History of Manufactured Gas - Appliances and Machinery of The Historic Gas-works - Retort Bench

The retort bench was the construction in which the retorts were located for the carbonization (synonymous with pyrolysis) of the coal feedstock and the evolution of coal gas. Over the many years of manufactured gas production, advances were made that turned the retort-bench from little more than coal-containing iron vessels over an open fire to a massive, highly efficient, partially automated, industrial-scale, capital-intensive plant for the carbonization of large amounts of coal. Several retort benches were usually located in a single "retort house", which there was at least one of in every gas-works.

Initially, retort benches were of many different configurations due to the lack of long use and scientific and practical understanding of the carbonization of coal. Some early retorts were little more than iron vessels filled with coal and thrust upon a coal fire with pipes attached to their top ends. Though practical for the earliest of gas-workings, this quickly changed once the very early gas-works served more than a relatively few customers. As the size of such vessels grew – the need became apparent for efficiency in refilling retorts – and it became known that though while filling one-ended vertical retorts may be easy – removing the coke and residues from them after the carbonization of coal was far more difficult than filling them was. So gas retorts were transitioned from vertical vessels to horizontal tubular vessels.

Retorts were usually made of cast iron during the early days of manufactured gas. Early gas engineers experimented extensively with the best shape, size, and setting to adopt. No one form of retort initially came to dominate, and many different cross-sections remained in use; however, after the 1850s, retorts generally became made of fire-clay due to greater heat retention, greater durability under heat, longer life with proper care, and other positive qualities. Cast-iron retorts were still used in the small gas-works, due to their compatibility with the demands of such, with the cast-iron retort's lower cost, ability to heat up quickly to meet transient demand, and "plug and play" replacement capabilities outweighing the disadvantages of shorter life-time, lower temperature margins, and lack of ability to be manufactured in non-cylindrical shapes. Also, general gas-works practice following the switch to fire-clay retorts favored retorts that were shaped like a "D" turned 90 degrees to the left, sometimes with a slightly pitched bottom section.

With the introduction of the fire-clay retort, higher heats could be held in the retort benches, leading to faster and more complete carbonization of the coal within. As higher heats became possible, advanced methods of retort bench firing were introduced, catalyzed by the development of the open hearth furnace by Siemens, during a period from around 1855–1870, leading to a revolution in gas-works efficiency.

Specifically, the two major advances were:

• The introduction of the "indirectly fired" retort bench. The early "directly fired" retort bench consisted of retorts suspended over a coke fire, which heated the retorts and drove the carbonization of coal within to coke, and the evolution of gas. The introduction of indirect firing changed this – instead of the retorts being heated directly by the fire – the fire was placed a ways below and to one side of the retorts, brought to a very high heat, while the air supply was reduced and a small amount of steam introduced. Instead of evolving large quantities of heat to directly heat the retorts, the fire now evolved heated gasses – specifically carbon monoxide and – due to the steam – a small amount of hydrogen gas as well, which are both highly combustible. These gasses rise from the fire into a channel which soon brings them to the "tuyeres" – small holes similar to "nostrils", located adjacent to the retorts, which shoot the "furnace-gasses" out of them. Adjacent "tuyeres" emit a large amount of "secondary air", which is preheated air, that, upon mixing with the furnace gasses, causes them to ignite and burst into flame and bathe the exterior of the retorts in heat.
• The introduction of heat recuperation for the preheating of the air of primary and secondary combustion. By causing the exhaust of the retort-bench to pass through a labyrinthine maze of refractory brickwork, substantial quantities of heat can be extracted from it. On the other side of the exhaust channels are channels for the passage of the air of combustion. The bricks thus transfer the heat of the exhaust to the air of combustion, preheating it. This provides for a much greater degree of thermal efficiency in the retort-bench, causing it to be able to use far less coke – as air that is preheated by waste heat is already hot when it enters the fire to be burnt, or the "tuyere" to fuel secondary combustion.

These two advances turned the old, "directly fired" retort bench into the advanced, "indirectly fired", "regenerative" or "generative" retort bench, and lead coke usage within the retort benches – at least in the larger works – to drop from upwards of 40% of the coke made by the retorts to factors as low as 15% of the coke made by the retorts, leading to an improvement in efficiency of an order of magnitude. However, these improvements imparted an additional capital cost to the retort bench to incorporate them, which caused them to be only slowly incorporated in the smaller gas-works, if they were incorporated at all.

Further increases in efficiency and safety were seen with the introduction of the "through" retort, which had a door at both its front and its rear. This provided for greater efficiency and safety in loading and unloading the retorts, which was a labor-intensive and often dangerous process. Coal could now be pushed out of the retort – rather than pulled out of the retort. One interesting modification of the "through" retort was the "inclined" retort – coming into its heyday in the 1880s – a retort set on a moderate incline, where coal was poured in at one end, and the retort sealed; following pyrolysis, the bottom was opened and the coke poured out through means of gravity. This was adopted in some gas-works, but the savings in labor were often offset by the uneven distribution and pyrolysis of the coal as well as clumping problems leading to failure of the coal to pour out of the bottom following pyrolysis that were exacerbated in certain coal types. As such, inclined retorts were rendered obsolescent by later advances, including the retort-handling machine and the vertical retort system.

Several advanced retort-house appliances were introduced for improved efficiency and convenience. The compressed-air or steam-driven clinkering pick was found to be especially useful in removing clinker from the primary combustion area of the indirectly fired benches – previously clinkering was an arduous and time-consuming process that used large amounts of retort house labor. Another class of appliances introduced were apparatuses – and ultimately, machines – for retort loading and unloading. Retorts were generally loaded by using an elongated scoop, into which the coal was loaded – a gang of men would then lift the scoop and ram it into the retort. The coal would then be raked by the men into a layer of even thickness and the retort sealed. Gas production would then ensue – and from 8 – 12 hours later, the retort would be opened, and the coal would be either pulled (in the case of "stop-ended" retorts) or pushed (in the case of "through" retorts) out of the retort. Thus, the retort house had heavy manpower requirements – as many men were often required to bear the coal-containing scoop and load the retort.

(TBD: Brief description of advanced retort loading apparatus; more detailed description of retort-handling machine.)

Coming soon: The introduction of the coke-oven system, and, finally, the vertical retort system.