Coal and Coke
| Infobox on Coal and Coke | |
|---|---|
| Example of Coal and Coke |  |
| Facts | |
| Origin | - |
| Stowage factor (in m3/t) |
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| Humidity / moisture | - |
| Ventilation | See IMSBC Code |
| Risk factors | See text |
Coal and Coke
Description / Shipment - Storage / Uses
Coal, a fossil fuel, is the largest source of energy for the generation of electricity worldwide, as well as one of the largest worldwide anthropogenic sources of carbon dioxide emissions. Gross carbon dioxide emissions from coal usage are slightly more than those from petroleum and about double the amount from natural gas. Coal is extracted from the ground by mining, either underground by shaft mining through the seams or in open pits.
All classes of coal are liable to spontaneous combustion when carried in bulk, the softer types to a greater extent than others. Adequate ventilation of holds is essential to minimize the risk. If damaged by heating or combustion, the affected portion should be segregated from the remainder to prevent spread of the damage. Most classes of coal, particularly those to be used for coke-making, and of coke will suffer deterioration or depreciation in value by contact with salt water although quality, calorific value apart, is seldom affected by contact with fresh water. Particular care must be exercised in arriving at depreciation or loss arising from spontaneous combustion, heating or water damage. The effects can range from a decrease in calorific value to a significant change in the suitability of the coal/coke for its original purpose. The assistance of a qualified sampler/analyst is essential to determine the degree of deterioration or loss. Coal or coke shipped in a wet condition may be subject to loss of weight due to drainage during the voyage or by evaporation, up to say 3% depending on the quantity shipped. However, loading and discharging of these materials in heavy rain could result in an increase in discharged weight over shipped weight. Apart from the physical weighing of the commodity on loading and discharging, an approximate check of the lost or added weight can be made by using the immersion scale of the vessel or by comparison of the moisture analysis of cargo samples taken professionally during loading and discharging. Cargo weights are in certain cases ascertained at points remote from ship’s rail or alternatively at the time the cargo was put into stock on quay or in barge. It is in these cases that variation between the weight indicated as shipped and actually loaded will occur. The loading, transportation and discharge of coke results in some degree of breakage, leading to a smaller average size of lumps and a greater content of fine material, breeze and dust. Careful sampling and testing according to standardized procedure are necessary to determine the extent of these changes. Total quantities can be checked accurately only if the moisture content is known while the size analysis affects the value of coke for all saleable purposes.
In respect of coals liable to spontaneous heating, the IMO Code recommends that the ship’s hatches be closed immediately after completion of loading in each cargo space. The atmosphere spaces should be monitored and, if the carbon monoxide level shows a steady increase then the cargo spaces should be completely closed down. The covers could also be additionally sealed with suitable sealing tapes.
It should be noted that even well fitted hatch covers may be weather-tight to rain and seas over the deck. However, with various rolling movements of the ship, the covers may not be ‘airtight’. Leakage of air into the cargo space will then assist spontaneous heating of the coal. Subsequent heating of the coal will set up thermal movements within the cargo space, hot products of combustion out of the space and a fresh supply of oxygen into the space to assist further oxidation and heating of the coal.
Recommendations for overseas carriage of coal
1) On completion of loading, the cargo should be trimmed to a level surface. Pyramid stowage should not be permitted.
2) The water content of the coals should be checked during loading.
3) Bills of lading should be claused to reflect the condition of the coal.
4) Steam observed coming from the coal during loading is a warning of excess humidity and temperature, and should give cause for similar steps to be taken as at 2).
5) Condensation forming beneath hatch panels and deck beams should be controlled.
6) Checks should be made to ascertain whether the coal has sulphur marks (yellow colour spots), since sulphur may increase the risk of ignition.
7) If the temperature of the cargo increases to 90°C, inject CO2 or inert gas into the hold.
8) Water should not be used in order to reduce the temperature. This may in fact increase the temperature, and water spraying may additionally cause damage to the steel plating of the vessel. Foam or sand should be used, where necessary, to reduce temperatures
9) If temperatures increase to such an extent that the situation gets out of hand, coal experts advise that one solution may be to fill the hold with water.
