WO1995008514A1

WO1995008514A1 - Vitrification and power generation system

Info

Publication number: WO1995008514A1
Application number: PCT/GB1994/002011
Authority: WO
Inventors: Keith Russell Mcneill; Brian Jerome Boland
Original assignee: Vert Investments Limited
Priority date: 1993-09-18
Filing date: 1994-09-15
Publication date: 1995-03-30
Also published as: ZA947172B; CN1112524A; GB9319365D0; EP0739314A1; TW294646B; JPH09503738A; BR9407554A; AU7646194A; CA2171427A1

Abstract

This invention relates to a method and apparatus for vitrification and power generation. A vitrification and power generation apparatus comprises a glass-forming furnace (1) including inlets (3, 5) for feedstock and inlet gas including fuel gases or oil and pre-heated air, and a gas outlet (9) from which furnace flue gases are fed; heat recovery means (25) through which furnace flue gases are fed to reduce their temperature, the heat recovery means (25) also pre-heating the air fed to the furnace; and a boiler (11) coupled to the heat recovery means (25) including a gas inlet through which the furnace flue gases are fed to the boiler (11), a steam outlet (13) and a gas outlet (15) through which boiler flue gases are fed, the boiler flue gases for feeding to a gas treatment system (17) in which the boiler flue outlet (5) is coupled to two outlet paths (19, 21) and means for splitting the flow such that one outlet path (19) is for feeding boiler flue gases to a gas treatment system (17) and the other outlet (21) path leads to the furnace inlet (5), to the furnace outlet (9) or to the boiler inlet (10), whereby part of the flow of boiler flue gas is recirculated.

Description

VITRIFICATION & POWER GENERATION SYSTEM

Field of the Invention

The invention relates to a system for vitrification and power generation. The invention is applicable with particular advantage to systems which vitrify toxic waste substances and use waste oils as fuel. It is equally applicable to conventional gas furnace operations when the furnace feedstocks do not have their origins as waste.

Review of Prior Art

A variety of glass-melting furnaces are available and, many of them have been adapted to form glass from a wide range of toxic feedstocks. In order to accommodate a wide range of feedstocks whilst maintaining an output of glass the conditions within the furnace will be varied by varying the amount of fuel gases or oil supplied to the furnace.

To form and melt glass requires the glass to reach a temperature in the order of 1350°C. In order to achieve this the temperature of the flue gas has to be around 1550°C. Because of these high temperatures the amount of energy leaving the furnace in the flue gases can be very high. Thus traditionally the glass industry has attempted to recover some of this heat by using regenerative or recuperative air pre-heating systems. In such cases the flue gases are passed through a regenerator. Typically a regenerator comprises two chambers filled with fire bricks one chamber heating the incoming air whihc will be fed to the furnace, and one cooling the hot flue gases leaving the furnace. These chambers are periodically switched over because the passage of the flue gases through the chamber causes the bricks • o heat up. Once the temperature of the bricks has reached a predetermined temperature the flow through the regenerator is changed such that the gas being input in to the chamber is the combustion air for the furnace which is preheated by the hot bricks. Whilst this is proceeding the second regenerator is being used to remove heat from the output gases from the furnace. The passage through the two regenerative chambers is continuously alternated. Typical values of the air pre¬ heating temperatures are between 1200-1300°C with flue gas temperatures of 1450-1550°C at the top of the regenerator. Even with the air pre-heating temperatures so high the temperatures of the flue gas leaving the regenerator is in the order of 450-600°C. This is equivalent to an energy content in the flue gas of around 25% of the energy input of the fuel. The reason for this is that the ratio of the heat capacity flux at those temperatures to that of the flue gas to the air is above 1.2:1 which limits the efficiency of any regenerator to about 70 or 80%.

There are a number of different types of recuperator available. One such recuperator is of metallic construction although it is possible to construct one from refractory materials. In this recuperator the hot waste gases pass continuously through a flue which contains tubes of metal or refractory. Air is passed continuously through the tubes in a contraflow to the waste gases and is thus preheated.

Preheats of 650-700°C are the norm with an exit waste gas temperature of 950-1050°C. Therefore it can be seen that this recuperator is not as efficient as a regenerator. Thus they are not widely used for heat recovery in the glass industry.

