Rotary kilns are synonymous with cement making, being the workhorses of this industry. There are many types of rotary kiln arrangements for producing cement clinker with each incremental design goal aimed at improving energy efficiency, ease of operation, and product quality and minimizing environmental pollutants. Rotary cement kilns can be classified into wet-process kilns, semidry kilns, dry kilns, preheater kilns, and precalciner kilns. All of these are described in the book by Peray (1986) and many others, hence we will not dwell upon them here. Rather, we will briefly show the pertinent process chemistry and the heat requirements that drive them, so as to be consistent with the transport phenomena theme.
Rotary kilns have been used in various industrial applications (e.g., oil shale retorting, tar sands coking, incineration, cement production, etc.). The rotation of a cylinder-shaped vessel positioned longitudinally approximately 30 of the horizontal position ensures a continuous motion of catalyst between the entrance and exit of the kiln. With regard to the spent catalyst regeneration, the description of rotary kilns was given by Ellingham and Garrett . There are two types of rotary kilns, i.e., direct fire and indirect fire.
The direct fire is a single shell vessel with rings added inside to slow the catalyst as it tumbles from the inlet (elevated part) towards outlet (lower part). The oxidation medium flows countercurrent to catalyst movement. The O2 concentration in the medium will decrease in the same direction because of its consumption. Therefore, the zone in the vessel located near the inlet may function as a stripper of volatile components of coke. The kiln is fired by gas burners directly against the outer shell of the vessel. The temperature inside the kiln is controlled by adjusting the burner heat, varying concentration of O2 in the oxidizing medium and its flow. The indirect fire kiln comprises a double-shell cylinder vessel. The inner shell is similar as that of the direct fire kiln. The space between the shells is heated either by combustion gas or steam. In some cases, the inner cylinder shell is ebullated allowing hot gases or steam to enter and contact the tumbled catalyst. The catalyst temperatures are controlled by monitoring the temperatures of the inlet and outlet gases. It is believed that Eurocat process evolved from a rotary kiln process by be improving the control of operating parameters such as temperature, gas flow, speed of rotation, etc.
The rotary kiln is used to process the lead-containing components resulting from the breaking and separation of waste batteries. The main components of a rotary kiln are an inclined cylindrical, refractory-lined reaction shaft equipped to rotate over rollers and a burner. Process heat is generated by burning fine coke or coal contained in the charge and by the exothermic heat of the PbO reduction by CO. This process produces molten lead and a slag with 35% Pb. A drawback of this technology is the short life of refractory liners.
The rotary kiln is a long tube that is positioned at an angle near horizontal and is rotated. The angle and the rotation allow solid reactants to work their way down the tube. Speed and angle dictate the retention time in the kiln. Gas is passed through the tube countercurrent to the solid reactant. The kiln is operated at high temperatures with three or four heating zones depending on whether a wet or dry feed is used. These zones are drying, heating, reaction, and soaking. Bed depth is controlled at any location in the tube with the use of a ring dam.
The most common reactor of this type is the lime kiln. This is a noncatalytic reaction where gas reacts with calcium carbonate moving down the kiln. Other reactions performed in the rotary kiln include calcination, oxidation, and chloridization.
Use of rotary kilns for hazardous waste incineration is becoming more common for disposal of chlorinated hydrocarbons such as polychlorinated biphenyls (PCBs). Flow in these kilns is cocurrent. Major advantages include high temperature, long residence time, and flexibility to process gas, liquid, solid, or drummed wastes.
The rotary kilns used in the first half of the twentieth century were wet process kilns which were fed with raw mix in the form of a slurry. Moisture contents were typically 40% by mass and although the wet process enabled the raw mix to be homogenized easily, it carried a very heavy fuel penalty as the water present had to be driven off in the kiln.
