Cement kiln

Cement kilns are mechanical, industrial furnace used for the pyroprocessing stage of manufacture of portland and other types of hydraulic cement. The kilns use high heat to cook calcium carbonate with silica-bearing minerals to create the more reactive mixture of calcium silicates, called clinker, which is ground into a fine powder that is the main component of cements and concretes.
Kilns are relatively distributed technologies all over the world: over a billion tonnes of cement are made per year, and cement kiln capacity defines the capacity of the cement plants. The kilns is an integrated part of the cement plant, connected by a number of ancillary pieces of equipment, used to engineer an ideal flow of cement to the rest of the system. Improvement to kiln systems and ancillary equipment, such as heat recovery, can improve the efficiency kilns and reduce the cost of overall operation of a cement plan.
Emissions from cement kilns are a major source of greenhouse gas emissions, accounting for around 2.5% of non-natural carbon emissions worldwide. The emissions come from two sources: the fuel and the waste CO2 created from heating the silicate rocks. Conventional cement kilns burn fossil fuels or alternative fuels like tire waste, agricultural waste or other wastes, as a form of waste valorization. Because of the need to reduce emissions to mitigate climate change, multiple companies are investing in alternative fuel sources, including investigations of hydrogen or electricity based heating. Other mitigation approaches, include capturing carbon dioxide from the process at the exhaust stage of the kiln, and reducing use of clinker in final mix of concretes.
Kilns also produce other toxic emissions, such as particulates, Sulfer Dioxide, Nitrous dioxide and other industrial emissions. If not mitigated correctly at the emissions pipe, surrounding communities can have increases in air pollution.

The manufacture of cement clinker

A typical process of manufacture consists of three stages:

grinding a mixture of limestone and clay or shale to make a fine "rawmix" ;
heating the rawmix to sintering temperature in a cement kiln;
grinding the resulting clinker to make cement.

In the second stage, the rawmix is fed into the kiln and gradually heated by contact with the hot gases from combustion of the kiln fuel. Successive chemical reactions take place as the temperature of the rawmix rises:

70 to 110 °C – Free water is evaporated.
400 to 600 °C – clay-like minerals are decomposed into their constituent oxides; principally SiO₂ and Al₂O₃. dolomite decomposes to calcium carbonate, MgO and CO₂.
650 to 900 °C – calcium carbonate reacts with SiO₂ to form belite .
900 to 1050 °C – the remaining calcium carbonate decomposes to calcium oxide and CO₂.
1300 to 1450 °C – partial melting takes place, and belite reacts with calcium oxide to form alite .

Alite is the characteristic constituent of Portland cement. Typically, a peak temperature of 1400–1450 °C is required to complete the reaction. The partial melting causes the material to aggregate into lumps or nodules, typically of diameter 1–10 mm. This is called clinker.
The hot clinker next falls into a cooler which recovers most of its heat, and cools the clinker to around 100 °C, at which temperature it can be conveniently conveyed to storage.
The cement kiln system is designed to accomplish these processes.

Early history

Portland cement clinker was first made in a modified form of the traditional static lime kiln. The basic, egg-cup shaped lime kiln was provided with a conical or beehive shaped extension to increase draught and thus obtain the higher temperature needed to make cement clinker. For nearly half a century, this design, and minor modifications, remained the only method of manufacture. The kiln was restricted in size by the strength of the chunks of rawmix: if the charge in the kiln collapsed under its own weight, the kiln would be extinguished. For this reason, beehive kilns never made more than 30 tonnes of clinker per batch. A batch took one week to turn around: a day to fill the kiln, three days to burn off, two days to cool, and a day to unload. Thus, a kiln would produce about 1500 tonnes per year.
Around 1885, experiments began on design of continuous kilns. One design was the shaft kiln, similar in design to a blast furnace. Rawmix in the form of lumps and fuel were continuously added at the top, and clinker was continually withdrawn at the bottom. Air was blown through under pressure from the base to combust the fuel. The shaft kiln had a brief period of use before it was eclipsed by the rotary kiln, but it had a limited renaissance from 1970 onward in China and elsewhere, when it was used for small-scale, low-tech plants in rural areas away from transport routes. Several thousand such kilns were constructed in China. A typical shaft kiln produces 100-200 tonnes per day.
From 1885, trials began on the development of the rotary kiln, which today accounts for more than 95% of world production.

