WO1994009320A1

WO1994009320A1 - Furnaces

Info

Publication number: WO1994009320A1
Application number: PCT/GB1993/002096
Authority: WO
Inventors: Peter Reginald Piggin
Original assignee: Erithglen Ltd
Current assignee: Erithglen Ltd
Priority date: 1992-10-16
Filing date: 1993-10-11
Publication date: 1994-04-28
Anticipated expiration: 1995-04-16
Also published as: GB9221821D0

Abstract

A furnace for incinerating hazardous waste material includes a combustion chamber (4); an inlet (8) for introducing waste material into the combustion chamber (4); an outlet (10) for removing exhaust gases from the combustion chamber (4); a burner (18) for regulating the temperature inside the combustion chamber (4); one or more primary grate (14, 15) in the inlet (8); and a secondary grate (16) in the combustion chamber (4). The primary grates (14, 15) are located above the combustion chamber (4) so that the underside of waste material thereon can be ignited from the combustion chamber (4) below. Burning material falling from the primary grates (14, 15) to the secondary grate (16) is effectively oxidized, improving the quality of emissions and in particular reducing carbon monoxide concentrations.

Description

TITLE: Furnaces DESCRIPTION:

Field of the invention The invention relates to furnaces, particularly to furnaces for incinerating hazardous waste material such as human and animal tissue, blood or other body fluids, which are all classified as clinical waste, and drugs or other pharmaceutical products arising directly from manufacturing processes.

Background of the invention

All organizations that produce hazardous waste dispose of this waste by incineration, which will be the most cost efficient way of disposal. In order to recoup tne cost of fuel consumed by the incinerator, the furnace may be incorporated in a boiler capable cf using the generated heat to produce hot water for example for a central heating system. Such a waste-fired boiler is illustrated in the present Applicant's International Patent Application, published as WO 88/02834 on 19S8 April 21.

The allowable emissions from all incinerators are strictly regulated in many countries. In particular, tne British Environmental Protection Act 1990, Part One PG5/K91) Clinical Waste Incineration Processes Under One

Tonne an Hour, limits incinerator carbon monoxide

3 emissions to 100 mg/m and organic compounds excluding particulate matter must not exceed 20 mg/m^~ expressed as total carbon.

A proposal for a European Council Directive on the incineration of hazardous waste will further reduce tnese

3 3 values to 50 mg/m for carbon monoxide and 10 mg/m for organic compounds. These emission levels can only be met by complete combustion in the firing chamber. The only known way to meet these emission levels on conventional incineration plants is by having a minimum temperature of

1000°C for 2 seconds in tne secondary co oustion zone.

This can only be achieved by a large and expensive firing chamber.

The variable combustion properties of diverse waste materials provide an additional difficulty in ensuring consistently low emissions.

Summary of the invention The invention provides a furnace for incinerating waste material, including a combustion chamber, an inlet for introducing waste material to the combusticn chamber, an outlet for removing exhaust gases from tne combustion chamber, a burner for regulating the temperature inside the combustion chamber, a primary grate in the inlet and a secondary grate in the combustion chamber, CHARACTERIZED IN THAT the primary grate is located above the combustion chamber so tnat the underside of waste material on the primary grate can ce ignited from the combustion chamber.

The primary grate is preferaoly rotatable so as to drop waste material from the primary grate on the secondary grate. However, the primary grate may alternatively be fixed and may have gaps through which waste material can fall. Means may be provided for weighing the waste material on the primary grate. The secondary grate preferably takes the form of a pivotable trap door to allow the removal of ash and unburned waste from the combustion chamber.

A plurality of such primary grates arranged in a vertical series may be provided in the inlet.

A further burner may be provided in the outlet to effect more complete combustion of the emissions from the combustion chamber. The waste material may be fed to the inlet through a rotary air valve, which allows a limited supply of air into the inlet to assist combustion at the first grate. A pair of the furnaces may conveniently be supplied alternately with waste material through a single rotary air valve.

In a preferred embodiment, the furnace forms part of a boiler. For this purpose it may be surrounded by a water jacket and the conventional apparatus of a three pass boiler system.

The invention also provides a method of incinerating waste material, comprising the steps of: introducing the waste material onto a primary grate above a combustion chamber to ignite the underside of the waste material en the primary grate; causing ignited waste material to fall from the primary grate onto a secondary grate in the combustion chamber; and combusting the waste material en the secondary grate, , using a burner tc regulate the temperature in the combustion chamber.

The arrangement of a one or more primary grates witπir. the inlet, upon which the burning of waste material can commence from the underside and from which burning waste material can fall to a secondary grate, has been found to be most successful at reducing carbon monoxide emissions.

In tests of one prototvpe, the emissions were found to be

3 3 reduced from 11400 mg/m to an average of 41 mg/m . Eurning waste falling from the primary grate to the secondary grate is given an increased opportunity of oxidation as it passes through the air in the combustion chamber, the volatile part of the waste already being burnt off at a lower temperature on the primary grate or grates, thus allowing complete combustion to take place in the subsequent combustion zones.

The arrangement has also been found to reduce smoke emissions from the incinerator which are caused by incomplete combustion. No unburnt material can now be detected in the ash residue.

The drawing

The Figure illustrates, schematically and in vertical cross section, a furnace in accordance with the present invention.

Description of a preferred embodiment

The furnace of the invention is contained within an upright, generally cylindrical, refractory lined vessel 2. A space in the bottom of the vessel 2 provides a combustion chamber 4. A central wall 6 divides the upper part of the vessel 2 into an inlet 8 and an outlet 10. A chute 12 leads from a rotary air valve (not shown) to the inlet 8.