Bituminous coal or black coal is a relatively soft coal containing a tarlike substance called bitumen. It is of higher quality than lignite coal but of poorer quality than Anthracite.
Bituminous coal is an organic sedimentary rock formed by diagenetic and sub metamorphic compression of peat bog material.
Bituminous coal has been compressed and heated so that its primary constituents are macerals vitrinite, exinite, and so on. The carbon content of bituminous coal is around 60-80%; the rest is composed of water, air, hydrogen, and sulfur, which have not been driven off from the macerals.
The heat content of bituminous coal ranges from 21 million to 30 million Btu/ton (24 to 35 MJ/kg) on a moist, mineral-matter-free basis.
Bituminous coal is usually black, sometimes dark brown, often with well-defined bands of bright and dull material. Bituminous coal seams are stratigraphically identified by the distinctive sequence of bright and dark bands and are classified accordingly as either "dull, bright-banded" or "bright, dull-banded" and so on.
Bank density is approximately 1346 kg/m3 (84 lb/ft3). Bulk density typically runs to 833 kg/m3 (52 lb/ft3).
Bituminous coals are graded according to vitrinite reflectance, moisture content, volatile content, plasticity and ash content. Generally, the highest value bituminous coals have a specific grade of plasticity, volatility and low ash content, especially with low carbonate, phosphorus, and sulfur.
Plasticity is vital for coking as it represents its ability to gradually form specific plasticity phases during the coking process, measured by coal dilatation tests. Low phosphorus content is vital for these coals, as phosphorus is a highly deleterious (damaging) element in steel making.
Coking coal is best if it has a very narrow range of volatility and plasticity. This is measured by the free swelling index test. Volatile content and swelling index are used to select coals for coke blending as well.
Volatility is also critical for steel-making and power generation, as this determines the burn rate of the coal. High volatile content coals, while easy to ignite often are not as prized as moderately volatile coals; low volatile coal may be difficult to ignite although it contains more energy per unit volume. The smelter must balance the volatile content of the coals to optimize the ease of ignition, burn rate, and energy output of the coal.
Low ash, sulfur, and carbonate coals are prized for power generation because they do not produce much boiler slag and they do not require as much effort to scrub the flue gases to remove particulate matter. Carbonates are deleterious as they readily stick to the boiler apparatus. Sulfide contents are also deleterious in some fashion as this sulfur is emitted and can form smog, acid rain and haze. pollution. Again, scrubbers on the flue gases aim to eliminate particulate and sulfur emissions.
Smithing coal is a type of high quality bituminous coal ideally suited for use in a coal forge. It is as free from ash, sulfur, and other impurities as possible. The constituents of the coal should be as follows:
| Constituent | Percentage |
| Sulfur | Not over 1% |
| Ash | Not over 7% |
| Carbon | Not less than 70% |
| Moisture | Not over 12% |
Coke is the solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Cokes from coal are grey, hard, and porous. While coke can be formed naturally, the commonly used form is man-made.
Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. It is there to reduce the Iron Oxide (haematite) in order to collect iron.
Since smoke-producing constituents are driven off during the coking of coal, coke forms a desirable fuel for stoves and furnaces in which conditions are not suitable for the complete burning of bituminous coal itself. Coke may be burned with little or no smoke under combustion conditions, while bituminous coal would produce much smoke.
Types of coal
As geological processes apply pressure to dead biotic material over time, under suitable conditions it is transformed successively into:
- Peat, considered to be a precursor of coal, has industrial importance as a fuel in some regions, for example, Ireland and Finland. In its dehydrated form, peat is a highly effective absorbent for fuel and oil spills on land and water
- Lignite, also referred to as brown coal, is the lowest rank of coal and used almost exclusively as fuel for electric power generation. Jet is a compact form of lignite that is sometimes polished and has been used as an ornamental stone since the Upper Palaeolithic
- Sub-bituminous coal, whose properties range from those of lignite to those of bituminous coal are used primarily as fuel for steam-electric power generation. Additionally, it is an important source of light aromatic hydrocarbons for the chemical synthesis industry.
