Energy from Wood Wastes and Its Utilization!
Advocates of alternate energy sources have begun to examine the wood processing industry as an untapped energy reservoir. Each year tremendous quantities of timber are harvested across the country to be processed into lumber, plywood, or pulp. These wood operations generate large volumes of wood and bark residues.
ADVERTISEMENTS:
Bark, chips, edgings, sawdust, sanding dust, trimmings are by-products of primary manufacturing. These by-products are comprised of more than half of the volume of the original log. Some utilisation of these residues has been accomplished through reprocessing into pulp, pulp products, composition board, and sometimes fuel.
Raw material for pulp plants in Oregon is supplied almost entirely by the wood residues of sawmills and plywood plants. Wastes from the logging operations are almost entirely neglected except for disposal concerns. Logging wastes are those wood wastes effected by timber harvesting operations.
These wastes are generated in appreciable amounts. Milling wastes are those wood wastes generated during the processing of the logs into products. Usually these quantities can amount to 1-1/4 dry tons per 1000 feet of lumber cut and 1/2 dry ton per 1000 feet by 3/8 inch plywood produced.
Other wood residues that are potentially available include – (i) Fire damaged trees, (ii) Insect and disease ridden trees, (iii) Driftwood, (iv) Construction wood wastes, and (v) the wood portion of municipal solid wastes.
These wood and bark residues can have widespread usage in a variety of applications. The demand for products which can utilise wood residues as a raw material is continuously rising. Depending on the residue size and quality, they can be used in the manufacture of plywood, lumber, paper and pulp and building boards.
Miscellaneous round-wood products such as poles, piling, mining timbers, shingles and board products and wood for fuel are expected to have very little change in their demand in 2000 over what it is today. With increased housing requirements the domestic demand for lumber is expected to increase dramatically.
ADVERTISEMENTS:
The accelerated construction pace and continued rise in the manufacture of wood products will result in plywood and building board usage to more than double and triple, respectively, by 2000. Therefore, the more virgin wood used, the more wood wastes generated. A significant portion of the increased composition board demand is expected to be satisfied through the use of wood residues as raw materials.
Wood wastes are also expected to help satisfy increase in paper, cardboard and pulp products. Wood residues are available and will continue to be available throughout this century. They have been utilised to some extent, and are expected to have increased usage in the future.
However, their utilisation is somewhat dependent on the consumer industry’s proximity to the waste reservoir. The paper and pulp industry can readily use the waste wood resource because large quantities of milling and logging residues are generated in this region.
Although these wood and bark residues can and do have widespread usage in a variety of applications, the ultimate determining factor in their usage is cost. The economics may prove to be unfeasible for a relatively small milling plant to re-use or have its wood wastes reused.
ADVERTISEMENTS:
Furthermore, the large plants which generate huge volumes of wood residues may not have an outlet to accept these wastes and may find they have to transport long distances to be utilised. When the utilisation of wood and bark residue becomes uneconomical, a disposal problem is created. The use of these materials as a fuel may prove to be a viable alternative.
Before examining wood residue characteristics any further, their units of measure are discussed to familiarise the reader with the terminology:
(i) Cubic foot – The volume of logs or chunks are typically expressed in solid cubic feet. The gross cubic feet are the total volume of solid material and the net cubic feet term describes the portion of the total that is considered usable.
(ii) Tons (wet) – The moisture content of the wood can be as much as 50% by weight. When converting from tons (wet) to tons (dry) a water content of 50 pounds per cubic foot of wood and bark is often assumed. This is often referred to as the wood’s green weight.
ADVERTISEMENTS:
(iii) Tons (dry) – To measure the weight of wood residues of varying densities and water contents the bone dry weight of the material is used. It is the basic unit of wood residue weight measurement.
(iv) Unit – Wood wastes in the form of particles such as sawdust, hogged fuel, or chips are usually measured in bulk. A unit of wood residue is the amount contained in a volume of 200 cubic feet. Units vary with respect to solid wood content which is dependent on wood type, moisture content, and compaction.
