4.2 Wood for carbonisation
4.3 Agricultural residues for carbonisation
4.4 Bark waste
The raw material for carbonisation, its gathering and preparation constitute the single most important aspect of charcoal manufacture no matter what method of carbonisation is used. In this chapter raw materials are divided into two groups:- those derived from trees, i.e. wood in some form or other and those derived from agriculture, the so-called agricultural residues. Not all raw materials produce quality charcoal and the quality of the charcoal produced by a particular raw material should be checked to see if it fits proposed markets or end uses before any investment is made. There is an important point to take note of. Unlike coking coal which when it is turned into coke by carbonisation fuses together to form strong aggregates even if powdered coal is used, woody type raw materials when carbonised do not fuse and the charcoal has the form of the original raw material, i.e. Lump wood produces lumps of charcoal and powdered agricultural waste or sawdust produces charcoal in the form of powder which is unsuited for many industrial charcoal uses even if it is briquetted in the usual way.
The mechanical strength of charcoal be it lump or powder also depends on the raw material. To possess a high crushing strength as is needed for use in blast furnaces the raw material must contain lignin and other lignin like extractives. These substances when carbonised give strength to the charcoal. High strength charcoal requires wood or nut shells as raw material. If lump charcoal is needed then wood is practically the only material though coconut shells produce strong charcoal suited for gas absorbtion purposes in a size adequate for this application.
All other raw materials produce fine charcoal which must be briquetted and is only economic when sold into the barbecue market to middle class consumers.
4.2.1 The unit operations in wood harvesting
4.2.3 Felling and skidding
4.2.4 Blocking and drying
4.2.5 Haulage to carboniser
4.2.6 Final block preparation and stockpiles
This is the most important raw material by far and produces excellent charcoal. There are two broad types of wood, hardwoods produced by broadleaved species and softwoods produced by conifers. Both produce charcoal but hardwood charcoal is usually stronger than softwood charcoal.
There are also differences between the liquid condensate produced when the two types of wood are carbonised. Softwoods because of their resin content tend to produce more tars and there is less acetic acid and similar products in the pyroligneous acid fraction. Often where pine-wood is available as waste it is more rational to extract first the rosin content of the wood by solvent extraction after reducing the wood to chips. The extracted wood may be then used for pulp, fuel or made into charcoal and the resulting fine charcoal briquetted. (Fig. 9)
Wood occurs as natural forest or in man-made plantations. The management of natural forest and plantations is a technique requiring knowledge of forest science and cannot be dealt with here. More detailed information from the point of view of charcoal production is given in (12, 13,14, 15).
The harvesting problem when making charcoal on a large scale, is the same as for the gathering of pulpwood except for the need to dry the wood as much as possible before it is carbonised.
The unit operations in harvesting wood are:- roading, felling, skidding, blocking, drying, transport to carboniser, stockpiling and final block preparation. These unit operations are discussed in the following paragraphs. (See also Chapter 7 and Figs. 9 and 14).
First the forest area which will provide the raw material for the life of the enterprise must be defined and secured in a legal sense. Then a roading and harvesting plan must be drawn up which will allow the wood to be felled, dried and extracted at minimum cost over the life of the enterprise. The road system needs to be planned to minimise haulage distance and decisions made as to where the wood is to be pi led for drying, at or near the stump or at the charcoal making enterprise itself.
If the wood is obtained from plantations then the harvesting scheme must be coordinated with the plantation management and regeneration plan.
The quality of the road system needed depends on the type of logging trucks to be used. If as is common in charcoal wood gathering either flat top trucks or combinations of agricultural tractor and trailer are to be used a simple roading system will be adequate. But if large logs in full lengths are to be extracted as may apply if the charcoal making forms part of a larger integrated logging system then heavier roads and bridges will be needed.
Fig. 9 Charcoal Making Installation - Lambiotte Process - General View
1. Wood preparation
2. Carbonisation retort
3. Charcoal processing and packing
The wood drying system also determines road layout. The best system is to allow maximum drying to be done as near the stump as practical as this reduces capital tied up in stock and reduces the weight of wood to be transported quite substantially. In plantations with good access right to the stump the trees are merely felled lying in the one direction and allowed to dry before being crosscut. This may be 3 to 12 months.
Otherwise the full length log may be skidded to the roadside and either loaded onto truck there or crosscut into blocks and pi led for drying at the road side. In all cases the road system needs to be. brought to about 100-200 m of the stump if the topography allows.
