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Oxygen supply for oxyfuel CO2 capture
International Journal of Greenhouse Gas Control 5S (2011) S194–S203
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Oxygen supply for oxyfuel CO2 capture Paul Higginbotham a,∗ , Vince White a , Kevin Fogash b , Galip Guvelioglu b a b
Air Products PLC, Hersham Place Technology Park, Molesey Road, Walton-on-Thames, Surrey KT12 4RZ, UK Air Products & Chemicals, Inc, 7201 Hamilton Blvd, Allentown, PA 18195, USA
a r t i c l e
i n f o
Article history: Received 13 January 2011 Received in revised form 14 February 2011 Accepted 15 March 2011 Available online 22 April 2011 Keywords: Air Products Oxygen Oxyfuel Oxycoal CO2 capture Air separation unit
a b s t r a c t This paper presents the results of a study to develop Air Products’ air separation unit (ASU) offerings for oxyfuel coal CO2 capture projects. A scalable “reference plant” concept is described to match particular sizes of power generation equipment, taking into account factors such as safety, reliability, operating flexibility, efficiency, and low capital cost. We describe the selection of a process cycle to exploit the low purity requirements, as well as the options for compression machinery and drivers as the scale of the plant increases and the sizes of referenced equipment limit the possibilities. We also explore integration with other elements of the system, such as preheating condensate or heating and expanding pressurised nitrogen. In addition, we consider how the ASU affects the flexibility of the oxyfuel system and discuss how its power consumption can be reduced during periods of high power demand. Finally, the advantages and disadvantages of different execution strategies for air separation unit projects are discussed, as well as alternative commercial models for the supply of oxygen. © 2011 Published by Elsevier B.V.
1. Introduction Since Air Products began working on the supply of oxygen for oxyfuel coal combustion nearly twenty years ago (Allam and Spilsbury, 1992), the necessity of reducing carbon dioxide emissions has been increasingly widely recognised. We are now seeing the initiation of projects for the design and construction of the first large-scale oxyfuel coal combustion demonstration plants of around 200–300 MWe scheduled to be on stream from 2015. The building of the first generation of commercial oxyfuel coal plants in multiples of around 500–600 MWe is expected to start between 2015 and 2020 to meet increasing carbon dioxide reduction targets in Europe and around the world. Although several different air separation processes are commercially available, the only one currently proven and economically viable at the scale required for oxyfuel coal combustion is cryogenic distillation, and so that is the focus of this paper. In the future, processes such as those using ion transport membranes (ITMs) will be scaled up and proven at larger scale and may provide a more economic oxygen supply, but in the meantime cryogenic distillation will be important for at least the first generation of commercial oxyfuel coal combustion plants. It is important that the imminent demonstration projects are able to follow as closely as possible the concept for the full-scale
∗ Corresponding author. Tel.: +44 1932 249506. E-mail address: [email protected] (P. Higginbotham). 1750-5836/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.ijggc.2011.03.007
commercial plants both in technology selection and execution strategy to provide the most benefit to future projects. Air Products has designed a scalable reference plant concept for the supply of oxygen to oxyfuel coal power plants taking into account the unique requirements of this application. We will now describe the development of the process and plant design considering the product requirements, process selection, equipment design and execution strategy.
2. Requirements for oxyfuel coal combustion The reason for combustion of coal in oxygen rather than air is to remove nitrogen from the combustion air so that very little remains in the flue gas and the CO2 is easy to purify. In order to use existing boiler designs with minimal modifications, flue gas is recycled so that CO2 replaces the nitrogen in air and the oxygen concentration during combustion is selected to suit the operating and materials constraints in the boiler. The oxygen requirements for oxyfuel combustion in a coal-fired power station are different from those of other oxygen users in several ways. Because the boiler operates at a slightly sub-atmospheric pressure there is always some leakage of air into the boiler, and so there will always be a need to remove some nitrogen and argon from the carbon dioxide. This means that the oxygen purity can be low (<97%) as the remaining impurities will be removed in the carbon dioxide purification process. The oxygen pressure is also low, close to atmospheric pressure, to match the combustion air pressure in a conventionally-fired boiler. The flow of oxygen required is
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large, requiring multiple world-scale plants for a typical boiler size, for instance a 500 MWe power plant will consume around 10,000 tonnes per day of oxygen. There is normally no significant associated demand for any gaseous or liquid co-products, such as argon or nitrogen, except for relatively very small quantities of utility nitrogen or instrument air and liquid for back-up storage. Many coal-fired power stations that might be converted to oxyfuel combustion are located inland without easy access to rivers or the sea, and so the dimensions of equipment that can be shipped to the sites are limited. Depending on the particular location, options may be needed to use multiple parallel process units within a single train or even multiple smaller trains. The parasitic nature of the power means that high efficiency is important, but the large scale means that capital cost should be minimised. This unique combination of factors opens up new possibilities to optimise the ASU process cycle and machinery configuration to minimise the power consumption of the air separation unit without compromising the requirement for low capital cost. 3. Development of air separation units for oxyfuel coal combustion Multiple factors must be taken into account in the design of an air separation plant to match a particular set of product requirements including safety, efficiency and capital cost (Allam et al., 2003; White et al., 2009). A particularly important parameter is the capitalised value of power, i.e. the additional capital investment worthwhile to save a unit of power, normally expressed as $/kW. This determines whether a more costly, high efficiency plant or a less costly, low efficiency plant is more appropriate. In the case of oxyfuel combustion, the power value is expected to be relatively high; any saving in ASU power will increase the power output from the power station and so reduce the specific capital cost of the rest of the power plant and lower the cost of CO2 capture. Air Products has proposed the use of a triple column cycle to minimise the parasitic power consumption of the air separation unit for oxyfuel power generation (Dillon et al., 2005), and others have put forward alternative solutions (Darde et al., 2009; Beysel, 2009; Tranier et al., 2009). In developing a range of reference plants for this application, we have evaluated several alternative cycles on a comparable basis to confirm that this is an appropriate cycle choice. 3.1. ASU cycle selection Over the 100 or so years since the double column air separation process was invented, many modifications have been developed to supply different purity, pressure and flow requirements of gaseous and liquid oxygen, nitrogen and argon products. In most cases, the double column system still provides an optimum balance of efficiency and capital cost where high purity oxygen is required and power costs are moderate. Oxygen is considered to have a low purity if it is less than about 97%, where almost all the nitrogen has been removed but little of the argon. Because it is more difficult to separate argon from oxygen than nitrogen from oxygen, the separation energy is less for low purity oxygen than high purity oxygen. Numerous process cycles have been developed for producing low purity oxygen, all capable of realising the potential theoretical power saving in different ways and to varying extents. Despite this, surprisingly few low purity oxygen plants have actually been built because of the value of argon that can be co-produced when the oxygen purity is increased, even if the actual requirement is for a low purity. A notable exception is where relatively large quantities of oxygen are needed without a correspondingly large adjacent argon market, for example IGCC schemes, and in future oxyfuel combustion applications.
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Because of the low purity and pressure of the oxygen, the specific power of a plant designed for oxyfuel coal combustion will be very low and the problem arises of how to design a cycle to process all the air required with a low enough net power input. There are broadly three approaches that can be taken: 1) Minimise total power input by reducing the feed air pressure, producing each product at no more than its specified pressure and venting waste nitrogen to the atmosphere. 2) Minimise net power input by maintaining a normal feed air pressure but recovering some compression energy as a pressurised product, and if necessary recovering excess compression energy by expansion. 3) Minimise net power input by increasing the operating pressure of the ASU and producing all the waste nitrogen at high pressure and using or recovering the compression energy it contains. In general, approaches 2 and 3 are only worthwhile when there is actually a need for the compressed product stream so that the avoided product compression power can be credited against the input power to give a low net separation power and the capital cost of the product compressor can be reduced. If there is no use for the pressurised product, only part of the energy can normally be recovered by expansion, and additional capital equipment is required in the form of an expander. A number of ASU processes were compared following all three approaches, including: 1) The three column cycle proposed by Air Products (Dillon et al., 2005, Fig. 1, cycle 1 in Table 1). 2) A conventional double column cycle (Fig. 2, cycle 2 in Table 1). 3) A dual reboiler cycle (Fig. 3, cycle 3 in Table 1). 4) The three column cycle with elevated column pressures (Fig. 4, cycle 4 in Table 1). 5) An elevated pressure dual reboiler cycle with a low pressure column condenser (Fig. 5, cycle 5 in Table 1). In comparing process cycles, it is important to optimise each one with appropriate comparable pressure drops and temperature differences and to consider the costs associated with the differences in pipe and heat exchanger sizes. Possible machinery options and compressor sizes must also be taken into account. Table 1 compares the oxygen separation shaft power (i.e. the part of the shaft power for producing oxygen at 1.013 bar(a)) for the different cycles at ISO conditions (Darde et al., 2009) for a plant optimised for a high power cost making 5400 tonnes per day contained oxygen at 95% purity and 1.1 bar(a) pressure. Motor and transformer losses, cooling system and molecular sieve regeneration energy are all excluded and there are no liquid or gaseous co-products, so that the basis is the same as Darde et al. (2009). Each process was also evaluated with pressurised gaseous nitrogen (GAN) produced directly from the column system, and the specific powers for oxygen production compared for two such cases. In the first case, the GAN is expanded to recover power after heating with heat from the air compressor only. In the second case, the GAN is used as a pressurised product and so the power to compress it from atmospheric pressure to the pressure at which it leaves the process is credited against the oxygen production power (since this is the avoided power of the GAN compressor that would otherwise have been needed). 3.1.1. Three column “dual high pressure column” cycle In IEA GHG report 2005/9 (Dillon et al., 2005), Air Products described a triple column cycle for producing low purity oxygen for oxyfuel combustion (a variant of which is illustrated in Fig. 1). In that report, the specific electrical power consumption for a plant
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Fig. 1. Three column (dual high pressure column) cycle.