What has become evident is that wood residues vary in size and composition. Sizes of wood residues range from dust particles generated by sanding to large chips and slabs. The moisture content of the sander dust is relatively small as opposed to the moisture content of some woods which exceeds 50%. These wood wastes also vary with respect to appearance.
To utilise wood residues for fuel or any application, they must first be reduced to a size that can be handled conveniently. Large volumes of wood and bark residues can be effectively reduced in size by a “hog” machine. The particles produced by this piece of equipment are commonly referred to as hogged fuel or hog fuel.
Hogged fuel can include any portion of wood or bark residues in a reduced particle size. Additives to the hogged fuel may also include sawdust and wood shavings and ground bark. Hogged fuel may be a conglomerate of many types of wood wastes.
Wood Waste as a Fuel:
Hogged fuel, is usually sold in bulk volumes or units. A unit of hogged fuel roughly contains a ton (dry) of wood material. This dry weight can vary from 2600 pounds for hogged Douglas fir bark to 1900 pounds for sawdust to 1200 pounds for Douglas fir shavings.
The moisture content of a particular hogged fuel varies with the wood type, season, size of a hogged wood particle, and whether the original logs were dry or wet handled. Bark from Douglas fir logs that were handled in ponds contains moisture in quantities almost equal to its dry weight. A unit of hogged Douglas fir bark from logs handled in such a manner might have a wet weight of approximately 5200 pounds.
The quantities of the various wood residues generated by the manufacture of lumber and plywood can be estimated by the conversion factors given in Tables 5.26 and 5.27. These are average values for the estimation of wood wastes from Oregon mills only, since these factors vary from mill to mill depending on parameters such as log size, quality and species, manufacturing equipment; final product and the mill’s quality control. Residue factors for each mill at a particular location must be determined based on that mill’s operating conditions.
Wood residues from Oregon sawmills and plywood operations that are used for fuel, the heat content of those residues used for fuel is 70 × 1012 BTU. Thus, availability of these wood residues is of such magnitude and considering the cost of disposal, that their utilisation as a fuel or fuel supplement is becoming imperative.
Wood at one time was the primary fuel of the country, but because of its relatively low heating value as compared to coal and oil, it dropped from use. However, rising gar, oil, coal and disposal prices has resulted in renewed interest in wood as a fuel.
Fuel Properties of Wood and Bark:
The basis elemental composition of most wood species is relatively constant and this is true of bark also. Table 5.28 gives the typical ultimate analysis of Douglas fir and western hemlock barks. Hogged bark from a saw-mill, which contained some quantities of wood (normally found in hogged fuel), are used for the analysis. Bark and wood fuels have negligible sulphur contents and unlike most heavy oils and coals, they do not effect sulphur compound air pollution problems.
Air pollution problems due to ash are also relatively low. Ash is the portion of the fuel which is non-combustible and normally must be separated from the combustion gas before they are exhausted out of stack. Wood contains small amounts of ash in the order of less than 1% by dry weight.
The non-combustible content of the wood fuel is often increased as a result of dirt and sand adhering to the bark during harvesting and handling. Ash contents of softwood barks can range from 0.6% for sugar pine to 2.5% for Engelmann spruce. Hardwood barks had ash contents ranging from 1.5% for paper birch to 10.7% for white oak.
From Table 5.28, it can be seen that bark generally has a higher fixed carbon. The heating value is one of the most important properties of a fuel. The heating value of most wood species is about the same, 8300 BTU per pound, provided they are moisture and resin free.
Woods with resin contents have higher heating values than non-resinous woods because the heating value of the resin is nearly 17,000 BTU per pound. The barks generally, because of high resin contents, have higher heating values than woods. Wood fuel is frequently sold by the volume unit rather than weight. These values may vary depending on the moisture content of the wood.