Chain saws are universally used for felling nowadays where the logging operation is of large size necessary for any retort based charcoal making operation. Clear felling is preferred rather than selective operations. Where natural forest is being logged it is best to arrange that all the logs suitable for sawmilling, etc., are first removed selectively and then the wood destined for charcoal is felled as a clean-up operation to prepare the forest for regeneration or for agriculture, etc. In this way, access and drying conditions are improved and the felled wood can be left to dry in the forest or along the access roads without interfering with other operations.
The logging waste from the extraction of the sawlogs is also recovered at the same time. In natural forests the logs are then skidded to roadside together with other logging waste and there they are crosscut and pi led for drying.
In plantation forests because of the easy access and the uniformity of growth it is often possible to fell the trees and allow them to rest on the branches and on the stump so that they dry out rapidly in this elevated position. The transpiration from the leaves which remain attached to the stem appears to aid the drying process. After sufficient time has elapsed the stems are skidded out to roadside and crosscut or may be cut into blocks at the stump and loaded directly into tractor drawn trailers.
The maximum amount of bark, leaves and twigs should be left on the forest floor in order to conserve nutrients and because this material cannot produce commercial charcoal.
The kind of retort system in use determines the length of block which can be used. The longer the block the lower the cost of this operation. Conventional brick kilns have here a great advantage over retort systems in that they can economically handle stems up to about 0.5 m diameter and 1.8 m long. Rinsing gas retorts require blocks about 0.3 m long maximum and about 0.1 m diameter. Fluidised bed systems require the wood to be in chips or sawdust which makes the preparation cost prohibitive. These systems are only operable with waste wood which has been size-reduced in connection with some other operation, for example sawdust or bark flakes from softwood debarking operations.
To minimise the amount of capital tied up in the drying wood stock it should be blocked into its final length as soon as possible after felling and as close as possible to the stump. It should be dried as close as possible to the stump.
A compromise is often necessary. About 12 months drying to get the moisture content close to about 18-20% is desirable for maximum fuel economy in retort operation. This works well in dry savannah woodlands but in the humid tropical high forest the wood may well be rotted after 12 months and three months may be about the maximum practicable drying time. Rotted wood produces only useless powdered charcoal. The time allowed for drying must be chosen so as to reduce rotting and loss of wood by insect attack to an acceptable level and yet lower the moisture content as much as possible to improve the thermal efficiency of the carbonisation process. locating the dumps for drying the wood so that insolation and air circulation are maximised to obtain the most rapid drying possible is well worthwhile.
Road transport of the wood to the carbonising site is usually carried out with either trucks where the transport distance is more than a couple of kilometers or using trailers and agricultural tractors where the haul is short. Where the wood is to be transported as logs then orthodox log trucks are the best solution. Despite the disadvantages if the trees are large then it may be necessary to establish a central block cutting and splitting unit.
In planning the road haulage system regard must be paid to obtaining a steady supply of wood during wet weather by reserving dumps of wood accessible during wet conditions.
The amount of blockwood which must be held at the carbonising site depends on the size of the logs which are processed, drying conditions of the zone which will determine if a block dryer is needed and the amount of blockwood which must be kept on hand to maintain the retort in operation during prolonged wet weather, seasonal shutdowns or labour disputes.
From a cost point of view the wood should be stored in the forest and only brought to the carboniser when it is ready for use. In practice some stockpiling at the site is unavoidable. The normal system of storage is to bulldoze a large area about the size of a football field with good drainage and exposed to sun and wind and fill it with randomly dumped blocks about one meter deep with access for front end loaders to handle the blocks as required. The stock should be kept rotating to avoid decay and insect attack problems and ensure that the driest blocks are being used at all times.
A special problem arises where the trees being carbonised are of large diameter. As pointed out the conventional brick kiln systems easily handle logs up to 0.5 m diameter without splitting. The length can be more than 1.5 m. Retorts cannot handle big blocks and splitting the large logs is a costly and difficult problem.
A successful system at Wundowie in Australia for splitting logs up to 1.5 m in diameter consisted of two units. The first was a large trough into which the large logs could be dumped by log forks off the log trucks. When the trough was full it rolled sideways on tracks past two large circular saws mounted one above the other and capable of cutting a log 1.5m diameter. At each pass of the carriage a plate pushed the logs forward along the trough the thickness of the discs of wood required. The cut discs fell onto a heavy conveyor which carried them under a reciprocating press armed with a wide wedge which split the discs into blocks of a size which could be fed directly into the rinsing gas retorts.
Preparing blocks for retorts in the huge volume needed can be a costly operation without a high level of mechanisation. This is especially so where large diameter trees are to be processed. The attraction of processing these trees is that they are often dry having died many years ago and useless for sawmilling.
