TECHNICAL FIELD

The invention relates generally to Stirling engine cogeneration systems for combined generation of heat and electrical power, and particularly to an improved cogeneration system having a low temperature combustion system and low pollutant exhaust emissions with minimal detriment to electrical generation efficiency.

BACKGROUND

In a cogeneration system a heat engine driving an ac or dc generator is used to generate electricity. An exhaust gas heat exchanger is used for recovering heat from the exhaust of the engine. Heat from the Stirling engine cooling fluid is also recovered.

Fuel (such as LPG or CNG or diesel) is burnt to generate heat to drive the engine and generator. Nitrogen oxides (NOx) are present in the exhaust gases which (after heat extraction) generally exit to the environment, and it is desirable to minimise NOx emissions (and it is obligatory that NOx emissions are within limits specified by regulation in many countries).

It is an object of the invention to provide a co-generation system with low NOx emission and optimal thermal energy recovery whilst maximising electrical efficiency of the co-generation system, or to at least provide the public with a useful choice.

SUMMARY OF THE INVENTION

In broad terms the invention provides a cogeneration system including: a combustion chamber and burner for supplying heat to an external combustion engine arranged to drive a generator for producing electricity wherein the burner comprises a radiative surface burner (as defined herein), and the system also comprises a pre-heater heat exchange arrangement for exchanging heat between hot exhaust gases and inlet air or pre-mixed inlet air and fuel.

The invention also provides a method of operating a cogeneration system comprising a combustion chamber and burner for supplying heat to an external combustion engine arranged to diive a generator for producing electricity, comprising preheating inlet air or pre-tnixed inlet air and fuel by exchanging heat from hot exhaust gases to the inlet air or pre-mixed inlet air and fuel, and operating a surface burner within the combustion chamber in radiant mode.

We have found that providing a radiative surface burner across the combustion chamber lowers the temperature of combustion within the combustion chamber and distributes heat evenly to the hot-end heat exchangers of the engine. The lower flame temperature reduces the production of thermal NOx from fuel combustion and therefore NOx emissions in the exhaust gases from the co-generation unit.

In a preferred form, the pre-heater for exchanging heat between hot exhaust gases from the combustion chamber and inlet air or pre-mixed inlet air and fuel (herein:pre-mixture) comprises a series of shells over the hot end of the engine, which define the combustion chamber and the pre- heater. The exhaust gases and inlet air or pre-mixture flow through passages defined between the shells to exchange heat from the exhaust gases to the inlet air or pre-mixture, before the inlet air or pre-mixture enters the combustion chamber and the exhaust gases exit the burner assembly. The shell structure over the combustion chamber also cools the external surface of the burner assembly.

By "radiative surface burner" is meant any component, of for example metal mesh or gauze or any perforated metal or ceramic component, which is provided to extend across the combustion chamber and has the effect of extending the combustion flame over a major part of the cross- section of the combustion chamber, over the hot end of the engine.

The term 'comprising' as used in this specification and claims means 'consisting at least in part of, that is to say when interpreting independent claims including that term, the features prefaced by that term in each claim will need to be present but other features can also be present.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of example only and with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of a cogeneration system of the invention.

Figure 2 is a perspective view from the exterior of the upper end of a preferred form co- generation unit of the invention and in particular of a burner assembly over the hot end of the external combustion engine,

Figure 3 is an upper end-on view of the preferred form burner assembly,

Figure 4 is a cross-section through the burner assembly and the hot end of the external combustion engine along line B-B of Figure 3, and in particular through the combustion chamber and a surrounding inlet air preheater,

Figure 5 is a cross-section through the burner assembly along line A-A of Figure 3 showing the inlet air flow path from an air inlet, through an inlet air preheater, and past a gas inlet to a plenum above the combustion chamber, showing a surface burner operating in radiant mode within the combustion chamber, and showing the flow path of hot combustion products from the combustion chamber through the preheater, to an exhaust outlet.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to Figure 1 of the drawings, a cogeneration system includes a Stirling generator 1 for generating electricity and providing heat for an external thermal load, such as for water or space heating. The Stirling generator includes an external combustion (heat) engine, such as a Stirling engine Ia, having a combustion chamber 2 with a burner 3 within a burner assembly over the hot end of the engine. Air and fuel are supplied through inlets 4 and 5 respectively. The Stirling engine Ia is coupled to an electric generator Ib for producing electricity. The generator output may be alternating or direct current and may be supplied to the electricity network, or to an electric energy storage device such as a battery, for example.

An engine exhaust passage 7 connects the combustion chamber 2 to an exhaust gas heat exchanger 9. A cooling or heat transfer medium circulates through the exhaust gas heat exchanger 9 to take up exhaust gas heat. The cooling medium passes through the heat exchanger via inlet 11 and outlet 12 (or inlet 12 and outlet 11). The cooling medium very preferably also passes through the engine for cooling the engine and taking up engine heat, and may pass through the engine before or after passing through the heat exchanger 9, or in parallel split flow. An induced draught fan 10 draws flow from the inlet, through the burner 2, hot end heat exchangers, an inlet air preheater as will be further described, exhaust gas heat exchanger 9, and to a flue 13.

Referring to Figures 2 to 4, in the preferred embodiment, the heater head or hot end heat exchanger(s) 6 of the Stirling engine are positioned at the lower end of the combustion chamber 2 (see Figure 4) and comprise fins provided radially about the upper end of each cylinder within the combustion chamber to maximise heat exchange to the cylinders. In the preferred embodiment illustrated, the Stirling engine is a multi-cylinder engine, and has four cylinders 6 positioned in a square or other symmetrical arrangement about a central axis.

