3.1 Introduction
3.2 the Waggon or Arkansas retort
3.3 The reichert retort system
3.4 The Lambiotte or SIFIC process
3.5 The rotary hearth furnace
3.6 Fluidised bed and similar carbonisers
3.7 Recovery of by-products from carbonisation
3.8 Criteria for choosing carbonisation systems
Wood distillation carried out by heating wood in a closed retort and recovering the vapours given off during the process became important with the growth of the chemical industry in Europe in the nineteenth century. At the time there was no shortage of charcoal or wood for making it. The interest was in the chemical products which could be derived.
The original systems used a cast iron vessel which could be sealed with a bolted lid, set in a wood or coal fired heating furnace. Cast iron worked well as it resisted the corrosive fumes of the acetic acid produced during distillation. It was soon found that the non-condensible wood gas given off towards the end of "the cycle could be burned for heating the retorts. Further, where about six units were operated as a group it was found that the gas from those retorts approaching the end of carbonisation could be burned under others at an earlier stage of the cycle. This improved overall fuel efficiency and lowered the cost of recovering the chemical raw materials.
The need for these raw materials persisted into the twenties of this century and then began to decline with the growth of the oil industry with it's alternative methods of producing the basic products: acetic acid, acetone and methanol. By the end of the Second World War the decline of the wood distillation industry was complete.
However, the demand for charcoal in industrial countries for making activated charcoal, as a chemical reduction agent and in metallurgy continued and this kept up a demand for charcoal. It became worthwhile to produce charcoal from scrap and waste wood for the chemical industry even though the recovery of the by-products was of declining importance as an opportunity for new capital investment. The continuing need for high grade industrial charcoal focussed attention on finding ways to produce charcoal which were more efficient in conversion ratio and which were less labour intensive and polluting than kiln and mound techniques. Out of this need were developed the continuous and semi-continuous rinsing gas retorts of the Lambiotte and Reichert types described in this chapter. The Waggon or Arkansas retort arose earlier from attempts to reduce the labour intensity of traditional methods and collect chemical by-products.
A new factor arose with the need in the USA during the sixties to find a way to convert finely divided bark and wood waste from large pine sawmills to charcoal and sell this in the form of briquettes to a growing urban recreational market. From this need sprang the application of the Herreshoff multiple hearth roasting furnace to converting fine woody waste materials to charcoal. Recovery of chemical by-products was no longer of importance and the off-gases were merely burned to waste or passed through boilers to generate power.
The various systems which have proved commercially viable are now described. It is important to consider the particular circumstances under which each system developed because a system suited to one situation may be quite unsuited to another.
The waggon or Arkansas retort used to be widely used in Europe and the USA. The process lost ground in the thirties due to the development of semi-continuous retorts which showed lower overall operating costs. The waggon retort was particularly noted for high maintenance costs on the steel waggons and the shell of the retort itself. Nevertheless, a couple of plants have survived in Europe despite high operating labour costs. (8,23,31). However, the process is now of mainly historical interest.
The operating principles and layout of the waggon retort are shown in Fig. 1. The usual raw materials are roundwood, split roundwood and slabs from sawmilling. Average length is about 1 to 1.2 metres. Some shorter material can be used but it tends to fall out of the waggons. The wood should be dried to about 25% moisture content maximum for good results. A year of air drying is good practice in a suitable climate.
The wood is charged into steel waggons with slatter sides. The waggons fit the dimension of the retort rather closely to ensure maximum volumetric efficiency. The waggons roll into and are removed from the retort on steel rails which connect with a cooling chamber of the same dimensions as the retort and built directly facing it so that the waggons after carbonisation can be drawn quickly into the cooling chamber and sealed for cooling.
The minimum number of sets of retorts and coolers to ensure a steady supply of wood gas for retort heating is six but much larger numbers were not uncommon. Railway tracks and transfers connect the retorts with the wood storage yard. large systems had several kilometers of track and small locos for hauling the waggons. All this complication tended to multiply maintenance, supervision and operating costs.
