GB2092283A - Boiler System for Energy Recovery from Refuse and Coal - Google Patents

Abstract

A unified combustion system is provided for energy recovery from refuse and coal in which refuse is partially burnt by the starved air principle in a primary zone comprising a refuse fired furnace, the combustible gases evolving from this furnace being ducted and burnt completely within a secondary zone comprising the integral furnace of an adjacent boiler, itself burning coal on a grate.

Description

SPECIFICATION Boiler System for Energy Recovery from Refuse and Coal This invention relates to a boiler system capable of generating efficiently either steam or hot water from single or dual firing on refuse and coal. The system design permits any proportion of the total energy output from an installation to be supported by refuse as fuel.

The essence of the invention lies in its unified combustion system, as follows. A water tube boiler, having a comparatively large combustion chamber integral with the heat exchange surfaces, is utilised to burn coal mechanically and conventionally fed to the boiler. A separate furnace, operating on the starved air principle, is utilised to burn refuse mechanically or manually fed to the furnace. Volatile matter evolving from the refuse has had an inadequate supply of primary air for its combustion and, consequently, is rich in combustible gases still; these gases are ducted from the refuse fired furnace to the combustion chamber of the water tube boiler wherein additional secondary air supplied to the boiler ensures their complete burnout along with any volatile matter evolving from coal.

The starved air combustion principle, embodied in this invention for burning. refuse, always utilises two combustion zones designated primary and secondary. The primary zone contains the refuse fuel bed whose combustion takes place in a quiescent atmosphere deficient in oxygen, primary air being around 70-80% of stoichiometric requirement. This quantity of air is sufficient to totally oxidise the fixed carbon contained in the refuse but only partially oxidise the volatile matter content evolving at the high temperature of the combustion reactions in the primary zone.Leaving the primary zone are, consequently, clean combustible gases at a temperature or around 600-8000C. Their combustion in the separate and distant secondary zone, in this case the combustion chamber of the water tube boiler, with additional air elevates their temperature to around 1 2000C. Thus the starved air combustion principle embodies very low gas velocity in the primary zone, avoiding particulate matter carry over into heat exchange surfaces of the boiler, and complete combustion in the secondary zone, avoiding smoke formation. This invention differs materially from other furnaces operating on the starved air principle by token of its secondary zone being integral with the combustion chamber of a separate water tube boiler, and not integral with the furnace itself.

Consequently, the principles embodied in this invention constitute a truly unified combustion system.

The principle of the secondary zone of the refuse fired furnace being integral with the combustion chamber of the boiler has additional merit, as follows. Once the combustion chamber of the boiler is up to operating temperature, the presence of a coal bed within this combustion chamber is not essential to the efficacy of the unified combustion system. Consequently, the invention permits little or much coal to be burnt simultaneously with refuse, thereby permitting any proportion of the total energy output from an installation to be supported by refuse as fuel.

Moreover, as long as refuse is burnt in its furnace, a radiant flame is always present within the combustion chamber of the boiler.

Consequently, grades and types of coal which are comparatively difficult to burn when fired singularly, e.g. low grade anthracite of low volatile matter content, may now be fed to a grate within the boiler to be favourably exposed to radiation from the aforementioned flame which greatly assists the ignition, combustion, and burnout without smoke of such coal. This invention thereby permits hitherto unsaleable coal to be usefully employed to generate steam or hot water.

Lastly, this invention allows a modular approach to energy recovery from refuse and coal.

Its flexibility affords an initial low cost installation consisting of a single boiler and furnace to which, at subsequent times, additional refuse fired furnaces may be added, along with any other system refinements. Thereby, a heat recovery system tailored to the energy demand can be built up gradually, in the light of the availability and cost of refuse and coal, in the light of the most cost-effective seasonal operation of the installation, and in the light of experience and confidence gained particularly when making the transition from oil or gas to solid fuel firing.

Mating of two starved air refuse fired furnaces to a single coal fired boiler is illustrated in Fig. 1, a plan view of a typical installation. The system shown in Fig. 1 shows a single conveyor belt conveying refuse to hoppers feeding each of the two refuse fired furnaces, diverters in turn directing the refuse into whichever hopper requires replenishing. Coal is likewise conveyed to the hopper feeding the boiler, in this example the same conveyor belt as carries the refuse being used on a batch basis to a Fraser type water tube boiler. Shown in Fig. 1 is the duct from the refuse fired furnaces conveying combustible gases from the primary zone to the secondary zone integral with the combustion chamber of the Fraser boiler.

Also shown in Fig. 1 is the ash conveyor continuously removing ash from both the refuse fired furnaces and the coal fired Fraser boiler.

Each refuse fired furnace employs a stepped hearth shown in Fig. 2. A battery of rams at the base of the each refuse filled hopper pushes intermittently refuse onto the stepped hearth, whereupon the refuse gently tumbles down the hearth exposing material for activation by oxygen.

At the same time, the fall of material is controlled else the disturbance may cause undue ash and other particulate matter to be carried over in the gas stream into the secondary zone integral with the Fraser water tube boiler. Air flow through each furnace is induced to maintain a depression within the furnace and prevent fire entering the refuse hopper. Air entering each furnace is preheated by counterflow with ash leaving.

Throughout this specification mention has been made of a water tube boiler used in conjunction with the unified combustion system which is the claim of this invention; particular reference has been made of the Fraser water tube boiler. However, this invention requires only that the combustion chamber volume, integral with the boiler. is sufficiently large to efficiently burnout all volatile matter. Boiler types other than water tube may have an adequately large combustion chamber and, consequently, this specification embraces all boilers possessing an integral combustion chamber which can suitably serve as the secondary zone of a starved air refuse fired furnace coupled to the boiler, essentially as embodied and described above.

The above specification claims a starved air refuse fired furnace fitted with a stepped hearth and fed with refuse from a hopper by a battery of rams. However, the principle of the unified combustion system claimed is served when other types of furnace operating on the starved air principle, e.g. rotary kiln and Dutch oven, are used. Consequently, this specification embraces all refuse fired furnaces operating on the starved air principle, essentially as embodied and described above.

Claims (3)

Claims

1. A unified combustion system for energy recovery from refuse and coal in which refuse is partially burnt by the starved air principle in a primary zone comprising of a refuse fired furnace itself, the combustible gases evolving from this said furnace being ducted and burnt completely within a secondary zone comprising of the integral furnace of an adjacent boiler, itself burning coal on a grate.

2. A refuse fired furnace operating on the starved air principle as aforementioned containing a stepped hearth upon which the refuse burns, an external refuse filled hopper with a battery of rams at its lower end pushing refuse intermittently into the furnace, and fitted with an ash pit and ash removal conveyor from whence primary air for combustion is induced into the furnace in counterflow to ash leaving. A water tube boiler generating steam or hot water and fitted with a secondary zone integral with the combustion chamber of the boiler into which the combustible gases evolving from the refuse fired furnace are ducted and completely burnt.

3. Any refuse fired furnace operating on the starved air principle as aforementioned coupled to any boiler fitted with an integral furnace and into which combustible gases evolving from the refuse fired furnace are ducted and completely burnt.