WO2013088100A1

WO2013088100A1 - Energy recovery system

Info

Publication number: WO2013088100A1
Application number: PCT/GB2012/000865
Authority: WO
Inventors: Andrew William STAPLETON
Original assignee: Qinetiq Limited
Priority date: 2011-12-14
Filing date: 2012-11-28
Publication date: 2013-06-20
Also published as: GB201121438D0

Abstract

The invention provides a system for recovering energy from a waste pyrolyser, the system (1) comprising a combustor unit (2) and a heat exchanger (5), wherein the combustor unit receives a flow of syngas (8) and produces combustion gas (12) therefrom, and wherein a gas dilution chamber (4) is positioned between the combustor unit (2) and the heat exchanger (5) to reduce the temperature of the combustion gas (12).

Description

Energy recovery system

The present invention relates to an energy recovery system for a thermal processing system, particularly for a pyrolysis or gasification system, and a related method. The invention also relates to an energy from waste system comprising said energy recovery system, preferably a waste pyrolysis system.

Pyrolysis is a thermochemical decomposition reaction of organic material (biomass) at elevated temperatures in the absence of oxygen, a thermal processing method. Pyrolysis may occur under a range of pressures and at operating temperatures between 400°C to 750°C. Pyrolysis may occur at pressures that are a few mbar (e.g. 5-10mbar) below atmospheric pressure. In general, pyrolysis of organic substances produces gas and liquid products and leaves a solid residue richer in carbon content (char). Pyrolysis is a well-known method of destroying waste, including household and/or industrial waste, and it is highly desirable to make use of the hot synthesis gas, or "syngas", which is produced by the pyrolysis technique. One way of using syngas is to derive or recover energy therefrom. Small scale pyrolysis and gasification plant having a capacity of less than around 1000 kg/hour typically utilise gas engines for energy recovery. For example, the syngas can be used to drive a reciprocating gas engine to generate electricity. Although this is, in theory, an efficient method for generating electricity from the syngas, the use of a gas engine typically requires significant gas "clean-up", i.e. the undesirable tars and other long chain hydrocarbons need to be removed from the syngas before introduction into the engine. Gas clean-up can be complicated, difficult and costly, leading to poor reliability and increased maintenance costs for the energy recovery system. This is highly undesirable for a small scale pyrolysis or gasification plant. In contrast, energy recovery systems for larger scale pyrolysis and gasification plant having a capacity of more than around 1000 kg/hour are typically based upon combusting the syngas, and then using a high pressure water/steam boiler to remove the available thermal energy from the exhaust stream. However, the complexity of such a system makes it prohibitive for small scale, low cost systems.

It is an object of the invention to provide an improved energy recovery system. According to a first aspect of the invention, there is provided a system for recovering energy from a waste pyrolyser, the system comprising a combustor unit and a heat exchanger, wherein the combustor unit receives a flow of syngas and produces combustion gas therefrom, and wherein a gas dilution chamber is positioned between the combustor unit and the heat exchanger to reduce the temperature of the combustion gas.

The temperature of the combustion gas leaving the combustor unit will generally be higher than that of the syngas, as a result of the combustion process.

The invention provides an alternative way of extracting energy from syngas using a combustion unit in conjunction with a dilution chamber and a heat exchanger (preferably a low pressure heat exchanger), the purpose of the dilution chamber being to reduce the temperature of the combustion gas so that it is compatible with the working temperature of the heat exchanger. Although this reduces the overall efficiency of the energy recovery process compared with some known systems (in terms of overall percent conversion of available chemical energy from the syngas to thermal and/or electrical energy), the inventors have realised that, particularly for small scale systems, an energy recovery system can be provided that is nevertheless simple, reliable, relatively low cost and still reasonably efficient. Importantly, the requirement for gas clean-up is avoided, and the invention allows financially viable, small scale, de-centralised, preferably pyrolysis- based, "energy from waste" plants to be constructed.

By a "small scale" energy from waste system or plant is generally meant a system having a waste capacity of less than around 1000 kg/hour, more preferably less than around 500 kg/hour. Usually, the capacity of the plant is greater than or equal to 50 kg/hour. For the reasons outlined above, the invention is particularly advantageous for deriving energy f om small scale energy from waste plant. Ideally, the invention is suitable for use with a pyrolysis unit having a capacity of 50 to 000 kg/hour, more preferably 50 to 500 kg/hour.

The purpose of the combustor unit is to oxidise syngas produced by a system such as a pyrolysis or gasification unit (which is typically at a temperature of around 400-450°C), thereby producing combustion gas at a yet more elevated temperature. It is generally required that the combustion gas reaches a particular temperature, which temperature depends upon factors such as legislative requirements (for example, around 850°C for 0086S

household waste, or around 1100°C for hazardous waste). The temperature which can be attained by the combustion gas depends on factors such as the fuel content of the syngas and may be variable. Accordingly, the temperature is preferably monitored and controlled by one of the preferred features described below.

