Coal, slow combustion

It has been suggested that, just as in the combustion of carbon and the slow combustion of hydrocarbon gases, the first step in the slow combustion of coal consists in the formation of an additive compound or complex, consisting of oxygen and one or more of the substances present in coal.10... 

Coal char combustion Phenomenological aspect In the regime of "low temperature," the chemical reaction rate is slow compared with the diffusion through the pores, because the O2 completely penetrates the char matrix. In this case the rate controlling the regime of char combustion is kinetically limited. 

Spontaneous combustion may develop due to the heating of coal particles and slow combustion. Toxic material may be released with the oxidation of compounds present in the waste material. Runoffs, with minimal iron pyrites content, from the tip surface, promote hazards and the acid compounds may leach into domestic water supplies. All these effects may be partly or wholly overcome with proper management of the planning and design of waste heaps and good operating practices. Moreover, encouragement of the commercial use of waste material is beneficial. 

Ib., 1808, xix, 148-53 (dated 10 January) Phil. Trans., 1808, xcviii, 282 Description of an Apparatus for the Analysis of Compound Inflammable Gases by Slow Combustion with Experiments on the gas from Coal, explaining its Application. 

The irradiated coal sample is placed in a ceramic combustion boat with added mercuric oxide carrier and burned in a slow stream of oxygen in a simple apparatus consisting of a straight quartz combustion tube connected to a straight condenser surrounded by a trap of liquid nitrogen. The ash and the tube are then heated to about 800°C to drive all volatile material into the cold trap. The products are dissolved in nitric... 

Thus the reaction rate sought is not constant and, for example, in very wide tubes (underground vaults) the combustion energy of coal dust may be used in the wave, although in laboratory equipment combustion of dust proves so slow that detonation is out of the question. 

In both methods it is possible to lose some of the chlorine during combustion unless necessary precautions are taken. Thoroughly mixing the coal sample with Eschka mixture and covering this carefully with additional Eschka mixture minimizes the loss of chlorine. In bomb combustion methods, the ammonium and sodium carbonate solutions in the bomb are used to absorb the chlorine as it is released in the combustion. The 10- and 15-minute waiting periods and the slow release of pressure on the bomb help to prevent the loss of chlorine as well. 

The velocity of the combustion front is determined by the volatile matter content of the raw coal, Wv. If the volatile matter content of the coal is high the frontal velocity will be slowed. With low volatility coals the frontal velocity will be relatively fast for the same excess energy production in the com-bustion/gasification regions of the bed. The velocity vc is set by the requirement that the volatiles content of the coal ahead of the devolatilization zone be essentially that of the raw coal. From an analysis of the energy balance in the region near the front of the devolatilization zone, it can be shown that a particular root of the relation below establishes v (3) ... 

Pulverizers The pulverizer is the heart of any solid-fuel suspension-firing system. Air is used to dry the coal, transport it through the pulverizer, classify it, and transport it to the burner, where the transport air provides part of the air for combustion. The pulverizers themselves are classified according to whether they are under positive or negative pressure and whether they operate at slow, medium, or high speed. 

Coal gas is the product of slow distillation of coal with exclusion of air. Blast-furnace gas results from the combustion of coal in a restricted supply of air its ideal composition would be, by weight, CO, 34.4 N, 65.6. Water gas is made by the action of steam on incandescent coal, and should consist ideally of H and CO only the process is necessarily intermittent. Producer gas is a combination of blast-furnace gas and water gas, the supply of steam being so limited as to permit of a continuous process. In all of the manufactured gases, the hydrocarbons (CH4, CeHe, C2H4) come from the coal either directly or as the result of a breakdown of other hydrocarbons. The hydrogen in coal gas and blast-furnace gas has a similar origin. The CO and CO2 in coal gas are due to a partial (and objectionable) combustion of the carbon in the coal. 

Probably the rotary horizontal kiln is the most versatile, since it allows a feed of lumps or fines of limestone or marble, or wet or dry calcium carbonate sludges (Fig. 7.1). The main component of this calcination system is a 2.5- to 3.5-m diameter by 45- to 130-m long firebrick-lined inclined steel tube. Heat is applied to the lower end of this via oil, gas, or coal burners [7]. The feed to be calcined is fed in at the top end. Slow rotation of the tube on its axis gradually moves the feed down the tube, as it tumbles countercurrent to the hot combustion gases. In this way, wet feed is dried in the first few meters of travel. Further down the tube, carbon dioxide loss begins as the temperature of the feed rises. By the time the solid charge reaches the lower, fired end of the kiln it reaches temperatures of 900-1,000°C and carbon dioxide evolution is virtually complete. Normally the temperature of the lower end of the kiln is not allowed to go much above this as it reduces the life of the kiln lining. It also adversely affects the crystal structure of the lime product since it produces a dead-burned or overburned lime. Overburned lime is difficult to slake to convert it to calcium hydroxide and raises... 

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