Method and Apparatus for Producing Gas From Solid Municipal Waste

Mansfield APPARATUS AND METHOD FOR PRODUCING GAS, filed concurrently herewith.

Distillation: Apparatus, Horizontal, Plural gas and/or air admission.

Mansfield U.S. Pat. No. 3,434,932; Kay 1,913,396; Wagner 3,126,846; and Kato et al 3,863,578; "Purox" and "Andco-Torrax" systems.

The broad objectives of the invention are three-fold, the first being to reduce solid municipal waste to sterile ash, second, to produce a hot low BTU gaseous by-product utilizable in adjacent boiler and, third, to avoid the formation of localized "hot spots" in the material being treated, thereby avoiding the formation of clinkers while avoiding unnecessary burning of the volatile matter in the material being treated. To accomplish the foregoing, the raw material is first transported horizontally in static-bed form through part of a hot carbonizing furnace, e.g., on a chain grate through which limited amounts of air are fed so as to maintain limited combustion of the material in the bed and to limit the temperature rise of the material, comparably with part of the coking process of Mansfield (supra). Within the same hot carbonizer furnace chamber is a horizontal reciprocating step grate upon which the partly carbonized material from the chain grate is dropped. As the then slow-burning material cascades down the step-grate, being crowded off one step onto the other by the reciprocating movements of the steps, hot pre-treated air is fed upwardly through the agitated material in closely-controlled, very limited amounts. Enough, but only enough burning is permitted, by close control of the airfeeds, to drive off hot-Btu gases from the material without burning any more of them than is necessary, while burning the hydrocarbons in the material so as to reduce it to sterile ash; and by keeping the material constantly on the move and shifting while it undergoes the hottest treatment, hot spots and resultant localized burning and formation of clinkers by glass and other meltable content of the material is avoided.

These and other objects will be apparent from the following specification and drawing, in which:

FIG. 1 is a diagrammatic vertical cross-section through the apparatus; and,

FIG. 2 is an enlarged fragmentary view illustrating the operation of the grate and air feed therethrough.

Referring now to the drawing, in which like reference numerals denote similar elements, the hot carbonizer furnace 2 has ceramic walls, a raw material infeed 6 which includes double air-lock gates 7 and a treated material outlet 8 which also has double air-lock gates 9 so that, as will be detailed hereinafter, a very limited and closely controlled feed of air to the material being treated can be maintained. Low Btu gas at about 1800.degree. F is exhausted via the flue 10 to a furnace chamber 12 for a boiler 14. An air feed 15 is provided for burning with the hot Btu gases which are exhausted via a conduit 16. Part of the still hot spent exhaust gases are bled off and fed back via a line 18 to the input end of the furnace for preheating and driving the moisture off the incoming material as described hereinafter. The remainder of the hot spent exhaust gases are cooled while passing through a heat exchanger 20 to the atmosphere. Atmospheric air which is heated in the heat exchanger 20 is fed via a line 22 to a manifold 24 and thence through control valves 26 to the zones 28 of an airbox from which air is fed upwardly through a horizontal chain grate 30 to the material being transported thereon through the furnace. Other of preheated air is fed to a manifold 32 and thence through control valves 34 to the zones of an airbox 36 which underlies a reciprocating-step stoker grate 38, on which final treatment of the material takes place.

The raw material incoming to the furnace is deposited on chain grate 30, on which it is spread by a vertically adjustable conventional spreader gate 40 to form a bed, preferably about four feed in thickness. However, before passing beneath spreader gate 40 the incoming material is preheated and all the moisture is driven off by hot spent exhaust gases from line 18 which are fed downwardly through a flue 42 and down-drafted through the incoming material. Valves 44 control the flow of downdraft gases to a manifold 46 from which they are drawn by fan 48 and fed through an electrostatic precipertator 50 to the atmosphere.

As the material progresses through the furnace chamber on chain grate 30, sufficient preheated air is fed upwardly therethrough from the air box zones in quantities to ignite the bed completely through, from top to bottom, by the time the material reaches the end of the chain grate, so as to drive off volatile matter from the material in the form of low BTU gases. Solid municipal waste is known to have a value of about 5000 Btu per pound. By closely controlling the air feed through the air box zones, in both the chain grate and the stepped stoker grate phases of the treatment, the formation of zones of intense burning in the material is avoided, this being desired so that the glass and metal content of the material do not melt and form clinkers.

The incandesent material dropping off the end of chain grate 30 cascades downwardly onto the reciprocating ceramic grate steps 52 which have air passages 54 for transmitting air from the airbox zones 36 to the material cascading down the steps. The ceramic steps are preferably provided with metallic bearing surfaces 56 so as to reduce wear as they reciprocate upon one another and upon supporting cross beams 58. The ceramic steps are reciprocated by rotating crank shafts 60 which they are connected by rods 62 which extend from gear 64 on the steps to the throws of the crank shafts 60. Stokers of this general type are known in the art. In this case, the air feed to airbox zones 36 is controlled so as to complete the burning of the solid material and reduce it to sterile ash. However, this is a "starved" air feed so as to reduce to the greatest extent possible the burning of gases within the furnace 2 and thereby preserve the burnable gases for combustion in the boiler chamber 12.

Screw conveyors 66 are provided in the lower corners of the zones of airbox 36 so as to carry off ash residue which works its way through the reciprocating steps of grate 38.

Assuming a value of about 5000 Btu per pound of solid municipal waste entering the furnace, about 120,000 pounds of steam per hour is recoverable, assuming an overall conversion efficiency of about 73.6% to usable steam, or about 92% to a low Btu gas. A boiler efficiency of about 80% is assumed for converting the low Btu gas to steam. The make-up of the low Btu gas is as follows:

The solid material emerging through outlet 8 is in the form of sterile refuse which can easily be disposed of. The gases exhausted to the atmosphere are cool and relatively devoid of noxious odors or gases as compared with incinerators, and the capital cost of the system is sufficiently low as to render it economically feasible. It is estimated that a plant wherein the grates are about 12 feet wide and wherein the chain grate is about 15 feet long running at 10 to 15 inches per minute can handle about 400 tons of solid municipal waste refuse per 24 hours.

Alternate fuels, such as bagasse, lignite, wood chips, peat, sub-bituminous and bituminous fuels may be used in the apparatus. Where moisture content of the starting material is so low to present no problem of reporting into the output gas stream, the downdrifting step may be eliminated.