6.1 Introduction
6.2 Sources of available wood residues
6.3 The fuel value of wood residues
6.4 The preparation of wood waste fuel
6.5 Applications for waste-based energy
6.6 Combustion
6.7 Cogeneration
Unlike most other industries, the forest industries are fortunate to be able to use their waste to help meet their energy needs. In mechanical wood processing the greater part of the thermal energy requirements can be met from the available residues, in fact, the sawmilling industry has the potential to produce both a surplus of heat and electricity and therefore could support other energy deficient conversion processes in an integrated complex producing, for example, lumber, plywood and particleboard or, in the rural areas, to supplying energy for the needs of the surrounding community.
Over the years many mills have regarded wood waste as a troublesome by-product of the sawmilling operation, resulting in its being disposed of as landfill or incinerated in Wigwam burners or the like. However, both have recently become contentious environmental issues and, combined with the rising costs of energy, mill owners have been forced to seriously consider the merits of using the residues as an alternative fuel source this has also coincided with the increase in demand for the residues as furnish for paper-pulp and panel board manufacture, due to the rising cost and increased competition for solid wood.
Nowadays most wood processing plants being built in developed countries incorporate hog fuel burners in order to safeguard against certain and costly fossil fuel supply. In instances where the amount of residues produced are insufficient to meet the plant's thermal needs, purchased hog fuel and/or fuel oil are used to make-up the balance. However, little use is made of the energy potential of sawmilling residues in developing countries, this being partly due to the minimal use of kiln drying and the investment capital involved in the installation of the heat generating plant.
Although the heat produced from wood residues is less than that from oil or gas, its cost compared to fossil fuels makes it an attractive source of readily available heat or heat and power. In spite of the growing competition for the residues for other uses, its projected increase in price over the coming years will undoubtedly be less than that expected for traditional fuels. Although the handling, processing and combustion of the residues may involve a higher capital outlay, considerable developments in new and improved technology and plant design are now rendering it an economically attractive fuel source.
The most effective utilization of wood residues, particularly in the sawmilling and plywood industry, plays an important role in energy efficient production and it can often prove to be an important factor in determining whether a sawmill is to operate at a profit or loss, especially if lumber is manufactured from marginal logs. However, when contemplating the use of wood residues as an energy source, whether it only be to provide heat for kiln drying or both heat and power for use in an integrated complex, the following items will need to be examined in detail as they can influence the economic viability of the venture:
(a) present day and projected future costs of traditional energy sources and their availability;(b) energy requirements of the plant (heat and electricity);
(c) availability and reliability of residue supplies, their cost, type, size, moisture content and proportion of contraries;
(d) the capital cost of equipment needed to collect, process and combust the wood residues;
(e) disposal cost of residues;
(f) resale value of the residues as a raw material for panel board or pulp manufacture, etc.
It is only by undertaking a professional study of the above, as well as the most appropriate type and size of plant and the best use of the surplus heat and power, that an efficient waste handling, treatment and combustion system can be designed in which the return on investment would warrant capital expenditure. Obviously it would not be logical to invest in a plant in which the capital and operating costs exceed the gains from using the residues as fuel.
Although residues may represent a free source of readily available fuel, it is a misconception to believe that it is a free source of energy. The cost of waste handling, treatment and combustion equipment, together with labour and maintenance can be a costly adjunct to a plant's operating costs and capital outlay, and may prove to be excessive for some small mills. Also, this particularly applies to on-site power generation, which, due to the high cost of steam raising and power generating plant, would not be considered an economically viable investment for most small- and medium-sized install
6.2.1 Forest residues
6.2.2 Mill-site generated wood waste
6.2.3 Integrated production
6.2.4 Alternative uses of residues
The residues generated from the forest products industry may be divided into two parts; that which results from harvesting and extracting logs from the forest, and generally considered of no economic use for further processing, and that which is generated by the forest industries themselves during the process of manufacturing timber, plywood, particleboard and the like (refer to Figures 1, 2 and 3), namely:
|
Source |
Type of residue |
|
Forest operations |
Branches, needles, leaves, stumps, roots, low grade and decayed wood, slashings and sawdust; |
|
Sawmilling |
Bark, sawdust, trimmings, split wood, planer shavings, sanderdust; |
|
Plywood production |
Bark, core, sawdust, lillypads, veneer clippings and waste, panel trim, sanderdust; |
|
Particleboard production |
Bark, screening fines, panel trim, sawdust, sanderdust. |
In general it may be said that of a typical tree, less than two-thirds is taken from the forest for further processing, the remainder being either left, burnt or collected as fuelwood by the local inhabitants. After processing, only 28 percent of the original tree becomes lumber, the remainder being residues, as indicated in Table 7.
