Patent Application
20140223909
The invention relates to a thermal power plant having either an oxyfuel firing capability or a partial oxyfuel firing capability, and preferably combined with biomass firing technology, and to a gas supply system for, control system for and method of operation of the same. The invention in particular relates to a gas supply system for, control system for and a method of operation of a thermal power plant suitable for flexible operation in response to varying demand.
Most of the energy used in the world today is derived from the combustion of fossil fuels, such as coal, oil, and natural gas, for example in thermal power generation plants. The combustion of such fossil fuels produces a large volume of CO2 which is conventionally vented to atmosphere. Atmospheric CO2 is recognised as a significant greenhouse gas. It has been established that one of the main causes of global warming is the rise in greenhouse gas contamination in the atmosphere due to anthropological effects. The limitation of further release of greenhouse gases and into the atmosphere is generally recognised as a pressing environmental need. The successful implementation of strategies to reduce atmospheric CO2 emissions from the combustion of fossil fuels is important if the continued use of fossil fuels in many applications, including power generation, is to be possible.
Oxyfuel firing is a means of firing the fuel with an oxygen enriched comburant gas. In conventional fossil fuel fired combustion equipment for example in boilers for steam generation the oxygen required to burn the fuel is supplied by using atmospheric air as a comburant gas. In the case of oxyfuel firing a supply of gas with a higher oxygen content, and in particular a mixture of substantially pure O2 and recycled CO2, is used as a comburant gas. The oxyfuel combustion process seeks to produce combustion products that are highly concentrated in CO2 and in particular consist essentially of CO2 and water to facilitate carbon capture and mitigate the CO2 emissions. To effect this, the combustion air supply must first be is separated prior to supply to the furnace in a suitable air separation unit (ASU). Only the separated gaseous oxygen is intended for supply to the combustion process. The separated nitrogen/argon mix may be expanded and vented to atmosphere or stored in storage tanks for later expansion and venting. Within the air separation unit processes the liquid oxygen may be cryogenically stored in an embedded or external liquid oxygen (LOX) storage facility. Liquid air may be stored in an embedded or external liquid air (LA) storage facility.
In a conventional Oxyfuel plant where combustion furnace uses as comburant gas mixture of CO2 rich recycled flue gas and Oxygen, the operation of the air separation unit (ASU) coupled to that of the boiler and to that of the compression and purification unit (CPU) the ASU and CPU are designed for operation at 100% nominal boiler demand.
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In all cases the prior art systems to provide Oxyfuel combustion in mixture of flue gas and gaseous oxygen require the plant to have an ASU sized at least to 100% nominal boiler comburant demand. Indeed, in each case it is known additionally that the ASU could be oversized to accommodate a maximum boiler demand that is higher than nominal demand and/or that the LOX storage could be oversized to shorten ASU response time to accommodate rapid Boiler/Turbine Unit oxygen gas demand changes.
Fossil fuel thermal power plants have a particular role in a practical mixed supply grid. Typically they are not run continuously at full load. Instead, their output will vary, partly in response to changes in supply or demand within the grid so that the grid supply is maintained. Operation in this way in response to diurnal and seasonal changes in demand, and periods of downtime, can reduce load factors over a period to 80% or less. If the ASU is designed for operation at 100% nominal boiler demand this leads to an excess of capacity, and increases both capex and opex costs for the plant.
In accordance with an aspect of an exemplary embodiment, a comburant gas supply system for a combustion boiler/turbine of a thermal power plant is provided that includes an air separation module to separate and output an oxygen rich gas from an input air supply; a comburant gas storage module fluidly connected to the output of the air separation module for storage in liquid state of separated oxygen rich gas; a comburant gas supply module to supply the oxygen rich gas to the combustion boiler selectively from the air separation system and/or the comburant gas storage system; wherein the air separation module has an oxygen rich gas output capacity based on one or both of:
Similarly, in accordance with a further aspect of the invention, a combustion boiler/turbine system of a thermal power plant is provided comprising a combustion boiler for combustion of a fuel in the presence of an oxygen rich comburant gas; a steam turbine unit driven thereby; and a comburant gas supply system as above described fluidly linked to supply comburant gas to the combustion boiler to support combustion of the fuel.
Similarly, in accordance with a further aspect of the invention, a combustion boiler/turbine system of a thermal plant is provided comprising a combustion furnace for combustion of a fuel in the presence of an oxygen rich comburant gas to provide heat input to the industrial process; and a comburant gas supply system as above described fluidly linked to supply comburant gas to the combustion furnace to support combustion of the fuel.