producing 95% oxygen at 1.6 bar(a) was stated as 201.3 kWh per tonne of contained oxygen (kWh/t) under ambient conditions of 9 ◦ C, 1013 mbar, 87% RH and 12 ◦ C cooling water. At the same time, approximately 128 kWh heat per tonne of contained oxygen is exported to the condensate preheat system which allows additional power of about 24.6 kWh per tonne of contained oxygen to be generated by the LP steam it displaces. Table 2 shows how this power can be adjusted to ISO conditions of 15 ◦ C, 1013 mbar and 60% RH and 15 ◦ C cooling water and how it can be split up into its individual components. First, the power is increased to take account of the warmer inlet air and cooling
water temperatures, and then reduced to exclude the cooling system power and electrical losses, since we are calculating the shaft power only. Finally, the power to compress the oxygen from atmospheric pressure to 1.6 bar(a) is deducted and the power credit for the additional power generated by the exported heat is applied. This leaves the contained gaseous oxygen (GOX) separation shaft power on the basis used by Darde et al. (2009) of 162.4 kWh/t (0.232 kWh/Nm3 ). With intercooled air compression and no heat integration, there is no power credit from the exported heat, but the compressor power is reduced, so the specific GOX separation shaft power increases to 177.4 kWh/t (0.253 kWh/Nm3 ). The table also
Table 1 Comparison of different process cycles for 5400 TPD contained oxygen at 95% purity at ISO conditions. Cycle number Description
Low pressure column pressure Oxygen recovery Input power GAN power credit (avoided compression power) GAN power credit Net GOX Power Specific shaft separation power % GAN credit to equal (1a) Specific power (GAN power recovery, no external heat)
1a Three column cycle
1b Three column cycle + MPGAN
2a Dual column cycle
2b Dual column cycle + MPGAN
3a Dual column dual reboiler
3b Dual column dual reboiler + MPGAN
4 EP three column cycle + MPGAN
5 EP dual column dual reboiler + MPGAN
bar(a)
1.2
1.2
1.2
1.2
1.2
1.2
2.7
4.2
% kW kW
97% 36,410 0
93% 40,900 7107
99% 42,767 0
92% 47,960 11,540
90% 38,256 0
83% 45,351 10,566
89% 58,716 26,907
87% 72,586 42,949
% power in 0% kW 36,410 kWh/t 158
17% 33,793 147
0% 42,767 187
24% 36,420 158
0% 38,257 167
23% 34,785 151
46% 31,809 138
59% 29,637 128
%
63%
N/A
100%
N/A
85%
83%
84%
166
177
190
kWh/t
N/A
157
175
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Fig. 2. Conventional double column cycle.
Fig. 3. Dual reboiler cycle.
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Fig. 4. Elevated pressure three column cycle.
Fig. 5. Elevated pressure dual reboiler cycle.
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Table 2 Breakdown of specific power for the three column cycle in Dillon et al. (2005) for comparison with Darde et al. (2009). kWh/t contained O2 Total specific power at ambient conditions 9 ◦ C 87%RH and 12 ◦ C cooling water (Dillon et al., 2005) Adjusted to ISO conditions 15 ◦ C, 60%RH and 15 ◦ C CW Front end cooling pumps Motor and transformer losses GOX compression (1.013–1.6 bar(a)) Credit for additional power generated by heat integration Equivalent GOX separation specific shaft power at ISO conditions with adiabatic compression and heat integration Adjustment for intercooled compression and no heat integration Equivalent GOX separation specific shaft power at ISO conditions with intercooled compression and no integration Additional specific power: TSA regeneration (heat duty) Electrical equivalent of TSA regeneration
indicates how the electrical equivalent of the average temperature swing adsorber (TSA) regeneration heat duty can be accounted for in the calculation of overall power consumption, assuming a thermal efficiency of about 25% for power generation from medium pressure steam. Further optimisation of this process to reduce pressure drops and temperature differences in the plant has resulted in a further 10% reduction in unintegrated GOX separation shaft power from 177 kWh/t to about 158 kWh/t (0.226 kWh/Nm3 ) (see Table 1, cycle 1a), with the potential for further reductions from integration or by trading off further increased capital cost for reduced power consumption at higher power values. With adiabatic compression and heat integration, this would lead to a value around 143 kWh/t. If low purity, medium pressure (MP) gaseous nitrogen (GAN) is required at about 2.5 bar(a) and its avoided compression power is credited against the oxygen separation power at the same efficiency as the air compression, this is reduced by a further 7% to 147 kWh/t (cycle 1b) (or 132 kWh/t with heat integration). The input power increases by around 12% as the oxygen recovery is reduced and the feed pressure increased, but this is outweighed by the nitrogen power credit of about 17% of the total input power. Care must be taken when comparing powers incorporating such credits that equal efficiencies are used for air and nitrogen compression – if a lower nitrogen compressor efficiency is assumed than that of the air compressor, the separation power could appear artificially reduced. Table 1 shows that for this cycle, if less than 63% of the nitrogen compression power can be recovered, the net power is higher than the base cycle with no MP GAN. The typical power recovery rate from an expander with no external heat input (i.e. only heating nitrogen with heat from the compression system) is about 66%, which gives a specific power of 157 kWh/t. This is virtually the same as the base cycle, so attempting power recovery in this way is not worthwhile. 3.1.2. Conventional dual column cycle A similar optimisation process for the conventional dual column cycle (2a) illustrated in Fig. 2 leads to an oxygen separation shaft power of 187 kWh/t (about 18% higher than the triple column cycle) when no pressurised product is taken from the high pressure column, but the capital cost is reduced because the main heat exchanger volume is reduced as a result of excess refrigeration from the high expander flow and pressure ratio. However, if pressurised nitrogen is taken from the high pressure column (cycle 2b) and credited with the avoided compression power, the specific power may be reduced to 158 kWh/t (as the expander flow and excess refrigeration are reduced). This is the same as the triple column cycle with no GAN production, and narrows the gap with the triple
kWh/t contained O2
kWh/Nm3 contained O2
201.3
0.288
+4.0 −1.0 −4.1 −13.2 −24.6
205.3 204.3 200.2 187.0 162.4 162.4
0.293 0.292 0.286 0.267 0.232 0.232
+15.0
177.4 177.4
0.253 0.253
16.2 4.1
0.023 0.006
column with GAN production to 7%. But the power credit accounts for 24% of the input power and all of it must be recovered to equal the base triple column cycle, whereas only about 66% can actually be recovered by internal heating and expansion. The specific power with such GAN power recovery is 175 kWh/t, 11% higher than the base cycle. 3.1.3. Dual reboiler cycle An alternative cycle for low purity oxygen production is the dual reboiler cycle illustrated in Fig. 3. Like the triple column cycle, this cycle has two reboilers in the low pressure column, but instead of being reboiled with nitrogen, the lower one is boiled by condensing air. This reduces the oxygen recovery, as there is less liquid nitrogen reflux for the low pressure column. With no GAN production from the high pressure (HP) column (cycle 3a), the oxygen separation shaft power is 167 kWh/t, 5% higher than the triple column cycle. With GAN production and full power credit (cycle 3b), this is reduced to 151 kWh/t, still 3% higher, and the air flow required (and therefore plant size) is about 10% bigger. The power recovery needed from the GAN for this cycle to match the unintegrated triple column cycle is 85%; with the achievable level of 66% power recovery, the specific power of 166 kWh/t is virtually the same as when no GAN is produced. 3.1.4. Elevated pressure three column cycle If the column pressures in the three column cycle are elevated so that all the waste nitrogen leaves the plant at about 2.5 bar(a) and the oxygen is boiled with the medium pressure feed rather than the high pressure feed (Fig. 4 and cycle 4 in Table 1), the GOX separation shaft power can be reduced a further 6% to about 138 kWh/t if full credit is taken for the nitrogen compression power. But now the input power is further increased so that it is 85% more than the net power and 46% of it has to be recovered as nitrogen compression credit. If there is no use for the pressurised nitrogen, the required power recovery rate to equal the base cycle increases to 83%. This type of process in not suitable for unintegrated power recovery as its specific power is then 177 kWh/t, 12% higher than the base cycle. 3.1.5. Elevated pressure dual reboiler cycle As the pressures of the columns of the dual reboiler cycle are increased, eventually the pressure of the low pressure column becomes high enough for nitrogen from it to boil the oxygen product at the required low pressure (Table 1, cycle 5 and Fig. 5). This is a particularly efficient configuration because extra liquid nitrogen reflux for the low pressure column can be generated in this oxygen reboiler to make up for that lost by condensing air in the low pressure column reboiler. If a full power credit can be applied for all