Table 7. Division of a typical tree harvested for sawntimber
|
Tree part or product |
Portion |
|
(%) |
|
|
Left in the forest: |
|
|
Top, branches and foliage |
23.0 |
|
Stump (excluding roots) |
10.0 |
|
Sawdust |
5.0 |
|
Sawmilling: |
|
|
Slabs, edgings and off-cuts |
17.0 |
|
Sawdust and fines |
7.5 |
|
Various losses |
4.0 |
|
Bark |
5.5 |
|
Sawn timber |
28.0 |
|
Total |
100.0 |
Sources: (37) (144)
It is only in the last few years that, due to the economics of rapidly rising fuel and wood costs, industry in the developed countries have invested in ways and means to extract the maximum quantity of recoverable wood during logging operations. Although this document draws attention mainly to the energy value of residues produced during the manufacturing operations, consideration should be given to the potential industrial use of residues left in the forest.
It is not uncommon for some 60 percent of the total harvested tree to be left in the forest and for non-commercial species to be subjected to slash and burn, or merely felled and left to rot so as to make access easier for logging. Such practices as sawing and squaring logs in the forest, rather than at the sawmill, wastes a further eight to ten percent and 30 to 50 percent respectively (29).
Proper training and provision of appropriate tools and logging equipment can do much to improve the methods of harvesting so as to substantially reduce the excessive wastes, which could otherwise represent a higher yield of solid wood or a source of fuel.
However, although forest residues may appear to be an attractive fuel source, collection and handling costs must be taken into consideration, as well as its loss as a valuable soil nutrient. The viability of its use may be improved if collection be undertaken at the same time as log extraction, with shared equipment and management, whereby logging slash and marginal timber may be collected and chipped using portable or semi-portable chippers placed in the immediate logging areas. By ensuring that leaves, bark and thinnings are left behind, the soil's nutrients would not be depleted.
Transport costs are also a critical factor in the use of forest residues, due to the low heat values of such bulky material, for which reason distances are to be kept low so as not to incur unnecessary expense if the waste is to remain economically attractive as a fuel source. Chipping of the residues does afford some degree of compaction, also several processes are in operation which further compress the waste into more manageable forms, such as pellets, thus improving their bulk handling characteristics. However, due to the high capital and operating costs involved, densification tends to be only financially viable when the waste needs to be transported over long distances.
Whilst regarding wood as a renewable energy resource, consideration should also be given at regional or national level to encourage the collection and use of logging residues, be they branches, tops or whole-tree utilisation, to the establishment of energy plantations using quick growing species especially selected for their value as a fuel.
If one considers that approximately 45 to 55 percent of the log input to a sawmill or plywood plant is to become waste, it would be illogical not to maximize its use as a fuel source, if no other profitable market outlet can be found.
The actual production of residues, or waste, generated from the manufacture of wood products, differs from plant to plant and depends on several factors, from the properties of the wood to the type, operation and maintenance of the processing plant. However, mean averages apply to each type of industry, which, for developing countries have been detailed in Tables 1, 2 and 3 of Appendix VI, and summarized in Table 8.
Table 8. Proportion of residues generated in selected forest products industries 1/
|
|
Sawmilling 2/ |
Plywood Manu. |
Particleboard Manu. |
Integrated Operations |
|
% |
% |
% |
% |
|
|
Finished product (range) |
45-55 |
40-50 |
85-90 |
65-70 |
|
Finished product (average) |
50 |
47 |
90 |
68 |
|
Residues/Fuel |
43 |
45 |
5 |
24 |
|
Losses |
7 |
8 |
5 |
8 |
|
Total |
100 |
100 |
100 |
100 |
1/ Excluding bark
2/ Air-dried
All wood waste and bark, which is also commonly referred to as hog fuel due to the process of reducing the residues in size in a "hogger", has a value as a fuel, although it is produced in a wide range of sizes with varying moisture contents, as shown in Table 10, and comprises mainly of the following:
- Bark, which makes up some 10 to 22 percent of the total log volume depending on size and species, can in itself represent a serious waste disposal problem unless it can be used as a fuel or removed prior to log preparation;- Coarse residues, such as slabs, edgings, off-cuts, veneer clippings, sawmill and particleboard trim, when reduced in size, make ideal fuel, especially when dry. They also have a resale value as pulp and particleboard furnish;
- Cores, from plywood peeler logs, are generally sold to sawmills or lumber or as pulp chips;
- Sawdust, being a product of all mechanical wood processing operations, particularly sawmilling, is generally not regarded as a prime pulping material due to its small size, although it proves to be acceptable for the manufacture of particleboard;
- Planer shavings result from dimensioning and smoothing lumber, plywood and particleboard with planers during the finishing stage. They are considered ideal for particleboard production and are particularly good for heating kilns and dryers;
- Sanderdust is produced during the abrasive sanding of lumber, plywood and particleboard during the finishing stage. Due to its size and very low moisture content it is well suited for direct firing;
- Particleboard waste, being in the order of five percent, is negligible compared to that generated in other mechanical wood-based industries, as it is largely recycled within the production process. In fact the waste from sawmilling and plywood manufacture make up a large part of particleboard furnish.