The amount of water included with the dry wood is also an important property of wood and bark fuel. In order to evaporate moisture, heat is required, which is then lost to the evaporating moisture. Therefore, moisture has a negative heating value. Table 5.29 illustrates the relationship of bark moisture to heat content. As the moisture content is increased the heat content is proportionately decreased.
The moisture content of these wood materials can be expressed in several manners. The forestry industry usually expresses moisture in terms of a ratio of water weight in a material to its dry weight. This ratio is usually expressed as a percentage. A material with equal weights of dry substance and water would have a moisture content of 1:1% or 100% (dry basis).
The fuel and combustion industry expresses moisture as the ratio of water weight to the total weight. A material with equal weights of water and dry substance would have a moisture content of 1:2 or 50% (wet basis).
The following equations can be used to convert from wet to dry basis or vice versa –
M.C. (wet) = 100 M.C. (dry)/[100 + M.C. (dry)]
M.C. (dry) = 100 M.C. (wet)/[100 – M.C. (wet)]
where M.C. = moisture content in per cent.
The moisture content of wood and bark fuels varies widely. Hogged fuel generally has a moisture content of around 50% (wet basis) most of which is contained in the cellular structure of the wood.
Combustion of Wood and Bark:
In the combustion of wood and bark three processes occur, consecutively at first, but then simultaneously:
(i) First, heat must be supplied to evaporate the water in the wood fuel in order to effect combustion.
(ii) Volatile hydrocarbon gases are then evolved and mixed with oxygen giving off heat.
(iii) More heat is released and combustion completed with the reaction of oxygen with the fixed carbon at high temperatures. Initially these processes occur in succession, but as heat is generated the wood eventually begins to sustain its own combustion and all processes occur at once.
The stochiometric air requirement of a combustion process is that amount of air necessary to burn the carbon and hydrogen in the fuel completely to carbon dioxide and water. The Douglas fir bark fuel analysis of Table 5.28 is used to calculate stochiometric air requirements for its combustion.
About 6.5 pounds of air is necessary to completely burn 1 pound of dry fuel. Excess air quantities for the complete burning of 1 pound of dry Douglas fir bark fuel with upto 100% excess air are illustrated in Fig. 5.17.
Depending on the moisture content, stochiometric air requirements vary and naturally so does the weight of the stack gases evolved. Fig. 5.18 gives the weight of the stack gases evolved from the complete combustion of 1 pound of dry Douglas fir bark with various moisture contents and quantities of excess air.
As the temperature of the stack gases increases their volume increases. Based on the burning of 1 pound of dry Douglas fir bark with 40% excess air Fig. 5.19 shows the volume of stack gases as a function of stack temperature and wood moisture content. 40% excess air is a representative air requirement of a boiler plant burning wood fuel.
Figs. 5.17, 5.18 and 5.19 could be utilised by a wood power plant designer to determine equipment criteria such as fan size, ductwork and stack design.
Wood Burning Plants:
Many firms commonly use wood and bark residue fuels. These residues are also burned for home heating in stoves, furnaces and fireplaces. The heat of combustion of sander dust is used in veneer and wood particle drying processes and for the production of steam.
The latter process effects the largest industrial use of wood and bark fuels. Steam is produced for heating, processing and power generation through electricity. Hogged fueled steam plants range in steam capacity from 10,000 pounds per hour for a small plant to over 500,000 pounds per hour.
A steam power plant to utilise wood and wood products for fuel would typically consist of the following:
(i) A mill or hog to process large wood scraps into a size more readily acceptable for combustion
(ii) Wood storage facilities to meet peak steam demands and during interruption of wood fuel deliveries
(iii) Equipment to control the flow of fuel into the furnace
(iv) A furnace for the combustion process
(v) A boiler for steam generation
(vi) Air pollution control devices
(vii) A network of conveyors or other systems to conveniently handle the wood fuel and resultant ash
(viii) Controls to effect automatic operation of the system.
(ix) A fuel pre-drying process may be necessary for the combustion of wood residues with high moisture contents.