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the nitrogen compression, the GOX separation shaft power of this cycle can be reduced to 128 kWh/t, 7% lower than triple column cycle with column pressures elevated to a lesser extent. However, the drawback of this low net power consumption is that 59% of the power has to be recovered in the compressed nitrogen. Like the elevated pressure triple column cycle, more than 84% of this theoretical nitrogen compression power must be recovered to result in a lower specific separation power than the base triple column cycle. This is not attainable with unintegrated power recovery, when the specific power is 190 kWh/t, 20% higher than the base cycle and even higher than the conventional cycle. 3.2. Integration We have seen that ASU cycles that rely on power recovery from pressurised nitrogen are not worthwhile when no external heat is available, as the input power is significantly increased but the theoretical power credit from the products cannot be fully realised. Another approach (Tranier et al., 2009) is to use the pressurised nitrogen to run a Brayton power generation cycle alongside the steam cycle by taking heat from the boiler. This only provides a net benefit if the Brayton cycle is more efficient than the steam cycle at converting this heat to power. For this to be the case, the nitrogen must be heated to at least about 600 ◦ C so that the temperature ratio between compression and expansion is high enough. This requires modification to the boiler superheat zone, and if it leads to a reduction in available steam superheating it would compromise the steam cycle efficiency and could offset any potential gain. It also requires the development of high temperature, high flow expanders. For this reason, such a process was not selected for the first generation of oxyfuel coal plants which aim to demonstrate and then apply the technology with minimum modifications to existing air-based boiler equipment. The ASU is therefore designed for maximum efficiency as a stand-alone unit. However, as indicated by Dillon et al. (2005), the selected triple column cycle is still ideally suited to integration with the power plant by the use of adiabatic air compression with low temperature heat recovery into the steam system, and there is also the option to produce as much low purity, medium pressure nitrogen at 2.5 bar(a) as can be effectively used. 3.3. Machinery and drivers The air compressors and their drivers are a significant component of the cost of an ASU and are responsible for most of the power consumption. So it is critical that their efficiency is optimised. The size of air compressors available will typically be a limiting factor on ASU train size, although it is possible to use multiple compression trains for a single cold box. Centrifugal air compressors are generally lower cost than axial machines and have been increasing in size over recent years, so that they are now available for air flows corresponding to up to around 5000 tonnes/day oxygen (by using parallel first stage impellers). Axial compressors are available up to about 8000 tonnes/day oxygen equivalent, and have been used for many years in the steel industry as blast furnace blowers. Axial compressors derived from gas turbine compressors are being developed, and will soon be offered at even larger sizes. The main drive options for ASU compressors are electric motors or steam turbines. Gas turbine drives are occasionally used, but are not suitable for oxyfuel coal combustion applications. Steam turbine drives can have the advantage of lower mechanical and electrical losses, but steam turbines designed for mechanical drives are smaller and generally less efficient than those designed for power generation. They will also result in more complex opera-
Table 3 Air Products’ reference plant sizes. Plant size – contained O2 tonnes/day
Machinery options
Approximate power consumption, MW
3000–4000
Centrifugal 1 or 2 train or axial 1 train Centrifugal 1 or 2 train or axial 1 train Centrifugal 2 train or axial 1 train Centrifugal or axial 2 train
22–33
4000–5500 5500–7000 7000–10,000
30–45 41–58 53–82
tions, as steam must be available before the ASU can be started. Electric motor drives mean that there will be additional losses due to the conversion of steam to electricity in the turbine and generator and back to shaft power in the motor, but the gain in steam turbine efficiency is similar to these extra losses. Driving the compressors with electric motors simplifies the start up and operation of the ASU, allowing it to be almost independent of the power plant, and will improve its availability. Because of the low power consumption of the chosen process, the drivers will always be within referenced power limits, even for the largest ASU sizes. 3.4. Plant sizes Taking into account the anticipated oxygen requirements of oxyfuel coal combustion power plants, Air Products has developed conceptual designs for a scalable plant, covering the approximate ranges of contained oxygen production shown in Table 3. All these plant sizes use columns that are no bigger in diameter than those currently operating or, for the largest size, within proven scale-up and existing manufacturing capabilities. The use of multiple columns where necessary helps this scale-up in single cold box train capacity and does not add significantly to the cold box cost, which is in any case a relatively minor part of the total. In full scale commercial applications, multiple plants will be required, and, depending on location and access for transport, it may be necessary to select more, smaller plants. 3.5. Safety The very low pressure of oxygen needed for oxyfuel coal combustion means that the oxygen boils at a relatively low temperature and the solubilities of CO2 and N2 O in liquid and vapour oxygen are therefore low. So, even with very high removal rates in the front end purification system, oxygen cannot be boiled to dryness in the main heat exchanger without exceeding the vapour solubilities and risking precipitation of these contaminants leading to a blockage and potential accumulation of hydrocarbons. As a result, the oxygen is boiled in a separate reboiler unit, from which sufficient liquid purge must be withdrawn to limit the contaminant concentrations. The required purge rate is minimised by designing the front end purification system for high removal rates and by monitoring the concentration of contaminants in this reboiler during operation. 3.6. Front end air purification Air Products’ advanced, low-energy, multi-layer horizontal adsorbers are used for the front end clean-up to give the most cost-effective control of contaminants. The design allows the use of lower pressure steam or hot condensate for regeneration heating to minimise the impact on the power generation cycle, and ensures that the regeneration heat is used effectively. To minimise regeneration energy and capital cost and to maximise transportability, two adsorber systems are used, one at the higher feed air pressure
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(about 4.5 bar(a)) and one at the lower pressure (about 3 bar(a)). As well as being efficient and effective, the proven mechanical design is robust and low cost.
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tive control (MPC) will be used to optimise efficiency and minimise purity deviations during ramping. 3.10. Back-up system
3.7. Heat exchangers The low operating pressures of the chosen cycle mean that there is a wide choice of suppliers for the brazed aluminium plate-fin cores used for the main heat exchangers. Larger numbers of these cores are used than in a conventional air separation plant because of the high value of power, but they can be easily manifolded together in the conventional manner. For the distillation column reboilers, a downflow (falling film) design is selected to minimise the temperature approaches and optimise power consumption. Their operating safety is assured by the high liquid purge rates from these reboilers and by the removal of contaminants to very low levels in the front end air purification system. However, for the oxygen product reboiler, a thermosyphon reboiler is used because it can safely operate closer to the solubility limits of CO2 and N2 O, as described in Section 3.5. 3.8. Distillation columns All the distillation columns use Air Products’ proprietary highcapacity structured packing to minimise the column diameters needed and allow shipping of complete columns within specified transport limits. The low pressure drop of structured packing helps to achieve the very low specific power offered by this type of plant. Air Products’ proprietary distributor design and programme of testing each individual distributor ensures good performance over a wide operating range even at the large diameters required for these plants. Two parallel low-pressure distillation columns are used where necessary to permit the columns to remain within scale-up, manufacturing or shipping limitations. 3.9. Operating flexibility The turndown of an ASU is limited by the air compressors rather than the cold box. For maximum efficiency, the normal operating range of the reference plants is 75–100%. If dual machinery trains are used, it is possible to turn down to 50% to run with a single compression train, but it will generally be impossible to run between 50% and 75% without venting air, as both machine trains will need to run in this range. The operating range can be increased by modifying the machine design, but there will be a penalty in efficiency at the design point. The range can also be increased by using more compression trains, for example 3 × 33% or 4 × 25% compressors, and stopping and starting compressors as needed. For very large commercial systems with multiple plants and compressors, the operating range will naturally be more continuous, as it is possible to turn compressors on and off on some plants and not on others. Dual machinery trains can be advantageous here, as all the cold boxes can be kept operational as compressors are stopped and started. In integrated schemes, it may be necessary to incorporate additional equipment for start-up, shut-down or off-design operation to ensure that integrated heat loads always remain balanced. For example, heat supplied by the compressors will not be available until the ASU is started up, and in certain turndown cases it will increase in relation to the condensate preheat demand as the compressors have to vent excess air, and some of the heat may need to be removed by cooling water. The ASU has been designed to allow rapid ramping at 5% of design capacity per minute and a dynamic model of the process has been developed to demonstrate this capability. Model predic-