In the second half of the twentieth century significant advances were made which have culminated in the development of the precalciner dry process kiln. In this type of kiln, the energy-consuming stage of decarbonating the limestone present in the raw mix is completed before the feed enters the rotary kiln. The precalcination of the feed brings many advantages, the most important of which is high kiln output from a relatively short and small-diameter rotary kiln. Almost all new kilns installed since 1980 have been of this type. Figure1.4 illustrates the main features of a precalciner kiln.
The raw materials are ground to a fineness, which will enable satisfactory combination to be achieved under normal operating conditions. The required fineness depends on the nature of the raw materials but is typically in the range 1030% retained on a 90 micron sieve. The homogenized raw meal is introduced into the top of the preheater tower and passes downwards through a series of cyclones to the precalciner vessel. The raw meal is suspended in the gas stream and heat exchange is rapid. In the precalciner vessel the meal is flash heated to ~900C and although the material residence time in the vessel is only a few seconds, approximately 90% of the limestone in the meal is decarbonated before entering the rotary kiln. In the rotary kiln the feed is heated to ~ 1500C and as a result of the tumbling action and the partial melting it is converted into the granular material known as clinker. Material residence time in the rotary kiln of a precalciner process is typically 30 minutes. The clinker exits the rotary kiln at ~ 1200C and is cooled to ~60C in the cooler before going to storage and then being ground with gypsum (calcium sulfate) to produce cement. The air which cools the clinker is used as preheated combustion air thus improving the thermal efficiency of the process. As will be discussed in section1.5, the calcium sulfate is added to control the initial hydration reactions of the cement and prevent rapid, or flash, setting.
If coal is the sole fuel in use then a modem kiln will consume approximately 12 tonnes of coal for every 100 tonnes of clinker produced. Approximately 60% of the fuel input will be burned in the precalciner vessel. The high fuel loading in the static precalciner vessel reduces the size of rotary kiln required for a given output and also reduces the consumption of refractories. A wider range of fuel types (for example, tyre chips) can be burnt in the precalciner vessel than is possible in the rotary kiln.
Although kilns with daily clinker outputs of ~9000tonnes are in production in Asia most modem precalciner kilns in operation in Europe have a production capability of between 3000 and 5000 tonnes per day.
A rotary kiln is a physically large process unit used in cement production where limestone is decomposed into calcium oxide which forms the basis of cement clinker particles under high temperatures. The modelling of rotary kilns are well documented in literature. Mujumdar et al. 2007 developed an iteration based rotary kiln simulator (RoCKS), which integrates models for a pre-heater, calciner, kiln and clinker cooling that agreed well with observations in industry. The model takes complexities in reactions and heat transfers with different sections into account by coupling multiple models with common boundaries regarding heat and mass communications. Other work (Ngadi and Lahlaouti, 2017) neatly demonstrates an experimentally proven kiln model being applied for screening of combustion fuel used for kilns, and how it may impact the production. This contribution coupled modelling of reactions and heat transfer in the bed region and another model for combustion and heat transfer in the freeboard region.
While modelling of these processes with varying degree of complexity has been performed, proper uncertainty and sensitivity analysis of these models have not been given due importance/consideration. As the use of computer aided process engineering tools increases, the need for robust uncertainty and sensitivity analysis frameworks becomes more important. There are several frameworks of uncertainty and sensitivity analysis applied for different problems, from good modelling practice (Sin et al., 2009) to process design and product design (Frutiger et al. 2016). These frameworks typically include the following steps (0) problem statement, (i) identification of input sources of uncertainties, (ii) sampling (iii) Monte Carlo simulations and (vi) sensitivity analysis. The purpose of this work is to perform a systematic uncertainty and sensitivity analysis of rotary kiln process design in order to address the following: (1) Given a certain base case design, what is the impact of uncertainties in the model and measurements on the key process design metrics (minimum required reactor length and degree of conversion), and, (2) given a certain source of uncertainties, what is the robust design to ensure process performance with 95 % confidence.
