5.1 Safety precautions in charcoal operations
5.4 Hazards to the public
5.5 Safety devices and equipment
5.6 Precautions for charcoal storing
5.7 General safeguarding of charcoal plants
5.8 Environmental considerations for the charcoal maker
Whether charcoal is made in the traditional way or by industrial methods, three hazards are always present: fire of stored charcoal, gas and dust explosions and carbon monoxide poisoning.
Accidents can be greatly reduced by use of safety devices and safe working habits. In all plants where high-temperature operating conditions with combustible materials are normal, carelessness can be ruinous. Production hazards increase, and undesirable or dangerous conditions can arise if the operator neglects to pay close attention to such vital operational factors as the converter temperature, pressure indicators, structural conditions of the production equipment and storage bins. There are ample records of plant damage and destruction because of just such neglect and oversight.
The causes of such accidents are often not clear but are probably most often due to mixture of pyrolysis gases with air.
In a batch-wise operated retort the presence of gas in the retort is greatest at the end of the cycle, the admittance of air can form highly explosive gas mixtures. In a continuously operated converter, the admittance of more air than needed may cause first a sudden rise in temperature and then formation of explosive mixtures in the off-gas system.
Fire can result from the admission of large quantities of air to the retort, converter or off-gas system through cracks or badly closed doors. In other cases fire can result from explosions. Such events can arise through the operator's unfamiliarity with proper operating procedures or simple carelessness. In high temperature operations there is always the danger of wall swelling and the occurrence of unnoticed air inlets. The inflow of excessive amounts of air could easily change the temperature pattern. This may cause very high retort temperatures, either gradually or rapidly, creating a serious fire condition. The operator's familiarity with his equipment and necessary counter measures are the best insurance for safe practice and satisfactory production. Well established, periodic inspection of the industrial charcoal-making plant will determine the corrective measures necessary for proper control and reduce the possibilities of fire damage.
Improper sealing or structural leakages of air in the charcoal cooling bins during the cooling period can also lead to considerably reduced yields production of poisonous carbon monoxide gas and equipment damage. Such conditions may occur even in a well established operating plant and the importance of inspecting and maintaining cooling bins during the cooling cycles, controlling operational conditions and following safe practices cannot be overemphasized.
Fire and gas leaks whether controlled inside the retorts or converter or uncontrolled, constitute a potential hazard for the public. Unauthorised persons should not be permitted in the plant unless guided. Safety helmets are a necessity for both workmen and visitors. Transport of wood, other raw material, charcoal handling and other essential work gives rise to operational hazards and safety measures and safe work habits must be considered of prime importance.
5.5.1 Pressure-relief doors
5.5.2 Automatic temperature shutdown
5.5.3 Electric power failure devices
5.5.4 Temperature indication and control
Explosions are always possible when handling a dusty material or one which contains combustible gases and vapours. Thus, the feed and storage bins are designed with explosion blow out panels which will vent gases when the internal pressure goes above 350 to 400 mm water. In addition, dead-weight relief doors are usually incorporated on the top of the bins and they lift at an even lower internal pressure.
In the event that temperatures inside the retorts, coverter or the off-gas system exceed preset limits, the air supply, the heating fans of the retorts or the fan of the converter gas system should be shut down. At the same time, the air supply of continuously operated converters should be cut off and the converter isolated. Residual gases should be vented through the emergency flare.
If a total power failure occurs, the air supply and the draft fan stop and all shut-off gates remain in safe condition. When the process becomes dormant the system is isolated from other plant units.
Equipment that indicates the operating temperatures and controls the safety devices has to be selected very carefully. Regular inspection and proper maintainance are a necessity.
Customarily, charcoal demand is seasonal which makes it necessary to stockpile a considerable inventory.
A great deal of care must be taken in storing freshly produced charcoal. It has a tendency to absorb the oxygen from the air. Rapid absorption creates considerable heat, which builds up to a point where the stock-pi led char will start burning.
Tightly packed masses of charcoal fines and charcoal with a high content of volatiles are more subject to spontaneous combustion than larger pieces of lump charcoal.
Self ignition may even occur if charcoal has been watersprayed for better cooling.
It is therefore advisable to place freshly discharged charcoal in the open, separate from previously cooled and conditioned charcoal for at least 24 hours. During this time, the char should be exposed to air circulation and protected from rain and wind, preferably in an open shed rather than under a tarpaulin. If there is no evidence of heat or active fire after 25 hours, the charcoal can be considered safe for storing in a specially designed warehouse (15, 28).
5.7.1 Water supply
5.7.2 Detection of poisonous gases
5.7.3 Safety manual
5.7.4 First aid accessories
A water supply is highly important to any charcoal plant. A hose with a nozzle should be kept ready for immediate use at assigned points of the plant. Back-pack water pumps or large capacity fire extinguishers provide some measure of fire protection.
