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heat loss. The tarry products do not give trouble in gas producers when 
the gas is burned hot in ovens or furnaces. If however, the gas is allowed 
to cool, these products condense and stop up the piping, and unless re- 
moved, will clog up the engine valves if used for power purposes. The 
removal of these tarry products not only involves special and expensive 
apparatus and the expenditure of power, but also results in a loss of the 
available heat from the gas. 
The problem as here presented is the outgrowth of an attempt to adapt 
the suction gas producer to the use of bituminous fuel. The type of pro- 
ducer used is what is known as the re-circulating producer such as is rep- 
resented by the Whitfield and Pintsch patents. In this type of producer an 
attempt is made to draw off the tarry vapors from the top of the fuel 
column and introduce them agin into the fire at the very bottom of the 
producer. The finished gas is drawn from the central portion of the fuel 
column. This location must be chosen with at least three points in mind: 
(1) It must be far enough down in the fuel column to be below the 
point at which the last trace of tar is driven off from the coal. 
(2) It must not be any farther down than is necessary, or the loss 
due to the sensible heat in the gas will be excessive. This loss, in per- 
centage of the total heat value of the coal, will equal approximately the 
number of hundreds of degrees F. at which the gas leaves the producer. 
Thus, if the gas leaves at 1,200°F., the loss will approximate 12 per cent. 
of the total heat value of the coal burned. 
(3) The point of exit must be high enough above the bottom to 
allow ample opportunity for the CO, and H.O resulting from the com- 
bustion of the distilled volatile matter to be reduced to free H, and CO. 
To fulfill these several requirements, it is necessary to know exactly 
when each factor is operative. The depth of the incandescent zone that is 
necessary for a producer of a given size and capacity, and the precautions 
that are necessary to prevent a concentration of draft at any part of the 
producer, are fairly well known from practice. The most important item 
that is left for investigation is therefore to ascertain the exact tempera- 
ture at which the last trace of tar is driven off from the coal. 
An attempt was made to follow the temperature conditions met with 
in the gas producer, in these laboratory tests. This made it necessary to 
place the following list of requirements on the laboratory apparatus: 
(1) The coal must be heated very slowly and at a uniform rate. 
(2) The heat must be conducted from the outside to the center of 