The rotary kiln is often used in solid/liquid waste incineration because of its versatility in processing solid, liquid, and containerized wastes. The kiln is refractory lined. The shell is mounted at a 5 degree incline from the horizontal plane to facilitate mixing the waste materials. A conveyor system or a ram usually feeds solid wastes and drummed wastes. Liquid hazardous wastes are injected through a nozzle(s). Non-combustible metal and other residues are discharged as ash at the end of the kiln. Rotary kilns are also frequently used to burn hazardous wastes.
Rotary kiln incinerators are cylindrical, refractory-lined steel shells supported by two or more steel trundles that ride on rollers, allowing the kiln to rotate on its horizontal axis. The refractory lining is resistant to corrosion from the acid gases generated during the incineration process. Rotary kiln incinerators usually have a length-to-diameter (L/D) ratio between 2 and 8. Rotational speeds range between 0.5 and 2.5 cm/s, depending on the kiln periphery. High L/D ratios and slower rotational speeds are used for wastes requiring longer residence times. The kilns range from 2 to 5 meters in diameter and 8 to 40 meters in length. Rotation rate of the kiln and residence time for solids are inversely related; as the rotation rate increases, residence time for solids decreases. Residence time for the waste feeds varied from 30 to 80 minutes, and the kiln rotation rate ranges from 30 to 120 revolutions per hour. Another factor that has an effect on residence time is the orientation of the kiln. Kilns are oriented on a slight incline, a position referred to as the rake. The rake typically is inclined 5 from the horizontal.
Hazardous or non-hazardous wastes are fed directly into the rotary kiln, either continuously or semi-continuously through arm feeders, auger screw feeders, or belt feeders to feed solid wastes. Hazardous liquid wastes can also be injected by a waste lance or mixed with solid wastes. Rotary kiln systems typically include secondary combustion chambers of afterburners to ensure complete destruction of the hazardous waste. Operating kiln temperatures range from 800C to 1,300C in the secondary combustion chamber or afterburner depending on the type of wastes. Liquid wastes are often injected into the kiln combustion chamber.
The advantages of the rotary kiln include the ability to handle a variety of wastes, high operating temperature, and continuous mixing of incoming wastes. The disadvantages are high capital and operating costs and the need for trained personnel. Maintenance costs can also be high because of the abrasive characteristics of the waste and exposure of moving parts to high incineration temperatures.
A cement kiln incinerator is an option that can be used to incinerate most hazardous and non-hazardous wastes. The rotary kiln type is the typical furnace used in all cement factories. Rotary kilns used in the cement industry are much larger in diameter and longer in length than the previously discussed incinerator.
The manufacture of cement from limestone requires high kiln temperatures (1,400C) and long residence times, creating an excellent opportunity for hazardous waste destruction. Further, the lime can neutralize the hydrogen chloride generated from chlorinated wastes without adversely affecting the properties of the cement. Liquid hazardous wastes with high heat contents are an ideal supplemental fuel for cement kilns and promote the concept of recycling and recovery. As much as 40% of the fuel requirement of a well-operated cement kiln can be supplied by hazardous wastes such as solvents, paint thinners, and dry cleaning fluids. The selection of hazardous wastes to be used in cement kiln incinerators is very important not only to treat the hazardous wastes but also to reap some benefits as alternative fuel and alternative raw material without affecting both the product properties and gas emissions. However, if hazardous waste is burned in a cement kiln, attention has to be given to the compounds that may be released as air emissions because of the combustion of the hazardous waste. The savings in fuel cost due to use of hazardous waste as a fuel may offset the cost of additional air emission control systems in a cement kiln. Therefore with proper emission control systems, cement kilns may be an economical option for incineration of hazardous waste.