Fig. 1 The Waggon Retort Plant
1. Retort
2. Waggons
3. Siding
4. Charcoal cooling cylinder
5. Water cooler
6. Scrubber for residual gas
7. Pyroligneous acid vat
8. Oil pump
9. Intermediate Tank
10. Preheater for distillation column
11-13. Fractionation of pyrolisis acid
14. Settling vat for tar
Retorts were typically about 8 meters long holding. two waggons but retorts up to double this length were used. The retorts were approximately cylindrical in shape to fit the loaded waggons as closely as possible. In fact it was found in Australia that despite the lower volumetric efficiency it paid to make the retorts circular in section and fit them with circular doors. The equalisation of heating stresses reduced maintenance markedly.
Also by burning the wood gas in a separate heating stove and circulating the hot gas around the circular retort in a helical duct heat transfer was improved and retort maintenance reduced. However, the weaknesses of the waggons remained and maintenance costs still tended to be excessive.
Waggon retorts were heated by burning wood gas and air under the retort. The gas was obtained from retorts close to the end of a carbonisation cycle and it is this need to obtain wood gas from other retorts which necessitates operating them in groups. The wood gas was obtained by drawing off the gases and vapours from the retorts by means of a fan and passing the gas through water cooled condensers which condense first the tar and then the pyroligneous acid. The non-condensible wood gas passes on and is directed to another retort for burning with air to provide heat for carbonisation. The tar and pyroligneous acid is stored in a tank to be separated and worked up for by-products in a refinery.
The carbonisation stage takes about 22 hours and as soon as it is complete the retort is opened and the waggons drawn across the short space to the cooler which is sealed to extinguish the charcoal which takes fire immediately it is removed from the retort. Cooling takes from 24 to 48 hours depending on climate and whether the walls of the cooler are sprayed frequently with water. This retort produces very small amounts of fines but careless handling of the finished charcoal can lift the total to 5% to 10%.
Summarized data on the retort system is as follows:
Retort dimensions:
8 to 16 m long by 2.5 m diameter.
Capacity:
35 to 60 m3.
Wood input:
9 -18 tons of drywood per charge or 270 tonnes per month.
Wood dimensions:
1 to 1.2 m long. Maximum diameter or thickness 12 cm.
Moisture content:
25%
Average yields:
Charcoal 30 - 33%, pyroligneous acid 20 - 25%
Photo. 1. Wood residues used for charcoal making through the waggon retort system.
Photo. 2. Waggon retort system pre-drier
Photo. 3. The carbonisation chamber in the waggon retort system (Sweden)
If the tar and pyroligneous acid are burned along with the wood gas there will be ample heat to operate the retorts and leave an excess for other purposes. Where by-products are recovered some additional heat inputs to the wood gas may be needed.
As in all charcoal making enterprises the logistics of the wood supply is the dominating factor in the cost of production. Additionally with this system the maintenance costs of the retorts and associated equipment can be a large component of costs.
The waggon retort system does have some advantages if maintenance costs could be controlled by better design of the retort. It is technically simple and unskilled personnel soon learn the operating skills. It produces lump charcoal with only a small amount of fines. Large pieces of wood can be used which reduces the cost of wood preparation considerably.
The Reichert retort represents the first successful attempt to eliminate the difficulties of transmitting heat to the charge inside the retort through the metal walls of the retort itself. It is this problem which is the root cause of the heavy maintenance costs of the waggon retort and its predecessors.
In the Reichert retort the heat is transferred to the wood by blowing recirculated heated gas through the charge inside the retort. The gas is either inert or reducing in nature to prevent the wood igniting and is typically a heated fuel gas of some kind. Flue gas is also quite suitable but the advantage of using a combustible gas is that the wood gas given off from the charcoal when combined with the heating gas does not dilute the mixture beyond the point of easy combustibility so that it can be subsequently burned to generate steam or other process heat. In this way the necessity to bleed off gas from the system without burning it is eliminated. The problem in most situations is to find a source of suitable fuel gas. Usually the wood gases and vapours from the retort are used and surplus gas is constantly bled off to keep the circulating gas volume constant (4).
