Lime kiln dust (LKD) and cement kiln dust (CKD) are by-products of manufacturing and processing of lime and cement, respectively. CKD contains mostly dried raw materials such as limestone, sand, shale, and iron ore. Nearly 4 million tons of CKD is disposed of every year in the United States (Miller and Zaman, 2000). Due to its pozzolanic activator attribute, CKD has been successfully implemented in road soil stabilization (Miller and Azad, 2000). CKD creates a low-ductile asphalt binder, waterproofing, and protection. CKD-treated soils exhibit reduced liquid limit and reduced plasticity indices.
CKD inclusion in soil increases unconfined compressive strength, stiffness, and durability compared to untreated soils (Miller and Azad, 2000). Although LKD and CKD additives are not considered hazardous by environmental regulatory agencies, proper supervision and handling are needed when these additives are used in the field application.
The lime kiln calcines the calcium carbonate in lime mud to produce quicklime. Several modifications are possible to reduce energy consumption in the kiln. High-efficiency filters can be installed to reduce the water content of the kiln inputs, thus reducing evaporation energy. Higher-efficiency refractory insulation brick or chains can be installed to increase heat transfer in the kiln. Heat can also be captured from the lime and from kiln exhaust gases to preheat incoming lime and combustion air. Average savings achieved by these measures is approximately 0.46GJ/t pulp (Elaahi and Lowitt,1988; Grace,1987; Grace etal.,1989; Byrne and Larsen,1997; Lewko,1996; Pearson and Dion,1999). These improvements can also improve the rate of recovery of lime from green liquor. This will reduce the plants requirement for additional purchased lime. Based on an analysis of kiln modifications in cement production, Martin etal. (2000) assumed an investment cost of $2.5/t pulp. One study indicated that newer high-performance refractories can lead to lime kiln energy savings of up to 5%. Heat can also be recovered from the lime and from kiln exhaust gases to preheat incoming lime and combustion air (Kramer etal.,2009).
Materials contributing to air pollution include particulate matter from the boilers, lime kiln, and smelt dissolving tank; gaseous combustion pollutants such as carbon monoxide, NOx, and volatile organics from power boilers, recovery furnaces, and the lime kiln; odor consisting of reduced sulfur compounds in the kraft process arising from the digesters, evaporators, recovery boiler, smelt dissolving tank, and lime kiln; sulfur dioxide from the recovery furnace and boilers using fuel containing sulfur; and volatile organic chemicals from miscellaneous sources. In the sulfite process the primary pollutants are organics, oxides of sulfur, and particulates.
Lime is produced by calcining limestone at 825C in a lime kiln. The cost of this processing is reflected in the cost of the lime produced which is considerably more than that of limestone. The cost of transporting lime is also higher than that of limestone as lime must be protected from moisture. Karlsson and Rosenberg (1980a) estimate the cost ratio of lime to limestone on a molar basis to be between 2 and 4 depending on the transportation distance.
Lime has certain advantages over limestone in FGD applications. Cases of pH instability have been reported for limestone due to its relatively slow rate of dissolution. Limestone has a greater liquid-side resistance to mass transfer. It is also claimed that an unsaturated mode is more readily attained with lime (Karlsson and Rosenberg, 1980a). Despite these relative advantages FGD systems based on lime are becoming less popular than those based on limestone. Recent advances in the reliabilities of both lime and limestone systems have resulted in the cost of reagent being the overriding consideration. Of the plants under construction or contracted in the USA twice as many are based on limestone than on lime. Very few, if any, lime systems are under construction for projected plants (Karlsson and Rosenberg, 1980a).
Whether lime or limestone is employed, efficient utilisation of reagent is important from an economic point of view. Single loop, calcium based FGD systems must operate at pH values of about 6 to 6.5 to obtain adequate removal of SO2. The concentration of reagent, particularly when using limestone, in the solution at these values of pH can be as low as 2 to 4%. At these low solubility values part of the reagent may be discharged with the slurry. When 90% or greater removal of SO2 is required limestone utilisation may be as low as 70% (Braden, 1978) but is typically between 75 and 90% (Karlsson and Rosenberg, 1980a). With double loop operation reagent utilisation approaches 100% because conditions are such that any unused reagent discharged from the absorber loop is dissolved in the quench loop.
As of January 1980 there was a total of 35 680 MW of plant in operation or committed to limestone or lime scrubbing processes in the USA. This represented over 70% of the total FGD capacity in the USA. Operating and maintenance experience is well documented (e.g. Spring, 1980; Hewitt and Saleem, 1980).
