Biofuel for the 1st stage
In modern cement kilns, the rotary kiln itself operates at a high temperature, from about 900 C and upwards of 1500 C in the process of forming clinker. Finely ground clinker is the cement we are familiar with. ..But before the kiln is a tower pre~heater (precalciner) that goes from ambient temperature up to circa 900 C. ..Here a biofuel could be employed.
Calcium butanoate would be useful replacing much, if not all, of the limestone. ..At a high temperature this salt breaks down to calcium oxide (CaO) plus one CO2 molecule and combustible gases equivalent to the ketone it otherwise may form. Limestone merely breaks down into CaO and CO2.
It is important to recognise that the CO2 emitted is from the biofuel and thus is biological in origin, whereas that from limestone is not. When the CO2 from the ferment to butanoic acid, neutralised by limestone to obtain its calcium salt, is sequestered, this includes CO2 from the limestone. Clearly this way of sequestering the CO2 from limestone is far cheaper than doing so from the preheater of the kiln at a cement plant.
Calcium butanoate is readily obtained by direct fermentation of agricultural material such as straws, maize cobs and the like. According to an old British Patent [565,773] this can be a septic ferment using a mix of unselected wild microbes plus one butanoic producing bacteria, which benefits from the enzymes excreted by the others. One which converts lactic acid directly to butanoic acid would be especially suitable. Such a bacteria which does so in preference to any other energy source (even glucose) is described in an old USPatent [4138.498].
Given time to develop, the endpoint of the ferment will be almost all butanoic acid. It is easily enough isolated as its Ca salt which is not very soluble. As instance, adding NaCl or KCl or sea salt to the batch ferment at its conclusion should be sufficient to cause the Ca butanoate to drop out. It is then obtainable by filtration or centrifuging and dried,.. or handled as a slurry. (Other means of forcing it to crystalise may instead be used.)
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