- Bituminous coal, dense sedimentary rock, black but sometimes dark brown, often with well-defined bands of bright and dull material, used primarily as fuel in steam-electric power generation, with substantial quantities also used for heat and power applications in manufacturing and to make coke
- Steam coal is a grade between bituminous coal and anthracite, once widely used as a fuel for steam locomotives. In this specialized use it is sometimes known as sea-coal in the U.S. Small steam coal (dry small steam nuts or DSSN) was used as a fuel for domestic water heating
- Anthracite, the highest rank; a harder, glossy, black coal used primarily for residential and commercial space heating. It may be divided further into metamorphically altered bituminous coal and petrified oil, as from the deposits in Pennsylvania
- Graphite, technically the highest rank, but difficult to ignite and is not so commonly used as fuel: it is mostly used in pencils and, when powdered, as a lubricant.
Uses
Coal is primarily used as a solid fuel to produce electricity and heat through combustion.
When coal is used for electricity generation, it is usually pulverized and then combusted (burned) in a furnace with a boiler. The furnace heat converts boiler water to steam, which is then used to spin turbines which turn generators and create electricity. The thermodynamic efficiency of this process has been improved over time. Simple cycle steam turbines have topped out with some of the most advanced reaching about 35% thermodynamic efficiency for the entire process. Increasing the combustion temperature can boost this efficiency even further. Old coal power plants, especially "grandfathered" plants, are significantly less efficient and produce higher levels of waste heat. At least 40% of the world's electricity comes from coal, and in 2008 approximately 49% of the United States' electricity came from coal. The emergence of the supercritical turbine concept envisions running a boiler at extremely high temperatures and pressures with projected efficiencies of 46%, with further theorized increases in temperature and pressure perhaps resulting in even higher efficiencies.
Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents are driven off by baking in an oven without oxygen at temperatures as high as 1,000 °C (1,832 °F) so that the fixed carbon and residual ash are fused together. Metallurgical coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. The coking coal should be low in sulphur and phosphorus so that they do not migrate to the metal. The product is cast iron and is too rich in dissolved carbon, and so must be treated further to make steel.
Why is it necessary to convert coal into coke for use in smelting steel?
Iron ore that is mined contains iron oxide and some impurities. To obtain iron, it is necessary to get rid of the oxygen and impurities. The easiest way to do this is to heat the ore along with a carbonaceous material like charcoal, coal, or coke, in a blast furnace. The carbon serves as the fuel and burns when air is blown into the blast furnace, thus heating it to high temperatures. The carbon chemically "grabs" the oxygen away from the iron, and becomes the gas carbon dioxide which goes away. (Actually, it is carbon monoxide that does the reacting with the iron). If limestone (calcium carbonate) is added, the calcium reacts with the silica impurities to form slag, which separates from the iron. Presto - lots of Pig Iron with a fairly high carbon content and still some sulfur.
You may ask why air is blown into a blast furnace when the idea is to remove the oxygen, not add more? Well, high temperatures are needed, and there is ample amount of coke present to react with the oxygen and the iron ore, so too much oxygen is not a problem.
In fact, when the iron comes out, it is loaded with carbon and still contains some sulfur. The next step is to remove some of the carbon and sulfur to make steel. So steel is made by blowing oxygen into molten iron. The oxygen reacts with the carbon to form carbon dioxide which escapes as a gas. The sulfur is removed by adding "fluxes" that react with the sulfur and other impurities to form a slag that floats on the top.
Yes, it seems strange to first add carbon, and then take away carbon, but attempts at making one-step steel have not worked well.
Up until about 1750, charcoal was used in iron production. Charcoal works well, but it comes from wood, and wood is too expensive to use making iron. It takes 100 kg of charcoal to make 1 kg of steel. The forests were being wiped out.
Coal was tried, but it did not work well because impurities in the coal are transferred to the metal. This is especially bad for sulfur. Sulfur in iron makes it very weak.