The syngas typically undergoes oxidation by mixing the syngas with an oxidising gas such as air or oxygen, preferably with air. Accordingly, the combustor unit usually comprises an inlet nozzle for delivery of the syngas and a gas inlet valve for the oxidising gas. The combustor unit may also comprise a burner to provide additional heat for use when the calorific value of the syngas is too low to sustain the minimum combustion gas temperature.

The feedstock entering a waste pyrolysis unit is generally not homogeneous, and may vary in composition and/or water content. As a result, the syngas entering the combustor unit may have a variable fuel content. Preferably, therefore, the gas inlet valve is adjustable in order to maintain a constant combustion gas temperature.

Preferably, the temperature of the combustion gas is monitored as it leaves the combustor unit, to ensure that the temperature meets the relevant legislative and/or other requirements. If necessary, the temperature of the combustion gas can be adjusted either up or down by varying the air flow through the gas inlet valve. Alternatively, or in addition to the above, the temperature of the combustion gas can be controlled by adjusting the calorific value of the fuel (i.e. by increasing or decreasing the flow of the syngas). If the gas inlet valve is already at a minimum or near minimum setting and the combustor outlet temperature is too low, the optional burner can be used to provide the additional heat required. The burner can also be used to start up the combustion process.

The combustion gas may leave the combustor unit (or - if included - the below- mentioned optional residence chamber) at a temperature that is greater than can be accommodated by the heat exchanger used to extract the energy. This is particularly likely if the heat exchanger is a low pressure heat exchanger. Accordingly, a dilution chamber is incorporated into the system between the combustor unit and the heat exchanger, which has the purpose of diluting the combustor gas with a cooler, preferably inert, diluent gas. The dilution chamber typically comprises a closed chamber and a diluent gas valve, through which the diluent gas can be introduced. In this way, the diluent gas valve can be used to control the temperature of the combustion gas entering the heat exchanger. Conveniently, the diluent gas is air, and more conveniently, the diluent gas is air at substantially ambient temperature.

Hence, the dilution chamber provides control of the combustion gas temperature prior to heat exchange, and typically reduces the temperature of the combustion gas to at or around the maximum inlet temperature of the heat exchanger. This is particularly important when a low pressure heat exchanger is used.

The heat exchanger can be any suitable heat exchanger, but is preferably a heat exchanger which comprises a working fluid such as water or thermal oil and recovers heat from the hot combustion gas stream by transferring the heat to the working fluid. More preferably, the heat exchanger is a low pressure heat exchanger, by which is meant that the pressure of the heat exchanger fluid (working fluid) is less than 30 bar, more preferably less than or equal to 20 bar, even more preferably less than or equal to 10 bar and most preferably less than or equal to 6 bar. Although heat exchangers have previously been used in conjunction with combustor units, they are usually high pressure systems used with high capacity pyrolysis units having a capacity of more than about 1000 kg/hour; in that case, the object is to generate steam for a steam turbine or suchlike, and the fluid pressure is likely to be greater than 30 bar, more typically greater than 40 bar.

The heat exchanger is used to remove the thermal energy which is available from the combustion gas, after it has passed through the dilution chamber. The output from the heat exchanger (which is normally in the form of a heated fluid, preferably heated water) can be used in any convenient way, for example as a direct source of thermal energy, or as a heat supply for an electrical generator such as, for example, an Organic Rankine Cycle engine.

It is important that the temperature of the combustion gas at the inlet of the heat exchanger does not exceed the maximum continuous operating temperature of the heat exchanger and/or that the temperature of the combustion gas at the outlet from the heat exchanger does not exceed the maximum continuous operating temperature of the below-mentioned optional fan. If the temperature is too hot in either or both positions, the extent of dilution by the gas dilution chamber can be increased (preferably by opening or further opening the diluent gas valve). If the diluent gas valve is already fully open and the combustion gas temperature is still too high at either or both positions, the calorific value of the syngas can be reduced (at least temporarily) by reducing or stopping the flow thereof. In other words, the flow of syngas is preferably adjusted if the temperature of the combustion gas at one or both of the inlet of the heat exchanger or the outlet of the heat exchanger exceeds one or more threshold temperatures.

Advantageously, the combination of combustor unit, dilution chamber and heat exchanger components together provide a system which can be optimised for the specific energy recovery requirements of a particular pyrolysis or gasification application, and to adjust, in operation, to transient changes in those requirements. Accordingly, in addition to the above-mentioned advantages (financial viability, small scale etc), the invention provides a flexible energy recovery system which can independently control and optimise parameters including; the thermal energy released by the energy recovery system, the temperature of the combustion process and/or the temperature at the inlet of the heat exchanger.