From the fuel requirement point of view, rotary kilns are the most flexible of all lime kilns (Oates, 1998). They are successfully fired with natural gas, fuel oil, and pulverized fuels of all types including coal, coke, and sawdust. According to Boynton (1980), the United States is by far the world's leader in rotary kiln lime production with about 88% of its commercial and about 70% of captive plant capacity provided by kilns. The conventional rotary lime kiln has a length-to-diameter (L/D) ratio in the 3040 range with lengths of 75500ft (22.7152.5m) and diameter of 411ft (1.23.3m). Lime kilns are usually inclined at about 35 slope with material charged at the elevated end and discharging at the lower end. The degree of fill is relatively deep, about 1012%. Owing to its low thermal conductivity, limestone with a large diameter of about 2 in (5cm) results in higher effective bed heat conduction than smaller stones. The larger feed material sizes tend to have larger pore volume in the bulk and thereby maximize the particle-to-particle heat transfer, which is usually dominated by radiation at the dissociation temperatures. The smaller feed stones tend to pack themselves upon rotation and render the bed a poor conductor of heat. For many years, most long kilns operated with deplorable fuel efficiencies because of poor or lack of heat recuperation such as coolers and preheaters (Figure10.4) with thermal consumption as high as 1215 millionBtu/ton (33364170kcal/kg) of lime. Thanks to ingenious heat recuperation systems such as coolers, preheaters, and lifters, today, thermal efficiencies of rotary lime kilns are in the 68 millionBtu/ton range (16682224kcal/kg), using fuel at about half the rate of early long kilns.
Some rotary lime kilns operate under reducing conditions by curtailing the combustion air to substoichiometric levels so as to volatilize any sulfur that may be in the limestone in order to meet the stringent sulfur specifications imposed by steel and chemical users. For most operations except for dead burnt dolomite, the burner tip velocities can range between a low of 25m/s and a high of about 60m/s. These are significantly lower than the velocities of cement kilns, which operate around 80100m/s. The momentum ratio and associated CrayaCurtet parameter is usually lower than 2, which means that the burner jet recirculation will have eddies and that fuel/air mixing is moderate and the flame is less intense than that in dead burnt dolomite kilns or cement kilns. A simple heat and mass balance for the kiln section of a lime-making process is shown in Figure10.5.
Borate autocausticizing makes it possible to produce sodium hydroxide directly in the recovery boiler and improves the lime kiln and recausticization operations by reducing causticizing loads and the amount of lime processed through the system. The major function of the recausticizing plant in a pulp mill is to regenerate the caustic. Caustic is typically recovered from the spent pulping chemical in the following stages:
Autocausticizing could be an attractive alternative for kraft mills because it allows higher caustic production without increasing lime demand and can even eliminate lime demand (Kochesfahani and Bair,2002).
The partial borate autocausticizing process occurs when sodium borates are added to the kraft liquor at substoichiometric levels (Bjrk etal.,2005). A portion of the sodium carbonate is causticized in the recovery boiler. The causticization of the remaining sodium carbonate is completed in a conventional recausticizing plant of the pulp mill with a reduced quantity of lime. The technology may appear as an attractive option particularly for kraft pulp mills where incremental causticizing and lime kiln capacity are required. Mill-scale trials have shown that there are no major side effects on the mill operations. The major findings of the studies suggest that borate present in cooking liquor presents several advantages (Table11.19).
Borate autocausticizing technology uses Neobor, a form of sodium borate to replace lime. Each ton of Neobor added to the pulping process replaces 10 to 30 times its weight in lime (RTM,2010). The main autocausticizing reaction that occurs in the recovery boiler is between sodium metaborate and sodium carbonate in the molten smelt, which forms trisodium borate. The trisodium borate reacts with the water in the smelt-dissolving tank to form sodium hydroxide and regenerate sodium metaborate (Bjrk etal.,2005). Sodium metaborate stays in solution and circulates through the chemical recovery cycle to continue forming caustic in the recovery boiler. The borate compounds remain in the liquor cycle, self-regenerating to be used again in producing caustic. Sodium metaborate drives autocausticizing reactions in the recovery boiler and forms sodium hydroxide in the smelt-dissolving tank without the use of lime or additional recovery processes, so this technology reduces energy consumption and increases causticizing and calcining capacities. For kraft and soda pulp mills, reducing the lime kiln load not only translates to lower operating costs, energy consumption, and emissions, but also significantly reduces the amount of lime mud that requires special handling and disposal (ITP,2011b).
