Heat usage

In the secondary reformer, process air is admitted to the syngas via a special nozzle system that provides a perfect mixture of air and gas. Subsequent high-pressure steam generation and superheating guarantee maximum process heat usage to achieve an optimized energy efficient process. 

Reactor volumetric solids loading 3 to 12% heat usage 25 to 60 kW/m of kiln volume solids capacity for cement 0.4 to 1.1 Mg/d m kiln volume solids capacity pigments, dolomite 0.1 to 2 Mg/d-m kiln volume. 

Some producers add 1 to 2 % of quicklime to the raw mix, to improve agglomeration and produce stronger green pellets. This increases the output of the sinter strand and reduces heat usage (see section 27.2). 

As coal generally contains less volatile matter (and less water) than wood, more of the heat released when it burns is high-grade . The resulting kiln heat usages per tonne of lime are significantly lower than with wood. The emission of volatiles to atmosphere, however, still results in the emission of copious quantities of smoke. 

There are, however, two major disadvantages of using producer gas. The first is the high total heat usage per tonne of lime, as a result of heat losses in the gas producer and of the low proportion of high-grade heat produced by the gas. The second is the additional capital and operating costs associated with the gas producer. 

The heat usages of oil-fired shaft kilns can be quite low (1,100 to 1,250 kcal/kg), although where the gasifiers are water-cooled, the usage is about 10 % higher. 

Natural gas (calorific value about 8,600 kcal/Nm ) is introduced via ports in the walls of the kiln. It burns with a longer, lazier flame than oil or coal and more readily produces medium reactivity lime. As it contains insignificant amounts of sulfur and produces no ash, the resulting lime is not contaminated by the fuel in any way. Moreover, natural gas does not readily produce smoke, so that the exhaust gases have a low opacity. The heat usages of gas-fired shaft kilns are generally about 1,150 kcal/kg. 

As coke starts to bum at 800 °C, relatively little of its heat is lost as low-grade heat. The net heat usage of coke-fired mixed feed kilns can be as low as 830 kcal/ kg [14.11]. The disadvantages of coke are its high cost per unit of heat and the relatively low availability of suitable grades. 

Anthracite (see Table 14.3) typically contains 4 to 10% of volatile matter, which results in higher heat usages than coke and in the emission of dark smoke. It needs to be strong of the correct particle size and have a high ash fusion temperature. 

In practice, however, the achieved heat requirement can be much lower than the above value and modern lime kilns achieve net heat usages (see section 14.3.6) below 900 kcal/kg CaO. The above calculation makes two major omissions. 

Heat usages were generally high at about 1350kcal/kg net of saleable lime. There was a considerable loss of volatile matter in the exhaust gases, particularly... 

Coke-fired mixed-feed kilns can have the lowest heat usage of all kilns. Net heat usages of about 850kcal/kg are reported [16.4,16.6], with 950 to 1,100 being more typical of routine operation. 

Anthracite (see section 14.3.3.3) is widely used in mixed-feed kilns. As much of the volatile organic matter distils at below the dissociation temperature, part of the calorific value is lost and the total heat usage is increased (typically coke usage is in the range 130 to 150 kg/t lime). The volatiles also cause the emission of smoke, which is increasingly becoming environmentally unacceptable. 

Ref. no. Kiln design Fuels Output range (t/d) Range of stone size (mm) Net heat usage (kcal/kg) Power usage (kWh/t)... 

Reported long-term net heat usages at design output for these types of kiln generally lie in the range 1,000 to 1,200 kcal/kg. 

The net heat usage is about 1,030 kcal/kg. A range of solid, liquid and gaseous fuels can be used, although they should be selected with care to avoid excessive build-ups caused by fuel ash and calcium sulfate deposits. 

Cooling air is preheated by lime in the cooling zone and is withdrawn, dedusted and re-injected via the combustion chambers. A feature of the kiln is that the temperature of the lower combustion chambers can be varied to control the reactivity of the lime over a wide range. The kiln can be fired with solid, liquid and gaseous fuels (or a mixture) and the net heat usage is about 1000 kcal/kg. It can accept stone with a minimum size of 20 mm up to a maximum size of 150 mm. Further details are available in references [16.19-16.23]. 

Rotary kilns fitted with pre-heaters (type K in Table 16.1 Figs. 16.16 and 16.17) are generally considerably shorter than the conventional rotary kiln (e.g. 40 to 60 m). The heat usage decreases because of reduced radiation and convection losses as well as the increased heat recovery from the exhaust gases. Thus, with coal firing, net heat usages below 1250 kcal/kg are reported. 

For most lime producers, the production cost is dominated by the cost of fuel per tonne of lime. As a result, efforts to reduce heat usage frequently results in worthwhile savings. 

Having obtained a heat balance, it is possible to identify and quantify where reductions in unit heat usage may be made. Areas meriting consideration include ... 

Light-burned dolomite (or dolime) is produced in either rotary or shaft kilns. The principles are similar to those of making high calcium quicklime. Heat usages are presumably somewhat lower, owing to the lower heat of calcination of dolomite and its lower dissociation temperature. 

The most widely used unit for the specific heat usage of lime kilns is kcal/kg. lOOOkcal/kg = lOOOcal/g... 

Use alternate separation techniques and/or more efficient heat usage techniques such as heat cascading (Branan, 2012)... 

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