The main unit of the system is a large steel vertical retort (Fig. 2) with a charge capacity of about 100 m3. The raw material is wood of maximum length about 30 cm and 10 cm thick. The wood must be small enough to 'flow' into the retort and yet not be so small as to impede the gas circulation. Large pieces of wood carbonise more slowly than small and this irregular rate of carbonisation tends to slow up production preventing optimum carbonisation time being achieved. There has to be a compromise between the cost of wood preparation and drying and the carbonisation time and hence the production rate achieved by the retort.
Whatever gas is used to heat the charge it must be burned with air under carefully controlled conditions in a special stove and its temperature raised above 450 - 550°C. It is then blown into the retort and gives up its heat to the wood. The gas being given off by the carbonising wood and the water vapour from drying and pyrolysis is added to the heating gas on its passage through the retort. For this reason it is necessary to bleed some of the gas to waste to maintain the volume of circulating gas constant. It is advantageous to pass the gas after it emerges from the retort to be reheated through tar scrubbers and condensers to remove the condensible portion of the gas as this can be burned as liquid fuel providing the wood charged to the retort is not too high in moisture content. By this means one avoids bleeding off a valuable fuel component.
When carbonisation is complete the bottom of the retort is opened and the charcoal dropped into closed steel containers to cool. If rapid cooling of the charcoal is required it can be cooled before discharge by passing cold inert gas through the retort.
This type of retort has been in successful commercial use for more than forty years. Providing adequate mechanisation of the handling of the wood and charcoal and the gas circulation controls are automated, the operating costs are satisfactory. However, the investment cost remains high and overall they are probably not as economic as the continuous rinsing gas retorts described in the next section.
This process originally developed by Lambiotte in the early 1940's (25) is perhaps the most successful technology for the continuous carbonisation of slab and roundwood to produce conventional lump charcoal useable for all purposes. (See References 1, 4, 9, 10, 16, 25 for more information on this system). The general arrangement of such a plant is shown in Fig. 4.
Fig. 2 The Reichert Retort Process
1. Large space retort
2. Tar stripper
3. Water cooler
4. Scrubber for residual gas
5. Combustion chamber and heat exchanger
6. Off-gas fan
7. Combustion air fan
8. Forced gas circulation fan
9. Dust collector
10. Wood preparation
11. Charging conveyor
12. Charcoal cooler
13. Charcoal conveyor
Continuous carbonising systems for the same capital investment give an increases output compared with complex technology batch type systems for the same investment, a higher yield than simple technologies and definite savings in fuel needed for carbonisation compared to any other system.
The operating principles (Fig. 3) are as follows. The predried wood is lifted to the top of the retort by means of a skip-car or conveyor and is dumped into a double bell gate which allows the wood to enter and yet prevents escape of significant quantities of retort gases. The level of the wood in the retort is monitored and loading controlled to keep the level constant by means of automatic controls.
During carbonisation the wood moves slowly down the retort encountering a rising countercurrent flow of inert hot gas which dries the wood and raises it to carbonising temperature. Typically the wood takes about eleven hours to pass through the retort and emerge as charcoal from the base through a pair of hydraulically operated interlocking gates which allow the finished charcoal to be removed in small batches about every twenty minutes. The time to pass through the retort and the intervals between removals of charcoal at the base are under the control of the operator and can be varied to suit the moisture content of the wood, required rate of charcoal output and fixed carbon requirement in the product. The movement of the charge down the retort leads to formation of undesired fines but by unloading intermittently as described above this effect is reduced and the percentage of fines is about the same as charcoal produced in conventional brick kilns.
The charcoal has to be cool when it leaves the retort as otherwise it would ignite on contact with the air. The cooling is accomplished by blowing either cool inert or combustible gas into the bottom of the retort and as it rises it extracts the heat from the finished charcoal as it passes down the retort to the discharge gates. The heated gas is drawn off at the middle of the retort just below the point where the hot gas for converting the wood to charcoal is blown in.
The correct circulation of the gas streams is assured by close control of the pressures at critical points (4). The hot gas is usually produced by burning combustible gas in a stove with air and this hot neutral gas at around 900°C is blown in just above the exit point of the cooling gas stream. This strips the remaining tar from the charcoal at this point and completes the carbonisation step. The gas passes up the retort, giving up its heat to the descending charge and picking up and 'rinsing' away the volatiles being given off by the descending wood.