To ensure that this heat is not lost the energy in the flue gas from the furnace could be used to heat the feedstock, heat boilers or used in other waste heat recovery systems which are not connected with glass melting- furnaces.

Unfortunately the use of the flue gas to pre-heat the furnace feedstock conventionally brings the flue gas into direct contact with the feedstock allowing the water in the feedstock to be driven off with the hot flue gas in its way to the chimney. Where toxic wastes are contained in the feedstock, organic toxins would also be 'riven off with the water and international legislation requires exposure of any toxic gases to elevated temperatures (at present a temperature of at least 1200°C) in the presence of surplus oxygen for a given period of (at present more than • two seconds) to destroy them which means that this option is not available where toxic wastes form part of the furnace feedstock.

One method to utilize this energy is to use it to heat a boiler. The boiler can be used to create steam to drive a turbine to proc a power. However a conventional glass furnace would r ise at best 1.5 MW of power and it is likely that it would prove difficult to use a conventional glass furnace to provide a constant output of power. If this power is to be used efficiently the amount of power produced by the apparatus needs to be constant and predictable.

Problems arise when attempting to increase the output of the boiler. There are limits to the temperature to which the furnace can be raised since at too high temperatures the refractory lining of the furnace will be destroyed. Similarly the flue gas from the furnace cannot be fed directly to the boiler without the temperature being lowered since there is a limit to the temperature at which boilers can operate. For example, for some flue gases boilers cannot operate at temperatures higher than 1200°C. This is particularly so when the gases contain volatile heavy metals and acid gases. If more gases are supplied ie by increasing the air supplied into the fuel thereby reducing the furnace temperature this would mean that the total gases being output by the boiler would be similarly increased. However it is important that the amount of gas being fed from the boiler is kept to a minimum because all such gases have to be treated to remove any remaining toxins which are not removed in the vitrification process.

The apparatus and processes involved in the treatment of such gases is expensive and it is therefore important to minimize the volume of gas which requires to be treated.

Hitherto it has proved impossible to balance the three requirements ie:

(i) to maximize the production of glass from the furnace; (ii) to produce a predetermined constant amount of power from the boiler; and (iii) to minimize the amount of flue gases from the boiler.

Summary of the Invention

According to the invention there is provided a vitrification and power generation apparatus, comprising, a glass-forming furnace including inlets for feedstock and inlet for gas including fuel gases or oil and pre¬ heated air, and a gas outlet from which furnace flue gases are fed; heat recovery means through which furnace flue gases are fed to reduce their temperature, the heat recovery means also pre-heating the air fed to the furnace; and a boiler coupled to the heat recovery means including a gas inlet through which the furnace flue gases are fed to the boiler, a steam outlet and a gas outlet through which boiler flue gases are fed, the boiler flue gases for feeding to a gas treatment system; in which the boiler flue outlet is coupled to two outlet paths and means for splitting the flow such that one path is for feeding boiler flue gases to a gas treatment system and the second path leads to an inlet of the furnace, to the furnace outlet, or to the boiler inlet, whereby part of the flow of boiler flue gas is re-circulated. A method in accordance with the invention comprises the steps of feeding to a glass-forming furnace feedstock, fuel gases or oil and pre-heated air; feeding furnace flue gases from the furnace through heat recovery means, where the temperature of the flue gases are reduced, to a boiler which produces steam and outputs boiler flue gases, the boiler flue gases being fed towards a gas treatment system, whereby part of the flow of boiler flue gases are re¬ circulated into the furnace inlet, the furnace outlet or the boiler inlet.

By taking a portion of the boiler flue gas from the outlet this reduces the total volume of gas being fed to the gas treatment system and therefore reduces the costs of the final gas treatment.

If the flue gas is fed to the furnace inlet this increases the volume of gas being fed through the furnace which means that the amount of air which needs to be added to the fuel gases can be reduced. This reduces the total amount of air being fed into the system.

Alternatively the boiler flue gas is recirculated to the furnace outlet so that it mixes with the furnace flue gas before it enters the regenerator. Alternatively it may be fed to the furnace flue gas as it leaves the regenerator at the boiler inlet.