The optional back-up system will provide instantaneous additional oxygen supply in the event of a plant or compression train trip or peak shaving requirement (e.g. at extreme ambient conditions) and may be used to assist ramping at even faster rates. The quantity of back-up storage installed is flexible depending on customer availability targets, but due to the large size of these plants it is likely to cover only a few hours of operation at full rate. Since oxygen is only required when the power plant is operating, warm cooling water is used to vaporise the liquid oxygen when required. The ASU is capable of supplying liquid when necessary to refill the back-up tanks in a specified time period. 3.11. Electrical load management A further optional feature is the provision of liquid air storage in addition to liquid oxygen storage. Since liquid oxygen is produced from the distillation column system and evaporated against condensing air, storing both liquids allows the supply of oxygen from the cold box to be decoupled from its production by the distillation columns. If the oxygen production is less than the demand, it can be supplemented from storage and the resulting additional liquid air sent to storage with no net refrigeration requirement or disturbance to the column system. All that is needed is an increase in air flow from the compression system to provide the extra air to boil the oxygen. Conversely, if the ASU is producing more oxygen than required, liquid air can be injected from storage to provide refrigeration to produce liquid oxygen that can then be stored rather than evaporated. In this case, less air has to be provided by the compression system, since part of the feed is provided as liquid air. As well as adapting to variations in oxygen demand, the system can also be used to maintain the oxygen supply rate whilst the ASU is turned down to reduce its power consumption during peak power demand periods and turned up during off-peak periods. The effective cost of the power provided to the ASU can therefore be reduced and the maximum output of the power station increased. In practice, a combination of power management and variable oxygen supply will be likely as the boiler’s oxygen demand will be reduced during periods of low power demand and increased when power demand is high. In assessing this option, it is important to understand the likely operating profile of the power station and evaluate the potential benefits of load management against any additional costs associated with the ASU, compressors and storage. For example, it may be worthwhile to oversize the ASU to allow load management even at full oxygen output, or it may only be required when operating with average production rates below design capacity. Table 4 provides an idealised example in which the ASU is sized for 112% design capacity with two compression trains each operable between 40% and 56%. The columns also operate between 40% and 112%. Air flows are relative to 100% air flow and oxygen flows are relative to 100% oxygen flow. At oxygen demands below 40% of design capacity, the ASU is fully turned down and operates continuously at this rate. At higher oxygen demands, the ASU supplies the specified oxygen rate continuously, but the distillation columns are turned down for part of the time (low power mode) and turned up for the rest of the time (high power mode). In low power mode, only a single compression train operates, and in high power mode both are running. The relative air flows in the example also give an indication of the relative powers in the different cases. The fraction of the time that the ASU can operate in low power mode is determined by its sizing and the required output, and with the 112% ASU
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Table 4 Example of electrical load management scheme using liquid oxygen/liquid air swap. Relative oxygen demand
Relative MAC air flow in low power mode
Relative MAC air as liquid to storage in low power mode
Relative liquid oxygen from storage in low power mode
Relative MAC air in high power mode
Relative MAC air as liquid from storage in high power mode
Relative liquid oxygen to storage in high power mode
Fraction of time in low power mode
0.40 0.50 0.56 0.60 0.70 0.80 0.90 1.00
0.40 0.43 0.44 0.45 0.48 0.51 0.54 0.56
0.00 0.03 0.04 0.05 0.08 0.11 0.14 0.16
0.00 0.10 0.16 0.20 0.30 0.40 0.50 0.60
N/A 0.95 0.97 0.98 1.01 1.03 1.06 1.09
N/A 0.17 0.15 0.14 0.11 0.09 0.06 0.03
0.00 0.62 0.56 0.52 0.42 0.32 0.22 0.12
1.00 0.86 0.78 0.72 0.58 0.44 0.31 0.17
in the example this is one sixth of the time at 100% output, increasing to nearly a third at 90% output. The example also shows that the ASU can operate more efficiently in the 56–80% range by switching between the two modes than by operating continuously with both compressors turned down to minimum and venting air or oxygen. 3.12. Rare gas production The rare gases xenon and krypton are normally produced from air separation plants, even though they are only present in the air at very low levels. Large-scale ASUs for oxyfuel combustion applications have the potential to become a significant source of these gases, whose production and sale could improve the economics of these projects. In the process selected, virtually all the xenon and a significant quantity of the krypton in the air are present in the liquid purge from the oxygen product reboiler and can be recovered relatively easily. If required, virtually all the krypton can also be recovered at low cost by adding a few distillation stages in the top of the reboiler vessel. 3.13. Execution strategy The plant is designed so that the main columns, tanks, adsorber vessels and heat exchanger assemblies are shop-fabricated and shipped to site as complete units. The final cold box assembly and other interconnections are completed on site. Multiple units are used where necessary to overcome shipping limits, and smaller air separation trains may even be selected if required. 4. Oxygen supply concepts Oxygen consumers may choose whether to buy an oxygen plant (a “sale of equipment” (SOE) contract) or to buy oxygen from an industrial gases supplier (a “sale of gas” (SOG) or “build own operate” (BOO) contract). There are advantages to each in particular situations, and each type of supply can be configured in different ways. 4.1. Sale of gas – oxygen supply In a sale of gas contract, the oxygen supplier owns, operates and maintains the air separation unit and supplies the oxygen through a pipeline to the consumer. The oxygen supplier is typically an industrial gas company, or a joint venture between the customer and an industrial gas company. To justify the large investment required to construct an oxygen plant, contracts must be long term, typically 15–20 years. Such contracts will have fixed and variable cost elements, and will include “minimum take or pay” provisions so that the capital investment is repaid. Power may be paid for by the customer or the supplier and the variable cost adjusted accordingly.
Sale of gas contracts provide the biggest benefit when the oxygen supplier can sell additional co-products such as nitrogen, argon or liquids to other long term customers or into the merchant market, as the costs of the plant can be shared. A further benefit is that operating expertise of the industrial gas companies allows them to make guarantees on oxygen availability, should that be important to the customer. In addition, any risk on the investment in the plant is borne by the supplier, and so the customer can have certainty on the cost of oxygen for the lifetime of the contract. If there is integration between the ASU and the customer’s process, for example in an oxyfuel power plant, the contract becomes more complicated, but integrated schemes (including transfer of steam and condensate) have been operated successfully by Air Products and other industrial gas companies over many years. 4.2. Sale of equipment – ASU supply If the oxygen consumer wishes to own the ASU, there are numerous possible contract structures, some of which are outlined below. The main differences are in the scope of supply and who will eventually operate and maintain the plant. 4.2.1. Lump sum turnkey In a lump-sum turnkey project, the supplier hands over a fully operational plant for a fixed lump sum at a particular date. A customer will opt for such a project to minimise his exposure to cost overruns. However although a large element of this risk is with contractor, as with all large projects it ultimately remains with the client if it escalates to a level that the contractor cannot bear. Lumpsum turnkey contracts inevitably include contingency to mitigate the contractor’s risk, and so may not be the lowest cost model. 4.2.2. Reimbursable turnkey In a reimbursable turnkey project, the client retains the cost risk but works closely with supplier to control it and pays the contractor for work done. This avoids the addition of contingency by the supplier, and potentially benefits both parties. 4.2.3. “Proprietary kit” model When an ASU makes up part of a much larger project, Air Products has developed the “proprietary kit” supply model which can provide a low cost solution to ASU supply. It may be advantageous for the overall contractor to do the bulk of the detailed engineering, procurement and construction (EPC) of those parts of the plant that are not proprietary. Air Products would supply the overall process design package and the “proprietary kit” consisting primarily of the air purification equipment and the cold box. Machinery, which makes up a significant proportion of the cost, may be purchased directly by the client, thus avoiding multiple mark-ups. The use of a single EPC contractor for the overall project can simplify the coordination of construction and reduce the overall project risk.