Similarly, in accordance with a further aspect of the invention, a combustion boiler/turbine system of a thermal power plant is provided comprising a combustion boiler for combustion of a fuel in the presence of an oxygen rich comburant gas; a gas turbine unit driven thereby; and a comburant gas supply system as above described fluidly linked to supply comburant gas to the combustion boiler to support combustion of the fuel.
The key to the invention is that the air separation module is not, as is conventional in the art, sized to the full nominal comburant gas requirement of the combustion boiler/turbine unit with which is it designed to be used. Instead, it is sized to a reduced comburant gas requirement less than the full nominal comburant gas requirement of the boiler/turbine unit running at full capacity, but rather based on an adjusted comburant gas requirement of the boiler/turbine unit that takes account of a boiler/turbine unit load factor over a predetermined operating period. The air separation module is sized to a percentage of the full nominal comburant gas requirement of the boiler/turbine unit to which it supplies oxygen rich comburant gas, which percentage is related to a predetermined operational load factor of the combustion boiler/turbine unit over a suitable operating period. Additionally the air separation module size may be changed to offer longer term energy storage than previous load factor based sizing.
Operation of the air separation module is decoupled from that of the boiler/turbine unit to which it ultimately supplies oxygen rich comburant gas. Therefore the ASU unit is utilizing electrical energy from grid that comes from renewable sources or other low cost sources. It in neither rated to the full nominal comburant gas requirement of the combustion boiler/turbine unit nor limited to operation during periods in the operational cycle when the boiler/turbine unit is operational.
Instead, the air separation module is supported, variation in comburant gas requirement of the boiler/turbine unit over the operating period is accommodated, and supply up to the nominal full load comburant gas requirement of the boiler/turbine unit is enabled by use of the comburant gas storage module selectively to store excess separated oxygen rich gas or be a source of supply of additional separated oxygen rich gas to the comburant gas supply module.
That is to say, when the combustion boiler/turbine unit is operating at an immediate comburant gas demand level that is less than the volume of oxygen rich gas produced by the air separation module, for example if the combustion boiler/turbine unit is operating at substantially less than full load or is not in operation, surplus oxygen rich gas is stored in the comburant gas storage module. When the combustion boiler/turbine unit is operating at an immediate comburant gas demand level that is greater than the volume of oxygen rich gas produced by the air separation module, for example if the combustion boiler/turbine unit is operating at substantially full load, a resultant shortfall in oxygen rich gas supplied by the air separation module to the comburant gas supply module is supplement by oxygen rich gas stored in the comburant gas storage module.
In this way, it is not necessary for the air separation module to be sized, as is conventionally the case, with respect to the full capacity nominal comburant gas demand level of the boiler/turbine unit to which it ultimately supplies oxygen rich comburant gas. It is merely necessary that it is sized in conjunction with a comburant gas storage module to be able to supply the comburant gas requirement of the boiler/turbine unit over a load cycle based on a typical load factor for the cycle. Both capex and opex costs can be reduced relative to air separation modules which are conventionally sized and coupled to the nominal operational load of the boiler/turbine unit.
Nitrogen/argon separated in the ASU unit can be stored in compressed form in tanks for later expansion and stored energy recovery on demand.
The air separation module produces an oxygen rich comburant gas from an input supply of air, in particular preferably to support oxyfuel firing of a carbonaceous fuel in the associated boiler/turbine unit, and the comburant gas storage module stores this gas if required. For example the air separation module may be provided with at least one and preferably a plurality of air separation unit compressors, for example producing an oxygen rich comburant gas by cryogenic separation. Although the term “oxygen rich gas” is intended to cover any gas having a proportion of oxygen which is greater than atmospheric air it will be appreciated that in practice for oxyfuel firing a comburant gas that is substantially free of nitrogen and in particular a comburant gas that is substantially pure oxygen will be preferred.
Preferably therefore the air separation module is adapted to produce and supply gas that is substantially free of nitrogen and in particular that is substantially pure oxygen to the comburant gas storage module and/or comburant gas supply module. For example the air separation module comprises one or more cryogenic air separation units as will be familiar.
The underlying inventive concept of the invention lies in the determination of a suitable size (that is, a suitable oxygen rich gas production capacity) for an air separation module in accordance with the invention, which is determined not with reference to 100% nominal boiler comburant demand for a steady state operation (and still less with reference to a higher maximum boiler demand) but is rather determined with reference to an adjusted demand that takes account of a boiler load factor over a suitable pre-determined period of time and/or that takes account of an energy storage demand.
