Integrated Burner and Heat Exchanger in a Combined Heat and Power System.

Field of the Invention The present invention relates to a combined heat and power generating system having a burner, which is integrated with a heat exchanger.

Background of the Invention A heating system comprising a burner and one or more heat exchangers can be used for indirect heating of a fluid. The fluid can be used for heating purposes and/or be the working fluid of a heat engine cycle, e.g. a Brayton cycle, which in combination with a generator can produce electricity. Moreover, it is important for a Brayton cycle to have a high working temperature, since this means high efficiency and hence electrical power. The heat exchangers used in the cycle have thus to be designed for sustaining very high temperatures and if the heat transfer primarily is accomplished with convection, it also leads to complicated channel shapes in the heat exchanger. Heat exchangers with complicated channel shapes are very difficult and expensive to manufacture in these high temperature materials. It is an object of the present invention to provide a combined heat and power generating system having a burner being integrated with a heat exchanger in such a way that a substantial part of the heat transfer from the burner to the Brayton cycle is accomplished by radiation at the same time as the exhaust heat from said burner is recovered in an efficient way, for reducing at least some of the problems of the prior art.

This system furthermore reduces the amount of high temperature material that is needed and makes it possible to manufacture this high temperature heat transferring

system part in a simple way. It also makes it possible to use solid fuel in the burner since there are no gas passages for the combustion gas that can get contaminated by matters that can be found in the combustion gases from solid fuels. This is a large problem with the use of normal heat exchangers placed after combustion gases from most solid fuels.

Summary of the Invention

The present invention solves these and other problems by providing a combined heat and power generating system comprising a compressor, a turbine and a generator. These components are connected to a main shaft, and the compressor communicates with the turbine through at least one pipe. The system also comprises a recuperator, an integrated burner and heat exchanger, in which the heat from the combustion is transferred by radiation from the flame to the heat exchanger part, which on the heat- exchanger side is connected to a working flow from the recuperator to the turbine. On its other side, the burner is supplied with fuel and air for being combusted in a combustion area of the integrated burner and heat exchanger, wherein the other side of the recuperator is connected to an outlet of the turbine further comprising a recup for preheating air before it enters the heat- exchanger part of the integrated burner and heat exchanger. The heat generation is accomplished by recovering of heat in the combustion gases leaving the integrated burner and heat exchanger.

According to one embodiment of the invention a part of the air leaving the recuperator could be used for heating purposes, either in a heat exchanger or by direct use of the hot air.

In order to get a preheated air-flow for the integrated burner, a part of the air leaving the recuperator or the turbine could be supplied as combustion air to the burner. In one embodiment of the invention, a part of the air leaving the recuperator is mixed with exhaust gas from the burner before heat recovery is taken place.

In order to further enhance the heating efficiency of the system, the heat recovery could takes place in a system that includes condensation of the exhaust gases.

I order to facilitate use of any suitable fuel, the combustion area of the burner could be adapted for combustion of solid fuels, such as wood, grain, coal, or similar. In order to increase the heat transfer area, the heat transfer area of the integrated burner and heat exchanger could be arranged as plate and fins.

In another embodiment of the invention, the heat transfer area of the integrated burner and heat exchanger could be arranged as a substrate.

In still another embodiment of the invention, the heat transfer area of the integrated burner and heat exchanger could be arranged as a porous body.

This system furthermore reduces the amount of high temperature material that is needed and makes it possible to manufacture this high temperature heat transferring system part in a simple way. It also makes it possible to use solid fuel in the burner since there are no gas passages for the combustion gas that can get contaminated by matters that can be found in the combustion gases from solid fuels. This is a large problem with the use of normal heat exchangers placed after combustion gases from most solid fuels.

Brief Description of the Drawings

The invention will be more readily understood by- looking at the appended drawings, wherein

Fig. 1 is a side view of an integrated burner and heat exchanger in partial cross-section,

Fig. 2 is an end view of the integrated burner and heat exchanger of Fig. 1,

Fig. 3 is a schematic view of a combined heat and power generating system according to the present invention, Fig. 4 is a schematic view of a second embodiment of the heat and power generating system of the present invention and

Fig. 5 is a schematic view of a third embodiment of the heat and power generating system of the present invention.

Detailed Description of the Invention

A burner 10 with an integrated heat exchanger according to one embodiment of present invention is shown in Fig. 1. The burner and the heat exchanger are integrated in such a way that heat will radiate directly from the combustion flame towards one side of the heat exchanger, in the shown embodiment an inner surface 21 of the heat exchanger. The other side of the inner surface of the heat exchanger is designed in such a way that heat can be transferred to the working flow by convection. The material of the heat exchanger part of the system has high heat conductivity. Thus, the temperature of the heat exchanger will be rather constant and more or less independent of the temperature of the working fluid.

The burner system is designed in such a way that it will radiate heat from a relatively large surface. This is accomplished by use of different kinds of flame holders or use of porous material with injection of fuel and air in many locations. The shape of the system can be either

substantially cylindrical as in Fig. 1 or have a rectangular or spherical shape. It can also be designed in such a way that it is suitable for burning solid fuel. This means that it shall be possible to dispose of ashes preferable by the force of gravity.