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4.2.4. Operation and maintenance (O&M) contracts Air Products specialises in safe and efficient plant operation and maintenance, and so it is often called upon by clients to operate and maintain their plants according to a specified required performance, for example operating efficiency and product availability. Guarantees and associated bonuses and penalties may also be provided. This relieves the client of the need to operate and maintain a plant which might be quite different from others on his facility and unfamiliar to his operators. 5. Conclusions • Oxygen supply requirements for oxyfuel coal combustion are unique and enable the use of process cycles with particularly low power consumption. • Air Products has developed a state-of-the-art scalable ASU concept that uses an efficient and cost-effective three column cycle optimised to provide a low specific power for oxyfuel coal combustion applications whether or not it is integrated with the power generation cycle. • A number of processes were evaluated as part of the reference plant development, and it was found that although it is possible to achieve a lower specific separation power than the triple column process proposed by Air Products, this would require significant machinery development and modification of well-proven boiler equipment. • In comparing specific power consumption figures, care is needed to ensure that they are comparable and attainable within the combination of specifications for a particular project, such as operating range and level of integration. • The reference plant concept uses multiple individual process units where necessary but allows single train cold boxes to be built to supply up to around 7000 tonnes/day oxygen within the
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size of existing proven equipment and 10,000 tonnes/day oxygen with modest scale-up. Single train machinery is possible up to about 8000 tonnes/day. • The plant is capable of rapid load change to meet the requirements of the power generation system to modulate its output as necessary to meet the requirements of the electricity distribution network. • It is possible to configure the ASU to effectively store energy by swapping electrical load from one time period to another. To design the system appropriately, the expected average loading of the ASU and power plant must be considered and the benefits of increased stored energy weighed against additional ASU costs. • Heat integration of the ASU with the boiler system can significantly lower the effective energy consumption of the oxygen plant, however the usefulness of the heat from the ASU depends on its temperature level and the configuration of the power generation cycle. References Allam, R.A., Spilsbury, C.G., 1992. A study of the extraction of CO2 from the flue gas of a 500 MW pulverised coal fired boiler. Energy Convers. Manage. 33 (5–8), 373–378. Allam, R.A., Higginbotham, P., White, V., 2003. The use of oxygen in CO2 efficient power generation. In: International Congress of Refrigeration. Beysel, G., 2009. Enhanced cryogenic air separation – a proven process applied to oxyfuel. In: 1st Oxyfuel Combustion Conference, Cottbus, 8th Sep. Darde, A., Prabhakar, R., Tranier, J.-P., Perrin, N., 2009. Air separation and flue gas purification units for oxy-coal combustion systems. Energy Procedia 1, 527–534. Dillon, D.J., White V., Allam, R.J., Wall, R.A., Gibbins, J., 2005. Oxy combustion processes for CO2 capture from power plant. IEA GHG report 2005/9. Tranier, J.-P., Dubettier, R., Perrin, N., 2009. Air separation unit for oxy-coal combustion systems. In: 1st Oxyfuel Combustion Conference, Cottbus, 8th Sep. White, V., Armstrong, P., Fogash, K., 2009. Oxygen supply for oxyfuel CO2 capture. In: 1st Oxyfuel Combustion Conference, Cottbus, 8th Sep.
Contents lists available at ScienceDirect
International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Oxygen supply for oxyfuel CO2 capture Paul Higginbotham a,∗ , Vince White a , Kevin Fogash b , Galip Guvelioglu b a b
Air Products PLC, Hersham Place Technology Park, Molesey Road, Walton-on-Thames, Surrey KT12 4RZ, UK Air Products & Chemicals, Inc, 7201 Hamilton Blvd, Allentown, PA 18195, USA
a r t i c l e
i n f o
Article history: Received 13 January 2011 Received in revised form 14 February 2011 Accepted 15 March 2011 Available online 22 April 2011 Keywords: Air Products Oxygen Oxyfuel Oxycoal CO2 capture Air separation unit
a b s t r a c t This paper presents the results of a study to develop Air Products’ air separation unit (ASU) offerings for oxyfuel coal CO2 capture projects. A scalable “reference plant” concept is described to match particular sizes of power generation equipment, taking into account factors such as safety, reliability, operating flexibility, efficiency, and low capital cost. We describe the selection of a process cycle to exploit the low purity requirements, as well as the options for compression machinery and drivers as the scale of the plant increases and the sizes of referenced equipment limit the possibilities. We also explore integration with other elements of the system, such as preheating condensate or heating and expanding pressurised nitrogen. In addition, we consider how the ASU affects the flexibility of the oxyfuel system and discuss how its power consumption can be reduced during periods of high power demand. Finally, the advantages and disadvantages of different execution strategies for air separation unit projects are discussed, as well as alternative commercial models for the supply of oxygen. © 2011 Published by Elsevier B.V.
1. Introduction Since Air Products began working on the supply of oxygen for oxyfuel coal combustion nearly twenty years ago (Allam and Spilsbury, 1992), the necessity of reducing carbon dioxide emissions has been increasingly widely recognised. We are now seeing the initiation of projects for the design and construction of the first large-scale oxyfuel coal combustion demonstration plants of around 200–300 MWe scheduled to be on stream from 2015. The building of the first generation of commercial oxyfuel coal plants in multiples of around 500–600 MWe is expected to start between 2015 and 2020 to meet increasing carbon dioxide reduction targets in Europe and around the world. Although several different air separation processes are commercially available, the only one currently proven and economically viable at the scale required for oxyfuel coal combustion is cryogenic distillation, and so that is the focus of this paper. In the future, processes such as those using ion transport membranes (ITMs) will be scaled up and proven at larger scale and may provide a more economic oxygen supply, but in the meantime cryogenic distillation will be important for at least the first generation of commercial oxyfuel coal combustion plants. It is important that the imminent demonstration projects are able to follow as closely as possible the concept for the full-scale
∗ Corresponding author. Tel.: +44 1932 249506. E-mail address: [email protected] (P. Higginbotham). 1750-5836/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.ijggc.2011.03.007
commercial plants both in technology selection and execution strategy to provide the most benefit to future projects. Air Products has designed a scalable reference plant concept for the supply of oxygen to oxyfuel coal power plants taking into account the unique requirements of this application. We will now describe the development of the process and plant design considering the product requirements, process selection, equipment design and execution strategy.
2. Requirements for oxyfuel coal combustion The reason for combustion of coal in oxygen rather than air is to remove nitrogen from the combustion air so that very little remains in the flue gas and the CO2 is easy to purify. In order to use existing boiler designs with minimal modifications, flue gas is recycled so that CO2 replaces the nitrogen in air and the oxygen concentration during combustion is selected to suit the operating and materials constraints in the boiler. The oxygen requirements for oxyfuel combustion in a coal-fired power station are different from those of other oxygen users in several ways. Because the boiler operates at a slightly sub-atmospheric pressure there is always some leakage of air into the boiler, and so there will always be a need to remove some nitrogen and argon from the carbon dioxide. This means that the oxygen purity can be low (<97%) as the remaining impurities will be removed in the carbon dioxide purification process. The oxygen pressure is also low, close to atmospheric pressure, to match the combustion air pressure in a conventionally-fired boiler. The flow of oxygen required is
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large, requiring multiple world-scale plants for a typical boiler size, for instance a 500 MWe power plant will consume around 10,000 tonnes per day of oxygen. There is normally no significant associated demand for any gaseous or liquid co-products, such as argon or nitrogen, except for relatively very small quantities of utility nitrogen or instrument air and liquid for back-up storage. Many coal-fired power stations that might be converted to oxyfuel combustion are located inland without easy access to rivers or the sea, and so the dimensions of equipment that can be shipped to the sites are limited. Depending on the particular location, options may be needed to use multiple parallel process units within a single train or even multiple smaller trains. The parasitic nature of the power means that high efficiency is important, but the large scale means that capital cost should be minimised. This unique combination of factors opens up new possibilities to optimise the ASU process cycle and machinery configuration to minimise the power consumption of the air separation unit without compromising the requirement for low capital cost. 3. Development of air separation units for oxyfuel coal combustion Multiple factors must be taken into account in the design of an air separation plant to match a particular set of product requirements including safety, efficiency and capital cost (Allam et al., 2003; White et al., 2009). A particularly important parameter is the capitalised value of power, i.e. the additional capital investment worthwhile to save a unit of power, normally expressed as $/kW. This determines whether a more costly, high efficiency plant or a less costly, low efficiency plant is more appropriate. In the case of oxyfuel combustion, the power value is expected to be relatively high; any saving in ASU power will increase the power output from the power station and so reduce the specific capital cost of the rest of the power plant and lower the cost of CO2 capture. Air Products has proposed the use of a triple column cycle to minimise the parasitic power consumption of the air separation unit for oxyfuel power generation (Dillon et al., 2005), and others have put forward alternative solutions (Darde et al., 2009; Beysel, 2009; Tranier et al., 2009). In developing a range of reference plants for this application, we have evaluated several alternative cycles on a comparable basis to confirm that this is an appropriate cycle choice. 3.1. ASU cycle selection Over the 100 or so years since the double column air separation process was invented, many modifications have been developed to supply different purity, pressure and flow requirements of gaseous and liquid oxygen, nitrogen and argon products. In most cases, the double column system still provides an optimum balance of efficiency and capital cost where high purity oxygen is required and power costs are moderate. Oxygen is considered to have a low purity if it is less than about 97%, where almost all the nitrogen has been removed but little of the argon. Because it is more difficult to separate argon from oxygen than nitrogen from oxygen, the separation energy is less for low purity oxygen than high purity oxygen. Numerous process cycles have been developed for producing low purity oxygen, all capable of realising the potential theoretical power saving in different ways and to varying extents. Despite this, surprisingly few low purity oxygen plants have actually been built because of the value of argon that can be co-produced when the oxygen purity is increased, even if the actual requirement is for a low purity. A notable exception is where relatively large quantities of oxygen are needed without a correspondingly large adjacent argon market, for example IGCC schemes, and in future oxyfuel combustion applications.