Probably the most common hogged wood fuel burning process involves the use of a dutch oven. Fig. 5.20 shows a two-stage dutch oven furnace. In the first stage (dutch oven), water in the wood is evaporated and the fuel is gasified. In the secondary furnace combustion is completed. The system is gravity fed as the hogged fuel enters the dutch oven from above and forms a conical pile. Presently, more efficient and larger capacity systems are available.
The fuel cell process is also a two-stage furnace as illustrated in Fig. 5.21. The hogged wood fuel enters the primary furnace compartment from overhead. The gravity fed system allows the fuel to drop onto a water cooled grate. The wood is first gasified in the primary stage and the gases pass into a secondary combustion compartment for complete combustion. Hogged fuel boilers of this type are in rather widespread use throughout.
Labour operating costs are low because these steam plants are highly automated. Low operating pressures of about 25 psi gives these plants steam capacities ranging from 10,000 to 30,000 pounds per hour. When the moisture content of the fuel is above 100% (dry basis), hogged fuel dryers are necessary. Many of these wood steam plants have applications in drying kiln processes.
Many steam plants recently constructed to be wood or bark fueled are the spreader-stoker type. In the spreader-stoker system, a pneumatic or mechanical spreader feeds the hogged fuel from above onto a grate in the furnace. As the fuel falls to the grate it is partially combusted while in suspension.
Combustion of the fuel is completed on the grate. Fig. 5.22 is typical spreader-stoker steam plants currently in operation. This operation can be used with small plants effecting steam rates of about 25,000 pounds per hour and with large capacity plants of 500,000 pounds per hour.
Fig. 5.23 illustrates the inclined grate furnace. This system is very similar to many municipal solid waste incineration methods. At the furnace inlet the hogged fuel is deposited on the top section of the grate. The wood fuel then passes through the three zones of the grate. The first section dries the wood for combustion in the second section. In the third section the combustion process is completed and the ash is removed.
After the system start-up, no supplemental fuel is required to keep the wood burning and the system is generally automatically operated. Boiler efficiencies obtained from this fluidised bed heat recovery unit are comparable to conventional fuel equipment. Fig. 5.24 is a flow sheet of the system’s complete operation.
Wood Waste Boiler Performance:
The efficiency of a boiler system is dependent on the heat lost during the entire combustion and power generation cycle.
The heat balance includes:
(i) Heat transferred to the boiler fluid (i.e. steam).
(ii) Heat lost to gases exiting the stack.
(iii) Heat lost evaporating moisture in the fuel.
(iv) Heat lost from the formation of water from hydrogen in the fuel.
(v) Heat lost because of incomplete combustion.
(vi) Heat lost to radiation.
(vii) Heat lost to accounting procedures.
Heat losses resulting from conditions (v), (vi) and (vii) are small, usually in the range of 4%. Heat loss due to water formation from hydrogen in the fuel is dependent on the burning temperature and for Douglas fir bark fuel are about 7 or 8%.
Because of their high moisture contents, most wood and bark fuels have high heat losses due to water evaporation from the fuel. Fig. 5.25 shows the percentage heat loss of bark fuel as its moisture content is increased. On the average, a fuel with a 100% moisture content (dry basis) requires about 13% of the fuel’s total heat output to evaporate the moisture. Over one-quarter of the final fuel heat output is required at a moisture content of 200% (dry basis).
As the moisture content is increased the flame temperature is lowered and combustion is inhibited, reducing the steam output of the boiler. The wood’s combustion can no longer be self-sustaining as the moisture content approaches between 180% and 230% (dry basis). Excessively moist wood fuels would probably have to be dried prior to combustion or be burned along with supplemental fuels such as oil and coal.
The amount of excess air required for combustion of the fuel and the gas exiting temperature determine the boiler heat loss due to dry stack gases. The heat loss due to dry stack gases as a function of gas exit temperature and excess air used is shown in Fig. 5.26. By reducing the amounts of excess air used and by passing the stack gases through a heat recovery unit before they exit the stack, heat losses can be minimised.