It was discovered that when the coal was converted into coke (by heating in the absence of air) that many of the impurities would come out of the coal, either as gases or liquids. Even some sulfur comes out. The coke is then acceptable for smelting iron ore into iron.
The coke must be strong enough to resist the weight of overburden in the blast furnace, which is why coking coal is so important in making steel using the conventional route. However, the alternative route to is direct reduced iron, where any carbonaceous fuel can be used to make sponge or pelletised iron. Coke from coal is grey, hard, and porous and has a heating value of 24.8 million Btu/ton (29.6 MJ/kg). Some cokemaking processes produce valuable by-products that include Coal Tar, ammonia, light oils, and "coal gas".
Petroleum coke is the solid residue obtained in oil refining, which resembles coke but contains too many impurities to be useful in metallurgical applications.
Properties and characteristics
- Coals may emit methane, a flammable gas. A methane/air mixture containing between 5% and 16% methane constitutes an explosive atmosphere which can be ignited by sparks or naked flame, e.g., electrical or frictional sparks, a match or lighted cigarette. Methane is lighter than air and may, therefore, accumulate in the upper region of the cargo space or other enclosed spaces. If the cargo space boundaries are not tight, methane can seep through into spaces adjacent to the cargo space.
- Coals may be subject to oxidation, leading to depletion of oxygen and an increase in carbon dioxide or carbon monoxide concentrations in the cargo space. Carbon monoxide is an odourless gas, slightly lighter than air, and has flammable limits in air of 12% to 75% by volume. It is toxic by inhalation, with an affinity for blood haemoglobin over 200 times that of oxygen.
- Some coals may heat spontaneously and the spontaneous heating may lead to spontaneous combustion in the cargo space. Flammable and toxic gases, including carbon monoxide, may be produced.
- Some coals may be liable to react with water and produce acids which may cause corrosion. Flammable and toxic gases, including hydrogen, may be produced. Hydrogen is an odourless gas, much lighter than air, and has flammable limits in air of 4% to 75% by volume.
Additional information on the transport of coal
The carriage of coal is attended by the risk of both fire and explosion. Whilst many millions of tonnes of this commodity are safely carried annually, the attendant risks should not be forgotten. Coal is a mineral of organic origin, formed from the remains of vegetation which has, over the course of millions of years, been transformed by the effects of heat and pressure from overlying rock or water. As might be expected, therefore, the term "coal" covers a wide range of products with a correspondingly wide range of properties. In spite of this, coals of all types may be carried safely if they are handled, loaded and stowed properly with due regard being given to their particular properties. The principal hazards associated with coal which are of importance when shipping the commodityare:
1) The potential for self heating, in extreme cases to ignition
2) The potential to emit methane (a flammable gas)
3) The potential to cause corrosion to the ship's structure
Whilst all classes of coal are susceptible to self heating under appropriate conditions, some coals, particularly the lower rank (or geologically immature) coals, have a particular propensity in this regard: the blending of different types of coal may also enhance this property. Very little can be discerned about the potential properties of a cargo simply by visual inspection at the time of loading. Thus, given the wide variability in the properties of coal, it is essential that full details of the specific characteristics of the cargo to be carried, and recommended procedures for its safe carriage, are obtained from the shipper prior to loading. For cargoes which are described as "liable to self-heal" ventilation should be restricted to an absolute minimum, and it is particularly important that cargoes of this type are trimmed as level as is reasonably possible. An increase in temperature is clearly one manifestation of self-heating and temperature probes in the cargo can be of assistance in monitoring the process. However, the nature of the cargo is such that a hot spot in the stow is unlikely to be detected in the early stages unless there happens to be a temperature probe in precisely the right location. Evidence of self-heating ,therefore, is generally most readily detected in the early stages by monitoring carbon monoxide (CO) levels in the hold atmosphere (CO is a by-product of the self-heating process), and equipment suitable for measuring the gas should be carried on board: responsible members of the ship's crew should be trained in the use of the equipment. Because self-heating is an oxidation process the oxygen concentration in the hold atmosphere under conditions of restricted ventilation is likely to become depleted: it should also be noted that carbon monoxide is toxic. Coal emits a flammable gas, methane, particularly when newly worked or freshly broken. This gas, when mixed with certain proportions of air, will explode if brought into contact with a suitable ignition source such as an electrical or frictional spark, flame, or heated surface. Again, given the wide variability in the properties of coal, some cargoes will be particularly prone to this. For cargoes which are described as "liable to emit methane", therefore, the greater hazard is that associated with an explosion and precautions must be taken to avoid the accumulation of a flammable atmosphere. Adequate surface ventilation of the cargo will prevent the build up of a flammable atmosphere in the holds although excessive ventilation, particularly into the body of the coal, could promote self-heating. As with coals that are liable to self-heat, therefore, the cargo should be trimmed as flat as is reasonably possible, and equipment capable of measuring the concentrations of methane and oxygen in the holds, without requiring entry to the cargo spaces, should be employed to monitor the hold atmospheres. Whilst specific safety arrangements will vary with each ship, the following general precautions should be observed:
1) Warning notices against smoking and the use of naked lights should be posted at the entrance to cargo compartments and adjoining spaces where flammable gases may accumulate.
