Traditional kiln fuels are gas, oil or coat- The choice is normally based on price and availability. It must be noted, however, that fuels are usually priced in terms of gross heat (heat available assuming water in
combustion product is condensed to recover latent heat of vapourisa- tion). In practice, only the net heat is employed (assumes that water in combustion gas is released as vapour). The difference varies with fuel:
Gross kcal/kg Net kcal/kg Diff
Coal 5500-7100 5400-7000 2% Oil, #6 10200 9700 5% Natural Gas (kcal/M3) 6200 5600 10%
It should also be noted that the gas flame is of lowest emissivity and, requiring more combustion air per unit of heat, is the least efficient. Kiln production typically increases by 2-3% when gas is replaced by coal. On the other hand, gas is the cheapest and easiest fuel to handle and is conventionally billed after use rather than requiring advance purchase and inventory. Assuming 850kcal/kg clinker and 2% excess O2:
Flame temperature
Combustion gas NM3million Cal
Total exhaust gas NM3/tonne clinker
Coal 2250°C 1.23 1360
Oil 2350°C 1.31 1420 Gas 2400°C 1.45 1550
Coal, much more than oil or gas, is liable to compositional variation. The nature of production and handling by major suppliers should min- imize short-term fluctuation while long-term variation can be compen- sated by analysis and normal kiln control procedures. If, however, the supply is from small-scale or multiple suppliers, adequate blending must be effected prior to use.
Stockpiling of coal requires vigilance as spontaneous combustion is common, particularly with low-rank, wet or pyrites-containing coal. Smouldering coal should be dug out, the site spread with limestone dust, and the coal then compressed. If long-term storage is necessary, the pile should be compacted and scaled with coal tar emulsion. Thermocouples embedded 1-2M below the surface allow monitoring for
combustion.
Coal is usually dried, ground so that the residue on 200# (75µ) is not more than 0.5 x % volatiles, and injected with carrying air at a pressure of 120150g/cm2 and tip velocity of 60-SOM/scc. A mor e precise deter- mination of optimum fineness according to coal type has been described by Seidel (ZKC; 1/1995, pg 18). % retained on 50# should be <0.2% and on 100# <0.5%.
Oil may require preheating to reduce viscosity and is injected with a nozzle pressure of about 2Ukg/cm2 except for pressure atomised sys- tems which employ pressure to lOOkg/cm2.
Gas is usually received at "10-70kg/cm2. Primary air is not essential and
the gas is injected as axial, or a mixture of axial and swirl, flow at 3-lOkg/cm2 and a tip velocity of 300-400M/sec (injection will normally be limited by sonic velocity -430M/scc for methane at 0"C). Gas requires turbulent diffusion and its heat flux tends to be released more slowly than with oil or coal; peak heat release is usually about 20M into the kiln against 5-10M for oil. This results in slower response to control changes which makes for more difficult control of the kiln. It should also be noted that, with a higher ignition temperature than oil or coal, natural gas cannot be reliably reignited off hot kiln lining.
In recent years the cost of fuel, which, for most plants, is the largest sin- gle cost factor, has stimulated a search for low cost alternatives. Gaebel & Nachtwcy (WC; 4/2001, pg 59) review fossil fuel reserves and the future of alternative fuels.
Petroleum coke has certain advantages, particularly its very high heat content, but increasing price in some markets has reduced its attraction. The usually high sulphur content (3-6%) also limits rate of addition. It should be noted that there are two main types of pet-coke: 'delayed' and 'fluid'. The preponderant type comes from the delayed batch
process in which feedstock is heated under vacuum to about 500°C; the residue, 'green delayed coke' has typically 8-16% volatiles while cal- cining atabout 1700° yields less than 1% volatiles. Delayed coke may be 'sponge' or 'shot',can be milled with coal, and is commonly used to 60% of total fuel (to 100% has been claimed). Fluid coke consists of small
spherical particles resulting from a continuous coking process at about 650". Volatiles are typically 5-10% and the coke is too hard for conven- tional milling. Fluid coke is injected un-milled at 10-20% of total fuel (ICR;10/]993,pg55).
Numerous other by-product and waste fuels have been used and many command disposal fees. Progressively, however, source reduction is
diminishing the supply of easily handled liquid solvents and waste oils, and the available materials are, increasingly, solids, aqueous sludges, or scheduled hazardous materials involving onerous regulation. With such materials, both consistency and possible contaminants must be moni- tored. Tyres are potentially attractive though shredding or pyrolisis eliminates much of the cost benefit while fuels added discontinuously, such as whole tyres or containerised waste, de-rate the kiln since suffi- cient oxygen must be maintained to support the peaks of combustion.