Fig. 3 Hot Rinsing Gas Retort - Sectional View
1. Raw material.
2. Drying stage
3. Distillation stage
4. Carbonisation stage
5. Cooling stage
6. Charcoal
7. Retort gas
8. Hot stove gas
9. Cold inert gas
The gas issuing from the head of the retort is maintained at a temperature sufficient to prevent condensation of tars and other volatiles at the top of the retort and in the associated pipework. The high thermal efficiency of the lambiotte retort is due to the fact that the products of the system, charcoal and volatiles leave the retort at about the same temperature as the wood enters it. (4).
There are a number of options in disposing of the gas steam from the top of the retort. The most obvious one is to use this gas to heat the wood and to cool the charcoal. Providing the wood being carbonised is well dried there is enough heat in the effluent gas when it is finally burned with air to perform this (4).
The hot gas steam is divided, one part being burned directly in the stove to provide hot inert gas for heating the charge. The other portion is cooled and scrubbed to remove tar and then passed in at the base to cool the descending charcoal. This gas after leaving the retort is mixed with the rest of the gas and burned to produce the hot inert rinsing gas. Any deficiency. of heat caused by use of wet wood, etc., has to be made up by burning extra fuel such as oil or natural gas in the furnace.
Alternatively the whole of the hot effluent gas can be burned to produce steam for power generation and the heat needed for retort operation obtained by burning another fuel such as oil, coal or natural gas. The advantage of this procedure is that it avoids handling the retort off-gases and their associated tar and other liquids. Some of the advantage is lost if these alternative fuels are costly which is usually the case nowadays. The large retorts at the Wundowie ironworks in Australia burned the tar-laded off-gases for power generation and used clean cold blast furnace gas as the cooling and heating medium for the retorts. The best method to use at any particular site depends on local factors. But the heat inputs have to be paid for and efficiency depends in the long run on drying the wood as effectively as possible before it is charged to the retort. (See Chapter 4).
In the original installations of these retorts in Europe the head gas from the retort was processed to recover chemical by-products. Fig. 4-This kind of retort is suited for recovery of chemical by-products. Well dried wood is essential to minimise extra fuel inputs and avoid excessive dilution of the pyroligneous acid. This reduces costs by avoiding subsequent evaporation of water.
Fig. 4 Continuous Vertical Hot Rinsing Gas Retort - By-products recover
1. Wood elevator
2. Retort
3. Hot gas heating stove
4. Heating gas fan
5. Gas cooler
6. Pyroligneous acid cooler
7. Pyroligneous acid storage
8. Off-gas scrubber
9. Charcoal storage
EFFECT OF WOOD MOISTURE CONTENT ON ENERGY USE
|
Moisture % |
Energy used MJ/m3 |
Circulating gas m³ |
Electricity kWh |
|
5 |
35 |
210 |
2.5 |
|
10 |
40 |
270 |
3.2 |
|
15 |
154 |
490 |
4.4 |
|
20 |
293 |
770 |
5.2 |
|
25 |
460 |
1050 |
7.2 |
|
30 |
648 |
1400 |
9.0 |
The above data for European hardwoods shows the marked rise in heat and electricity use when the moisture content increases.
The sharp rise in energy consumption when the raw material moisture content rises is not the only problem. Increased moisture input to the system reduces the installed capacity of the plant and slows down production. These problems can be reduced if the wood is passed through a dryer before entering the retort. Raw material drying systems are described in Chapter 4.
Several attempts have been made to simplify the SIFIC process which is notable for its high investment cost. The most successful modification of the system is the smaller CISR-Lambiotte retort which has now been operating commercially for a number of years. Its major features are outlined below. (Photo 4).
The retort is heated by hot inert gas derived by burning part of the recycled retort gases and vapours. This provides the energy needed to complete the drying of the wood, raise it to spontaneous decomposition temperature, drive-off the surplus tar trapped in the structure of the charcoal and make up the various heat losses of the retort. The charcoal is cooled by passing the remainder of the retort gas after it has been cooled and cleaned into the base of the retort as in the larger SIFIC system. This gas becomes heated as it cools the charcoal and is mixed with the rest of the retort gas to be burned with air in a heating stove to provide the hot inert rinsing gas to be injected at the centre of the retort just above the exit point of the cooling gas.