The concept of borate autocausticizing was first investigated during the early 1980s in Europe. Rio Tinto Minerals developed partial borate autocausticizing (RTM,2010). They obtained support from the US DOE. This technology is being implemented in pulp mills worldwide to supplement conventional lime causticizing with almost no capital investment (ITP,2011b). A full-scale trial of partial autocausticizing with sodium borate was first conducted at Georgia-Pacific Camas mill, Washington State(United States), from 1999 to 2000 for a period of more than 16months (Hunter etal.,2001). In Europe, a full-scale partial borate autocausticizing trial was conducted in 2002 at the Stora Enso Norrsundet mill in Sweden (Bjrk etal.,2005). In this trial, the total lime requirement has been reduced by about 7%, and the autocausticizing level has typically been 911% during the 15-month period. There is some indication that borates may also improve pulp yield (Bjrk etal.,2005). Partial autocausticizing in the P.H. Gladfelter Co. mill in Spring Grove, Pennsylvania, in the United States increased production by about 5% in 2007 (ITP,2011b). However, full borate autocausticizing, which uses borates to drive all the causticizing reactions instead of just lime causticizing, is still undergoing further research and testing (ITP,2006c).
An air jet laden with particles such as that found in primary air issuing from a pulverized fuel pipe for combustion in cement and lime kilns may be synonymous with a jet of fluid with a density greater than that of air provided the particles are small enough that one can consider the fluid to be homogeneous. Under such conditions, the effect of the solid burden may be accounted for by simply assuming an increase in the gas density and a reduction in the kinematic viscosity. A concomitant result will be an accelerated turbulence and an intensification of mixing and the entrainment phenomena associated with it. Equation (3.32) applies in such situations whereby m0 might be increased by the factor 0/a owing to the presence of suspended solid so that the effective change in air entrained per unit volume of jet fluid might increase by a factor of (0/a)2. When the particles are not small enough to behave like a homogeneous fluid, a relative motion occurs between the particles and the surrounding air as a result of gravity or as a result of inertial forces resulting in the damping of the turbulence since the drag between the dust and the air will extract energy from the turbulent fluctuations. One important estimate is the distance at which a particle in a particle-laden jet will travel before coming to rest. This distance is defined as the range , a product of the initial velocity of the particle and the relaxation time R:
The relaxation time is defined here as the time taken for the relative velocity between particle and gas to fall to 36.8% of its initial value. For a perfect spherical particle, the relaxation time is defined as
where m and rp are the mass and radius of the particle, respectively, and is the dynamic viscosity of the surrounding fluid. With these definitions, one can estimate that coal particles with a diameter of 80m injected at 60m/s will have a range of about 150cm, some 150 nozzle diameters for a 1-cm nozzle pipe, and will have little effect on the jet (Field etal., 1967). However, if the particles were finer, for example, 40m in size, then the range would only be 30cm, which would have a damping effect on the jet due to turbulent energy transfer. The relaxation time is a measure of the shortest timescale of turbulence to which the particle could respond. As mentioned earlier, smaller eddies would have rapid velocity fluctuations and the particles would not have time to accelerate to the velocities within the eddies. However, if the eddies are large, then the particles can follow the streamlines without any appreciable slip and the suspension would tend to behave as a homogeneous fluid. It has been shown that increasing the fluid temperature shortens the relaxation time and thereby reduces the size of the eddy to which particles respond. When it falls within the same range as the timescale of the eddies, some damping of the turbulence can be expected, thereby reducing the eddy viscosity (Field etal., 1967). The concomitant result will be a decrease in the rate of entrainment and the rate of spread of the turbulent jet.
The Paraho Development Corporation developed new vertical shaft kiln hardware and process techniques and confirmed new technology in the 1960s by building three large commercial lime kilns. In the 1970s the company adapted their lime kiln technology to oil shale retorting. Paraho obtained a lease from the US Department of the Interior in May 1972 for the use of the US Bureau of Mines oil shale facility at Anvil Points near Rifle, Colorado, to demonstrate their retorting technology. The indirect combustion mode bums process gas in a separate furnace and hot gases carry heat to the retort. This retort can also be operated in a direct combustion (Section 4.16).
In the indirect mode Paraho retort (Fig.14.5), the portion of the vertical retorting chamber that was used for oil shale combustion in the direct mode is now the region of the retort chamber into which externally heated fuel gas is introduced. No combustion occurs within the retorting chamber. That separate combustion process is typically fueled by commercial fuels (natural gas, diesel, propane, etc.) that are often augmented with a portion of the fuel gas recovered from the retorting operation.
