Traditional building mortars were composed of rich mixes of hydraulic lime and aggregate. Mix proportions of 1:2½ to 1:3, sometimes those as rich as 1:2, were generally used. These hydraulic limes resulted from the burning of quarried material that was predominantly calcareous, e.g. chalk or limestone, but that contained silicious and/argillaceous or clay-like impurities. Modern lime production relies on winning product that is substantially free from clay contamination but historically, clay present in the calcareous feed stock resulted in the formation of calcium silicates that had the potential to react with water to produce a hydrated material, thus the nomenclature ‘hydraulic’ limes. The degree of hydraulicity in these limes varied greatly and some of the purer sources developed little hydraulic strength, relying instead on carbonation for minor strength development. Hydraulic limes are now manufactured in many European countries although often on a small scale, sometimes almost as a cottage industry. Hydraulic limes manufactured on the South Coast of England from the blue limestones found in parts of Dorset are termed blue lias limes and have a long record of use in masonry. The European Standard for lime permits the manufacture of materials known as artificial hydraulic limes Learn More
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These materials are not hydraulic limes at all but are mixtures of limes, usually pure or relatively pure substantially non-hydraulic materials, with cement and air entraining agent. Their name is misleading and they are often specified or used erroneously in the belief that they have a natural property that is in some way desirable. In reality, however, they only represent mixtures of binder and air entraining agent and do not possess the qualities of longer and slower strength gain associated with naturally occurring hydraulic limes
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 30–40 range with lengths of 75–500 ft (22.7–152.5 m) and diameter of 4–11 ft (1.2–3.3 m). Lime kilns are usually inclined at about 3°–5° slope with material charged at the elevated end and discharging at the lower end. The degree of fill is relatively deep, about 10–12%. Owing to its low thermal conductivity, limestone with a large diameter of about 2 in (5 cm) 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 (Figure 10.4) with thermal consumption as high as 12–15 million Btu/ton (3336–4170 kcal/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 6–8 million Btu/ton range (1668–2224 kcal/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 25 m/s and a high of about 60 m/s. These are significantly lower than the velocities of cement kilns, which operate around 80–100 m/s. The momentum ratio and associated Craya–Curtet 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 Figure 10.5
As already mentioned, power-to-gas systems allow for the option of producing methane by reducing carbon dioxide using the hydrogen generated. Thus, the term “power-to-gas” always implies a hydrogen path and the option of an additional methane path
In general, there are numerous sources and separation technologies available for the provision of CO2 for this methanation process, some of which are described in IPCC  or Li et al. . A large amount of CO2 can be obtained from the combustion of fossil fuels or renewable resources in power plants. Postcombustion, precombustion, oxyfuel, or chemical-looping technologies are used for the separation of the CO2. Depending on the separation technology a certain amount of heat from the power plant is needed, thus increasing the consumption of primary energy. This leads to an overall decrease in efficiency of the power plant by (7–10)%. Furthermore, between (20–40)% more primary energy per generated kilowatt-hour is needed [20,21]
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