Keys to Combustion Efficiency

Everyone knows that the kiln is the heart of the cement plant but not everyone appreciates the delicate balancing act between combustion, energy use, and product quality. Combustion is the process needed to transform the chemical energy locked up within the fuel into the heat needed to make cement. The more efficient we make this process, the more heat that we unlock, the less fuel that we waste, and the less ash that we produce. Here are a few basic things to consider in this intricate balance.

The right size.
The key to burning any fuel is to make sure that the particles are small enough that they can be burned quickly and easily. Think of this simple analogy. If we were to light a telephone book on fire with one match, we’d have a hard time getting combustion but if we instead ripped off each page separately and then lit them, we would get much more efficient combustion. If we shredded the book into confetti size material, we would get even more efficient combustion. Producing smaller particle sizes requires using more grinding energy. The balance then becomes a trade-off between how fine to grind and how much more efficient the combustion process becomes with a finer grind. Since a smaller particle size means the fuel burns quicker and easier, it also means there are more safety concerns with handling, conveyance and storage.

Where does all that heat come from?
The actual combustion process is an incredibly complex series of chemical reactions that start off with fuel, air, and an ignition source. In fact, there are actually more than one thousand separate reactions involved from the transition of fuel into the final combustion products of carbon dioxide and water. The kindling temperature for bituminous coal is roughly 300° C while the kindling temperature for petcoke is about 700° C. (Petcoke has a higher kindling temperature because as it is almost pure carbon and has very few volatiles compared to coal.) The combustion process continues provided there is enough fuel and air supplied. For solid fuels like coal and coke, that means roughly 11.5 pounds of air per pound of fuel.

If it’s mixed…it’s burned.
That old adage still holds true. Operators usually don’t have much ability to impact the mixing process except to make sure that the amount of primary air is consistently supplied. The actual physical mixing then becomes a function of burner design and flame geometry. Fuel that isn’t burned within the flame poses a critical safety concern because of its potential to accumulate and combust farther on down the kiln. Insufficient primary air can also cause accumulations of solid fuel in the burner pipe system with potentially disastrous results. Keep in mind that the rate of flame propagation in a coal air stream may be as high as 4,500 feet per minute. PCA’s Recommended Guidelines for Coal System Safety, SP 027 provides specific guidance on burner pipe tip velocity and air stream velocities for pipes pneumatically conveying coal.

Where does all that heat go?
The combustion process heats up unburned fuel, feed, coating, refractory, and the gases inside the kiln. That’s why it’s so important to maintain stable secondary air temperature. That can only be accomplished by maintaining consistent clinker cooler performance. Remember that fans are constant volume machines. One pound of air at a temperature of 70° F occupies just over 13 cubic feet of volume. Raise that same pound of air to 1500° F and now it occupies more than 49 cubic feet of volume. Yes, it does take more fan energy to move 49 cubic feet as compared to 13 cubic feet of air but cooler secondary air temperatures rob the entire kiln system of necessary process heat. Heat used to raise the temperature of the secondary air is heat that’s unavailable to raise the temperature of the kiln feed.

Flame stability.
Optimal flame length can promote rapid heating and cooling of the clinker. Flame stability also means reduced back end temperatures which in turn means lower heat losses from exit gases and shell radiation. The right flame length also optimizes the ID fan capability and reduces the potential for NOx formation. Unstable flames on the other hand means a varying ignition point, a variable stand off distance from the burner tip, high risk of flame out, and potential explosion risk.

NOx and SOx…SOx and NOx.
In a perfect world combustion products would be limited to just water and carbon dioxide. But we don’t live in a perfect world. Thermal NOx is generated in and around the flame at temperatures greater than 1200° C. A short hot burning zone can reduce the formation of thermal NOx. SO2 is formed as sulfide or elemental sulfur is oxidized at temperatures of 300 to 600° C. Limiting the source of sulfur or the necessary oxygen can limit the potential for SO2 formation. Carbon monoxide formation is another concern. CO is either formed because of incomplete combustion or the rapid cooling of combustion products below the ignition temperature of CO (610° C). Either situation is detrimental to optimizing the process.

Optimizing the combustion process is the key to optimizing kiln operations. Both processes are mutually dependent upon one another and, like most aspects of cement manufacturing that means tradeoffs; tradeoffs between constraints that can’t be changed with options that can be.

Information in this article was taken primarily from PCA’s new Innovations in Cement Manufacturing, CD 400 and also from presentations in PCA’s Kiln Process program.