A first primary grate 14 is rotatably mounted in an outer wall of the inlet 8. It is shown in solid lines at its normal operating position and in chain lines rotated through 90° from its normal position. The first primary grate 14 may be perforated by holes through which waste material can fall. A second primary grate 15 may optionally be provided in the inlet below the first primary grate 14, so that waste material falling from the first primary grate 14 will land on the second primary grate 15. The second primary grate 15 is rotatably mounted in the same manner as the first primary grate 14.

The base of the combustion chamber 4 comprises a pivotably mounted secondary grate 16. The secondary grate 16 is shown in solid lines at its normal operating position and in chain lines pivoted away from its normal position. Around the edge of the secondary grate 16 is a gap 17, tnrough which air for combustion may be blown into the combustion chamber 4.

A primary burner 18 is mounted in a wall of the combustion chamber 4. The burner 18 is fuelled by oil or gas and may be controlled in response to the measurements of a temperature sensor in the combustion chamber 4 to regulate the temperature. A secondary burner 20 in the outlet 10 effects further combustion of the gases from the combustion chamber 4 before they are finally emitted. The vessel 2 housing the furnace is surrounded by a water jacket and the conventional arrangement of pipes and pumps for a three pass boiler system (not illustrated).

During operation of the incinerator, waste material is fed, generally in bags, into the rotary air valve (not shown) , which upon being rotated deposits the waste in the inlet chute 12. The waste material slides down the chute 12 under gravity and lands on the first primary grate 14, which is in its horizontal position (as shown in solid lines). Here, the waste material is exposed to radiant heat from the combustion chamber 4 below, or from the second primary grate 15 if it is present, which causes the waste to ignite from the underside and releases any volatile waste slowly at the low temperature on the grate 14. The waste begins to burn on the first primary grate 14, supplied with air that seeps through the rotary air valve and down the inlet 8.

As combustion of the waste on the first primary grate 14 proceeds, the waste disintegrates and small pieces of burning waste drop through openings in the grate. If the second primary grate 15 is provided, the falling waste then lands on that second primary grate 15, which operates in a similar manner to the first primary grate 14.

As the pieces subsequently fall through tne main combustion zone of the combustion chamber 4, they burn fiercely because of their increased contact witn the surrounding oxygen. From time to time, each primary grate 14,15 is rotated tnrough 180° to prevent the accumulation of large, unburnt pieces of waste material.

The burning waste falling from the primary grates 14,15 lands on the secondary grate 16 in the base of the combustion chamber 4. There it is supplied with oxygen and kept agitated by the injection of air from below through the gaps 17. Depending on the calorific value of the waste, the temperature in the combustion cha oer 4 may tend to vary. The temperature sensor (not snown) therefore monitors the temperature and regulates the operation of the burner 18 to maintain the comoustion chamber temperature above a minimum value necessary to achieve effective combustion. Typically, the operating temperature in the combustion chamber 4 is aoove 1000°C. Periodically, the secondary grate 16 is pivoted to the position shown in chain lines to remove ash and other combustion residues from the chamber 4.

Depending on the waste material being incinerated, tne secondary burner 20 may be operated to maintain the combustion temperature in the outlet 10. Beyond the outlet 10, other means (not shown) may be provided for treating the emissions, such as the addition of lime to neutralize acid gases.

Claims

CLAIMS:

1. The invention provides a furnace for incinerating waste material, including: a combustion chamber (4); an inlet (8) for introducing waste material into the combustion chamber (4); an outlet (10) for removing exhaust gases from the combustion chamber (14); a burner (18) for regulating the temperature inside the combustion chamber (4); a primary first grate (14) in the inlet (18); and a secondary grate (16) in the combustion cnamoer (4) ;

CHARACTERIZED IN THAT the primary grate (14) is located above the combustion chamber (4) so that the underside of waste material on the primary grate (14) can oe ignited from the combustion chamber (4).

2. A furnace according to claim 1, wnerem the primary grate (14) is mounted in the inlet (18) for rotation about a generally horizontal axis.

3. A furnace according to claim 1 or claim 2, wherein tne primary grate (14) includes openings tnrougn wnich waste material may fall.

4. A furnace according to any preceding claim, further including means for weighing the waste material on the primary grate (14).

5. A furnace according to any preceding claim, lnludmg a plurality of primary grates (14,15) arranged in a vertical series in the inlet (8) such that the underside of waste material on eacn primary grate (1-,15) can be ignited from below.

6. A furnace according to claim 5, in vvnicr tne primary grates (14,15) comprise a mesh which becomes finer in each successive primary grate ⁽14,15⁾ down tne vertical series.

7. furnace according to any preceding claim, wherein the secondary grate (16) comprises a trap door in the bottom of the combustion chamber ⁽4⁾.

8. A furnace according to claim 7, wherein tne secondary grate (16) is mounted for pivotal movement about a generally horizontal axis.

9. A furnace according to any preceding claim, including a further burner (20) in the outlet (10).

10. A furnace according to any preceding clam, furtner including a rotary air valve through whicn waste material may be introduced into the inlet (8) and througn which air may bleed into the inlet.

11. A boiler including a furnace according to any preceding claim, for using heat generated by the furnace to heat water.

12. A boiler according to claim 11, being a three pass boiler.

13. A method of incinerating waste material, comprising the steps of: introducing the waste material onto a primary- grate (14) above a comoustion chamber (4) to ignite tne underside of the waste material on the primary grate (14) ; causing ignited waste material to fall from tr.e primary grate (14) onto a secondary grate (16) in tr.e combustion chamber (4); and combusting the waste material on tne seccndar' grate (16), using a burner (18) to regulate the temperature in the combustion chamber (4).

14. A method according to claim 13, wherein the step of causing ignited waste material to fall from the primary grate (14) includes rotating the primary grate (14) about a generally horizontal axis in a rocking motion.