Spent potliners from the aluminium industry are another potentially valuable fuel source (Kohnen; GCL; 6/2001, pg 8) comprising some 650,000T/Y world-wide. Their use hitherto has been constrained by a typically neurotic fear of fluoride and cyanide residues- A more signifi- cant limit to use for many plants will be the sodium content. Typical analysis is:
C 55% Al2O3 11% GCV 4700kca/kg Na2O 14% F 12%
Other wastes include:
Liquid waste fuels: tar chemical wastes
distillation residues waste solvents
used oil wax suspensions
petrochemical waste asphalt slurry
paint waste oil sludge
Solid waste fuels: paper waste rubber residues
pulp sludge used tyres
petroleum coke battery cases
plastic residues wood waste domestic refuse rice chaff refuse derived fuel nut shells oil-bearing earths sewage sludge
Gaseous waste: landfill gas pyrolysis gas
Kilns employing alternative fuels have detailed specifications to prevent operating or environmental problems, and each shipment is
sampled and checked before unloading to ensure compliance. Typically, heat va lue should not be less than 400Ukcal/kg, chloride is
limited to \% and most plants decline fuels with PCB content exceed- ing 50ppm. Waste fuels may be burned in the kiln, the riser, or the pre- calciner.
4.10 Coal Firing
Coal firing for cment kilns falls into two basic systems (Figure 4.5). Direct firing involves grinding of coal and feeding directly to the burn- er with all of the drying/carrying air entering as primary air (typically 15-30% of total combustion air).
Indirect firing involves intermediate storage of ground coal and sepa- rate cleaning and venting of the drying/carrying air. There are several variations on the two basic coal firing systems.
There is a common assumption that indirect firing yields higher ther- mal efficency by reducing primary air and by excluding the water vapour from coal drying. Such claims are largely invalid due to the poor fuel/air mixing of low primary air burners while water vapour in the flame has a catalytic effect on combustion. Of more importance is the ability of an indirect system with a single mill to supply two or more burners where a pure direct system requires one mill per burner. Note that significant volatile matter and, hence heat content of the fuel (up to 280kcal/kg), may be lost by venting the milling system.
Coal can be ground in most types of mill but roller mills probably pre- dominate. Tt may be noted that the Babcock E-Mill, although less common in cement applications, is used widely for coal grinding in the power industry (Floter & Thiel; ICR; 7/1992, page 22).
Generally, roller mills are designed with integral static classifiers though dynamic classifiers may be employed; dynamic classifiers allow fineness adjustment using rotor speed. Roll separation from grinding tables would be maintained at 5-10 mm and coal feed size should be 100% -25mm with approximately 30% +10mm. Rock and metal rejects fall from the table into the hut air plenum and are swept by a rotating scraper for discharge through an air-locked chute. Abnormal spillage (ie more than 2% of mill feed) may be due either to roll clearance of more than
15mm or to excessive clearance between table and louvre ring; if this clearance exceeds about 10mm, the required 25M/sec air velocity
through the louvre vanes cannot be maintained
Roller mills can dry coal of up to ]0% moisture beyond which the mill is de-rated according to manufacturers design data. Similarly mills are normally designed for 55 Hardgrove index and harder coals (lower HGI) w ill result in de-rating. Finally, a 10% fall in capacity between maintenance is assumed and allowed for in sizing a coal mill. Mills with common table and fan drives may be given separate drives, and capac- ity can then often be increased by raising the table speed.
Mill inlet temperature should not exceed 350"C and coal should not be dried to below \% surface moisture. Mill discharge temperature is limited to 65°C for indirect systems and 80°C for direct. Carrying air velocity must be maintained above 20M/sec to avoid dust settlement
(Recommended Guidelines for Coal System Safety; PCA; May 1983).
Vendors specify a minimum airflow, typically l-1.5kg (0.8-1.2NM3) air per kg coal, which must be maintained even when the drying require- ment is negligible. This airflow is required to ensure that coal does not remain above its ignition temperature long enough for auto-ignition. Fires are usually the result of rags or wood lodging within the mill and may be detected by an increasing discharge air temperature unrelated to increased inlet temperature or reduced feed rate. Fires in direct firing mills are extinguished by adding feed to act as a heat sink and lower- ing the mill inlet temperature. Mills in indirect firing systems conven- tionally employ CO monitoring to detect combustion (thermocouples are too slow to respond); extinction is effected either by water injection or, better, by C02 or N2 with C02 the more common.
Hot air for drying coal can come from cooler exhaust (normal air) or preheater exhaust (low oxygen). The inlet temperature to the mill is controlled to maintain the outlet temperature as described above and de-dusted in a cyclone. Tempe ring to about 370°C is effected by bleed- ing in cold air between cooler and cyclone. If preheater exhaust is used it will typically be at 300-350°C with 5% O2 and 6% moisture; the tem- perature and moisture must be considered in the system design. Although most mill fires occur on start-up or just after shut-down, the low oxygen atmosphere docs reduce the risk of a mill fire during normal operation.
In the United States all coal mil! systems are designed in accordance with NFPA Standard 8503 which requires the equipment to withstand pressures of 2.5 times the absolute working pressure; this equates to 3.5kg/cm2 (50psi). On pne umatic conveying systems, this requirement may be proportionately higher because the transfer system operating pressure can be l.5kg/cm2 (20psi).
Direct coal firing involves a single pipe burner through which the mill carrying air together with entrained coal are injected at a tip velocity of approximately 80M/sec. Tip velocity must always be substantially greater than the flame propagation velocity, which may be up to 25M/sec. Flames produced by coal nuzzle velocities in excess of 80M/sec are susceptible to severe instabilities. The pipe is usually nar- rowed near the tip to minimise parasitic pipe losses and convert the flow into the desired static pressure. For indirect firing, multi-channel burners of various designs are employed. One annulus is used for con- veying pulverised fuel from the mill and one or more separate streams are used to supply primary air fur controlling the flame (Figure 4.5). Typical specifications used by vendors for burners with indirect firing systems are as follows:
FLS 'Duoflex' Pillard 'Rotoflam' KHD 'Pyro-jet'
PF conveying air 2% 2% 3.8%
Total primary air (axial+swirl)
6-8% 8% 4.3%
Axial velocity, M/sec 140-160 200-230 350-450 Swirl velocity, M/sec (combined) 100-200 100-200
Hot air for drying coal can come from cooler exhaust (normal air) or preheater exhaust (low oxygen). The inlet temperature to the mill is controlled to maintain the outlet temperature as described above and de-dusted in a cyclone. Tempe ring to about 370°C is effected by bleed- ing in cold air between cooler and cyclone. If preheater exhaust is used it will typically be at 300-350°C with 5% O2 and 6% moisture; the tem- perature and moisture must be considered in the system design. Although most mill fires occur on start-up or just after shut-down, the low oxygen atmosphere docs reduce the risk of a mill fire during normal operation.
In the United States all coal mil! systems are designed in accordance with NFPA Standard 8503 which requires the equipment to withstand pressures of 2.5 times the absolute working pressure; this equates to 3.5kg/cm2 (50psi). On pne umatic conveying systems, this requirement may be proportionately higher because the transfer system operating pressure can be l.5kg/cm2 (20psi).
Direct coal firing involves a single pipe burner through which the mill carrying air together with entrained coal are injected at a tip velocity of approximately 80M/sec. Tip velocity must always be substantially greater than the flame propagation velocity, which may be up to 25M/sec. Flames produced by coal nuzzle velocities in excess of 80M/sec are susceptible to severe instabilities. The pipe is usually nar- rowed near the tip to minimise parasitic pipe losses and convert the flow into the desired static pressure. For indirect firing, multi-channel burners of various designs are employed. One annulus is used for con- veying pulverised fuel from the mill and one or more separate streams are used to supply primary air fur controlling the flame (Figure 4.5). Typical specifications used by vendors for burners with indirect firing systems are as follows:
FLS 'Duoflex' Pillard 'Rotoflam' KHD 'Pyro-jet'
PF conveying air 2% 2% 3.8%
Total primary air (axial+swirl)
6-8% 8% 4.3%
Axial velocity, M/sec 140-160 200-230 350-450 Swirl velocity, M/sec (combined) 100-200 100-200
Coal firing almost inevitably involves a normal operating condition where pulverised coal is in contact with air before reaching the burner; this is a condition that should, theoretically, be avoided due to its inher- ent risk of fire and explosion. The design of a coal firing system is, there- fore, critically important and should consider numerous factors including the following:
minimum ignition temperature of air/fuel mix; coal ignition temperatures range 200-750°C though the ignition temperature of volatile products may be lower
minimum explosive concentration of fuel in air is about 40g/M3 (note that dust suspensions are not homogeneous)
maximum permissible oxygen concentration to prevent ignition is 12%
the entire pulverised coal system must be designed to contain 3.5kg/cm2 (NFPA 85F) or with explosion relief as appropriate (NFPA 68).
Coal dust explosive tendency increases with volatiles content and with fineness, and decreases with water content and with inert dust diluent (eg limestone).
Coal dust is liable to spontaneous ignition which increases in risk with thickness of dust layer and with the presence of easily oxidised conta- minants such as pyrites (more than 2%). A smouldering fire can become explosive if disturbed.
Coal obviously can be handled safely but pulverised coal should always be considered as a potential explosive. Safety considerations are reviewed in a publication. Coal Fires and Explosions: Prevention, Detection
& Control; EPR1 Research Project 1883-1; Final Report, May 1986.