It is important to keep the moisture content of the wood entering the retort to around 30% or less. Otherwise the gas coming from the retort is difficult to burn and will not produce the hot inert heating gas needed. The amount of air mixed with the gases in the heating gas must also be carefully controlled to achieve maximum temperature. If care is not taken it is difficult to recover enough heat from the carbonising wood to keep the process going and additional fuel such as oil must be burned to make up deficiency. The carbonisation temperature in the retort is set by the quality of the charcoal to be produced. If this temperature is not reached the charcoal will be underburned and the retort will gradually stop operating properly.
Some typical features of the retort are given below:-
- size of the retort height 18 meters, diameter 3 meters- typical input approximately 7,000 tons of dry wood per year
- type of raw material roundwood and slabs. Length 350 mm, thickness 100 mm, moisture not exceeding 30%
- typical yield approximately 2,500 tons of charcoal per year
- power requirements for retort operation about 25 kw of electricity
- cooling water for gas coolers and scrubbers
-oil or natural gas for start-up and emergency heating of inert rinsing gas.
A problem affecting all carbonisers constructed of steel is corrosion by acetic and related acids evolved during carbonisation. It particularly affects the vertical continuous type retorts because there must always be some part of the retort at a temperature suitable for attack by the acidic vapours and the continuous movement of the material down the retort removes any protective layer which may form on the surface of the metal, constantly exposing it to fresh attack. The corrosion can be readily overcome but at considerable cost by using stainless steel in those parts of the retort where attack is rapid.
The rotary hearth furnace is a proven method for carbonising small particle size wood and bark. The charcoal is produced in powdered form. This type of furnace is also known as the Herreshoff roaster. It was developed in the metallurgical industry for roasting sulphide ores many years ago.
Unlike the other two carbonisers described earlier the Herreshoff furnace will not operate with wood in slab or round form. The feed must be in the form of moderately fine particles such as sawdust or bark fragments. Agricultural residues such as seed hulls and shell fragments can also be processed.
The furnace (see Fig. 5) consists of four to six circular re-factory hearths stacked one above the other. The hearths are about six to eight meters in diameter. The large furnaces (22) have six hearths and the smaller units four. The hearths are supported in a cylindrical refractory lined shell and are constructed with the underside domed so that the hearth is self-supporting. Each hearth has a central hole through which passes a hollow shaft and at each hearth, a set of rabble arms are fixed to it. These arms are hollow so that the whole raking system can be cooled by blowing air through it. The ploughs attached to the arms just clear the hearth surface and as the arms turn slowly, at one to two revolutions per minute, the ploughs turn the feedstock over and move it across the hearths. The ploughs are arranged so that those on one hearth move the material away from the centre towards the edge where it falls through an opening to the hearth below. The ploughs on this hearth cause the material to move to the centre where it falls through the central opening on to the hearth below. In this way the material slowly moves through the system whilst being constantly turned over to be exposed to the combustion air passing through the furnace from the bottom to the top.
The furnace is started using gas or oil burners on each hearth to raise the temperature to about 600°C causing the feedstock to ignite. when the furnace reaches normal operating temperature of 900 to 1000°C the burners are turned off. Once the furnace is lit it must operate continuously 24 hours per day. The rate of admission of air is regulated so that the wood carbonises and leaves the furnace as fine charcoal The gases produced are a mixture of wood gas, tar, pyroligneous acid condensibles and water vapour and as such form a highly polluting mixture. After it emerges from the top of the furnace it can be burned directly under boilers for process steam or power. Otherwise the gases must be burned and the flue gas vented to atmosphere in tall stacks to avoid environmental pollution since installations of this kind for economic reasons are almost always located in closely settled areas.
