The Rotary Kiln Evolution

Rotary kilns have been synonymous with cement and lime kilns probably because of the history of their evolution and development. It has been reported that cement deposits characterized by Israeli geologists in the 1960s and 1970s place cement making at 12,000,000 BC when reactions between limestone and oil shale occurred during spontaneous combustion to form a natural deposit of cement compounds (Blezard, 1998). Between 3000 and 300 BC the cement evolution had continued with the Egyptians who used mud mixed with straw to bind dried bricks to carry out massive projects such as the pyramids. This evolution continued with the Chinese who used cementitious materials for the building of the Great Wall. Projects such as the building of the Appian Way by the Romans later led to the use of pozzolana cement from Pozzuoli, Italy, near Mt. Vesuvius. However, it is reported that the technology that uses the burning of lime and pozzolan to form a cementitious admixture was lost and was only reintroduced in the 1300s. In the United States, projects such as the construction of a system of canals in the first half of the nineteenth century, particularly the Erie Canal in 1818, created the first large-scale demand for cement in this country that led to various cement production businesses to compete for the market share. By 1824 Portland cement had been invented and developed by Joseph Aspdin of England; this involving the burning of finely ground chalk with finely divided clay in a lime kiln yielding carbon dioxide as an off-gas (Peray, 1986). In these early days, stationary kilns were used and it is said that the sintered product was wastefully allowed to cool after each burning before grinding. The history of cement (Blezard, 1998) has it that in the late 1870s one Thomas Millen and his two sons, while experimenting with the manufacture of Portland cement in South Bend, Indiana, burned their first Portland cement in a piece of sewer pipe. This perhaps marked the first experimental rotary kiln use in America. By 1885, an English engineer, F. Ransome, had patented a slightly tilted horizontal kiln that could be rotated so that material could move gradually from one end to the other. The underlying principle of this invention constitutes the rotary kiln transport phenomenon we know of today.

Because this new type of kiln had much greater capacity and burned more thoroughly and uniformly, it rapidly displaced the older type kilns. It has been further mentioned that the factor that contributed to the tremendous surge of Portland cement between 1880 and 1890, reportedly from about 42,000 to 335,000 barrels, was the development of the rotary kiln (Blezard, 1998). Like most early inventions in the United States, it is said that Thomas A. Edison played a role in furthering the development of the rotary kiln. He is credited for introducing the first long kilns used in the industry at his Edison Portland Cement Works in New Village, NJ, in 1902. His kilns are believed to have been about 150 ft long in contrast to the customary length at that time of 60–80 ft. Today, some kilns are more than 500 feet long with applications ranging far wider than cement and lime making. By the 1900s, most of the advances in the design and operation of cement and lime kilns had undergone a systematic evolution since the days of the ancient Egyptians. By this time, almost countless variations of patented kilns had been invented and promoted although some of these never found useful applications. It is fair to say that kilns have evolved from the so-called field or pot kilns that were crudely constructed of stone and often on the side of hills, to vertical shaft and rotary kilns with each evolution step carried out with the improvement of labor intensiveness, productivity, mixing, heat transfer, and product quality in mind. Following cement, other industries also joined in the bandwagon. For example, the rotary kiln process for making lightweight aggregate (LWA) was developed by Stephen Hayde in the early 1900s in Kansas City, Missouri (Expanded Shale, Clay, and Slate Institute). In the expanded shale industry, natural lightweight aggregates had been used to make lightweight concrete since the days of the early Greeks and Romans, but it was not until the discovery of expanded shale, manufactured by the rotary kiln process, that a lightweight aggregate with sufficient strength and quality became available for use in the more demanding reinforced concrete structural applications. Currently, rotary kilns are employed by industry to carry out a wide array of material processing operations; for example, reduction of oxide ore, reclamation of hydrated lime, calcining of petroleum coke, hazardous waste reclamation, and so on. This widespread usage can be attributed to such factors as the ability to handle varied feedstock, spanning slurries to granular materials having large variations in particle size, and the ability to maintain distinct environments, for example, reducing conditions within the bed coexisting with an oxidizing freeboard (a unique feature of the rotary kiln that is not easily achieved in other reactors). The nature of the rotary kiln, which allows flame residence times of the order of 2–5 s and temperatures of over 2000 K, makes such kilns a competitive alternative to commercial incinerators of organic wastes and solvents. However, the operation of rotary kilns is not without problems. Dust generation, low thermal efficiency, and nonuniform product quality are some of the difficulties that still plague rotary kiln operations. Although the generally long residence time of the material within the kiln (typically greater than one hour) aids in achieving an acceptably uniform product as the early users had intended, there is considerable scope for improving this aspect of kiln performance. In order to achieve this improvement a more quantitative understanding of transport phenomena within the bed material is required; specifically of momentum transport, which determines particle motion and energy transport, which, in turn, determines the heating rate for individual particles. This book seeks to present the quantitative understanding of the transport phenomena underlying the rotary kiln.

Fundamentally, rotary kilns are heat exchangers in which energy from a hot gas phase is extracted by the bed material. During its passage along the kiln, the bed material will undergo various heat exchange

Figure 1. Schematic diagram of countercurrent flow rotary kiln

configuration.

processes, a typical sequence for long kilns being drying, heating, and chemical reactions that cover a broad range of temperatures. Although noncontact (i.e., externally heated) rotary kilns are employed for specialized work, most kilns allow direct contact between the freeboard gas and bed material as shown in Figure 1.1. The most common configuration is counter current flow whereby the bed and gas flows are in opposite directions although co-current flow may be utilized in some instances, for example, rotary driers.