Thus in accordance with the principles of the invention, a minimum size for an air separation module, and a minimum comburant gas supply capacity, can be determined as the product of a nominal steady state comburant gas demand for the associated boiler/turbine unit and a boiler/turbine unit load factor and the energy storage requirement. In a preferred case, this may be the optimal size, although some additional capacity may be provided for other operational reasons. This allows the air separation unit to be smaller than it would be if sized for full nominal boiler steady state demand and reduces both build and operational costs. For example, an air separation module in accordance with the invention may be sized to no more than 90% of the nominal boiler steady state operation comburant demand if the Boiler/Turbine Unit is operating over the period of time with load factor 0.8 and 10% of Boiler/Turbine Unit capacity is required as additional immediate demanded energy storage.
The invention does not exclude the possibility that an air separation module may still embody the principles of the invention but be sized to be larger than this, for example to accommodate a maximum boiler demand that is higher than the steady state demand and/or to enable the storage of an excess of comburant gas in the comburant gas storage module, which may for instance be used in conjunction with the air separation unit as a source of energy storage to provide demand. Both of these principles are known in the prior art. However, the essence of the invention remains that the operation parameters to be determined for the air separation module are de-coupled from those to be determined with reference to steady state operation of the boiler/turbine system, and instead adjusted to take account of the applicable load factor for the boiler/turbine system and/or required additional energy storage capacity.
In accordance with the principles of the invention, an air separation module is sized with reference to a nominal boiler comburant demand at steady state adjusted for a load factor over a period of time and/or required additional energy storage capacity. Its associated comburant gas storage module is sized accordingly to accommodate fluctuations in demand over that period of time and if required to provide long term comburant storage.
In accordance with the principles of the invention, the air separation module and associated gas storage module, should optimally be sized at least to a sufficient level to effect at least the following:
a) that the air separation module is capable of producing at least the total volume of comburant gas needed to meet the total demand of the associated boiler/turbine unit across the time period;
b) that in combination with the comburant gas storage module, supply of comburant gas is enabled which meets at least the nominal steady state demand of the associated boiler/turbine unit at any time during the said period when it is operating at full steady state load.
A load factor is determined across a suitable period of operation, for example over a full cycle to accommodate changes in daily/seasonal/annual demand, period of scheduled down-time etc.
An air separation module has a design capacity designed with reference to the nominal demand of a boiler/turbine system with which it is intended to be used adjusted to reflect a design load factor of the boiler/turbine system. The design process first involves determining such a load factor over a suitable period of time.
A suitable period of time might be a period from 24 hours up to a year, and might include periods in between.
Even when rated for relatively continuous operation changes in levels of demand over the course of a day and over the course of a year can reduce load factors over such a period to 80% or less. An air separation module in accordance with the invention might therefore have a gas output capacity based on no more than 80% of the nominal steady state demand rating of the combustion boiler/turbine. In many other systems, designed for less the continuous operation, load factors may be considerably less than 80%, for example as low as 50%.
The principles of the invention embody all such systems where the design output capacity of the air separation module is made with reference to the nominal steady state demand rating of the boiler/turbine adjusted to a realistically determined load factor and/or required energy storage capacity.
Preferably, the combustion furnace comprises one or more burners for the combustion of carbonaceous fuel for example including carbonaceous fossil fuel, for example including coal, and for example pulverised coal, but also for example including gas, and for example including oil, and for example including biomass, and for example including distillate, and any combination of same. The comburant gas supply module is adapted to supply comburant gas to the burners to support the combustion of the fuel in use. Suitable fuel supply means supply fuel to the combustion site for oxyfuel combustion in familiar manner.
The air separation module provides in the typical case the sole source of comburant gas supply to the combustion boiler/turbine system. In this context it will of course be understood that even where references made to a module or system in the singular the invention embodies all arrangements of apparatus, whether comprising single air separation units and single boilers or plural separation units and/or plural boilers working cooperatively together, where the air separation system is sized in accordance with the principles of the invention with reference to a demand rating for the combustion boiler/turbine system adjusted to a suitable load factor across a pre-determined operating period.
Preferably, the combustion boiler/turbine system of the invention, further comprises a carbon dioxide compression and storage module for the compression and storage of at least some of the carbon dioxide produced by combustion of fuel in the combustion boiler. Suitable compression and storage units will be well known from the art. Since it is a principle of the invention that the operational capacity and parameters of the air separation system have been decoupled from those of the boiler/turbine system, it follows that the specific operational parameters and capacity of the carbon dioxide compression and storage module are not specifically pertinent to the invention. However, in the preferred case, a carbon dioxide compression and storage module will be provided which has a compression capacity determined by and coupled to the boiler output, and for example to the output of combustion CO2 at least at nominal steady state capacity. Thus, the compression and storage unit is not decoupled in the same way from the boiler capacity but is preferably rated at least with reference to the required capture rate of nominal boiler demand.