The diversion of some of the flow from the boiler outlet away from the gas treatment feed reduces the amount of gas to be treated.

Although it is possible for the first and second aspects of the invention to the used in isolation such systems are not flexible and cannot be used with a wide variety of furnace feedstocks.

In accordance with a third aspect of the invention combines the first and second aspects of the invention in that the part of the boiler flue gas which is fed away from the glass treatment system is split into two flows one being fed directly into the furnace inlet and the second being fed into the boiler inlet. The system can be used with part of the flow leading to the furnace inlet, and part to the boiler inlet at the same time. However preferably a switching mechanism is included to switch between flow paths. This supplies an efficient system whereby for example a 1000 Tonne of glass can be produced while at the same time producing 36 MW of power and not producing an excessive volume of waste gases.

The boiler can be either a water tube or fire tube type depending on the size and power of the system.

The heat recovery means may be a conventional regenerator or recuperator but it is preferred that the regenerator is non-conventional in that the chamber is not packed with the most efficient volume of bricks rather only contains a few bricks so that only a small amount of heat is taken from the flue gases from the furnace. This has the added advantage that the regenerator volume not packed with bricks guarantees that the requirement for the 2 seconds at a temperature above 1200°C for the waste gases is met at all times.

It should be noted that conventionally the most efficient heat exchange is achieved by providing a volume of bricks equal to 3.3 times the cross-section of the furnace. This however would be too efficient for this system, and the bricks would take so much heat that the air would be preheated to a very high temperature - too high for the furnace. Thus by "few" bricks, it is meant that the regenerator has less than the optimum most efficient volume.

Brief Description of the Drawings

An example of a vitrification and power generation system in accordance with all four aspects of the invention will now be described with reference to the accompanying drawings in which: -

Figure 1 is a schematic flow diagram illustrating a furnace and boiler in accordance with the prior art; Figure 2 is a schematic flow diagram of a system in accordance with the invention; and

Figure 3 is a schematic section through a regenerator for use in the system. Description of the Preferred Embodiment

Figure 1 illustrates a furnace and boiler system of conventional type. A furnace (1) is a conventional glass- forming furnace which includes an inlet (3) for feedstock and an inlet (5) for gases including fuel gases or oil. When the glass has been formed by the furnace it is fed through glass outlet (7) . Flue gases from the furnace are fed through outlet (9) and then fed via regenerator (25) through to boiler (11) which produces steam through steam outlet (13). Flue gas from the boiler is fed through outlet (15) and this is then fed to gas treatment apparatus (17) shown schematically.

Air is fed from air supply illustrated schematically as 24 to one of the chambers 25A of regenerator 25 where it is preheated by the bricks within. The preheated air then flows to gas inlet 3. At the same time the furnace flue gases are fed through the outer chamber 25B of the regenerator to heat the bricks. Periodically inputs to chambers 25A and 25B are swapped.

The feedstock can be a mixture of sand limestone corundum toxic materials and water. Examples of toxic materials are asbestos, residues from incinerators, mining activities, spent catalysts, filtercake, sludges from rivers, sewage and contaminated soils, oil refinery residues, drilling residues and out of specification chemicals. Typically in order to achieve an output of 527GJ from the boiler (11) the heat flow into the furnace needs to be 1072GJ. Typically the heat lost (A) will be 24GJ and the heat within the glass output will be 218GJ leaving 830GJ within the flue gas. This means that the flue gas from the boiler will be 303GJ.

Figure 2 illustrates a system in accordance with the invention. Elements in common with the system illustrated in Figure 1 are shown with the same reference numerals since these elements are the same elements. The system has additional elements as illustrated.

