Saturday, June 11, 2016

Historical Manufacture of Charcoal - II

In our last post, we looked at the charcoal manufacturing process, as it was done from the 14th to the early 20th century. In today's post, we will look at some variations of the process.

As we saw in the last post, charcoal manufactured for the purposes of gunpowder had to be of a higher quality with uniform charring. Therefore it was manufactured in smaller batches using iron cylinders to heat the wood, instead of heating up large heaps. Our last post also described the process in England, where they used small iron cylinders, each holding about 80 lbs. of wood, being placed inside a furnace and heated. In today's post, we will study some variants of this basic method.

Instead of using fixed carbonizing cylinders, many black powder factories in England started switching towards using movable cylinders in the 19th century. Each furnace was provided with two cylinders, so that one could be refilled while the contents of the other were being carbonized. Each filled cylinder would be run into the furnace on rails, with the rails supporting them over the fire. An elaborate system of pipes and valves was used to distribute the gases and the wood distillation byproducts (wood gas, tar, volatile chemicals etc.), so that they could be redirected back to any one of the furnaces, or allowed to escape through the chimney.

The advantages of this process were:

  1. Uniformity of the charcoal being produced.
  2. The gases produced by distilling the wood could be reused to additionally heat the furnace, thereby saving on fuel costs
  3. The charcoal was cooled down out of contact with the air, which took away the possibility of the charcoal catching fire.
In some British factories, vertical movable cylinders were used instead. The advantages of this were that more cylinders could be fired at the same time and the moving of the cylinders to the cooling room was easier.

In Sweden, some factories used rotating cylinders, with each cylinder being rotated 90 degrees on its horizontal axis every 30 minutes. This allowed the heat to act upon each side evenly and this process gave a more uniform carbonization and saved fuel as well.

Another method of carbonizing wood used superheated steam to do the job. Pressurized steam was produced by passing water through a coil of wrought iron heated by a fire. For the production (from dogwood) of charbon roux (brown charcoal) containing 70% carbon, the temperature of the steam had to be around 280° Fahrenheit; by using steam heated to about 350°, charcoal containing about 77% carbon could be produced, and by heating both the cylinders and the steam to about 450° fahrenheit, charcoal of about 89% carbon content could be produced. The charcoal produced by this method was very uniform in composition, but the method was later abandoned because it gave a larger yield of charbon roux, but not so much black charcoal, as the ordinary method of carbonization using iron cylinders; and the lightly-burnt charcoal was only required for sporting powders. Also, the cost of production of charcoal using superheated steam apparatus was greater.

In 1887, one Mr. H. Guttler of Reichenstein, Germany, invented a process of carbonizing wood (he received British patent # 8929 on June 22nd, 1887 for his idea "Improvement in the Manufacture of Charcoal for Explosives and other Purposes, and Apparatus for that Purpose"). His idea consisted of putting the material to be carbonized into a suitable air-tight cylinder fitted with a pressure gauge and pyrometer and using an arrangement of two furnaces to heat it. One is a normal charring furnace and the other is a producer-furnace, which produces carbon dioxide gas by blowing air through burning coke using a fan. The heated carbon dioxide gas is then piped into the cylinder (similar to the superheated steam process we saw above) and the carbonization takes place. The pressure of the carbon dioxide in the cylinder can be varied as needed. The temperature is regulated by admitting cold air to the muffle and by varying the supply of heated gas into the cylinder. After the charring is completed, cooled carbon dioxide is passed through the charcoal, which rapidly cools and absorbs the carbon dioxide in its pores. The advantage of this process over using superheated steam was that it didn't leave the charcoal produced in a moist state, which the steam process did. It could also be used to produce charcoal from cheaper materials such as wood cuttings, pulp, straw, peat etc. Another advantage was that since it used carbon dioxide instead of air, the charcoal produced could not spontaneously ignite.

In the next post, we will look at the historical production methods of the third ingredient of gunpowder: sulfur.

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