The charcoal leaving the furnace must be cooled to avoid spontaneous combustion. This is carried out by passing the charcoal slowly through a horizontal steel cylinder equipped with paddles which lift the charcoal and allow it to contact the walls which are cooled with a water spray. The cooled charcoal is stored in a hopper sufficient to hold two or three days supply for the briquetting plant, a necessary adjunct to such a furnace.
Fig. 5 Rotary Hearth Retort (Herreshoff)
1. Air stack to cool central staff
2. Emergency by-pass stack
3. Main exhaust stack
4. Induced draft fan
5. Air pollution controls
6. Centre shaft drive
7. Cooling fan for central shaft
8. Charcoal cooler
9. Out hearth
10. In hearth
11. Rabble arm and teeth
12. Centre shaft
13. Feed conveyor
The amount of wood or other residues needed to keep the furnace going is quite large. A small unit requires about 4 tons of oven dry residue per hour and the larger units about ten tons per hour. Capacity is influenced by the moisture content of the feedstock as in all carbonising systems. The above quantities will yield about 1 to 2.5 tons per hour of charcoal providing the moisture content is about 45% of the green weight. This adds up to about 9000 to 21000 tons of waste per year yielding 2200 to 5200 tons of fine charcoal.
This fine charcoal has few uses in this form and hence must be briquetted. As a rough rule the briquetting operation doubles the cost of the fine charcoal (see Chapter 6 and Refs 3 and 22).
Herreshoff roasters have been successful when attached to large saw and plywood mills in the south of the USA where the conjunction of an adequate raw material supply in the form of bark and sawdust and the proximity of a sophisticated urban barbecue market provides a profitable combination.
Although it would be theoretically possible to recover the pyroligneous condensates from the off-gas stream for chemical recovery economic factors seem unfavourable and hence the gases are either burned for power or merely burned to waste.
Many proposals have been made for carbonising finely divided wood and agricultural residues by continuous methods which would operate on a smaller-scale than the well proven rotary hearth furnace and which would allow recovery of chemical by-products or at the very least waste heat for process heating or power generation. Despite sustained efforts and a good deal of promotion there has been little progress beyond the laboratory stage. Since these systems are not practical systems which can yet be recommended we cannot devote much space to them in a manual of this kind intended to be of immediate benefit to the developing world. Nevertheless, it is worthwhile to devote a few lines to the problem because those concerned with the fuel problems of particular developing countries are often assailed with proposals to invest in such schemes.
The appeal of such systems is understandable. They seem to offer a way, with relatively modest capital investment, of turning apparently useless wastes into useful charcoal plus important quantities of pyroligneous acid.
Since these systems produce charcoal in powdered form it is implied but not always emphasised that investment in such a carbonising system requires investment in a briquetting plant as well so that the charcoal can be marketed as household fuel.
To convert these wastes to charcoal requires that they be dried and their temperature raised to spontaneous decomposition point to form charcoal and then further heated to reduce the tar content to an acceptable level. Providing the residues are at least air-dry on entering the carboniser there can be a net positive output of heat but careful design and control is needed to realise these benefits.
The biggest stumbling block is the problem of separating the charcoal from the rest of the material in the apparatus and recovering it for briquetting for sale. This task has to be accomplished using a fairly simple system which does not require large energy inputs and conserves the heat outputs of carbonisation so that extra energy inputs from the burning of other fuels are not needed to carry the process on.
It is relatively easy to process finely divided wood and similar residues in continuously operating closed combustion apparatus where the objective is to merely produce hot gas for process heating. Here the problem of separating the charcoal from the not-yet-charcoal does not arise as the only solid residue is ash. A number of commercial units of this kind have been developed in the seventies and installed in the USA and other countries to provide alternative heating to replace oil and natural gas. The relative fall in oil energy prices however has taken away most of the incentive to use such systems but they remain a valid technical option despite the fact that they require electric power and fairly sophisticated instrumental control. Unfortunately, the same progress has not been made in the production of charcoal and we are not able to describe any commercially proven systems.
As pointed out in Chapter 2, the development of the petrochemical industry has made the recovery of chemical by-products from wood carbonisation in most situations an economically unattractive activity for new investment. But there is still sufficient demand in some developed countries for the special chemicals which can be recovered from wood distillation and this sector of the industry will probably continue for some years.