It will be understood in the art that some carbonaceous fuels such as fossil fuels are considered at contributing at 100% of CO2 volume produced to a nominal emissions measurement, whereas other carbonaceous fuels, such as bio-fuels, are considered to contribute at a zero emissions rate.
In a preferred case, the carbon dioxide compression and storage module is rated to a compression capacity which will enable a nominal carbon emissions rate of zero or less during steady state operation of the boiler/turbine.
It follows that in the case where a boiler is designed for firing using fossil fuels with nominal 100% emissions contribution, the compression and storage module should be rated for a compression volume that is equal to that of the total furnace CO2 emission volume at steady state operation. Where a boiler is rated for a fuel mix, such as a mixed fossil fuel and bio-mass firing, or a pure bio-mass firing, which has a reduced or zero nominal emissions contribution, two possibilities arise. A carbon dioxide compression and storage unit may be reduced in size accordingly, so as to compress and store just that quantity of carbon dioxide produced from the boiler which is sufficient to give a zero nominal emissions for the system, with the remaining CO2 being vented to atmosphere, for example via a stack, or the carbon dioxide compression and storage system may have a greater design capacity, producing a system with a negative emissions rate.
In accordance with the invention in a further aspect, a thermal power plant comprises a power generation unit having a comburant gas supply system in accordance with the first aspect of the invention and/or a combustion boiler/turbine system in accordance with the second aspect of the invention.
In accordance with the invention in a further aspect, a method of operation of a thermal power plant having an air separation module for the separation of an oxygen rich comburant gas supply for oxyfuel firing of fossil fuel an oxygen rich comburant gas storage facility is provided. The method includes: providing a combustion boiler/turbine system of a thermal power plant having a combustion boiler for combustion of a fuel in the presence of an oxygen rich comburant gas; determining for the said combustion boiler a nominal steady state comburant gas demand; determining for the combustion boiler a design load factor across a pre-determined operating period; and/or defining required energy storage capacity required to determine ASU unit and LOX storage size; providing in association therewith an air separation module to separate and output an oxygen rich comburant gas from an input air supply, a comburant gas storage module fluidly connected to the output of the air separation module for storage in liquid state of separated oxygen rich gas, and a comburant gas supply module to supply the oxygen rich gas to the combustion boiler selectively from the air separation system and/or the comburant gas storage system; wherein the air separation module is operated at an oxygen rich gas separation capacity based on the said comburant gas demand of the combustion boiler/turbine adjusted to take account of the said determined load factor and/or said required energy storage capacity.
Thus, in accordance with the method of the invention, an air separation module is provided and operated at an output capacity which is decoupled from the demand capacity of the boiler/turbine at steady state, and is less or more than 100% of the said steady state demand, but is instead adjusted to take account of the load factor and/or required energy storage capacity. In particular, a minimum output capacity of the air separation module is preferably determined as the product of the nominal steady state comburant gas demand and the load factor and the required energy storage capacity.
Thus, the method can in an alternative be seen as a method of determination of a design capacity of an air separation module as above described, with reference to the demand capacity of a combustion boiler which it is to supply with comburant gas, which includes: determining a nominal comburant gas supply level for steady state operation for the combustion boiler; determining a load factor for the combustion boiler across a pre-determined operating period; and/or defining a required energy storage capacity; determining a comburant gas output capacity for the air separation module from the nominal steady state demand rating adjusted with reference to the determined load factor and/or required energy storage capacity.
In particular, the method comprises determining a design output for the air separation module which is less than the nominal comburant gas demand of the combustion boiler at steady state, but which is at least the mean comburant gas demand of the boiler over the pre-determined operating period when due account is taken of the load factor and required energy storage capacity.
The method is in particular a method of operation of a thermal power plant comprises a power generation unit having a comburant gas supply system in accordance with the first aspect of the invention and/or a combustion boiler/turbine system in accordance with the second aspect of the invention, and preferred features will be understood by analogy.
In particular it follows that the step of determining a load factor adjusted demand is preferably determined as the product of the nominal steady state comburant gas demand and the load factor.
In particular it follows that the oxygen rich comburant gas is suitable for oxyfuel firing of carbonaceous fuel and is for example preferably substantially free of nitrogen and more preferably substantially pure oxygen.
