The present disclosure is directed generally to systems and methods for drying wood products in one or more kilns with recovered flue gases.
Drying is an important process in wood product manufacturing for a number of reasons including prevention of wood degradation (e.g., mold), reduction of weight for transport, production of a stiffer product, and reduction of fluctuations in dimensional changes. Most wood products are dried in some type of dry kiln. Modern dry kilns provide a controlled temperature and humidity environment and are equipped with fans and vent systems to force air circulation and ventilation.
Dry kilns may be categorized as one of two types: batch dry kilns or continuous dry kilns. In a batch dry kiln, stacked loads of wood products are loaded into the kiln to be dried in a stationary batch process in a single chamber. In a continuous dry kiln, stacked loads of wood products enter the green end of the kiln and are moved forward through multiple drying chambers until exiting the dry end of the kiln. Continuous kilns typically consist of three chambers: one main chamber and two conditioning chambers.
Heat to a kiln may be provided either by indirect means or by direct firing. When a kiln is heated directly, combustion (or flue) gases and/or hot air from a source exterior to the kiln are directed into the kiln. When a kiln is heated indirectly, a source exterior to the kiln typically produces a heated fluid which is circulated in equipment such as a heat exchanger. Both batch kilns and continuous batch kilns feature vent control systems, which let in dry air and discharge hot humid air.
Kiln drying is a high energy consuming process, which can account for approximately 70 to 90 percent of a facility's energy needs; therefore, heat recovery may be an attractive method for reducing a facility's drying energy costs. However, the value of energy saved is largely dependent on the differential temperatures between the exhaust temperature of the combustion unit and the kiln exhaust temperature, the efficiency of the recovery system, and its capital and maintenance costs. Thus, there is a need to develop new systems and methods for drying wood products in kilns.
The following summary is provided for the benefit of the reader only and is not intended to limit in any way the invention as set forth by the claims. The present disclosure is directed generally towards systems and methods for drying wood products in one or more kilns with recovered flue gases.
In some embodiments, a system for drying wood products using recovered flue gas includes one or more combustion devices and one or more indirect fired kilns. The one or more indirect fired kilns each include one or more chambers, one or more heating zones within each of the one or more chambers, one or more heating elements in the one or more chambers, one or more openings for receiving wood products, and one or more ventilation systems. A flue gas recovery mechanism is configured to deliver flue gas from the one or more combustion devices to the one or more indirect fired kilns before the flue gas exits through the exhaust stack.
Further aspects are directed towards methods for drying wood products with recovered flue gas. In some embodiments, such methods include firing a fuel in the one or more combustion devices, thereby producing a heated fluid and a flue gas. The heated fluid is circulated in the one or more heating elements to provide a primary heat source to the one or more indirect fired kilns. An amount of flue gas is recovered and transferred to the one or more kilns via the one or more of the ventilation systems. The recovered flue gas provides a secondary heat source to the one or more indirect fired kilns.
The present disclosure is better understood by reading the following description of non-limitative embodiments with reference to the attached drawings wherein like parts of each of the figures are identified by the same reference characters, and are briefly described as follows:
The present disclosure describes systems and methods for drying wood products in one or more kilns with recovered flue gases. Certain specific details are set forth in the following description and
In this disclosure, the term “wood” is used to refer to any organic material produced from trees, shrubs, bushes, grasses or the like. The disclosure is not intended to be limited to a particular species or type of wood. The term “wood product” is used to refer to a product manufactured from logs such as lumber (e.g., boards, dimension lumber, headers and beams, timbers, moldings and other appearance products; laminated, finger jointed, or semi-finished lumber (e.g., flitches and cants); veneer products; particle board; wood strand products (e.g., oriented strand board, oriented strand lumber, laminated strand lumber, parallel strand lumber, and other similar composites); or components of any of the aforementioned examples. The term “kiln” is used to refer to any type of thermally insulated chamber or series of chambers in which controlled temperature regimes are produced.
Embodiments according to the disclosure may include a single kiln and/or a single combustion device. Alternatively, embodiments of the disclosure may include multiple kilns and/or multiple combustion devices.
In
Referring back to
The indirect fired kiln 104 shown in
The indirect fired kiln 104 has an opening 116 for receiving wood products (not shown). In some embodiments, the opening 116 may be sealed by a door (not shown). The wood products may be delivered by a fork lift, a truck, or by any other method known to a person of ordinary skill in the art. A discharge vent 120 is arranged on the indirect fired kiln 104 to discharge humid air from the chamber 108. Fans or another mechanism (not shown) may also be employed to help force humid air out of the indirect fired kiln 104. The indirect fired kiln also has an inlet vent 122 arranged for receiving heated air. In some embodiments, a fan 124 may be used to suck air into the chamber 108. Although only a single fan 124 is shown, multiple fans may be installed. The discharge vent 120, inlet vent 122, and one or more fans (e.g., the fan 124) make up a ventilation system for the indirect fired kiln 104.
Methods for drying wood products using the system 100 include the step of firing a fuel in the combustion device 102. The fuel may be a natural gas, coal, oil, a bio residual fuel (e.g., hog fuel, pellets, shavings, sawdust, etc), or another type of heating fuel known to a person of ordinary skill in the art. Combustion of the fuel will produce a heated fluid and a flue gas. The heated fluid is transported in a first string of insulated piping 126 for circulation in a heating element 128 (e.g., a heat exchanger) in the chamber 108. Heat produced by the heating element 128 serves as a primary heat source for the indirect fired kiln 104. In some embodiments, heat produced by the heating element constitutes approximately 70% to 80% of the total heat source for the indirect fired kiln 104.