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Because of the low purity and pressure of the oxygen, the specific power of a plant designed for oxyfuel coal combustion will be very low and the problem arises of how to design a cycle to process all the air required with a low enough net power input. There are broadly three approaches that can be taken: 1) Minimise total power input by reducing the feed air pressure, producing each product at no more than its specified pressure and venting waste nitrogen to the atmosphere. 2) Minimise net power input by maintaining a normal feed air pressure but recovering some compression energy as a pressurised product, and if necessary recovering excess compression energy by expansion. 3) Minimise net power input by increasing the operating pressure of the ASU and producing all the waste nitrogen at high pressure and using or recovering the compression energy it contains. In general, approaches 2 and 3 are only worthwhile when there is actually a need for the compressed product stream so that the avoided product compression power can be credited against the input power to give a low net separation power and the capital cost of the product compressor can be reduced. If there is no use for the pressurised product, only part of the energy can normally be recovered by expansion, and additional capital equipment is required in the form of an expander. A number of ASU processes were compared following all three approaches, including: 1) The three column cycle proposed by Air Products (Dillon et al., 2005, Fig. 1, cycle 1 in Table 1). 2) A conventional double column cycle (Fig. 2, cycle 2 in Table 1). 3) A dual reboiler cycle (Fig. 3, cycle 3 in Table 1). 4) The three column cycle with elevated column pressures (Fig. 4, cycle 4 in Table 1). 5) An elevated pressure dual reboiler cycle with a low pressure column condenser (Fig. 5, cycle 5 in Table 1). In comparing process cycles, it is important to optimise each one with appropriate comparable pressure drops and temperature differences and to consider the costs associated with the differences in pipe and heat exchanger sizes. Possible machinery options and compressor sizes must also be taken into account. Table 1 compares the oxygen separation shaft power (i.e. the part of the shaft power for producing oxygen at 1.013 bar(a)) for the different cycles at ISO conditions (Darde et al., 2009) for a plant optimised for a high power cost making 5400 tonnes per day contained oxygen at 95% purity and 1.1 bar(a) pressure. Motor and transformer losses, cooling system and molecular sieve regeneration energy are all excluded and there are no liquid or gaseous co-products, so that the basis is the same as Darde et al. (2009). Each process was also evaluated with pressurised gaseous nitrogen (GAN) produced directly from the column system, and the specific powers for oxygen production compared for two such cases. In the first case, the GAN is expanded to recover power after heating with heat from the air compressor only. In the second case, the GAN is used as a pressurised product and so the power to compress it from atmospheric pressure to the pressure at which it leaves the process is credited against the oxygen production power (since this is the avoided power of the GAN compressor that would otherwise have been needed). 3.1.1. Three column “dual high pressure column” cycle In IEA GHG report 2005/9 (Dillon et al., 2005), Air Products described a triple column cycle for producing low purity oxygen for oxyfuel combustion (a variant of which is illustrated in Fig. 1). In that report, the specific electrical power consumption for a plant
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Fig. 1. Three column (dual high pressure column) cycle.
producing 95% oxygen at 1.6 bar(a) was stated as 201.3 kWh per tonne of contained oxygen (kWh/t) under ambient conditions of 9 ◦ C, 1013 mbar, 87% RH and 12 ◦ C cooling water. At the same time, approximately 128 kWh heat per tonne of contained oxygen is exported to the condensate preheat system which allows additional power of about 24.6 kWh per tonne of contained oxygen to be generated by the LP steam it displaces. Table 2 shows how this power can be adjusted to ISO conditions of 15 ◦ C, 1013 mbar and 60% RH and 15 ◦ C cooling water and how it can be split up into its individual components. First, the power is increased to take account of the warmer inlet air and cooling
water temperatures, and then reduced to exclude the cooling system power and electrical losses, since we are calculating the shaft power only. Finally, the power to compress the oxygen from atmospheric pressure to 1.6 bar(a) is deducted and the power credit for the additional power generated by the exported heat is applied. This leaves the contained gaseous oxygen (GOX) separation shaft power on the basis used by Darde et al. (2009) of 162.4 kWh/t (0.232 kWh/Nm3 ). With intercooled air compression and no heat integration, there is no power credit from the exported heat, but the compressor power is reduced, so the specific GOX separation shaft power increases to 177.4 kWh/t (0.253 kWh/Nm3 ). The table also
Table 1 Comparison of different process cycles for 5400 TPD contained oxygen at 95% purity at ISO conditions. Cycle number Description
Low pressure column pressure Oxygen recovery Input power GAN power credit (avoided compression power) GAN power credit Net GOX Power Specific shaft separation power % GAN credit to equal (1a) Specific power (GAN power recovery, no external heat)
1a Three column cycle
1b Three column cycle + MPGAN
2a Dual column cycle
2b Dual column cycle + MPGAN
3a Dual column dual reboiler
3b Dual column dual reboiler + MPGAN
4 EP three column cycle + MPGAN
5 EP dual column dual reboiler + MPGAN
bar(a)
1.2
1.2
1.2
1.2
1.2
1.2
2.7
4.2
% kW kW
97% 36,410 0
93% 40,900 7107
99% 42,767 0
92% 47,960 11,540
90% 38,256 0
83% 45,351 10,566
89% 58,716 26,907
87% 72,586 42,949
% power in 0% kW 36,410 kWh/t 158
17% 33,793 147
0% 42,767 187
24% 36,420 158
0% 38,257 167
23% 34,785 151
46% 31,809 138
59% 29,637 128
%
63%
N/A
100%
N/A
85%
83%
84%
166
177
190
kWh/t
N/A
157
175
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Fig. 2. Conventional double column cycle.
Fig. 3. Dual reboiler cycle.
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Fig. 4. Elevated pressure three column cycle.
Fig. 5. Elevated pressure dual reboiler cycle.
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Table 2 Breakdown of specific power for the three column cycle in Dillon et al. (2005) for comparison with Darde et al. (2009). kWh/t contained O2 Total specific power at ambient conditions 9 ◦ C 87%RH and 12 ◦ C cooling water (Dillon et al., 2005) Adjusted to ISO conditions 15 ◦ C, 60%RH and 15 ◦ C CW Front end cooling pumps Motor and transformer losses GOX compression (1.013–1.6 bar(a)) Credit for additional power generated by heat integration Equivalent GOX separation specific shaft power at ISO conditions with adiabatic compression and heat integration Adjustment for intercooled compression and no heat integration Equivalent GOX separation specific shaft power at ISO conditions with intercooled compression and no integration Additional specific power: TSA regeneration (heat duty) Electrical equivalent of TSA regeneration
indicates how the electrical equivalent of the average temperature swing adsorber (TSA) regeneration heat duty can be accounted for in the calculation of overall power consumption, assuming a thermal efficiency of about 25% for power generation from medium pressure steam. Further optimisation of this process to reduce pressure drops and temperature differences in the plant has resulted in a further 10% reduction in unintegrated GOX separation shaft power from 177 kWh/t to about 158 kWh/t (0.226 kWh/Nm3 ) (see Table 1, cycle 1a), with the potential for further reductions from integration or by trading off further increased capital cost for reduced power consumption at higher power values. With adiabatic compression and heat integration, this would lead to a value around 143 kWh/t. If low purity, medium pressure (MP) gaseous nitrogen (GAN) is required at about 2.5 bar(a) and its avoided compression power is credited against the oxygen separation power at the same efficiency as the air compression, this is reduced by a further 7% to 147 kWh/t (cycle 1b) (or 132 kWh/t with heat integration). The input power increases by around 12% as the oxygen recovery is reduced and the feed pressure increased, but this is outweighed by the nitrogen power credit of about 17% of the total input power. Care must be taken when comparing powers incorporating such credits that equal efficiencies are used for air and nitrogen compression – if a lower nitrogen compressor efficiency is assumed than that of the air compressor, the separation power could appear artificially reduced. Table 1 shows that for this cycle, if less than 63% of the nitrogen compression power can be recovered, the net power is higher than the base cycle with no MP GAN. The typical power recovery rate from an expander with no external heat input (i.e. only heating nitrogen with heat from the compression system) is about 66%, which gives a specific power of 157 kWh/t. This is virtually the same as the base cycle, so attempting power recovery in this way is not worthwhile. 3.1.2. Conventional dual column cycle A similar optimisation process for the conventional dual column cycle (2a) illustrated in Fig. 2 leads to an oxygen separation shaft power of 187 kWh/t (about 18% higher than the triple column cycle) when no pressurised product is taken from the high pressure column, but the capital cost is reduced because the main heat exchanger volume is reduced as a result of excess refrigeration from the high expander flow and pressure ratio. However, if pressurised nitrogen is taken from the high pressure column (cycle 2b) and credited with the avoided compression power, the specific power may be reduced to 158 kWh/t (as the expander flow and excess refrigeration are reduced). This is the same as the triple column cycle with no GAN production, and narrows the gap with the triple
kWh/t contained O2