It is a requirement of the invention that the comburant gas production capacity of the air separation module is decoupled from the demand requirement of the boiler and is instead reduced with reference to a load factor adjusted demand and/or increased to provide required energy storage capacity. In effect, the load factor and/or storage capacity adjusted demand sets a minimum requirement for the production capacity of the air separation module. The air separation module may still be provided with a higher capacity, for example to accommodate peak demand levels above nominal or to provide an energy storage flexibility in which the air separation module is run at a higher capacity during periods of lower power demand.
More specifically in this later case, the method of operation may additionally comprise: tending to reduce the works power of the air separation module in response to an increased grid demand and balancing the same by comburant gas from storage to make up the required supply for oxyfuel firing; or tending to increase the works power of the air separation module in response to a reduced grid demand and balancing the same by supplying the resultant excess to the storage.
Thus, the air separation system is operated at reduced power at times of higher grid demanded output, and this reduced power reduces the overall works power of the plant in order to supply additional power to the grid without the need to vary the power output of the generation plant.
In accordance with the invention the energy stored in compressed nitrogen/argon form in tanks could be recovered when demanded.
In accordance with the invention in a further aspect, a thermal power plant comprises a power generation unit having an oxyfuel firing system including an air separation system as above described.
The principles of operation of the invention will be described in greater detail by way of exemplification with reference to the accompanying drawings in which:
In general principle, a suitable minimum ASU size is determined by the formula:
ASU=(100*(BLF)*O2Dem)+AESC (1)
The ASU could be oversized to accommodate a maximum boiler demand that is higher than nominal demand. Additionally the LOX storage could be oversized to accommodate a maximum boiler demand that is higher than nominal demand.
A possible embodiment of the invention is presented in
Another possible embodiment of the invention is presented in
Another possible embodiment of the invention is presented in
Another possible embodiment of the invention is presented in
Although
Another possible arrangement of the invention showing various such combinations, the principles of each of which may be applied separately in a practical embodiment of the invention, is presented in
The illustrated embodiment has four Boiler/Turbine Units A, B, C, and D, and one common LOX storage.
Boiler/Turbine Unit A fires 50% of coal and 50% of biomass and operates with load factor 0.75 over an operating period. The Boiler/Turbine Unit A has one ASU unit and the ASU unit is sized to 75% of Boiler/Turbine Unit nominal Oxygen requirement and is supported by external to ASU LOX Oxygen storage. The CPU Unit is sized for full CO2 gas volume from firing the coal and the biomass in the Boiler/Turbine Unit A. Compressing and storing emissions from firing the coal and the biomass results in the unit having nominal negative emissions.
Boiler/Turbine Units B and C have one shared ASU unit sized to 75% of both Boiler/Turbine Unit B and C nominal Oxygen requirements and is supported by external to ASU LOX Oxygen storage.
Boiler/Turbine Unit B fires 50% of coal and 50% of biomass and operates with a load factor 0.75 over an operating period. The CPU Unit for Boiler/Turbine Unit B is sized for full from firing the coal only. CO2 gas volume attributable to firing the biomass is vented via the stack. Compressing and storing emissions from firing the coal results in the unit having nominal near zero emissions. Boiler/Turbine Unit C fires only coal and operates with load factor 0.75 over an operating period. The CPU Unit for Boiler/Turbine Unit C is sized for full CO2 gas volume from firing the coal. Compressing and storing emissions from firing the coal results in unit having nominal near zero emissions.
Boiler/Turbine Unit D has Oxygen supplied from multiple ASU units. The ASU units are different sizes, however the combined size of the ASU units is sized to 75% of Boiler/Turbine Unit D nominal Oxygen requirement and is supported by external to ASU LOX Oxygen storage. The Boiler/Turbine Unit D fires 50% of coal and 50% of biomass and operates with load factor 0.75 over an operating period. The CPU Unit for Boiler/Turbine Unit D is sized for full CO2 gas volume from firing the coal and the biomass. Compressing and storing emissions from firing the coal and the biomass results in unit having nominal negative emissions.
In all presented possible arrangements the electrical energy required to power the ASU unit is preferably supplied from a renewable energy source or a low cost energy source and is decoupled from the Boiler/Turbine Unit operation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1116157.7 | Sep 2011 | GB | national |
This is a National Stage of International Application No. PCT/GB2012/052300 filed Sep. 18, 2012, claiming priority based on GB Patent Application No. 1116157.7 filed Sep. 19, 2011, the contents of all of which are incorporated herein by reference in their entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2012/052300 | 9/18/2012 | WO | 00 | 3/19/2014 |