The biggest obstacle to success in by-product recovery is the relatively great cost of the necessary refinery and the relatively low prices offered for the basic chemical products such as acetic acid and methanol. Where the investment has been made in previous years and the plant amortized then the economics of by-product recovery are more favourable. Closeness to markets is very important in ensuring a profitable result and this operates against the developing countries where there is no developed market for the chemicals produced.
The basic system used to recover the chemicals is described in Chapter 2. In this chapter some possibilities of recovering something from the otherwise waster gases and vapours of carbonisation is explored.
When the off-gases of carbonisation are merely allowed to escape into the atmosphere as occurs in the simple technologies the thermal and chemical energy these gases contain is merely wasted and this reflects back on the efficiency and cost of the charcoal making process.
The simplest way to recover something from this waste gas is to collect condensed tar from the flues of the carboniser. This wood tar has a definite economic value and is relatively simple to collect. Some methods for doing this are described in (15). Unfortunately to recover more from the gases requires a major investment step in the form of a full scale by-product refinery.
Where complex retort systems are used for charcoal making the simplest approach is undoubtedly to use the whole of the off-gases as a source of heat for carbonisation and if there is an excess to burn the gases for power or process heat generation. Some of the retort systems described earlier in this chapter recover the heat in the off-gases this way. There are a number of advantages which can be mentioned. The capital cost is fairly low, the technology requirements are minimial, pollution of the environment is minimised and the saving in fuelwood allows more charcoal to be produced from the same amount of wood. And making charcoal for people's needs is what this publication is really all about.
The only difficulty in operating this kind of heat recovery system is in recovering enough heat to eliminate the need to burn costly oil or gas fuels as make-up heat to keep the retort operating. This can turn an otherwise profitable installation into a loss maker. The key to success is proper drying of the wood before it enters the retort.
This calls for optimum use of the sun's heat to dry the wood as detailed in Chapter 4 and good management practice in retort operation to conserve heat and gas. Investment in a dryer fired with off-gas for the wood or residues being used may well be worthwhile. In general if the moisture content of the feedstock to the retort can be maintained at 30% or less there will be enough heat in the recovered off-gas to maintain carbonisation without added heat inputs (4). The heat quantities involved are quite significant. Actual figures will vary with the nature and moisture content of the feedstock. Approximately from one thousand kilograms of dry wood the following heat quantities are available in megajoules (MJ) in the final products.
|
Charcoal |
9500 MJ |
|
wood Gas |
1500 MJ |
|
Condensibles including Tar |
8000 MJ |
The heat in the wood gas and the condensibles is made available for carrying out the carbonisation by burning them with air in a suitable stove forming part of the retort complex.
The retort off-gases with their content of tar are difficult to handle once they cool. The tar is sticky and clogs up gas mains and valves and cleaning them is difficult and costly. Where the off-gases are to be used for retort heating or for power generation it is essential to keep them hot as they emerge from the retort until they enter the furnace where they will be burned. This avoids deposition of tar. A temperature in the hot gas mains of about 300°C is typical and they should be as short as possible.
If recovery of chemical by-products is to be undertaken then the off-gases pass directly to the tar scrubbers and condensers. The liquid condensed with the tar at this stage is known as crude pyroligneous acid and it must be stored to allow the insoluble tar which it carries to separate so that it can be decanted. The wood gas passes on without change from the condensers and it merely carries some tar in the form of mist which may be separated in further tar scrubbers or the tar-laden gas may be burned directly for retort heating. Unfortunately the wood gas is of low heating value once the condensibles including the tars have been stripped from it. The total amount of heat it can yield is not enough, unless dry wood is being processed, to carry out the carbonisation process. This implies that additional heat inputs will be needed to operate the carbonisation process. The cost of these heat inputs whatever their source, wood, oil or natural gas has to be offset from the profits obtained from recovery of chemical by-products. Obviously before investing in by-product recovery careful studies are needed to ensure the financial viability of the investment. Fig. 6 gives a schematic outline of a charcoal plant with recovery of chemical by-products from pyroligneous acid and tar. The functions of the various units is described in the text below the figure. In Fig. 7 an integrated carbonisation process is shown.