As previously indicated, the sawmilling and plywood industries each produce between 40 to 55 percent of waste from their incoming wood supply, with heat values in the range of 17 to 23 MJ/kg (dry weight), more than sufficient to meet their own energy requirements. Nonetheless, it is considered uneconomical to generate their own electricity from residues unless they have an additional sales outlet for the surplus power.
However, particleboard production produces little waste, being in the order of five to ten percent, and insufficient to cover the needs for heat, yet, would be resolved in the case of an integrated operation of all three industries - market forces permitting (25).
Sawmilling, veneer, plywood and particleboard production lend themselves quite readily to integration, with the advantages of shared waste handling processing facilities and services, and the maximum benefit derived from the use of the residues as a raw material and fuel, whereupon the surplus energy could be fully and economically used to the best advantage. But, the scale of such a complex may be beyond the means- of some developing countries.
Residues derived from the forest industries normally do have alternative outlets, as chips for pulp manufacture, raw materials for particleboard and fibreboard manufacture and as fuelwood and building materials to local inhabitants - all dependent on market location and demand. Listed below are several outlet areas.
|
Sawmilling |
- edgings and slabs |
- low cost building materials, fuelwood and pulp manufacture |
|
- barked edging chips |
- pulp manufacture and fuelwood |
|
|
Plywood Manufacture |
- peeler log cores |
- lumber manufacture |
|
- core chips |
- pulp manufacture |
|
|
- veneer chipping and chips |
- fuelwood |
|
|
Particleboard |
- uses all the above mentioned residues as raw material for board manufacture, and the majority of its own residues are recycled within the process. |
|
Alternative markets and the sale value of wood residues must, of course, be taken into consideration when undertaking a feasibility study of a specific manufacturing plant, so as to assess its availability for fuel and to account for its opportunity value in manufacturing cost analysis.
Apart from the use of residues as a potential fuel source to meet a plant's own energy requirements, its direct sale, or as pellets or briquettes, as fuel to other industrial users or electricity generating companies is becoming an attractive venture for some mills in developed countries. However, one must take into account its historic use in certain regions, as being a basic fuel for domestic heating and cooking in the smaller cities, villages and rural areas.
In some countries the use of wood residues as a raw material for the production of say pulp and paper and particleboard, is deemed to be more beneficial to both the local and national economic and social well-being, than its use as a fuel. (100) This being due to the value added element in the form of labour and trade derived during the various stages of processing the residues into a saleable product, whereas its impact as an alternative fuel is solely to reduce oil imports - a debatable issue.
6.3.1 Heating value
6.3.2 Effect of moisture content and particle size on heat values
When evaluating the properties of a combustible material with respect to its use as a fuel, the heating value, expressed in this document as gross calorific values or higher heating values, is one of the most important factors, which indicates the amount of thermal energy which may be obtained by combusting one mass unit of the material.
The heating value of wood depends very much on the species and the part of the tree being used and varies between 17 to 23 MJ/kg of bone dry wood; generally softwoods have higher caloric values than hardwoods, with an average value of 21 MJ/kg BD for resinous woods and 19.8 MJ/kg BD for other woods being used. In fact, there is very little variation in the heating values of the wood substance itself, being some 19 MJ/kg BD, as it is, in fact, the variation in resin content, with a calorific value of 40 MJ/kg BD, which accounts for the differences in values between the species. It is for this reason that bark, with a high gum and resin content, tends to have a higher value than wood.
However, although the fuel value may be fairly consistent in bone dry wood, the heating value depends on several factors, namely moisture content, particle size, type and efficiency of combustion equipment being used and the level of its operation and maintenance. Hence, in order to put the heating values of various wood residues into perspective one must take into consideration the heat content per unit of waste according to its moisture content, together with the efficiency of the energy conversion process which, as indicated in Table 9, provides a comparative analysis to be made with other alternative fuels (refer to Tables 1 and 2 of Appendix IV).
Table 9. The effect of moisture content on the net heating value of wood compared to that of other fuels
|
Fuel |
As fired Gross calorific value |
Typical burner efficiency |
Useable Net heating value |
|
|
(MJ/kg) |
(%) |
(MJ/kg) |
||
|
Wood at 0% m.c 1/ |
19.8 |
80 |
15.8 |
|
|
|
10% m.c |
17.8 |
78 |
13.9 |
|
|
20% m.c. |
15.9 |
76 |
12.1 |
|
|
30% m.c. |
14.5 |
74 |
10.7 |
|
|
40% m.c. |
12.0 |
72 |
8.6 |
|
|
50% m.c. |
10.0 |
67 |
6.7 |
|
Anthracite |
31.4 |
83 |
26.1 |
|
|
Lignite |
26.7 |
80 |
21.4 |
|
|
Heavy fuel oil |
42.6 |
82.5 |
35.1 |
|
|
Light fuel oil |
43.5 |
82.5 |
35.9 |
|
|
Butane |
49.3 |
79.0 |
38.9 |
|
|
Propane |
50.0 |
78.7 |
39.4 |
|
1/ Wet basis
Sources: (22) (61) (82)
Wood at the time of logging generally has a moisture content of approximately 50 to 55 percent, although the amount varies according to species, age and the portion of the tree from which it originated, i.e. branches, trunk, etc. Further fluctuations from the mean are influenced according to the season it is cut and the manner in which it is transported to the mill site and stored; logs that are floated down stream, wet-debarked or left in conditioning ponds could have moisture contents as high as 65 to 70 percent, whereas that which is road-hauled and dry-debarked would be in order of 45 to 50 percent m.c. Spring and summer storage can bring about a moisture loss of 10 to 25 percent.