The burner with the integrated heat exchanger is only described as a typical example, and other similar solutions are possible where heat from combustion is transferred to an external airflow. One embodiment of a heat exchanger according to the invention is schematically shown in Fig. 1 and in cross- section in Fig. 2. The burner 10 has a nozzle 20, from which fuel and air are introduced for combustion in a combustion area 23, which is surrounded by an inner wall 21 of the heat exchanger. A fluid to be heated is directed to a space 24, which is formed by the inner wall 21 and by an outer housing 22. The space 24 contains fins 25 that are part of the inner wall 21. The fins 25 are designed in such a way that they increase the area for heat transfer as well as combines good heat transfer coefficient with low pressure drop of the air A that shall be heated. Heat is radiated from a flame 15 to the heat exchanger inner wall 21. The heat is also spread by conduction to the fins 25. Typical temperatures of the flame and of the wall 21 and the fins 25 are 1500 C and 1000 C, respectively. A typical temperature of the air A to be heated is 600 C when it enters the heat exchanger and 800 C when it leaves the heat exchanger after it has collected heat from the inner wall 21, the fins 24 and the housing 22. The combustion gas is exhausted through a downstream end of the combustion area 23, where its remaining heat can be recovered. The integrated burner and heat exchanger can be manufactured in different materials, such as heat- resistant steel and/or ceramics. Examples of alternatives

to the fins are a substrate similar in shape to exhaust catalysts for cars or a porous material.

As shown in Fig. 3, the integrated burner and heat exchanger is then used as heating means in a Brayton cycle system for combined heat and power generation. A compressor

1 and a turbine 2 are interconnected by a main shaft 4, which in turn is connected to a high-speed generator 3. The compressor 1 and the generator 3 are powered by the turbine

2 during normal operation. During operation, air enters the compressor 1 through a pipe a, and the compressor 1 supplies pressurized air through a pipe b to a recuperator 5, where the air is preheated to a higher temperature by- exchanging heat with hot air passing the other side of the recuperator 5. The air is then directed through a pipe c to the heat exchanger side 24 of the integrated burner and heat exchanger 10, where it is heated by the combustion inside the combustion area 23 of the integrated burner and heat exchanger. The heated air leaving the burner and heat exchanger 10 is brought through a pipe d to the turbine 2, in which the hot air is expanded, which results in mechanical work that drives the compressor and the generator via the main shaft 3. The hot air leaving the turbine 2 is directed through a pipe e to the recuperator 5, for preheating the air that passes on the other side thereof, such as been described above. The recuperator is preferable in this design, since it reduces the heat that must be supplied to the air in the burner and heat exchanger. In the example mentioned above, the air is heated from around 200 C to 600 C in the recuperator and from 600 C to 800 C in the burner and heat exchanger. This makes it possible to use stainless steel in the recuperator; a high temperature material has to be used only for the integrated burner heat exchanger.

The burner 10 is supplied with fuel and air through a pipe I. The exhaust gas from the combustion process, which

still has a rather high temperature, is directed through a pipe j to the a first side of a heat exchanger 7, through pipes m and n, where some heat is transferred to a second side of heat exchanger 7. A gas or a liquid is then directed through the second side of the heat exchanger 7, through pipes m' and n' , where it will collect heat; the collected heat could be used for heating purposes.

The heat in the combustion gas leaving the turbine could be recovered in a heat exchanger 6 or being directly fed into a process that has a need for hot air. No heat exchanger is needed in the latter case.

With reference to Figs. 4 and 5, the burner circuit and the working air circuits can be provided with valves 11 (fig. 4) and 12 (fig. 5) for allowing use of all or part of the working air leaving the turbine as burner airflow or as mixing flow for obtaining an optimum gas temperature before entering the heat exchanger 7. These valves should be arranged in such a way that no flow in the wrong direction can occur. The pipe e can also be provided with a valve (not shown) for directing a part of the airflow to the burner intake (as combustion air) or to the heat exchanger 7, for mixing with the exhausts from the burner 10.

One embodiment with a three-way valve is shown in Fig. 4, where the three-way valve 11 controls the flow of air leaving the recuperator 5. The air can either be directed through pipe g to a heat exchanger 6, or be supplied as combustion air through the pipe i to the burner 10. The airflow through the turbine 2 is typically much larger than the airflow for the combustion. One reason to use some of the air from after the recuperator as combustion air is that it has a higher temperature and thus can be used for controlling flame temperature for obtaining a good radiation. One example where this will be important is when burning certain solid fuels.

Another embodiment is shown in Fig. 5, where the additional three-way valve 12 is provided for controlling the airflow leaving the recuperator 5. The air can be directed to a third three-way valve (not shown) , where the hot exhaust gas from the burner 10 is mixed with the cooler air having exchanged heat in the recuperator 5. This makes it possible to control the temperature of the gases passing through the hot side of the heat exchanger 7. In one embodiment, the heat exchanger 7 is arranged such that condensation of the combustion gases takes place. This will give an increased total energy efficiency of the combined heat and power system.

The burner flame is typically very hot, more than 1400-1500 °C, and can be controlled by mixing air from after the recuperator, wherein the amount of air from after the turbine is used as combustion air, since the air after the turbine typically has a temperature of 600 ⁰ C.

It may be possible to burn solid fuels in the burner, or other fuels that can lead to highly polluted exhausts, since the exhausts are not directed through any sensitive parts, like the compressor 1, turbine 2 or recuperator 5. This makes the system more reliable. Exemplary fuels are wood, grain, coal, or similar, but gaseous or liguid fuels are of course also possible. The nozzle should be adapted to the specific fuel.

Even though the present invention is presented as a specific embodiment, it will be apparent to a person skilled in the art that alterations and modifications to the invention are possible without departing from the scope of the invention such as defined by the appended claims.