The potential use of wood residues for energy generation 6. 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.
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The potential use of wood residues for energy generation 6. 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. 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.
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. 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. 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 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.
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. Particleboard Manu. 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. 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 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. 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.
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. 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.
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 Table Although wood may be burnt at 55 percent m.
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. 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. Figure The effect of wood residue moisture content on combustion efficiency 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. 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.
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.
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. 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. 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 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.
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 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.
For the long term durability of historical wooden buildings, constructors and users who deal with this subject have to know wood properties exactly. Wood is an organic, hygroscopic and anisotropic material. Its thermal, acoustic, electrical, mechanical, aesthetic, working, etc. With other materials, it is almost impossible. But wood has some disadvantages too.
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Сначала из мира ушли люди с их короткими жизнями, и было что-то невероятно ироническое в том, что последним адептом мессии-гуманоида стало существо, совершенно непохожее на человека, Огромный полип стал последним учеником Мастера по причине весьма тривиальной: он был бессмертен.
Миллиарды индивидуальных клеток, из которых состояло его тело, естественно, умирали своим чередом, но, прежде чем тому произойти, они воспроизводили себе подобных. Через длительные интервалы чудище распадалось на мириады клеток, которые начинали жить автономно и размножались делением -- если окружающая среда оказывалась для этого подходящей. В такие периоды полип уже не существовал как сознательное, разумное существо-единство.
И это лишь первая из бесконечного ряда твоих жизней. В комнате стало тихо, так тихо, что Элвин слышал странные, заунывные крики неизвестных тварей где-то в полях.
А вот эти просмотровые мониторы позволяют увеличить любой требуемый участок Диаспара, посмотреть на него в натуральную величину или даже под еще большим увеличением. Ими пользуются, когда нужно внести какие-либо изменения в конструкцию города хотя никто не брался за это уж бог знает сколько времени.
Если ты хочешь узнать, что же это такое -- Диаспар, то нужно идти именно. Здесь в несколько дней ты постигнешь больше, чем за целую жизнь изысканий там, на улицах. -- Как замечательно. -- воскликнул Олвин. -- И сколько же людей-знают о существовании этого места. -- О, знают очень многие, да только все это редко кого интересует. Время от времени сюда приходит Совет -- ведь ни одно изменение в городе не может произойти, если члены Совета не присутствуют тут в полном составе.
Но даже и этого недостаточно, если Центральный Компьютер не одобрит предполагаемое изменение. Словом, я сильно сомневаюсь, что хоть кто-то приходит сюда чаще, чем два-три раза в год.
Первым делом следовало точно выяснить, что произошло в его отсутствие. Хедрон, как сообщила ему Серанис, исчез. В Диаспаре было полно мест, способных служить укрытием, а поскольку Шут знал город как никто другой, вряд ли его можно было обнаружить до тех пор, пока он сам не соизволит показаться.
Возможно, подумал Элвин, ему следует оставить сообщение там, где Хедрон его обязательно найдет, и назначить тому встречу. Впрочем, присутствие стражи может сделать это намерение неосуществимым.
Членом экспедиции, который резко воспротивился присутствию робота, оказался Криф. То ли он вообразил, что теперь у него появился соперник, то ли из каких-то более общих соображений неодобрительно отнесся к существу, которое может летать без крыльев, -- это было неясно. Когда никто на него не смотрел, он сделал несколько попыток напасть на робота, но тот привел его в еще большую ярость тем, что не обратил на эти наскоки ни малейшего внимания. В конце концов Хилвару удалось его успокоить, и, когда они уже возвращались в мобиле, Криф, похоже на то, примирился с ситуацией.
Робот и насекомое, словно какой-то эскорт, сопровождали мобиль, беззвучно скользящий по лесам и полям, и каждый держался стороны, где сидел его хозяин, делая вид, что соперника просто не существует. Когда мобиль вплыл в Эрли, Сирэйнис уже ждала .
В задумчивом молчании шли Хилвар с Олвином обратно, к ожидавшему их кораблю. Как только они взлетели, крепость стала темной тенью среди холмов, она быстро сокращалась в размерах, пока не превратилась в странный черный глаз без век, обреченный на пристальный, вечный взгляд вверх, в пространство, -- и вскоре они потеряли его в огромной панораме Лиза.
Ученым того времени были подвластны все силы природы, все тайны времени и пространства. Тогда как наши мысли являются продуктом неимоверно сложной структуры мозговых клеток, связанных друг с другом сетью нервных проводников, те ученые стремились создать мозг, компоненты которого не были бы материальны на молекулярном или атомном уровне, а состояли бы из элементов самого вакуума.
Хедрон многократно прогнал на мониторе вперед и назад краткий период истории, запечатлевший трансформацию города. Превращение Диаспара из небольшого открытого города в значительно более обширный и закрытый заняло чуть более тысячи лет. За это время, видимо, были разработаны и построены машины, столь верно служившие Диаспару, и в блоки памяти были помещены знания, необходимые для выполнения соответствующих задачи.
Его другом двигали силы, приведенные в движение давным-давно теми гениями, что спланировали Диаспар со столь извращенным умением - или еще более великими гениями, противостоявшими первым. Подобно любому человеку, Элвин был в какой-то степени машиной, действия его предопределялись наследственностью.