2) Electrical circuits in cargo compartments, mast houses, deck houses, hatch trunks or other spaces where gas may accumulate should be isolated. These circuits should not be reconnected until the space has been adequately ventilated and checked to ensure there is no danger.
3) Torches specifically designed for safe use in potentially flammable atmospheres should be carried on board.
Where cargoes have both a tendency to self-heat and emit methane the more immediate hazard should be considered to be that due to methane and the precautions appropriate to that risk should be adopted. Some coals, particularly those with a high sulphur content, may react with water to produce solutions which are corrosive to the ship's structure: this risk can be monitored by measuring the pH value (a measure of acidity/alkalinity) of cargo hold bilge samples. The Master of a ship carrying coal should ensure that the requirements of the relevant Authority are complied with. If the Master is not satisfied that he has been provided with sufficient information concerning the properties of the cargo, or has reason for concern about the safe carriage of the cargo, he should seek expert advice. Explosives are not permitted to be carried in a hold carrying coal in any compartment over that in which coal is carried, nor in one that is not separated from a coal compartment by a steel bulkhead. Vessels carrying part cargoes of coal, separated from other cargo by bulkheads, should have the latter in a perfect dust-tight condition, otherwise claims for coal dust damage will likely result. Cases have occurred where vessels have been declared "unseaworthy" on it being proven that bulkheads were not dust-proof and of proper construction at the time of loading the cargo. Coal shipped in a wet condition will turn out about 3% less in weight. B/Ls should adequately cover the ship against any claims arising from short delivery from this cause. "Pond Coal" is the term used to describe coal which has been reclaimed after having been abandoned and dumped into fresh water ponds. The moisture content is usually high, and it may also have a high sulphur content. This latter can lead to the release of sulphuric acid, with the attendant risk of damage to the carrying vessel. The Stowage Factor of coal, depending as much as it does upon trimming, varies very considerably for the same class and port. In respect of 2 and 3 deck vessels it is considerably higher than for single deck vessels or the large bulk carrier. Sample stowage factors are shown below, but these may be exceeded to a substantial degree:
ISO containers may be successfully used for the carriage of bagged coal. Specially treated coal, in multi-ply paper sacks for domestic use, has little risk off heating or sifting. If Open-top, Half-height, or even Closed Box containers are used for bulk (e.g. when otherwise empty containers would be moved), the containers should be properly lined against abrasive damage (by lining ends and walls with plywood sheets, hardboard, etc.) and against dirt lining the whole container with plastic sheet or similar.
Risk factors
Coal; may create flammable atmospheres, may heat spontaneously, may deplete the oxygen concentration, may corrode metal structures. Can liquefy if predominantly fine 75% less than 5 mm coal.
Always consult the IMSBC Code (International Maritime Solid Bulk Cargoes Code of the IMO (International Maritime Organisation); see IMO Code under the headings ‘General requirements for all coals’ and ‘Special precautions Self-heating coals’.
See also: