TIROXINA (T 4 )

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).

48•Cement Plant Operations Handbook

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.

Cement Plant Operations Handbook • 49

50• Cement Plant Operations Handbook

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.

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:

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.

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:

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.