In the process, finely-ground oil shale enters a feed hopper on top of the retort after which, in a continuous moving bed, the oil shale flows downward consecutively through the mist formation, retorting, combustion, and cooling zones. As the shale descends, heat is efficiently exchanged with a countercurrent flow of recycle gas, which is introduced into the retort at different levels by three specific-purpose gas-air and gas distributors. Near the top of the retort the ambient temperature shale is warmed by rising hot oil vapors and gas, which, in turn, are cooled to form an oil mist that is entrained in the gas.
While they are very similar in operation, the direct and indirect mode Paraho retorts offer sufficiently different operating conditions so as to change the composition of the recovered crude shale oils and gases. Oil vapors and mists leave the direct mode retort at approximately 60C (140F), while the vapors and gases in the indirect mode leave the retorting vessel at 135C (280F) and have as much as nine times higher heating values than gases and vapors recovered from the direct mode retort (102 and 885 Btu/ft3) oil vapor and mists recovered from the direct mode are diluted with combustion gases from the combustion of the spent shale at the bottom portion of the retort.
The characteristics of the recovered raw shale oil are somewhat different for the direct and indirect mode retorts, but each has characteristics similar to shale oils recovered from other retorts using similar shale heating mechanisms (direct vs. indirect). In addition, gases from the indirect mode retorts have much lower levels of carbon dioxide but generally higher levels of hydrogen sulfide, ammonia, and hydrogen, which are thought to be the result of the indirect mode retort having much less of an oxidizing environment than the direct mode retort (EPA, 1979).
Smelt is cooled and dissolved in water. Hydroxide sodium is regenerated from sodium carbonate reacting with calcium hydroxide in causticisers. The calcium carbonate produced is regenerated in the lime kiln by heating. Regenerated white liquor, containing sodium hydroxide and sodium sulphide, is then sent back to chip cooking.
Low-pressure steam demand for causticising is approximately 20MJ/ adt. Medium pressure (MP) steam demand for lime-kiln oil burners (atomizing steam) is approximately 20MJ/adt. Kiln fuel demand (heavy fuel oil, natural gas or bio-gas produced in a gasification plant using waste wood and bark) is approximately 2.02.8 GJ/adt. Total power demand, including clarifier, filters and the electrostatic precipitator for lime kiln, is approximately 40 kWh/adt.
Electricity is a wonderful utility, but can be dangerous if not approached carefully. There are three basic hazards that cause injury or death shock, arc-flash, and arc-blast. It is important to remember that even a small amount of current passing through the chest can cause death. Most deaths occurring for circuits of less than 600 volts happen when people are working on hot, energized equipment PLEASE DISCONNECT AND LOCK OUT ALL ELECTRICAL POWER BEFORE ATTEMPTING KILN REPAIRS!
An electrical shock is a current that passes through the human body. Any electrical current flows through the path of least resistance towards ground; if an external voltage contacts a human body, e.g. by touching a live wire with the hand, the voltage will try to find a ground, and a current will develop that flows through the bodys nervous system or vascular system, and exit through the closest part of the body to ground (e.g., the other hand which may be touching a metal pipe.) Nerve shock disrupts the bodys normal electrical functions, and can stop the heart or the lungs, or both, causing severe injury or death.
An arc-flash is an extremely high temperature conductive mixture of plasma and gases, which causes very serious burns when it comes into contact with the body, and can ignite flammable clothing. Arc temperatures reach up to 35,000F which is 4X the temperature of the suns surface!
Arc-blast is a pressure wave resulting from arcing, which can carry molten metal fragments and plasma gasses at very high speeds and distances. This can not only carry very hot shrapnel to injure a person, but can actually be strong enough to destroy structures or knock workers off ladders.
A special vent tube is available for the Rolling Stand. Be sure to use it instead of the vent tube that is used on normal stands. If you just feed the flexible aluminum through the hole in the stand it could tear.
A replacement DynaTrol control will typically have our standard configuration for Type K thermocouples, 18 Deg F thermocouple offset, and 3 zone control.THIS MAY NOT MATCH YOUR KILN! Here is what you need to check:
Welcome to The Lime Kiln! Located on the A372 between Langport and Podimore, we are a 17th-century cider house converted into a traditional country pub and restaurant with breathtaking views over the Somerset Levels.
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