The rotary kiln

The rotary kiln consists of a tube made from steel plate, and lined with firebrick. The tube slopes slightly and slowly rotates on its axis at between 30 and 250 revolutions per hour. Rawmix is fed in at the upper end, and the rotation of the kiln causes it gradually to move downhill to the other end of the kiln. At the other end fuel, in the form of gas, oil, or pulverized solid fuel, is blown in through the "burner pipe", producing a large concentric flame in the lower part of the kiln tube. As material moves under the flame, it reaches its peak temperature, before dropping out of the kiln tube into the cooler. Air is drawn first through the cooler and then through the kiln for combustion of the fuel. In the cooler the air is heated by the cooling clinker, so that it may be 400 to 800 °C before it enters the kiln, thus causing intense and rapid combustion of the fuel.
The earliest successful rotary kilns were developed in Pennsylvania around 1890, based on a design by Frederick Ransome, and were about 1.5 m in diameter and 15 m in length. Such a kiln made about 20 tonnes of clinker per day. The fuel, initially, was oil, which was readily available in Pennsylvania at the time. It was particularly easy to get a good flame with this fuel. Within the next 10 years, the technique of firing by blowing in pulverized coal was developed, allowing the use of the cheapest available fuel. By 1905, the largest kilns were 2.7 x 60 m in size, and made 190 tonnes per day. At that date, after only 15 years of development, rotary kilns accounted for half of world production. Since then, the capacity of kilns has increased steadily, and the largest kilns today produce around 10,000 tonnes per day. In contrast to static kilns, the material passes through quickly: it takes from 3 hours to as little as 10 minutes. Rotary kilns run 24 hours a day, and are typically stopped only for a few days once or twice a year for essential maintenance. One of the main maintenance works on rotary kilns is tyre and roller surface machining and grinding works which can be done while the kiln works in full operation at speeds up to 3.5 rpm. This is an important discipline, because heating up and cooling down are long, wasteful, and damaging processes. Uninterrupted runs as long as 18 months have been achieved.

The wet process and the dry process

From the earliest times, two different methods of rawmix preparation were used: the mineral components were either dry-ground to form a flour-like powder, or were wet-ground with added water to produce a fine slurry with the consistency of paint, and with a typical water content of 40–45%.
The wet process suffered the obvious disadvantage that, when the slurry was introduced into the kiln, a large amount of extra fuel was used in evaporating the water. Furthermore, a larger kiln was needed for a given clinker output, because much of the kiln's length was committed to the drying process. On the other hand, the wet process had a number of advantages. Wet grinding of hard minerals is usually much more efficient than dry grinding. When slurry is dried in the kiln, it forms a granular crumble that is ideal for subsequent heating in the kiln. In the dry process, it is very difficult to keep the fine powder rawmix in the kiln, because the fast-flowing combustion gases tend to blow it back out again. It became a practice to spray water into dry kilns in order to "damp down" the dry mix, and thus, for many years there was little difference in efficiency between the two processes, and the overwhelming majority of kilns used the wet process. By 1950, a typical large, wet process kiln, fitted with drying-zone heat exchangers, was 3.3 x 120 m in size, made 680 tonnes per day, and used about 0.25–0.30 tonnes of coal fuel for every tonne of clinker produced. Before the energy crisis of the 1970s put an end to new wet-process installations, kilns as large as 5.8 x 225 m in size were making 3000 tonnes per day.
An interesting footnote on the wet process history is that some manufacturers have in fact made very old wet process facilities profitable through the use of waste fuels. Plants that burn waste fuels enjoy a negative fuel cost. As a result, the inefficiency of the wet process is an advantage—to the manufacturer. By locating waste burning operations at older wet process locations, higher fuel consumption actually equates to higher profits for the manufacturer, although it produces correspondingly greater emission of CO₂. Manufacturers who think such emissions should be reduced are abandoning the use of wet process.

Preheaters

In the 1930s, significantly, in Germany, the first attempts were made to redesign the kiln system to minimize waste of fuel. This led to two significant developments:

the grate preheater
the gas-suspension preheater.