The boiler flue gas outlet (15) is coupled to divergent paths (19) and (21) and means for splitting the flow (20). The choice of a suitable switch or valve will be apparent to the skilled addressee of the specification. The first path (19) leads directly to gas treatment apparatus (17). The second path (21) feeds part of the flow of boiler flue gas to be re-circulated. The second path (21) leads to two switches (26, 28) for splitting the flow, and switching the flow between the three paths (27, 29 and 31), one of which (27) leads to a furnace inlet (5), one of which (29) leads to furnace outlet (9), and the other of which (31) leads to boiler inlet (10). Air is supplied from air supply (24) via condenser (23) and regenerator (25) to gas inlet (5) of the furnace (1) . The steam outlet (13) from the boiler (11) drives turbine (30) to generate power. The steam is exhausted through condenser (23) and the condensate is returned to the boiler (11). The heat generated by the condensation of the steam in condenser (23) is used to heat the air supply. The air is further pre-heated by one of the regenerator chambers (25A). The regenerator (25B) gains its heat by the furnace flue gas (9). By re-circulating the boiler flue gas the volume of gas which is to be treated is reduced. The choice of whether to recirculate the gas via paths 27, 29 or 31 is dependant on the feedstock used, and the wetness of the gas generated. The heat lost at the gas treatment is also reduced. The figures on Figure 2 illustrate the heat flows in GJ per hour. Because 47GJ per hour are provided by the boiler flue gas to the furnace the amount of GJ per hour fed by the fuel gases (5) can be reduced to 1025 to produce the same effect as required to form the glass and to form the steam to drive the turbine. With this arrangement 1000 Tonne of glass per day can be produced together with 36MW of power. One chamber of the regenerator 25 is illustrated schematically in figure 3 from which it can be seen that the chamber includes a few bricks, but is not packed with bricks. This gives some heat exchange, but not a very efficient heat exchange.

Claims

1 A vitrification and power generation apparatus, comprising, a glass-forming furnace (1) including inlets (3,5) for feedstock and inlet gas including fuel gases or oil and pre-heated air, and a gas outlet (9) from which furnace flue gases are fed; heat recovery means (25) through which furnace flue gases are fed to reduce their temperature, the heat recovery means (25) also pre-heating the air fed to the furnace; and a boiler (11) coupled to the heat recovery means (25) including a gas inlet through which the furnace flue gases are fed to the boiler (11), a steam outlet (13) and a gas outlet (15) through which boiler flue gases are fed, the boiler flue gases for feeding to a gas treatment system (17), in which, the boiler flue outlet (5) is coupled to two outlet paths (19, 21) and means for splitting the flow such that one outlet path (19) is for feeding boiler flue gases to a gas treatment system (17) and the other outlet (21) path leads to an inlet (5) of the furnace, to the furnace outlet (9), or to the boiler inlet (10) whereby part of the flow of boiler flue gas is recirculated.

2 Apparatus according to claim 1, in which the furnace includes a flue gas inlet and the other outlet path (21) leads to the flue gas inlet (5) of the furnace whereby part of the flow of boiler flue gas is recirculated into the furnace.

3 Apparatus according to claim 1, in which the other outlet path (21) leads to the furnace outlet (9), whereby part of the flow of boiler flue gas is recirculated into the furnace flue gas fed to heat recovery means (25) . 4 Apparatus according to claim 1, in which the other outlet path (21) leads to the boiler inlet (10) whereby part of the flow of boiler flue gas is recirculated into the boiler inlet (10).

5 Apparatus according to claim 1, in which the other outlet path (21) leads to two paths, one leading to the furnace inlet, and the other leading to the boiler inlet, whereby part of the flow of boiler flue gas may be recirculated into the furnace inlet, and part to the boiler inlet.

6 Apparatus according to claim 5, in which the outlet path (21) includes- switching means for switching the flow of boiler flue gas between the path leading to the furnace inlet (5) and the path leading to the boiler inlet (10) whereby the boiler flue gas is recirculated either to the furnace inlet (5) or to the boiler inlet (10).

7 Apparatus according to any one of the preceding claims in which the heat recovery means comprises a regenerator including within it a volume of bricks less than the optimum volume for the furnace.

8 A method of vitrification and power generation comprising the steps of feeding to a glass-forming furnace, feedstock, gases or oil and preheated air; feeding furnace flue gases from the furnace through heat recovery means, where the temperature of the flue gases are reduced, to a boiler which produces steam and outputs boiler flue gases, the boiler flue gases being fed towards a gas treatment system, whereby part of the flow of boiler flue gases are re-circulated into the furnace inlet, the furnace outlet or the boiler inlet.