This article was originally published in Electrochemical and Metallurgical Industry Publication of October 1907. Information within this article is therefore correct as of 1907. The publication of this material aims to provide historical insight on the subject and its place in industry.
In coal-fired muffle furnaces the combustion products escape at very high temperature after heating the muffle, which means a considerable loss of heat. Producer gas firing is much more economical in such a furnace, as the waste heat can be used advantageously for preheating the combustion air.
Figures 1,2,3 - Producer Gas Fired Muffle Furnace
Furthermore, the sensible heat of the brick walls can also be utilized for preheating air by providing suitable air slots.
Figs. 1, 2 and 3 show a producer gas-fired muffle furnace. Fig. 1 shows a vertical section, Fig. 2 a horizontal section through the muffle proper, Fig. 3 a horizontal section through the recuperating apparatus.
The furnace shown in the illustration is provided with two producers. The gas goes through openings (o) into the fire space (f) surrounding the muffle. At the entrance the gas is mixed with hot air, which is led to these points through channels between the gas channels from the recuperator. The gas flame rises upwards, plays around the muffle arch and goes downward, heating the bottom of the muffle. Then the products of combustion-still very hot-go through pipes (r) to the flues (ee) leading to the stack. The pipes (r) are built into the wider channels, into which air is led through channels (m). This air plays around the pipes (r) opposite to the flame direction, and is heated by absorbing through the pipe walls the heat of the combustion products. The highly preheated air finally goes through channels (n) into pipes (l), which lead to the fire space and are arranged between the gas pipes.
For effecting a uniform heating of the muffle, the producer gas can be led into the fire space also at a second and third place, and the sensible heat of the brick walls utilized for preheating the air necessary for oxidizing this quantity of gas. In the construction shown in the illustration, at half the way of the fire gases around the muffle, producer gas and preheated air is again allowed to enter. The air necessary for burning this gas enters through openings regulable by slides into the air slits, arranged behind the fire-brick wall of the furnace. The producer gas entering at the second place is also taken from the producers.
Image Credit: The Library of Congress