Where large diameter logs are blocked on a smaller scale it is usually done with chain saws but the work is very arduous and without front-end loaders, etc. to manipulate the blocks almost impossible. Splitting the wood is normally done with hydraulic splitters driven from the power offtakes of agricultural tractors.
The cost of blocking the wood into a suitable size to be charged to retorts is a major cost. In addition the wood haulage cost tends to rise sharply with time because the retorts cannot usually be moved closer to the wood resource as is harvested often makes the whole operation uneconomic. These problems are discussed in more detail in Chapter 7.
It is good practice where possible to allow extra drying at the carbonisation site before the wood is charged to the carboniser. This can take place naturally by allowing the wood to remain stockpiled for some months before charging. Where waste heat is available it may well be worthwhile to dry the blocks in a continuous tower dryer fired with waste gas from the retort system. Fig. 10 shows the principle of such a dryer, in this case developed for use with the Lambiotte type retort where the hot gases from the top of the retort are diverted and burned to produce the heat needed for the dryer. Dry wood produces a higher yield of charcoal and a greater throughput from the retort making drying one of the most beneficial options open to the charcoal maker. Figure 11 shows an installation designed by Lambiotte for carbonisation of pre-dried wood.
4.3.1 Processing agricultural residues
Agricultural residues attract interest as carbonisation raw materials because they are often available in large quantities around processing plants and appear difficult to utilise except as fuel. The use of these residues however is not without disadvantages for agriculture since using them this way removes organic and inorganic materials from the soil leading to impoverishment of farmlands and increasing the need for costly aritificial fertilisers.
Despite the above objections there are some situations where the use of agricultural residues is quite feasible. Nut shells are of particular interest because of the special properties of this charcoal. After activation (Chapter 6) it is an excellent adsorbent for gases and vapours. Coconut shells are the best known raw material of this type.
Except for nut shells agricultural residues are not a preferred raw material for charcoal making. Rather they are used because making them into charcoal seems to offer a method of realising a profit on an otherwise useless waste material. Generally the charcoal is produced as a powder and has a high ash content which limits its use for industrial purposes. The only important market for it is the barbecue market of the developed world. The high cost of ocean freight and the seasonal nature of demand tends to shut out charcoal of this kind made in the developing world. (For an analysis of this see Ref. 16). The increasing cost of fuels has tended to make it attractive to burn these waste materials directly as a substitute for oil and gas rather than convert them into charcoal and even when carbonised the offgas and condensibles are often just burned for process heating or steam generation.
Fig. 10 Mood Drier for Continuous Operation
1. Drying cylinder
2. Discharge locks for dry wood
3. Combustion chamber for retort gas
4. Heating gas fan
5. Off-gas fan
Fig. 11 Pre-dried wood continuous carbonisation system (Lambiotte)
1. Wood preparation
2. Continuous drier
3. Carbonisation retort
4. Hot gases recovery
5. By-products recovery
Not much is known about by-products from the condensate produced by specific agricultural wastes when carbonised. But maize cobs have been studied extensively since they are used to produce the solvent agent, fur-fural, when heated with sulphuric acid to moderate temperatures.
Generally, one would expect the condensate to have a similar composition to that produced from the carbonisation of wood since the composition of most agricultural wastes is broadly similar, the main differences being a higher ash content and a limited degree of lignification except in the case of nut shells.
The list of agricultural residues which have been considered for carbonisation is long but the level of commercial success is limited to a few special cases. As mentioned the only attractive raw materials are the nut shells particularly coconut because of the high priced charcoal which they can produce.
The following list gives an idea of some of the various agricultural residues which have been considered as possible charcoal making materials.
- nut shells and husks
- residues from farm crop processing and canning
- sugar cane bagasse
- bamboo, scrub and cactus
- garbage wastes
- straw and reeds
- industrial wastes as from carpet factories and pulp mills
- processing residues from coffee, cotton and fruit canning
As a rule these proposals have foundered on economic grounds or a higher return from alternative uses.
The main conditions for charcoal making to be economically possible from these materials are:-
The material must be of low or zero value and concentrated at the proposed point of processing. There must be a market able to pay top prices for either powdered charcoal or briquettes, as a high ash product, ruling out most industrial uses. Supply must be either year round or storage must present no problems or the processing method used must be so low cost that it can operate for only a short period of the year. There must be a continuous supply of material available for years ahead to allow the plant to be amortised successfully.