The moisture content of the manufacturing residues depend very much on at what stage of the process they are extracted and whether there has been any drying of the product before that stage. For instance, sanding dust from plywood or particleboard manufacture is taken from the plant after the driers and hot presses, where its moisture content could be as low as ten percent or less, as indicated in Table 10.
Table 10. Range of characteristics of typical wood residues
|
Residues |
Size |
Moisture content 1/ |
Ash & dirt content 2/ |
|
(mm) |
(%) |
(%) |
|
|
Sanderdust |
- 1 |
2 - 10 |
0.1 - 0.5 |
|
Shavings |
1 - 12 |
10 - 20 |
0.1 - 1.0 |
|
Sawdust |
1 - 10 |
25 - 40 |
0.5 - 2.0 |
|
Bark (hogged) |
1 - 100 |
25 - 75 |
1.0 - 2.0 |
|
Log-yard clean-up |
up to 100 |
40 - 60 |
5.0 - 50 |
|
Forest residuals |
needles to stumps |
30 - 60 |
3.0 - 20 |
1/ Wet basis
2/ By weight
Source: (56)
As mentioned previously, moisture content is a major determinant in the heating value of wood waste, which, from 19.8 MJ/kg at 0 percent m.c. drops to 10 MJ/kg at 50 percent m.c., as can be seen by refering to Figure 12. Although wood may be burnt at 55 percent m.c., and up to 58 percent m.c. with careful operator attention and boiler tuning, it is always better to aim for a moisture content of 50 percent or lower in order to achieve satisfactory and substained operation. When the moisture content rises to 60 percent, burning of the wood residues become difficult as its heating value drops dramatically, to the extent where, at approximately 68 percent m.c., "furnace blackout" occurs, being the point at which combustion can no longer be sustained, unless a supplementary fuel is used to maintain combustion.
A high moisture content not only lowers the as-fired heat value of wood waste, but seriously affects the overall combustion efficiency due to the large amount of energy needed to heat considerable quantities of excess air and to vapourise the moisture in the waste, which, together with the moisture formed by the combustion process itself is subsequently lost up the stack as latent heat. Hence, it stands to reason that wood waste at ten percent m.c., with an as-fired heat value of 17.8 MJ/kg and a combustion efficiency of some 78 percent is preferable to green wood at 50 percent m.c. with an as-fired heat value of 10 MJ/kg and 67 percent combustion efficiency.
Figure 12. The effect of wood residue moisture content on combustion efficiency (103)
The size and form of the wood particle is also critical in both the handling characteristics and burning efficiency of residues and plays a major role in their combustibility and the selection and operation of processing and combustion plant. Whereas fine sanderdust and wood shavings may be burnt in suspension, larger sized wood-waste, in the form of large chips, coarsely hogged waste and slabs need a longer dwell time to burn which is generally undertaken on grates.
Hence, all steps taken to reduce the moisture content and size of the residues to a minimum, pays dividends in energy generation. The provision of prepared storage, suitably protected against the elements, the use of flue gases to dry the fuel etc., all contribute towards maintaining low residual moisture and optimum combustion efficiency.
6.4.1 Collection and handling
6.4.2 Storage
6.4.3 Size reduction and screening
6.4.4 Fuel drying
6.4.5 Densification
The handling, treatment and storage of wood waste fuel is considerably more costly and troublesome than that required for traditional fossil fuels. Hence, the importance of a well conceived and equipped woodfuel preparation system cannot be over-emphasized so as to maximize the fuel potential of a plant's residues and to minimize handling and combustion problems.
The reduction of particle size and moisture content, together with the most appropriate storage and handling systems are necessary for an efficiently operated wood waste combustion system.. The waste preparation process generally involves hogging, dewatering, screening, size reduction, bulk storage, blending and drying prior to combustion so as to ensure a reliable and consistent supply of quality fuel to the burners. An equal amount of care and attention needs to be paid to the state of the wood waste used, as would normally be the case with any other fuel. The use of waste that is decayed, too wet or containing an excessive amount of contraries is false economy, due to the difficulty in handling and storing the wet residues, undue wear-and-tear on equipment and the detrimental effect on the overall combustion efficiency.