kWh/Nm3 contained O2
201.3
0.288
+4.0 −1.0 −4.1 −13.2 −24.6
205.3 204.3 200.2 187.0 162.4 162.4
0.293 0.292 0.286 0.267 0.232 0.232
+15.0
177.4 177.4
0.253 0.253
16.2 4.1
0.023 0.006
column with GAN production to 7%. But the power credit accounts for 24% of the input power and all of it must be recovered to equal the base triple column cycle, whereas only about 66% can actually be recovered by internal heating and expansion. The specific power with such GAN power recovery is 175 kWh/t, 11% higher than the base cycle. 3.1.3. Dual reboiler cycle An alternative cycle for low purity oxygen production is the dual reboiler cycle illustrated in Fig. 3. Like the triple column cycle, this cycle has two reboilers in the low pressure column, but instead of being reboiled with nitrogen, the lower one is boiled by condensing air. This reduces the oxygen recovery, as there is less liquid nitrogen reflux for the low pressure column. With no GAN production from the high pressure (HP) column (cycle 3a), the oxygen separation shaft power is 167 kWh/t, 5% higher than the triple column cycle. With GAN production and full power credit (cycle 3b), this is reduced to 151 kWh/t, still 3% higher, and the air flow required (and therefore plant size) is about 10% bigger. The power recovery needed from the GAN for this cycle to match the unintegrated triple column cycle is 85%; with the achievable level of 66% power recovery, the specific power of 166 kWh/t is virtually the same as when no GAN is produced. 3.1.4. Elevated pressure three column cycle If the column pressures in the three column cycle are elevated so that all the waste nitrogen leaves the plant at about 2.5 bar(a) and the oxygen is boiled with the medium pressure feed rather than the high pressure feed (Fig. 4 and cycle 4 in Table 1), the GOX separation shaft power can be reduced a further 6% to about 138 kWh/t if full credit is taken for the nitrogen compression power. But now the input power is further increased so that it is 85% more than the net power and 46% of it has to be recovered as nitrogen compression credit. If there is no use for the pressurised nitrogen, the required power recovery rate to equal the base cycle increases to 83%. This type of process in not suitable for unintegrated power recovery as its specific power is then 177 kWh/t, 12% higher than the base cycle. 3.1.5. Elevated pressure dual reboiler cycle As the pressures of the columns of the dual reboiler cycle are increased, eventually the pressure of the low pressure column becomes high enough for nitrogen from it to boil the oxygen product at the required low pressure (Table 1, cycle 5 and Fig. 5). This is a particularly efficient configuration because extra liquid nitrogen reflux for the low pressure column can be generated in this oxygen reboiler to make up for that lost by condensing air in the low pressure column reboiler. If a full power credit can be applied for all
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the nitrogen compression, the GOX separation shaft power of this cycle can be reduced to 128 kWh/t, 7% lower than triple column cycle with column pressures elevated to a lesser extent. However, the drawback of this low net power consumption is that 59% of the power has to be recovered in the compressed nitrogen. Like the elevated pressure triple column cycle, more than 84% of this theoretical nitrogen compression power must be recovered to result in a lower specific separation power than the base triple column cycle. This is not attainable with unintegrated power recovery, when the specific power is 190 kWh/t, 20% higher than the base cycle and even higher than the conventional cycle. 3.2. Integration We have seen that ASU cycles that rely on power recovery from pressurised nitrogen are not worthwhile when no external heat is available, as the input power is significantly increased but the theoretical power credit from the products cannot be fully realised. Another approach (Tranier et al., 2009) is to use the pressurised nitrogen to run a Brayton power generation cycle alongside the steam cycle by taking heat from the boiler. This only provides a net benefit if the Brayton cycle is more efficient than the steam cycle at converting this heat to power. For this to be the case, the nitrogen must be heated to at least about 600 ◦ C so that the temperature ratio between compression and expansion is high enough. This requires modification to the boiler superheat zone, and if it leads to a reduction in available steam superheating it would compromise the steam cycle efficiency and could offset any potential gain. It also requires the development of high temperature, high flow expanders. For this reason, such a process was not selected for the first generation of oxyfuel coal plants which aim to demonstrate and then apply the technology with minimum modifications to existing air-based boiler equipment. The ASU is therefore designed for maximum efficiency as a stand-alone unit. However, as indicated by Dillon et al. (2005), the selected triple column cycle is still ideally suited to integration with the power plant by the use of adiabatic air compression with low temperature heat recovery into the steam system, and there is also the option to produce as much low purity, medium pressure nitrogen at 2.5 bar(a) as can be effectively used. 3.3. Machinery and drivers The air compressors and their drivers are a significant component of the cost of an ASU and are responsible for most of the power consumption. So it is critical that their efficiency is optimised. The size of air compressors available will typically be a limiting factor on ASU train size, although it is possible to use multiple compression trains for a single cold box. Centrifugal air compressors are generally lower cost than axial machines and have been increasing in size over recent years, so that they are now available for air flows corresponding to up to around 5000 tonnes/day oxygen (by using parallel first stage impellers). Axial compressors are available up to about 8000 tonnes/day oxygen equivalent, and have been used for many years in the steel industry as blast furnace blowers. Axial compressors derived from gas turbine compressors are being developed, and will soon be offered at even larger sizes. The main drive options for ASU compressors are electric motors or steam turbines. Gas turbine drives are occasionally used, but are not suitable for oxyfuel coal combustion applications. Steam turbine drives can have the advantage of lower mechanical and electrical losses, but steam turbines designed for mechanical drives are smaller and generally less efficient than those designed for power generation. They will also result in more complex opera-
Table 3 Air Products’ reference plant sizes. Plant size – contained O2 tonnes/day
Machinery options
Approximate power consumption, MW
3000–4000
Centrifugal 1 or 2 train or axial 1 train Centrifugal 1 or 2 train or axial 1 train Centrifugal 2 train or axial 1 train Centrifugal or axial 2 train
22–33
4000–5500 5500–7000 7000–10,000
30–45 41–58 53–82
tions, as steam must be available before the ASU can be started. Electric motor drives mean that there will be additional losses due to the conversion of steam to electricity in the turbine and generator and back to shaft power in the motor, but the gain in steam turbine efficiency is similar to these extra losses. Driving the compressors with electric motors simplifies the start up and operation of the ASU, allowing it to be almost independent of the power plant, and will improve its availability. Because of the low power consumption of the chosen process, the drivers will always be within referenced power limits, even for the largest ASU sizes. 3.4. Plant sizes Taking into account the anticipated oxygen requirements of oxyfuel coal combustion power plants, Air Products has developed conceptual designs for a scalable plant, covering the approximate ranges of contained oxygen production shown in Table 3. All these plant sizes use columns that are no bigger in diameter than those currently operating or, for the largest size, within proven scale-up and existing manufacturing capabilities. The use of multiple columns where necessary helps this scale-up in single cold box train capacity and does not add significantly to the cold box cost, which is in any case a relatively minor part of the total. In full scale commercial applications, multiple plants will be required, and, depending on location and access for transport, it may be necessary to select more, smaller plants. 3.5. Safety The very low pressure of oxygen needed for oxyfuel coal combustion means that the oxygen boils at a relatively low temperature and the solubilities of CO2 and N2 O in liquid and vapour oxygen are therefore low. So, even with very high removal rates in the front end purification system, oxygen cannot be boiled to dryness in the main heat exchanger without exceeding the vapour solubilities and risking precipitation of these contaminants leading to a blockage and potential accumulation of hydrocarbons. As a result, the oxygen is boiled in a separate reboiler unit, from which sufficient liquid purge must be withdrawn to limit the contaminant concentrations. The required purge rate is minimised by designing the front end purification system for high removal rates and by monitoring the concentration of contaminants in this reboiler during operation. 3.6. Front end air purification Air Products’ advanced, low-energy, multi-layer horizontal adsorbers are used for the front end clean-up to give the most cost-effective control of contaminants. The design allows the use of lower pressure steam or hot condensate for regeneration heating to minimise the impact on the power generation cycle, and ensures that the regeneration heat is used effectively. To minimise regeneration energy and capital cost and to maximise transportability, two adsorber systems are used, one at the higher feed air pressure
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(about 4.5 bar(a)) and one at the lower pressure (about 3 bar(a)). As well as being efficient and effective, the proven mechanical design is robust and low cost.