The rotary kiln gasifier is used in several applications, varying from industrial waste to cement production and the reactor accomplishes two objectives simultaneously: (1) moving solids into and out of a high temperature reaction zone and (2) assuring thorough mixing of the solids during reaction. The kiln is typically comprised of a steel cylindrical shell lined with abrasion-resistant refractory to prevent overheating of the metal and is usually inclined slightly toward the discharge port. The movement of the solids being processed is controlled by the speed of rotation of the kiln.
The moving grate gasifier is based on the system used for waste combustion in a waste-to-energy process. The constant-flow grate feeds the waste feedstock continuously to the incinerator furnace and provides movement of the waste bed and ash residue toward the discharge end of the grate. During the operation stoking and mixing of the burning material enhances distribution of the feedstocks and, hence, equalization of the feedstock composition in the gasifier. The thermal conversion takes place in two stages: (1) the primary chamber for gasification of the waste (typically at an equivalence ratio of 0.5) and (2) the secondary chamber for high temperature oxidation of the synthesis gas produced in the primary chamber (Grimshaw and Lago, 2010; Hankalin et al., 2011).
The rotary kiln ICM/Phoenix Bioenergy demonstration gasifier was operated at a transfer station in Newton, Kansas from 2009 to 2012 for more than 3200h, testing various types of biomass, RDF, tire-derived fuel or automobile shredded residue mixed with RDF. The 150-t-per-day facility reported to have tested more than 16 types of feedstock listed in Table 3.2 .
The gasification process consists of a horizontal cylinder with an internal auger which slowly rotates  allowing feedstock to move through the reactor, whereas air is injected at multiple points. Only small portion of the syngas was used to produce steam, whereas the rest was flared (Fig. 3.2).
Unfortunately, ICM had to take down the demonstration gasifier at the transfer station, upon completion of the project and financing grant, declaring that the facility did not prove to be a viable solution for the county. Some of the problems that ICM mention  were related to the availability of feedstock of only 90t per day, whereas the prototype was designed for 150t per day, but also insufficient investment from financial partners due to the lower projected returns. ICM announced that through a contract with the City of San Jose, CA they will have the ICM demonstration gasifier at the San Jos-Santa Clara Regional Wastewater Facility . The facility will process 10 short tons per day of woody biomass, yard waste or construction and demolition materials mixed with biosolids from the WWT.
Options to use phosphogypsum as an alternate of gypsum in cement manufacturing.Feasibility concept of phosphogypsum calcination through kiln waste heat recovery (WHR).Estimate the resultant temperature profiles of heat exchanger to be used for WHR.Analyse and reconfirm chemical characteristics of sample phosphogypsum for thermal calcination and chemical kinetics.Explore design concept of rotary dryer for processing the phosphogypsum.
The cement industry nowadays is giving thrust to replace and reduce fossil fuel-based energy through recovery of waste heat, utilization of wastes (Alternative fuels) and adoption of renewable energy sources like wind energy, solar PV systems etc.
In cement plants, high temperature heat is released into the atmosphere through exhaust flue gases, radiation and convection heat from hot and exposed surfaces like rotary kiln. A lot has already been explored about recovery of waste heat from exhaust hot flue gases. Many Indian cement plants have installed Waste Heat Recovery Systems (WHRS) at their facility for generating electricity by using heat energy in hot flue gases. At present cumulative generation capacity is estimated as ~344MW from such WHRS facilities in Indian cement plants.
Indian cement industry is facing issues about limited availability of the mineral gypsum. This is going to be more critical with possible restrictions over mineral gypsum import in India. On the other hand, the huge inventory of phosphogypsum is a challenge for phosphoric acid manufacturing plants. Phosphogypsum cant be used without processing in the cement production process due to adverse impact on cement properties. Processing of Phosphogypsum requires high temperature and if the temperature is raised through fossil fuel combustion then it will add to extra cost and associated Green House Gas (GHG) emissions. Heat recovery from kiln hot shell surfaces can be explored for phosphogypsum calcination purpose here. A feasibility concept of phosphogypsum calcination through kiln waste heat recovery is discussed in this paper.