Waste collection and handling does not necessarily need to be either labour intensive or. involve costly and sophisticated mechanical handling plant, which could otherwise render the use of residues uneconomical. In small-scale forest industries in developing countries, the collection and handling of waste is predominantly manual, aided by a tractor or bulldozer to both convey and push the residues to a belt conveyor: system, thus avoiding the need for an enormous capital outlay and with maximized use of available labour.
The handling systems should be so designed as to afford the highest degree of flexibility to the operator and to be able to cater for the full range of sizes and moisture contents of was. e expected from outside and within the mill. It is the failure to attend to such aspects of design that invariably give rise to fundamental problems in operation.
Waste brought to the mill-site in the form of forest residues, or as purchased industrial wood waste, to supplement the plant's own wood-based fuel, may be delivered by road or rail. Methods of unloading range from the use of manual labour or a knuckle boom loader fitted with a clam-shell bucket, to live bottom vans or hydraulically elevated dump trucks, all of which are determined by economic considerations.
The manner in which mill residues are best removed and handled is again a matter of economics, availability of labour and the quantity and type of waste produced and is normally undertaken by a combination of belt. and pneumatic conveyors, front-arid loaders and trucks, with manual gathering predominating in mills with an input rapacity of 20 000 m3/A and below.
Generally slabs, edgings, peeler cores, veneer waste and trimmings would be transported by mechanical conveyors or carried manually to a chipper and, after screening, conveyed to storage piles for use as either furnish for pulp or particleboard manufacture or as fuel. Bark, panel trim and waste from ply glue spreaders would be hogged and conveyed to the hog-fuel storage area. Sawdust and sanderdust, depending on the quantities produced, would be pneumatically extracted and conveyed to a separate storage area (preferably covered). Retrieval is normally achieved by the use of belt, drag link, flight or pneumatic conveyors, in conjunction with front-end loaders, which may also be used to build-up the piles.
In order to safeguard against damage to moving parts, stone traps and magnetic separators need to be incorporated in the handling system, ahead of the reduction plant so as to remove all stones and tramp iron. Depending on the proportion of contraries normally expected in the fuel supply and the type of burning equipment employed, air classification may need to be employed in order to remove rocks and sand from the smaller-sized fuel particles, but only if they are comparatively dry.
The type of wood waste storage will be largely determined by:
- the form and moisture content of the residues;
- the frequency and reliability of year-round deliveries to the mill and production of residues;
- the availability of land;
- climatic conditions;
- the need for air drying;
- the volume of wood waste fuel involved;
- the system of waste handling and treatment adopted.
Storage systems may be divided into two distinct categories, namely:
Outdoor storage, in piles on prepared concrete or gravel pads to aid drainage and reduce the entrainment of contraries, is the least expensive means of maintaining stocks. This form of storage is generally suited for stocks of 20 to 30 day's capacity of green forest residues, bark, moist wood slabs or chips. However, unless adequate preparations and precautions are taken, deterioration and fires from overheating and biological action can take place. Hence, residues should be monitored and those that do not benefit from drying with time should have a fast turnaround and be used on a first-in-first-out basis (109).
In instances where a large variety of residue sizes are involved, it is always advisable to segregate according to size, either before or after storage, and, in most cases, it is preferable to reduce the larger-sized waste in hoggers or chippers at an early stage in order to facilitate handling. Mixing of wet and dry waste should be avoided, as such a practice will reduce the efficiency of combustion; it is far better to have dual storage and feed systems in order to segregate the feed to the burners according to moisture content.
Covered storage systems, to safeguard against loss and damage due to wind and rain, is normally provided for materials which are readily wind-borne or freely absorb moisture, such as dry sawdust, planer shavings and sanderdust.
Such storage systems as open-sided buildings, hoppers, bins or silos are usually located in the near vicinity of the combustion plant, with approximately 48 hours capacity so as to sustain continuous operation without being hampered by weekends or interruptions in the flow of supply from the processing plant.
Whereas sawdust, planer shavings and sanderdust may be burnt directly without the need for further processing, other forms of wood waste have to be reduced in size in order to facilitate handling, storage and metering to the combustion chamber. By achieving a uniform particle size, combustion efficiency will be improved due to the uniform and controlled fuel feed rate and the ability to regulate the air supply. Additionally, in the case of fuels with a high moisture content, the reduction process exposes a greater surface area of the particle to the heated gases, thus releasing the moisture more rapidly, thereby enhancing its heating value.
Size reduction may be carried out in several stages in a hog or attrition mill, with screening before and in between.
The hog basically comprises of a set of knives or swing hammers mounted on a rapidly rotating shaft within a robust casing. The impact of the rotating impellers on the wood waste against the breaking plate reduces it to a standard size of approximately 20 to 50 mm (100).