Photo. 5. Carbonisation furnace for agricultural residues
Photo. 6. Charcoal produced from rice husk and formerly used for briquetting
The three main reasons which attract attention to agricultural wastes are:- availability at apparently low cost, the material is dry, and transport costs are zero or very low since the material is concentrated at the point of crop processing.
Although not strictly an agricultural waste it is convenient to discuss here the use of bark waste from timber processing as a raw material for charcoal. logs typically carry about 10% of their volume as bark. Both softwood and hardwood bark can be made into charcoal, in both cases the charcoal is in the form of powder, and after briquetting can be sold for barbecue purposes.
The best known example of the use of bark waste is in the south-east of the United States in the southern pine processing belt. (22). In the large sawmills and other processing plants of the region it is the practice to debark the logs so that solid wood residues can be used for pulping and particle board production. Hence there is an accumulation of bark and sawdust which has rather limited economic outlets. The price of charcoal in the USA is high along the East coast and hence it is not surprising that making charcoal briquettes from this waste became economic. The market is largely a barbecue one and hence powder charcoal of high ash content is quite acceptable after briquetting. The carboniser used to produce the charcoal is the Herreshoff multiple hearth roasting furnace described in Chapter 3. The minimum output of this kind of furnace is about one ton of charcoal per hour operating continuously the whole year round. Hence the minimum quantity of bark waste required is about 100 tons per day or about. 5000 tons per year. This quantity of bark is produced from about 70000 m3 of logs per year which is the input to a very large sawmill. A combined sawmill and panel plant can easily yield the required quantity. The bark, and sawdust, if it is used as well is not usually dried before feeding to the furnace. The moisture content is typically about 40% based on the green weight of the feedstock.
This industry based on bark waste must always compare the relative profit of burning the bark directly for energy instead of turning it into charcoal briquettes because of the sharp rise in the price of oil which has occurred in recent years. But the proximity of a large high price market for barbecue charcoal tends to make the system still profitable even if coal is substituted for fuel oil. For economic studies on bark carbonisation see Ref. (22).
Under the conditions prevailing in most developing countries the major reason for considering complex systems for charcoal production is the increased yield which is possible with such systems. The increased yields need to be carefully verified and compared before committing expenditure. The present day brick kilns of the kinds recommended in (6, 7, 15) are capable under optimum conditions of achieving yields around one ton of charcoal to about 4.5 tons of air dry wood on a year round basis. The best of the complex technologies can do better than this. Well operated Lambiotte type systems can achieve a yield of one ton from 3.5 tons of air dry wood. But whatever the system, proper drying of the wood is essential for high yields and it may be better to spend money on proper organization of air drying and upgrading charcoal making to modern brick kiln methods than invest substantial sums in complex retort systems.
A further reason for choosing complex charcoal making systems could be the availability of an agricultural residue that might be processed to produce needed charcoal. The processing of finely divided residues calls for a complex technology system, the only proven one for fines at the moment being the rotary hearth furnace. But careful analysis of alternatives must be made. It may be better from an overall energy point of view in the country to burn the residues completely to produce electrical energy and concentrate on solving the charcoal problem by simpler means involving a much lower level of investment, more certain results and greater flexibility in land use and system operations. It must always be remembered that today's low cost residue immediately acquires an increased price as soon as it is to be used for some commercial purpose. Unless the carbonising plant has total control of its raw material resource it may find itself held to ransom over raw material supplies essential to obtain the return on investment required.
To summarize the position can be put in the following way. where finely divided raw material such as agricultural residues, sawdust, bark and so on is available then it must be processed by complex systems of which the rotary hearth furnace with associated briquetting plant is technically viable given a sufficient quantity of residue available 24 hours per day 360 days per year. (Not an easy condition to fulfil).
The alternatives to carbonisation are return of the agricultural residue and its nutrients to the soil as part of the cropping cycle rather than removing them and discarding them as ash somewhere else. The needed charcoal would then have to be produced from wood grown in fast growing man-made plantations or natural forests if they exist.
In the case of solid wood there is a trade-off between the saving in wood needed to produce a given amount of charcoal and a capital investment involving foreign currency and probably foreign loans. To achieve this result one foregoes the jobs and activity associated with the growing harvesting and carbonising of this extra wood. Where land for forest purposes is not a limiting resource the advantage for most countries probably lies with making the charcoal by the modern brick kiln technologies (1, 5, 6, 7,15). It is interesting to observe that despite the existence of the continuous vertical retort (4, 25) as a proven technology in the developed world for almost 40 years it has had almost no impact in the developing world even in such countries as Brazil (1) where huge amounts of charcoal are produced for the iron smelting industry and where investment in modern industrial methods is common-place.