In accordance with the invention the combustion chamber comprises a radiative surface burner across the combustion chamber between air and fuel inlet(s) or a pre-mixture inlet to the combustion chamber, and the hot end of the engine, and heat is exchanged from exhaust gases to pre-heat the inlet air or pre-mixture in a pre-heater. In the preferred form the pre-heater is formed by a series of concentric shells around the combustion chamber as will be further described. The shells define the combustion chamber over the hot end of the engine and define the pre-heater. Referring to Figures 2 to 4, this general burner assembly is generally indicated at 100 and is coupled to the casing of the engine, indicated at 101, over the hot end of the engine, about the lower periphery of the burner assembly. Figure 4 is a cross-section through the burner assembly and the hot end of the engine, along line B-B of Figure 3. Figure 5 shows the inlet air flow path from an air inlet, through the pre-heater, and past a gas inlet to the combustion chamber. Figure 5 is a cross- section view through the burner assembly along line B-B of Figure 3 showing where combustion occurs within the combustion chamber and the subsequent flow path of hot combustion products through the inlet preheater, and to an exhaust outlet.

Air enters the burner assembly through port 13, as indicated by arrow Al in Figure 5, and passes downwardly between outer most annular wall or shell 14 and next inner annular wall or shell 15 as indicated by arrow A2. The inlet air flow then turns around the lower annular edge of shell 15, and passes upwardly between shell 15 and next inner shell 16 and enters the combustion chamber defined over the engine hot end by shell 17, from above surface burner 25.

Fuel such as gas fuel enters through an inlet 21 as shown in Figure 4. In particular in the preferred form shown gas is injected through holes into a swirl generating mixer 20 where the gas meets and is mixed with the inlet air flow. An inner mixing tube 19 extends from the swirl component 20 into intake plenum 2a. A perforated outer mixing tube 18 surrounds the inner mixing tube 19 in the intake plenum. The upper end of the inner mixing tube 19 is open to the space between the shells 15 and 16 at the centre- top of the burner assembly. Inlet air rising between the shells 15 and 16 meets incoming gas at the swirl mixer 20, which imparts a tangential velocity to the air and fuel flows. The air and gas are mixed at the swirl mixer and in the inner and then outer mixing tubes 19 and 18. Specifically air and gas flow downwardly through the inner mixixig tube 19, to enter the intake plenum 2a. Some of the ait-gas flow will pass around the lower annular edge of the inner mixing tube 19, and upwardly between the inner mixing tube 19 and outer mixing tube 18, and enter the intake plenum 2a through apertures in the outer mixing tube 18.

Radiative surface burner 25 extends over the combustion chamber 2 between the intake plenum and the hot end heat exchangers 6. In the preferred form the surface burner 25 comprises a thin mesh mat or perforated ceramic or perforated metal plate. Combustion occurs at and below the burner mat 25, in the combustion chamber 2. The surface burner 25 is arranged to operate in radiant mode, to spread and stabilise the flame across a major part of and preferably approximately the full width of the combustion chamber, to both reduce the combustion temperature within the combustion chamber 2, and minimise localised combustion hot spots within the combustion chamber 2. The surface burner 25 operating in radiant mode also promotes an even combustion temperature within the combustion chamber.

In the preferred form shown the surface burner mat 25 is supported by a mounting dish 26 which is preferably formed of a ceramic material. The mount 26 comprises apertures which enable it to fit over the hot end heat exchangers 6 of the Stirling engine. The surface burner 25 is supported by the upper annular edge of the mount 26 and in turn the lower edge of shell 17 defining the combustion chamber is formed into an annular flange which engages the upper annular edge of mount 26.

Figure 5 also shows the path of combustion products from the combustion chamber. Hot combustion products flow from the combustion chamber 2 through the annular space between each cylinder head and it's respective aperture in the ceramic mount 26, in doing so passing between the fins of the heat exchangers 6 around each cylinder head. Combustion products then flow beneath mount 26 and upwardly between the mount 26 and the shell 16, before exiting the burner assembly through exhaust port 28. The hot combustion products transfer heat through shells 16 and 17 to pre-heat the inlet air (or pre-mixture).

Very preferably, the pre-heater acts to raise the temperature of the inlet air to a temperature of 150-200°C when inlet air and fuel are mixed. As the inlet air-fuel mixture enters the intake plenum 2a it is heated further, but the construction should be such that the air-fuel mixture present in the intake plenum 2a, on the inlet side of the surface burner 25, is kept below 400°C to prevent auto-ignition. Complete mixing of air and fuel should occur so as to prevent auto ignition in the intake plenum 2a and to ensure that combustion occurs only on the downstream side of the surface burner 25.

It has been found that the effect of both pre-heating the inlet air or air-fuel pre-mixture and providing a surface burner operating in radiant mode is to lower the temperature of combustion within the combustion chamber. In turn, the production of thermal NOx from fuel combustion and therefore NOx emissions in the exhaust gases from the cogeneration unit are reduced. Electrical efficiency of the co-generation unit is increased by recovering exhaust heat to add energy to the incoming air or pre-mixture, so mat less fuel and air are needed to raise the temperature of the hot end of the engine.

Referring finally to Figures 3 and 4, in the preferred form ignition and flame detection probes 40 and 41 enter the combustion chamber 2 below the surface burner 25 from one side as shown. This position is preferable to placing the probes 40 and 41 in the plenum area because it avoids creating a localised hot area in the burner cavity which could cause high temperature flame combustion resulting in higher NOx pollutant emission.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope hereof.