Screening, directly before or after the hog, separates the dirt and fines and conserves energy in the subsequent reduction stages by removing those particles of acceptable size which would otherwise be reprocessed.
Attrition mills are used to reduce residues still further in size, by passing them between a stationary and a rotating disc, each fitted with slotted or grooved segments. The particles produced may, after screening, then be burnt in direct-fired suspension burners to produce hot gases for drying lumber, plywood and particleboard furnish and other such heating requirements.
As previously mentioned, combustion efficiency, boiler control and the operator's ability to provide a quick response to changes in steam demand become seriously impaired by a combination of high and fluctuating moisture content of incoming fuel. This situation may be improved by drying the fuel, which will also effectively increase boiler capacity and lead to better emission control.
The moisture in residues may be reduced either by mechanical pressing, air-drying or the use of hot air dryers, or a combination of all three. It is common practice for mechanical presses to be used on bark and wood waste with moisture levels in excess of 70 percent in order to reduce it to 55 to 60 percent m.c., which would then enable the waste to be mixed with dryer incoming materials to produce a combustible fuel. However, in the event that sufficient supplies of wood waste are readily available to meet the plant's energy needs and the disposal of bark does not present a serious problem to the mill, then it is not considered economically justified for it to be pressed and dried in view of the maintenance, power demand and the high capital intensive plant involved.
Air-drying of logging residues, assuming the right climatic conditions prevail, can bring about a moisture loss of some 10 to 15 percent, and the level may even drop further to 25 percent (67) should the residues be left in clear-felled spaces open to the action of wind and sun. Air-drying of mill waste, time and space permitting; is preferable under covered well-ventilated areas, especially for the smaller-sized residues such as sawdust, which is more liable to absorb rainfall and takes longer to air-dry than say mixed wood waste.
Green whole chips and mixed waste, when stored outside in piles for several months, may lose up to 10 to 25 percent (105) of their moisture content by way of the drying effect of wind, sun and spontaneous internal heating due to bacteriological action on the materials in the interior of the pile (108).
The use of fuel dryers to dry fuel to approximately 30 percent m.c. using plant such as rotary drum, flash and cascade type dryers employing waste stack gases, direct combustion of residues, steam or hot water as heating sources, undoubtedly lead to better combustion efficiency and boiler utilization. Nonetheless, the use of fuel dryers in medium-sized installations is questionable as the heat energy gained would be off-set by that which would be needed to dry the fuel, added to which one must take into consideration the high capital and operating costs involved.
A growing awareness has developed in recent years in the use of compacted wood waste, in the form of briquettes, pellets or "logs", as a domestic or industrial fuel.
Briquettes or logs are generally formed by forcing dry sawdust or shavings through a split cylindrical die using a hydraulic ram. The exerted pressure, of some 1 200 kg/cm2, and the resultant heat generated bonds the wood particles into "logs".
The production of pellets involves the reduction of wood waste to the size of sawdust, which is then dried to approximately 12 percent m.c., before being extruded in specially adapted agricultural pellet mills to form pellets of some 6 to 18 mm diameter and 15 to 30 mm long, with a density in the range of 950 to 1 300 kg/m3 1/. Drying of the furnish prior to extrusion is usually undertaken in rotating drum dryers, fired by approximately 15 to 20 percent of the plant's pellet production.
1/ Bulk density being 480 to 640 kg/m3 (106)
Although pelleting produces a product with excellent handling and storage characteristics, with four times the energy concentration of woodfuel, thus greatly reducing transport costs and improving boiler efficiency, it has been found that high capital investment in the processing plant and operating costs only prove economically attractive if transportation distances of the fuel exceed 250 km from the source of the raw material, and is normally not warranted for site-generated fuels.
A mill or integrated complex, with a readily available supply of hog-fuel, has several operations open to it as to the manner in which it may convert its waste into useable energy. However, before embarking upon a general description of the proven methods of combustion and combustion plants, a brief outline of several alternative applications for the recovered heat has been listed below. By examining Figures 5, 7 and 9 in Chapter 2, it becomes apparent which production centres will gain the most from either onsite heat or power generation or both.
The choice of the most efficient and cost-effective use of waste-based energy, and the selection of appropriate heating mediums would need to be studied on a case-by-case basis, in view of the individuality of each mill.