Optionally, the energy recovery system comprises a residence chamber positioned between the combustor unit and the dilution system, preferably immediately downstream of the combustor unit. The residence chamber generally takes the form of an enclosed chamber having an inlet and an outlet, through which the combustion gas flows, with the purpose of maintaining the combustion gas above a required temperature for a required time period. This allows the decomposition of chemicals such as VOCs and dioxins, thereby meeting legislative emissions requirements. Because the temperature needs to be maintained above a certain level, it is desirable that that the gas temperature at the outlet of the residence chamber is monitored. If the temperature drops below a threshold limit, then the inlet temperature can be increased (for example, by adjusting the combustor unit as described above). In other words, the temperature of the combustion gas at the outlet of the residence chamber is preferably monitored so that the temperature of the combustion gas in the residence chamber is maintained above a threshold limit. The residence chamber is usually insulated.

Generally, the dimensions of the residence chamber (specifically the enclosed volume) are selected to maintain the combustion gas above the required temperature for the specified time. Ideally, the temperature of the combustion gas at the outlet of the residence chamber is substantially the same as the temperature at the inlet. Ideally, the flow of combustion gas through the energy recovery system is controlled, thereby ensuring that the system is optimised and that the thermal limit of the system is not exceeded. For this reason, a fan is preferably arranged to pull the syngas through the system from the combustor unit to the heat exchanger. Any suitable tan can be used, one preferred example being an induced draft fan. Preferably, the fan is positioned downstream of the heat exchanger.

Typically, but not necessarily, the energy recovery system operates at a pressure which is a few mbar (e.g. 5-10mbar) below atmospheric pressure.

Because an important feature of the invention is its ability to control the temperature of the combustor gas, the system preferably comprises one or more temperature sensors to measure the combustion gas temperature at various locations in the system. Ideally, one or more temperature sensors are located in at least the following positions; at the outlet of the combustor unit and at the inlet of the heat exchanger. If a residence chamber is present, the system preferably comprises one or more additional temperature sensors at the outlet of the residence chamber.

According to a second aspect of the invention, there is provided a method of recovering energy from a synthesis gas, the method comprising the steps of: introducing the synthesis gas into a combustor unit to produce combustion gas; introducing the combustion gas into a dilution chamber so as to reduce the temperature thereof; and

passing the cooled combustion gas through a heat exchanger.

Preferably the heat exchanger comprises a working fluid (such as, for example, water or thermal oil) and recovers heat from a hot gas stream by transferring the heat to the working fluid. More preferably, the heat exchanger is a low pressure heat exchanger.

According to a third aspect of the invention, there is provided an energy from waste system comprising an energy recovery system as described above and a pyrolysis unit. The skilled person will be well aware of other possible components of an energy from waste system, such as, for example, a feedstock hopper, conveyor and/or char removal system. For convenience, the term "combustion gas" has been used to describe the gas leaving the combustor unit and flowing through the entire energy recovery system. It is not intended to imply that the physical and/or chemical composition of the combustion gas is constant throughout the system, and the skilled person will be aware - for example - that the chemical composition of the combustion gas is likely to change in the optional residence chamber.

Any feature in one aspect of the invention may be applied to any other aspects of the invention, in any appropriate combination. In particular system aspects may be applied to method aspects and vice versa. The invention extends to a system and method substantially as herein described, with reference to the accompanying drawings.

The invention will now be described, purely by way of example, with reference to the accompanying drawings, in which;

Figure 1 is a schematic representation of a preferred embodiment of the invention.

Figure 1 is a schematic representation of a preferred embodiment of the invention. Energy recovery system 1 comprises a combustor unit 2, a residence chamber 3, a dilution chamber 4, a heat exchanger 5 and an induced draft fan 6. It is important that the dilution chamber 4 is positioned between the residence chamber 3 and the heat exchanger 5, so that it acts to cool the combustion gas leaving the residence chamber.

The combustor unit comprises an inlet 7 for syngas 8, and a combustor air valve 9 to introduce air 10 into the unit as an oxidising agent. The combustor also comprises a burner 1 1 which can be used to provide additional heat and/or start up the combustion process. Syngas 8 is typically at a temperature of around 400-450°C.

Syngas 8 undergoes combustion in the combustor unit 2 and the hot combustion gas passes into residence chamber 3. The temperature of the combustion gas 12 leaving the combustor unit and passing through the residence chamber depends upon the precise legislative requirements of the system. For example, if the syngas is derived from the pyroiysis of household waste, the temperature needs to be maintained at least 850°C, whereas if the syngas is derived from the pyroiysis of hazardous waste, the temperature needs to be maintained at least 1 00°C. The volume of the residence chamber is chosen such that the temperature of combustion gas 12 is maintained at the required level for a certain duration, typically at least 2 seconds. In one example system, the volume of the residence chamber is around 10 m³ for an energy recovery system designed for use with a pyrolysis unit processing Municipal Solid Waste at a throughput of about 250 kg/hour.