Poisonous gas (carbon monoxide) is present in retorts and storage bins that have just been cooled down or in the off-gas system that has just been shut off. There are portable detectors on the market which indicate the concentration of gas present in the production apparatus.
However, the best prevention is to ensure good ventilation before workmen or maintainance crews enter the bins or retorts and in the working environment.
All safety instructions and changes to them have to be made known to the workmen. In all well organized plants every workman has to attend quaterly training classes, given by the plant engineer.
However, common sense remains most important in dealing with harzardous situations in the plant.
Adequate first aid kits including dust and gas (carbon monoxide) masks, should be available in a centrally located position.
5.8.1 General considerations
5.8.2 Raw material preparation
5.8.3 Retorts and converters
5.8.4 Char handling
5.8.5 Retort condensates and gas
5.8.6 Waste water
During the last two decades environmental control legislation has become an important factor for charcoal makers. There are numerous cases where plants operating for more than half a century, had to be shut down or completely altered as a result of legislative pressure.
Therefore, these aspects should be taken into consideration by the potential charcoal maker when the charcoal plant is designed.
The combustible gas generated by industrial systems will burn cleanly to waste in a stack, if not utilized otherwise for additional energy generation.
Another feature in favour of the industrial charcoal-plant with energy co-generation is the minimal liquid effluent discharge. Waste water does not occur except in plants where the by-products are recovered from the original condensates by distillation.
The environmental aspects of industrial charcoal making depend very much on the type of system, and the type of raw material. In general, the most important problems are odour and dust. The others can be eliminated.
Hogging machinery is usually noisy and may exceed local decibel allowances. Enclosure of the machinery reduces noise, and in residential areas machinery should not be operated during the night. It is good practice from all points of view to locate carbonisation plants at least 1 km from residential areas and preferably down-wind of them to avoid odour dust and noise pollution.
Dryer exhausts contain dust and the amount depends on the size of the feed (fines). Cyclonic equipment should be installed to reduce the dust exhaust and at the same time conserve raw material.
This equipment if well designed and built, does not present problems, but occasional escape of gases can be expected from industrial retorts when they are opened. In continuous operations due to the feed system and the fact that the retort or converter is opened only during the shut-down period, gas escape is minimal and therefore no preventive measures are necessary.
Conveyer belts are normally enclosed to prevent the wind scattering dust. The fumes of dust from the char hoppers are drawn by exhaust fans through bag filters which capture the dust. The dust is periodically shaken from the bags into the collecting bins attached. But disposing of the fine dust is a problem unless it can be mixed with the feedstock to a briquetting plant. The problem is more serious in smaller plants as it is not economic to install equipment to burn the fines as is sometimes done by blowing them into the base of blast furnaces or using them in a sinter plant in charcoal iron plants for example. On a small scale the fines may be wetted down with water and detergent and buried as land fill.
The direct loss of gas and vapour from the retorts must be avoided as much as possible to prevent direct air pollution. The condensates (pyroligneous acids and tars) are difficult to dispose of simply except where by-products are being recovered. The simplest way is to burn them for energy generation or to waste when they are converted into non-polluting carbon dioxide and water vapour. It is best to burn these substances as they emerge as hot gas from the retort before they condense. Where small amounts of condensate arise they can be collected in ponds and subsequently burned after the water they contain has largely evaporated. This can prove impossible however in areas of high rainfall.
Retorting systems are designed so that in the case of an unusual pressure rise within the system as may result from a fire the vapours are vented into the atmosphere. This is done for the safety of personnel and equipment. Based on long experience these events are uncommon and when they do occur they last only for a few seconds. Since the venting is to the open air the vapours quickly disperse.
The control of the discharge of waste water from carbonisation plants is of the highest importance as untreated waste water can be a serious environmental pollutant in streams used for drinking, stock watering and fishery.
A properly designed plant will include a waste water recovery system which meets the local pollution standards.
There are two sources of waste water; that used for cooling and that which is produced directly from the breakdown of the wood substance during drying and carbonisation. In the simple technologies this water is released directly into the air as steam and is dispersed along with the acidic vapours and tars. The problem with condensed water in the complex technology plant is that it is usually contaminated with acids and tars condensed from the retort off-gases and hence cannot be allowed to escape into streams without treatment.
Process water should be recirculated to the maximum extent so that the amount of water which has to be disposed of is limited. The process water system has to be monitored closely and surplus water should be allowed to evaporate wherever possible. Control is easier where the net evaporation in the zone is greater than net precipitation. The waste water can be accumulated in settling ponds and allowed to evaporate. The tarry residues remaining are periodically burned-off. Where there is a net gain of water in the zone through precipitation disposal is more complex. As much as possible of the tar is separated by settling and burned and the acidity of the clear waste water adjusted by lime treatment. After settling and dilution where possible with clean water it may be safe to run this water to waste. Specialist advice on the best methods of waste water disposal must be sought as each plant has its own special problems depending on the process used and the location.