|
Heating medium |
Broad outline of possible applications |
|
Hot air |
For direct drying of: |
|
(a) lumber; |
|
|
(b) plywood veneer; |
|
|
(c) particleboard furnish; |
|
|
Hot water and thermic oil |
As an indirect means to supply heat for: |
|
(d) log conditioning; |
|
|
(e) lumber and veneer drying; |
|
|
(f) glue and resin preparation; |
|
|
(g) hot pressing of ply and particleboard; |
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(h) space heating; |
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Steam |
May be used as a heating medium in all the above-mentioned applications, as well as: |
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(i) to provide transmission power to process plant through the use of a system of line-shaft and belt drives. (In the past, many sawmills were powered in this way, a large number of which are still operating successfully); |
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(j) to directly drive plant, such as boiler feed water pumps, induced draft fans, large air compressors, etc., by way of small steam turbines; |
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(k) steam which is surplus to the mill's requirements may be sold to neighbouring consumers for industrial, commercial and community use; |
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(l) to produce electricity by way of a turbine-generator to help meet the power demand of the integrated complex; |
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(m) in the case of non-integrated sawmills and plywood plants, in which their residue production far exceeds their actual heat energy needs and market demand, consideration may be given to on-site power generation to meet their own requirements, with the sale of the surplus to the public utilities. |
6.6.1 Firetube and watertube boilers
6.6.2 Pile burners
6.6.3 Suspension and cyclone burners
6.6.4 Fluidized-bed combustors
The range of combustion systems now available to the forest products industry is quite considerable, with a large choice of equipment for each category. Apart from the end use of the heat, particle size plays an important role in influencing the combustion plant. Whereas fine sanderdust and wood shavings may be burnt in suspension, larger-sized wood waste, in the form of chips, coarsely hogged residues and slabs need longer to burn which is generally undertaken on grates. The decision whether to select the combustion plant to suit the available fuel or, process the fuel to the requirements of the preferred plant, can only be made after a thorough analysis has been carried out.
Traditional methods of burning hogged fuel for steam or hot water production has been with the use of firetube and watertube boilers employing the pile burning method of combustion on a grate. The difference between standard oil or gas fired boilers and those for firing wood waste in that the slow-burning characteristics of wood, together with its high moisture content, necessitates a larger combustion chamber capacity with a high furnace so as to create low upward velocities and cater for the longer residence time needed to burn the wood fuel (16).
The necessity of a larger-sized boiler, together with the need for waste handling plant involves up to 1.5 to 4 times the investment cost of oil fired package boilers. As indicated in Table 9, combustion efficiencies of 65 to 75 percent may be expected when burning wood waste, compared to 80 percent obtained from gas or oil fired units. The difficulty of automatic firing, slow response to peak demand and the need for ash removal and disposal are other disadvantages which must be carefully weighted up when considering the use of, what may at first appear to be, a cheap fuel.
The direct firing of sanderdust and pulverized wood waste presents a somewhat simpler way in which to use the fuel's energy value to heat drying kilns, air heaters and boilers. However, there is still some hesitation in the industry to adopt the various direct-fire systems and other newer technologies, such as wood gasifiers, until they have been adequately proven, though they do hold promising prospects.
A brief description of the basic combustion systems, with the exception of gasification and pyrolysis, is given, without attempting to make any comparisons as each system has its own champions and needs to be assessed according to the circumstances of the mills. It must be pointed out, however, that the capacity ranges, technology and cost of several of the mentioned plants are inconsistent with medium-sized mills in developing countries.
Boilers fall into two categories, being firetube and watertube. Firetube boilers are principally used where steam pressures of not more than 20 kg/cm, (12) are required in small to medium-sized operations, and are well suited for the heat requirements of the mechanical wood-based industry. They are relatively inexpensive and operate on the principle of hot combustion gases passing through steel tubes set in a water jacket.
Watertube boilers consist of tubes welded together in such a manner as to form complete walls enclosing the combustion chamber, through which flows the water to be heated. By virtue of its construction, the watertube boiler is almost exclusively used where steam pressures above ten kg/cm2 (12) are employed, especially in providing motive power to turbine-generators.
Both types of boiler may be further sub-divided into those which arrive as a package of field-erected on-site. Package boilers are generally shop assembled units, thus allowing them to be readily shipped, installed and operated and tend to be less than 22 500 kg/hr steam capacity. Whereas all the component parts of the field-erected units are entirely assembled and welded on-site.
Pile burners, as the name implies, burn the fuel in piles either on a refractory floor or grate, and may be divided into two classes, namely:
Heaped pile burning furnaces, such as the tee-pee burners and clutch ovens, which are fed with fuel from the top of the furnace in batch form via chutes located across the grates or, continuously by way of variable screw feeders, to be burnt in a pile on a refractory floor or grate. Primary air for combustion is introduced through ports located on all four sides of each chamber and the heat transmitted to the boiler surface situated above and behind the combustion chamber.
Such furnaces may be used to burning fuels of up to 65 percent m.c., regardless of size or shape (98), although they do require a lot of attention and considerable time to build-up and burn down the pile and have low efficiencies in the order of 50 to 60 percent. In some furnaces, fuel of high moisture content may be added to the base of the pile by hydraulic rams, thus allowing the waste to burn more slowly and completely. The provision of more than one chamber permits ash to be removed in one compartment whilst the other is being fired.
Thin pile furnaces burn hogged fuel, up to 55 to 60 percent m.c., as a thin bed spread across the grate. Sloping grates, pinhole grates, travailing grates are but some of the systems currently in use enabling the fuel to progressively advance along the grate through the combustion chamber, whilst being exposed to primary air from below, to be then discharged by an assortment of removal systems as ash.
