein is made up of amino acids, which in turn have high levels of nitrogen. Further, agricultural materials, like corn, have significantly more sulfur, chlorine, phosphorous, and potassium than wood. It is speculated that the higher moisture content of corn as compared to typical wood pellets may also contribute to the corrosion problem due to the increased potential of condensed water (with a low pH and corrosive salts in solution) being in prolonged and direct contact with pipe surfaces.
Also contributing to corrosive nature of corn combustion emissions are the elevated phosphorous levels in corn producing phosphoric acid and phosphate salts upon combustion and the elevated levels of ammonium salts in the air emissions, most likely originating from the incomplete combustion of amino acids.
Conditions that increase the potential for condensation include corn with unusually high moisture content, a long chimney allowing for emissions to cool, and a cold climate with a high relative humidity, or a combination of them. Similarly, corn with a high moisture content, a high salt content, a high heat content (creating more nitrogen oxide and perhaps some direct hydrochloric acid emissions), or the combustion of expired seed corn containing fertilizers or pesticides may be all or in part responsible for the corrosion complaints.
Once the corrosive components of the corn were known, seven different metal coupon samples were tested in corrosive solutions designed to mimic the exhaust corrosion. These include: type 304 stainless steel; type 316L stainless steel; Allegany Ludlum material AL29-4C® (steel); type 444 stainless steel; type 436S steel; type 430 steel and copper.
Grade 304 is a versatile and most widely used stainless steel, available in a wide range of products, forms and finishes. An exemplary composition of Grade 304 stainless steel includes:
Grade 316 is a standard molybdenum-bearing grade stainless steel. The molybdenum gives 316 better overall corrosion resistant properties than Grade 304, particularly higher resistance to pitting and crevice corrosion in chloride environments. Grade 316L is a low carbon version of 316 and is resistant to sensitization (grain boundary carbide precipitation). Thus it is extensively used in heavy gauge welded components (over about 6 mm). An exemplary composition of Grade 316L is as follows:
The alloy designated AL 29-4C® is a stainless steel developed by Allegheny Ludlum. The alloy has excellent resistance to brackish, polluted or high chloride waters, e.g., seawater. AL 29-4C® alloy is known to provide the following advantages over other competitive materials: high resistance to severe chloride environments, such as seawater; higher resistance to vibration damage than titanium; higher resistance to erosion-corrosion than titanium and copper based alloys; better heat transfer properties than austenitic stainless steels; and low cobalt content and cost effectiveness.
A typical composition of AL 29-4C® is given in Table 6.
AISI Type 436 stainless steel is a ferritic stainless steel. The test sample was the Allegany Ludlum designation AL 436S® alloy is known to have improved general corrosion and pitting resistance compared to ferritic steels having less chromium. Like all ferritic stainless steels the AL 436S® alloy provides resistance to stress corrosion cracking in the presence of chlorides.
A typical composition of AL 436S® is given in Table 7 below.
Type 430 stainless steel combines good corrosion resistance with good formability and ductility. It is a ferritic, non-hardenable plain Chromium stainless steel with excellent finish quality. Grade 430 also has excellent resistance to nitric attack, which makes it well suited to use in chemical applications. An exemplary composition of Grade 430 is shown in Table 8:
To determine the best material suitable for use in biomass fuel appliance, and in particular a corn burning appliance, a testing program was implemented involving subjecting the metal samples to corrosive aqueous solutions at varying concentrations and temperatures. The aqueous solution (solution 1) was designed to mimic the liquids condensing on a chimney, attached to an appliance combusting corn. The principal agents of the solution being: nitric and sulfuric acid along with ammonium and potassium, inorganic salts. To accelerate the corrosion testing, a concentrated version of the solution (solution 2) was used, which subjected the metal coupons to a more harsh environment than would be found in a chimney as a consequence of corn combustion. The concentrated solution was five times as concentrated as the first solution. Table 9 lists the composition of the solutions:
The test solution was derived from an analysis conducted on corn combustion emissions. The highly corrosive environments created in the concentrated solutions would likely cover any bio-fuel combustion: switch grass, grape seeds, cherry pits, bark, or other agricultural byproducts. The test setup consisted of placing sheet metal coupons in beaker/watch glass cells with 50 milliliters of test of solution. More than 60 coupon samples subjected to three temperatures (room temperature, 200° F. (93° C.) and 570° F. (299° C.)) were used in the corrosion testing. The elevated temperature sets cycled from room temperature to testing temperature each day of testing. In the high-temperature set, salt deposits were rinsed from the coupons and beakers before adding new solution. And the mid-temperature set was changed as needed, due to less evaporation. The room temperature samples were not changed, as little was lost from evaporation. It was apparent from the testing that some metals showed extreme signs of corrosion and others showed little or no visible signs of corrosion. The corrosion signs observed included: dissolving metal, pitting, and/or salts forming on metal surfaces. In addition, many of the solutions changed color due to the leaching of metal components into solution.
After 110 hours of testing at 200° F., corrosion is evident on all but the AL29-4C® and 444 samples. After 41 hours of corn corrosion screening at 313° F. (156° C.), then increased to 570° F. (299° C.) for 20 hours, some corrosion is evident on all samples. After 1344 hours (56 days) of corn corrosion screening at room temperature—between 70° F. (21° C.) and 100° F. (38° C.), mild corrosion is evident on all samples except the 444 and AL29-4C® samples.
Although the 444 and AL29-4C® samples performed equally well, it was determined that the 444 material would make a superior venting system for a biomass fuel having the exhaust characteristics including the compounds in relative percentage as that of the test solutions. The 444 material provides a superior resistance to heat and is more cost effective. The UL standard for venting used on corn-burning appliances requires that the vent be tested to a continuous temperature of 570° F. with brief (10 minutes) forced-fire to 1700 F. The Allegheny Ludlum specification sheets for AL29-4C note that the maximum use temperature should be restricted to 600 F. Also, the Allegheny Ludlum spec sheet for 444 material notes that 444 may experience embrittlement if subjected to long-term exposures of 885° F., but was able to handle “many years” of exposure at 650° F. without degradation. Additionally, any embrittlement could be reversed with short exposures of around 1200 F.
The foregoing detailed description of the technology has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the technology to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The described embodiments were chosen in order to best explain the principles of the technology and its practical application to thereby enable others skilled in the art to best utilize the technology in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the technology be defined by the claims appended hereto.