The overall efficiency of a wood burning boiler system can be calculated by evaluating the heat losses. In Fig. 5.27, a steam plant which burns Douglas fir bark fuel with 40% excess air has its overall efficiencies given as a function of fuel moisture content and stack gas temperature. (40% excess air is a normal operating condition for a representative wood burning plant).
Fig. 5.27 can be used to correct when larger excess air volumes are needed for complete combustion. A boiler efficiency of nearly 70% can be expected from stack temperatures of 400-500°F and moisture contents of 100% (dry basis). These high efficiencies usually require fuel pre-treatment (i.e. pre-drying) and stack gas heat utilisation via exchangers.
Air Pollution Control:
Wood and bark fuels have relatively small amounts of sulphur. Sulphur emissions are usually well below governmental regulations. Of more concern to wood and bark fueled power plants are visible plume and particulate matter emission standards. Oregon’s regulations for new boiler units allow 0.1 grain of particulate matter per standard cubic foot of gas. Particulate emissions effected by bark fueled furnaces normally range from 0.5 to 5.0 grains per standard cubic foot.
The amount and type of particulate matter generated by wood and bark furnaces is dependent on the fuel burned. Ash contents vary with wood and bark type. They are generally higher in bark which also accumulates large quantities of dirt and sand from handling operations. These emissions are comprised of dirt, sand and char. The sands and dust are relatively large particles and are the nearly invisible component of the emissions. The char, or unburned carbon, is relatively small and highly visible.
Depending on particle size, power facility size, quantities of exhaust gases and emission rates, air pollution control devices to collect these particles can be installed. Larger particles can be efficiently collected by mechanical cyclones or a series of cyclones. For smaller particles electrostatic precipitators may be necessary to effect their removal from exiting gases.
These pollution devices may require large capital and operating investments. However, depending on the char quality, the collection equipment can be cost effective. By first passing the stack gases through a screening process to effect removal of the larger sand and dirt particles, the char can be re-injected into the furnace to complete combustion with a minimum increase in emissions.
Collection devices for wood and bark fueled power plants usually include a two-stage collection system. However, because there is relatively little experience with control devices for wood fired furnaces many collection systems have not been wood fuel proven. Baghouses, wet scrubbers and electrostatic precipitators can be employed depending upon the plant’s requirement and their economic feasibility at that plant.
Steam plants that utilise hogged fuel usually cost more than a conventional fuel fired plant because of the high moisture content and handling problems of hogged fuel. Studies have shown that a boiler system burning hogged fuel alone or with a supplemental fuel would cost twice as much than a boiler that burns oil only.
However, although the wood and bark fueled plant requires twice the initial capital investment over conventionally fueled plants, other cost effective factors should be considered including:
(i) Lower hogged fuel costs resulting in long-term savings
(ii) Less air pollution emissions.
(iii) Increasing conventional fuel costs.
(iv) Conventional fuel shortages.
(v) Use of hogged fuel with supplemental fuel.
(vi) Wood and bark waste disposal must be effected.
More Wood Derived Waste Disposal and Utilisation:
Wood wastes can be utilised in a number of ways as an energy source. Large power plant installations are not the only means of energy recovery. Logs for home use are made from sawdust, wood chips and other combustibles mixed with wax. Logs made from leaves have currently become a profitable enterprise. Not only are the leaves effectively disposed, but their energy is recovered.
The leaves are separated, dried and shredded before they are mixed with wax to aid combustion. They are then compressed and formed into 16 inch long by 4 inch diameter logs.
All wood wastes could be treated in a similar manner to alleviate just a fraction of the energy pinch. These wastes are available all around us and are usually obtainable to the fuel processor at no cost. Sometimes the waste producer will even pay the fuel processor to remove his wastes. There are large profits to be made in waste recycling and many individuals have begun to work at it.