Spreader stokers, using pneumatic or mechanical spreaders, are used in the larger-sized furnaces to evenly meter and distribute hogged wood or particles, of up to 45 to 50 percent m.c., into the firing zone of the combustion chamber, allowing the finer particles to burn in suspension and for the larger-sized fuel to fall to the grate where it is burnt.
Suspension and cyclone burners are proven technology, having been successfully used with pulverized coal for several years and adapted for use with woodfuels.
Suspension burners, as the term suggests, burns fine wood particles in suspension, in either special combustion chambers or boiler fireboxes, in a highly turbulent environment caused by forced combustion air. In order to operate efficiently the wood particles need to be no more than 6 mm in size and a maximum moisture content of 15 percent (98). Such units are particularly well-suited for use with lumber kilns, veneer and particleboard furnish dryers and boilers.
Apart from combustion efficiencies of approximately 75 percent, they often have a quick response to swing loads with high turndown ratios, although provision must be made to safeguard against the risk of explosions due to the nature of the fine fuel particles used.
In the case of cyclone burners, pulverized wood fuel, of a maximum 3.5 mm size and 12 percent m.c., is mixed in the first stage of the burner and combusted in an external cyclone burner.
Fluidized-bed combustors are capable of burning untreated hog fuel, with moisture levels as high as 55 to 60 percent (100), in a turbulent mixing zone above a fluidized bed of inert silica sand. The fuel is maintained in suspension during combustion by a high velocity of air forced through the bed of sand, which results in the sand adopting free-flowing and fluidized properties.
6.7.1 Restrictive regulations and penalties
6.7.2 Economic considerations
The simultaneous production of both electrical power and a useable form of thermal energy, such as steam, is termed cogeneration. This may be achieved by generating high pressure steam in a hog-fuel boiler, which would then be passed through a turbine generator for power before being used as exhaust steam in drying or process heating. Therefore, rather than just generating electricity from wood waste with a conversion efficiency of 25 to 30 percent, cogeneration raises the efficiency of energy utilization to some 75 percent (46).
The use of a condensing turbine generator with single or double automatic extraction or a back-pressure turbine generator are options available to the forest products industry, though the latter is often favoured. Although the fuel potential of the residues generated from sawmilling and plywood manufacture exceed the plants' heat and power requirements, energy self-sufficiency in electricity in an integrated operation, including particleboard production, is more difficult to attain due to the fact that there is a given ratio between the power output and the output of industrial heat generated by a back-pressure power plant (approximately 1:1.5) (25). This shortfall may be overcome by the following alternative solutions: (25) (47)
- to design a power plant in which the ratio between power and heat production meets that of the integrated complex's consumption ratio. However, this option involves both costly and sophisticated plant and therefore is not considered suitable for developing countries;- to supplement the plant's own hog-fuel production with purchases wood and wood residues or fuel oil. Yet this option either places on increased load on existing waste treatment and combustion systems or necessitates larger plant capacity, with heat production being surplus to needs;
- to make-up the balance of the mill's power requirements by either using diesel generating sets or purchasing power from the national grid.
In certain countries legislation and regulations discourage the generation of power for sale, and those mills that meet their own needs for power are often penalized by having to pay higher rates for their purchased power in the event of a shortfall or shut-down in their on-site power supply (58).
Although such regulations may well have been established to dissuade mills from investing in small unviable power plants, they do not take into account the potential for hog-fuelled back-pressure power. Hopefully, in the light of the anticipated rise in fuel costs such restrictive tariffs shall be removed.
Although it is technically feasible to use wood waste as fuel for power generation, it is the economics that invariably prove to he the limiting factor in most cases. Whereas there are obvious benefits to be gained by burning wood residues to reduce a manufacturer's fuel oil and electricity bill, they may be off-set by the high capital costs involved, low plant efficiencies and increased manning levels. Of course the economics of wood waste energy generation becomes more attractive as traditional fuel prices increase, though the real value of the wood waste as a fuel source must take into account its available heat content, the investment and operating costs of the plant needed to handle and convert it to useable energy, before any worthwhile comparative studies can be made.
It must be noted that the capacity of most energy generation plant available to the industry, especially for cogeneration, exceeds that which can be economically utilized by most mills and integrated units. Additionally, the limited finance available to the small- and medium-scale mills tend to be a major deterrent in their contemplating cogeneration as an option worthy of consideration, regardless of the possible long-term gains.
Hence, it is for these reasons that, in spite of their self-sufficiency in self-generated fuel, it is generally not considered economically justified for the individual sawmill or plywood plant of less than 150 000 m3/A log input capacity (34) to generate their own power, unless they be part of an integrated production unit consisting of sawmilling, plywood and particleboard manufacture, etc., with shared services.