Combustion gas 12 leaves the residence chamber at substantially the same temperature as it enters. The gas then passes into dilution chamber 4 where it is cooled by the introduction of air 14 using diluent gas valve 15. Combustion gas 12 leaves the dilution chamber 4 at a temperature which is compatible with the operating temperature of heat exchanger 5. Typically, this is around 600°C.

Heat exchanger 5 is a low pressure heat exchanger which transfers thermal energy from the combustion gas to a fluid (typically water). The water from the heat exchanger is typically around 80 to 100°C.

Gas is drawn through the system by means of induced draft fan 6. The combustion gas leaving the fan by way of exhaust is typically at a temperature of around 250°C. An important feature of the system is the presence of one or more temperature sensors (not shown) which are used to measure the combustion gas temperature at various locations in the system. Ideally, one or more temperature sensors are located in at least the following positions; between the combustor unit and the residence chamber (point A), at the outlet of the residence chamber (point B) and between the dilution chamber and the heat exchanger (point C).

The invention has been described with specific reference to energy recovery systems for "energy from waste" plant. It will be understood that this is not intended to be limiting and the invention may be used more generally in applications that will occur to the skilled person. In particular, the "syngas" can be any gas from which energy can be derived by a combustion process.

Claims

A system for recovering energy from a waste pyrolyser, the system comprising a combustor unit and a heat exchanger, wherein the combustor unit receives a flow of syngas and produces combustion gas therefrom, and wherein a gas dilution chamber is positioned between the combustor unit and the heat exchanger to reduce the temperature of the combustion gas.

A system according to claim 1 , wherein the system further comprises a residence chamber positioned between the combustor unit and the gas dilution system.

A system according to claim 2, wherein the residence chamber comprises an inlet and outlet for the combustion gas, and the dimensions of the residence chamber are selected such that combustion gas temperature at the inlet and outlet are substantially the same.

A system according to claim 2 or claim 3, wherein the residence chamber comprises an inlet and outlet and the temperature of the combustion gas at the outlet of the chamber is monitored so that temperature of the combustion gas in the chamber can be maintained above a threshold limit.

A system according to any proceeding claim, wherein the combustor unit comprises a gas inlet valve for the oxidising gas.

A system according to claim 5, wherein the gas inlet valve is adjustable so as to maintain a constant combustion gas temperature.

A system according to claim 5 or claim 6, wherein the combustion gas temperature at the outlet of the combustor unit is controlled by adjusting the gas inlet valve and/or the flow of syngas.

A system according to any preceding claim, wherein the dilution chamber reduces the temperature of the combustion gas, preferably to less than or equal to the maximum operating temperature of the heat exchanger. _*

9. A system according to any preceding claim, wherein the dilution chamber comprises a diluent gas valve which is used to control the temperature of the combustion gas entering the heat exchanger. 10. A system according to any preceding claim, wherein the flow of syngas is adjusted if the temperature of the combustion gas at one or both of the inlet of the heat exchanger or the outlet of the heat exchanger exceeds one or more threshold temperatures. 1 1. A system according to any preceding claim, additionally comprising a fan arranged to pull the syngas and combustion gas through the system.

A system according to any preceding claim, wherein the heat exchanger is a low pressure heat exchanger.

A system according to any preceding claim, wherein energy from the heat exchanger is used as a source of thermal energy.

A system according to any preceding claim, wherein the system is suitable for use with a pyrolysis unit having a capacity of 50 to 1000 kg/hour.

A system according to any preceding claim, wherein one or more temperature sensors are provided to measure the combust/on gas temperature at various locations in the system.

A system according to claim 15, or claim 15 when dependent on any one of claims 2 to 4, wherein the one or more temperature sensors are located in one, both or all of the following positions; at the outlet of the combustor unit, at the inlet of the heat exchanger and/or at the outlet of the residence chamber.

A method of recovering energy from synthesis gas, the method comprising the steps of: introducing the synthesis gas · into a combustor unit to prodw combustion gas; introducing the hot combustion gas into a dilution chamber so as to reduce the temperature thereof; and

passing the cooled combustion gas through a heat exchanger.

A method according to claim 17, said method comprising an additional step (iv) of passing the combustion gas through a residence chamber positioned between the combustor unit and the dilution chamber.

19. A method according to claim 17 or claim 18, wherein the synthesis gas is produced by a pyrolysis unit.

20. A method according to any one of claims 17 to 19, wherein the heat exchanger comprises a working fluid and recovers heat from a hot gas stream by transferring the heat to the working fluid.

2 . A method according to any one of claims 17 to 20, wherein the heat exchanger is a low pressure heat exchanger.

22. A waste pyrolysis system comprising an energy recovery system according to any one of claims 1 to 16.