Solvents calorific values

Gallwitz (Ref 16) reports the following data on the influence of the heat of explosion upon the bore wear. With a nitroglycerine powder containing no solvent and giving a heat of explosion of 950 kcal, the barrel stands up to 1700 rounds while with a similar powder giving a heat of explosion of 820 kcal, it withstands 3500 rounds. The reduction of the calorific value of the powder by 130 kcal therefore doubles the useful life of the barrel... 

The rotary kiln design allows for accepting a mix of high-chlorinated wastes (solvents, chlorinated tars, plastics). Such kilns are usually designed in relation to a specific optimal calorific value in the input. The input mix should be set in such a way that this optimal composition is approached (e.g., PVC waste and other waste streams with a lower calorific value). It is likely that a 100% input of PVC would lead to all kind of problems of temperature control due to its relatively high calorific value. Chlorine contents of over 50% can easily be accepted. A final demand is that the particle size should be 10 x 10 x 10 cm at maximum. This implies that sometimes waste has to be shredded before it can be put into the kiln. Other acceptance criteria have not been published in literature. 

One of the most satisfactory uses of waste lubricants is as a fuel extender in cement production, subject to control over metal content. The advantages of this disposal route are the low-cost, or free, calorific value of the waste as a fuel to the cement manufacturer and the wastes which might otherwise be vented to the atmosphere with the flue gases are retained within the cement product with no adverse environmental consequences. There are, of course, limits to the cement production capacity which can use this waste as a fuel, particularly as cement kilns are seen as the ideal disposal route for other hazardous wastes such as contaminated solvents. The capacity of cement plants to dispose of combustible wastes is also dependent upon the level of demand for cement, which reduces when the building industry is in recession. 

Burning of used solvents usefully as a fuel for cement manufacture or as support fuel for an incinerator can be justified logically particularly for hydrocarbon-based solvents since they are the cheapest and have high calorific values. When used as a fuel, hydrocarbons are only used once unlike their use as a solvent with subsequent use as a fuel. 

About 15 years ago the use of cement Idlns to destroy in an environmentally satisfactory way used solvents while, at the same time, using their calorific value became established. In the USA solvent recoverers were the natural collecting point to make suitable fuel blends and to incorporate in these blends the residues they had from the refining of the more valuable solvents. 

A minimum temperature of 1100°C, combined with a residence time of 4 s, is needed in the high-temperature zone to ensure satisfactory destruction of organics. To achieve the minimum temperature a lower calorific value of about 5000 kcal/kg (9000BTU/lb) is needed for the feed. This is easily reached for hydrocarbons and other solvents containing little water. However, if much water is present, additional support fuel may be necessary. 

In all cases the heat of combustion of the solvent to be destroyed needs to be known. In almost every case the water generated in the destruction will be discharged as water vapour and so the lower or net calorific value is the appropriate one to use and it is the one quoted here. The only common solvent which has no hydrogen to convert to water and therefore has identical higher and lower calorific value is carbon disulphide. 

Combustible The net heat of combustion and the calorific value help to estimate the potential energy which can be recovered from burning used solvents. In addition, the composition of the combustion products is considered to evaluate potential corrosiveness and the effect on the environment. 

The net calorific values for acetone, methanol and IPA is 6,962, 5,238 and 7,513 kcal/kg, respectively, and the simple mean value of their mixture is 6,600 kcal/kg. Let us assume that solvents are generated from a process at a maximum rate of 160 kg/h. The total heat generation of the VOC oxidation unit is as follows ... 

Production speeds for coil lines can be as high as 120m/min for steel with peak temperatures during cure of up to 250°C for periods of up to 1 minute. Such curing schedules require that low volatility solvents are used. Most coil coaters currently use thermal oxidisers to recover the calorific value of the released solvents. This plays an important role in the economics of such a high-temperature curing process. 

It is surprising to many that the method of application can have such a large effect on odour. Even with waterborne systems, incineration may still be required to reduce or remove odour. In this instance, it may not be economically viable to use waterborne systems, because solvents have calorific values which means that they can fuel or at least partially fuel an incinerator, thereby reducing the need for expensive fuel oil. It may be that a smaller incinerator can be used with waterborne systems compared to a solvent based system, with associated lower capital costs. The overall comparisons are not straightforward and all factors must be considered in the overall cost equation. 

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