Fig. 6 Charcoal Plant with Refinery for Recovery of Chemical By-Products
I. Carbonisation
II. Pyrolygneous acid recovery
III. Crude methanol plant
IV. Acetic acid concentration1. Crude methanol
2. Crude acetic acid
3. Methanol
4. Acetic acid
5. Methylating spirit
6. Tar
7. Waste water disposal
Since tar can be recovered during carbonisation with comparatively little equipment it is worth saying something about the products which can be recovered from it.
First of all there are two kinds of tar produced - water insoluble tar and water soluble tar. Most of the water soluble tar is recovered from the base of the primary still where the methanol, acetic acid, acetone, etc. is separated from this tar as explained in Chapter 2. Unfortunately soluble tar is of little commercial use. Insoluble tar mostly separates out when the retort vapours first pass through the condensers and it is recovered by decantation from the mixture of water soluble tar and pyroligneous acid. Small amounts of this tar are recovered from various parts of the off-gas handling system and added to the rest.
The equipment needed to refine the tar if it is not to be sold simply as crude tar are two stills and a rectifying column. Referring to Fig. 8 it can be seen that the tar is separated into three fractions. I is only suitable as fuel. II and III contain useful materials and these can be separated in the tar distillation plant. This is a complex chemical process and requires skilled operatives and management and significant volumes of raw material to make the process worthwhile. It is for this reason that wood tar distillation plants buy in tar from a number of producers to increase the amount available for processing and hence tend to be found only in those countries which have a well developed chemical industry.
Choosing a retort type carbonisation system requires great care because of the capital investment involved and the number of years for which the equipment must operate at a fixed location. If the resource within economic range of the plant proves to be inadequate then the investment may prove very ill-advised.
To provide background for selecting a carbonising system the relevant features of the systems are summarised below and the factors which must be considered are listed.
First of all certain technical and social factors must be considered. Presumably a decision has been taken to produce charcoal and that the situation rules out the use of the simple technologies which are dealt with in detail in (15).
Fig. 7 Integrated Carbonisation Process - Schematic Outline
Fig. 8 Tar Fractional ion -Schematic Outline
I. Light tar oil
II. Heavy tar oil
III. Residual tars from still
A decision of this kind cannot be undertaken lightly since in order that the investment may have a chance of being recovered, the resource area must be committed for twenty to thirty years into the future. The resource must also be managed in such a way that the needs of the charcoal retort system are adequately serviced. This precludes other uses for the land, a difficult decision where populations are rising and pressure on the land increasing. The use of complex retort systems introduces a degree of inflexibility in resource allocation which governments may find difficult to live with.
An important factor to be considered is the cost of transporting the wood from the furthest points of the allocated area to the carbonisation site. Retorts in general cannot be economically moved around the site as pits, mounds and brick kilns can and this may have a drastic effect on production costs.
Another factor is the type of material to be carbonised. Is it solid wood, finely divided residues, bark or agricultural wastes. Only solid wood can produce lump charcoal, other types of raw material with the possible exception of coconut shells will produce powdered charcoal usually requiring briquetting before it can be marketed. This means additional investment in a briquetting plant plus the cost of the binder and the briquetting operation. A supply of binder usually a starch of some type in quantity equal to about 8-10% of the weight of the charcoal produced must be available at the site. If the zone is not agricultural and/or food shortages occur, obtaining the binder can prove difficult and this adds another element of inflexibility to the overall system.
The next factor in making a decision is whether by-products are to be produced from the retort gases. If yes then finance to build the refinery is required. Under today's plant costs and the competition from the petrochemical industry a decision to produce by-products is rather unlikely. A refinery requires specialist technicians and plant operators and considerable periods of training before profitable operation could be achieved. Without careful planning and committment the refinery could well prove to be a lossmaker. It also introduces a further element of inflexibility into the system. It is therefore likely that the decision will be taken to merely use the off-gases as fuel either to directly assist carbonisation or to generate power or process steam. Fortunately a decision on a refinery unlike the briquetting plant can be deferred in most cases. The refinery can be added on at a later stage if shown to be worthwhile.