Flue gas produced during combustion is usually composed of carbon dioxide (CO2) and water vapor as well as nitrogen and excess oxygen. In
In embodiments according to the disclosure a flue gas recovery mechanism 134 is used to recover an amount of flue gas before it is discharged up the exhaust stack 106. The flue gas recovery mechanism 134 in
Referring to
The combustion device 202 may be, for example, a steam boiler, a water boiler, or a thermal fluid heater. Additionally other types of combustion devices that would be obvious to a person of ordinary skill in the art are envisioned as part of the disclosure. As shown in
The first indirect fired kiln 204 and the second indirect fired kiln 206 are both continuous kilns. Continuous kilns are well known to a person of ordinary skill in the art; therefore, the specific structures and systems associated with continuous kilns are not discussed in this disclosure. An example of a continuous dry kiln is described, for example, in U.S. Published Patent Application No. 2006/0272172, which is hereby incorporated by reference. Additionally, in other embodiments according to the disclosure, the one or more of the indirect fired kilns could be batch kilns.
The first indirect fired kiln 204 shown has three chambers: a first main chamber 210, a first conditioning chamber 212, and a second conditioning chamber 214. Continuous kilns according to the disclosure may have different numbers of chambers or configurations of chambers that differ from those shown in
The ventilation system for the first indirect fired kiln 204 includes a first inlet vent 216, a first opening 218, and a second opening 220. In addition, other vent mechanisms and/or fans may be included as part of the ventilation system. The first inlet vent 216 is configured to let air into the first main chamber 210. The first opening 218 and the second opening 220 are the openings by which wood products are fed into the first indirect fired kiln 204. The openings serve the dual functions of allowing wood products to enter and exit the kiln and providing a mechanism for discharge of hot, humid air. Although not shown explicitly in
The first main chamber 210 is equipped with one or more heating elements. In
The second indirect fired kiln 206 may have generally the same structural and operational elements as are the first indirect fired kiln 204. The second indirect fired kiln 206 includes three chambers: a second main chamber 224, a third conditioning chamber 226, and a fourth conditioning chamber 228. The ventilation system for the second indirect fired kiln 206 includes a second inlet vent 230, a third opening 232, and a fourth opening 234. In addition, other vent mechanisms may be included as part of the ventilation system. The second inlet vent 230 is configured to let air into the second main chamber 224. The third opening 232 and the fourth opening 234 are the openings by which wood products are fed into the second indirect fired kiln 206. The openings serve the dual functions of allowing wood products to enter and exit the kiln and providing a mechanism for discharge of hot, humid air.
The second main chamber 224 is equipped with one or more heating elements. In
Methods for drying wood products using the system 200 are similar to the methods described with respect to
In some embodiments, the combustion device 202 includes an electrostatic precipitator 240 for removing particulates from the flue gas before it is sent up the exhaust stack 208. An induced draft fan 242 may be used to assist in forcing the flue gas up the exhaust stack 208.
In embodiments according to the disclosure, a flue gas recovery mechanism 244 is used to recover an amount of flue gas before it is discharged up the exhaust stack 208. The flue gas recovery mechanism 244 in
From the foregoing, it will be appreciated that the specific embodiments of the disclosure have been described herein for purposes of illustration, but that various modifications may be made without deviating from the disclosure. For example, modifications to the kiln design and configurations that would be obvious to a person of ordinary skill in the art may be made. Although
Aspects of the disclosure described in the context of particular embodiments may be combined or eliminated in other embodiments. For example, some of the features shown in embodiments using multiple kilns may be used in embodiments where only a single kiln is used. Further, while advantages associated with certain embodiments of the disclosure may have been described in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the disclosure. Accordingly, the invention is not limited except as by the appended claims.
The following examples will serve to illustrate aspects of the present disclosure. The examples are intended only as a means of illustration and should not be construed to limit the scope of the disclosure in any way. Those skilled in the art will recognize many variations that may be made without departing from the spirit of the disclosure.
Systems and methods according to embodiments of the disclosure were compared to conventional methods for drying wood products using modeling techniques. A model of the total energy balance for both a continuous kiln and a batch kiln was constructed. Lumber drying operations were simulated for each of the systems modeled. Exemplary models for the continuous kilns are shown in
The model was used to simulate drying of nearly 160,000 pounds of Hemlock lumber.
The model was used to simulate drying of nearly 160,000 pounds of Hemlock lumber.
The results of the simulation show that systems and methods according to the disclosure provide a viable alternative method for drying wood products in kilns. Based upon the models, the amount of steam required to dry a fixed amount of lumber may be reduced using systems and methods according to the disclosure. In the case of a batch kiln, the reduction in steam required may be approximately 20%. In the case of a continuous kiln, the reduction in steam required may be approximately 37%. These results may differ based on the particular application and conditions; however, systems and methods according to the disclosure may be effective to save on energy and costs associated with constructing additional boilers and other equipment for drying wood products. Although the simulations were run using lumber as the wood product, systems are methods according to the disclosure are applicable to drying of wood products other than lumber.
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Number | Date | Country | |
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20110219640 A1 | Sep 2011 | US |