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tive control (MPC) will be used to optimise efficiency and minimise purity deviations during ramping. 3.10. Back-up system
3.7. Heat exchangers The low operating pressures of the chosen cycle mean that there is a wide choice of suppliers for the brazed aluminium plate-fin cores used for the main heat exchangers. Larger numbers of these cores are used than in a conventional air separation plant because of the high value of power, but they can be easily manifolded together in the conventional manner. For the distillation column reboilers, a downflow (falling film) design is selected to minimise the temperature approaches and optimise power consumption. Their operating safety is assured by the high liquid purge rates from these reboilers and by the removal of contaminants to very low levels in the front end air purification system. However, for the oxygen product reboiler, a thermosyphon reboiler is used because it can safely operate closer to the solubility limits of CO2 and N2 O, as described in Section 3.5. 3.8. Distillation columns All the distillation columns use Air Products’ proprietary highcapacity structured packing to minimise the column diameters needed and allow shipping of complete columns within specified transport limits. The low pressure drop of structured packing helps to achieve the very low specific power offered by this type of plant. Air Products’ proprietary distributor design and programme of testing each individual distributor ensures good performance over a wide operating range even at the large diameters required for these plants. Two parallel low-pressure distillation columns are used where necessary to permit the columns to remain within scale-up, manufacturing or shipping limitations. 3.9. Operating flexibility The turndown of an ASU is limited by the air compressors rather than the cold box. For maximum efficiency, the normal operating range of the reference plants is 75–100%. If dual machinery trains are used, it is possible to turn down to 50% to run with a single compression train, but it will generally be impossible to run between 50% and 75% without venting air, as both machine trains will need to run in this range. The operating range can be increased by modifying the machine design, but there will be a penalty in efficiency at the design point. The range can also be increased by using more compression trains, for example 3 × 33% or 4 × 25% compressors, and stopping and starting compressors as needed. For very large commercial systems with multiple plants and compressors, the operating range will naturally be more continuous, as it is possible to turn compressors on and off on some plants and not on others. Dual machinery trains can be advantageous here, as all the cold boxes can be kept operational as compressors are stopped and started. In integrated schemes, it may be necessary to incorporate additional equipment for start-up, shut-down or off-design operation to ensure that integrated heat loads always remain balanced. For example, heat supplied by the compressors will not be available until the ASU is started up, and in certain turndown cases it will increase in relation to the condensate preheat demand as the compressors have to vent excess air, and some of the heat may need to be removed by cooling water. The ASU has been designed to allow rapid ramping at 5% of design capacity per minute and a dynamic model of the process has been developed to demonstrate this capability. Model predic-
The optional back-up system will provide instantaneous additional oxygen supply in the event of a plant or compression train trip or peak shaving requirement (e.g. at extreme ambient conditions) and may be used to assist ramping at even faster rates. The quantity of back-up storage installed is flexible depending on customer availability targets, but due to the large size of these plants it is likely to cover only a few hours of operation at full rate. Since oxygen is only required when the power plant is operating, warm cooling water is used to vaporise the liquid oxygen when required. The ASU is capable of supplying liquid when necessary to refill the back-up tanks in a specified time period. 3.11. Electrical load management A further optional feature is the provision of liquid air storage in addition to liquid oxygen storage. Since liquid oxygen is produced from the distillation column system and evaporated against condensing air, storing both liquids allows the supply of oxygen from the cold box to be decoupled from its production by the distillation columns. If the oxygen production is less than the demand, it can be supplemented from storage and the resulting additional liquid air sent to storage with no net refrigeration requirement or disturbance to the column system. All that is needed is an increase in air flow from the compression system to provide the extra air to boil the oxygen. Conversely, if the ASU is producing more oxygen than required, liquid air can be injected from storage to provide refrigeration to produce liquid oxygen that can then be stored rather than evaporated. In this case, less air has to be provided by the compression system, since part of the feed is provided as liquid air. As well as adapting to variations in oxygen demand, the system can also be used to maintain the oxygen supply rate whilst the ASU is turned down to reduce its power consumption during peak power demand periods and turned up during off-peak periods. The effective cost of the power provided to the ASU can therefore be reduced and the maximum output of the power station increased. In practice, a combination of power management and variable oxygen supply will be likely as the boiler’s oxygen demand will be reduced during periods of low power demand and increased when power demand is high. In assessing this option, it is important to understand the likely operating profile of the power station and evaluate the potential benefits of load management against any additional costs associated with the ASU, compressors and storage. For example, it may be worthwhile to oversize the ASU to allow load management even at full oxygen output, or it may only be required when operating with average production rates below design capacity. Table 4 provides an idealised example in which the ASU is sized for 112% design capacity with two compression trains each operable between 40% and 56%. The columns also operate between 40% and 112%. Air flows are relative to 100% air flow and oxygen flows are relative to 100% oxygen flow. At oxygen demands below 40% of design capacity, the ASU is fully turned down and operates continuously at this rate. At higher oxygen demands, the ASU supplies the specified oxygen rate continuously, but the distillation columns are turned down for part of the time (low power mode) and turned up for the rest of the time (high power mode). In low power mode, only a single compression train operates, and in high power mode both are running. The relative air flows in the example also give an indication of the relative powers in the different cases. The fraction of the time that the ASU can operate in low power mode is determined by its sizing and the required output, and with the 112% ASU
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Table 4 Example of electrical load management scheme using liquid oxygen/liquid air swap. Relative oxygen demand
Relative MAC air flow in low power mode
Relative MAC air as liquid to storage in low power mode
Relative liquid oxygen from storage in low power mode
Relative MAC air in high power mode
Relative MAC air as liquid from storage in high power mode
Relative liquid oxygen to storage in high power mode
Fraction of time in low power mode
0.40 0.50 0.56 0.60 0.70 0.80 0.90 1.00
0.40 0.43 0.44 0.45 0.48 0.51 0.54 0.56
0.00 0.03 0.04 0.05 0.08 0.11 0.14 0.16
0.00 0.10 0.16 0.20 0.30 0.40 0.50 0.60
N/A 0.95 0.97 0.98 1.01 1.03 1.06 1.09
N/A 0.17 0.15 0.14 0.11 0.09 0.06 0.03
0.00 0.62 0.56 0.52 0.42 0.32 0.22 0.12
1.00 0.86 0.78 0.72 0.58 0.44 0.31 0.17
in the example this is one sixth of the time at 100% output, increasing to nearly a third at 90% output. The example also shows that the ASU can operate more efficiently in the 56–80% range by switching between the two modes than by operating continuously with both compressors turned down to minimum and venting air or oxygen. 3.12. Rare gas production The rare gases xenon and krypton are normally produced from air separation plants, even though they are only present in the air at very low levels. Large-scale ASUs for oxyfuel combustion applications have the potential to become a significant source of these gases, whose production and sale could improve the economics of these projects. In the process selected, virtually all the xenon and a significant quantity of the krypton in the air are present in the liquid purge from the oxygen product reboiler and can be recovered relatively easily. If required, virtually all the krypton can also be recovered at low cost by adding a few distillation stages in the top of the reboiler vessel. 3.13. Execution strategy The plant is designed so that the main columns, tanks, adsorber vessels and heat exchanger assemblies are shop-fabricated and shipped to site as complete units. The final cold box assembly and other interconnections are completed on site. Multiple units are used where necessary to overcome shipping limits, and smaller air separation trains may even be selected if required. 4. Oxygen supply concepts Oxygen consumers may choose whether to buy an oxygen plant (a “sale of equipment” (SOE) contract) or to buy oxygen from an industrial gases supplier (a “sale of gas” (SOG) or “build own operate” (BOO) contract). There are advantages to each in particular situations, and each type of supply can be configured in different ways. 4.1. Sale of gas – oxygen supply In a sale of gas contract, the oxygen supplier owns, operates and maintains the air separation unit and supplies the oxygen through a pipeline to the consumer. The oxygen supplier is typically an industrial gas company, or a joint venture between the customer and an industrial gas company. To justify the large investment required to construct an oxygen plant, contracts must be long term, typically 15–20 years. Such contracts will have fixed and variable cost elements, and will include “minimum take or pay” provisions so that the capital investment is repaid. Power may be paid for by the customer or the supplier and the variable cost adjusted accordingly.
Sale of gas contracts provide the biggest benefit when the oxygen supplier can sell additional co-products such as nitrogen, argon or liquids to other long term customers or into the merchant market, as the costs of the plant can be shared. A further benefit is that operating expertise of the industrial gas companies allows them to make guarantees on oxygen availability, should that be important to the customer. In addition, any risk on the investment in the plant is borne by the supplier, and so the customer can have certainty on the cost of oxygen for the lifetime of the contract. If there is integration between the ASU and the customer’s process, for example in an oxyfuel power plant, the contract becomes more complicated, but integrated schemes (including transfer of steam and condensate) have been operated successfully by Air Products and other industrial gas companies over many years. 4.2. Sale of equipment – ASU supply If the oxygen consumer wishes to own the ASU, there are numerous possible contract structures, some of which are outlined below. The main differences are in the scope of supply and who will eventually operate and maintain the plant. 4.2.1. Lump sum turnkey In a lump-sum turnkey project, the supplier hands over a fully operational plant for a fixed lump sum at a particular date. A customer will opt for such a project to minimise his exposure to cost overruns. However although a large element of this risk is with contractor, as with all large projects it ultimately remains with the client if it escalates to a level that the contractor cannot bear. Lumpsum turnkey contracts inevitably include contingency to mitigate the contractor’s risk, and so may not be the lowest cost model. 4.2.2. Reimbursable turnkey In a reimbursable turnkey project, the client retains the cost risk but works closely with supplier to control it and pays the contractor for work done. This avoids the addition of contingency by the supplier, and potentially benefits both parties. 4.2.3. “Proprietary kit” model When an ASU makes up part of a much larger project, Air Products has developed the “proprietary kit” supply model which can provide a low cost solution to ASU supply. It may be advantageous for the overall contractor to do the bulk of the detailed engineering, procurement and construction (EPC) of those parts of the plant that are not proprietary. Air Products would supply the overall process design package and the “proprietary kit” consisting primarily of the air purification equipment and the cold box. Machinery, which makes up a significant proportion of the cost, may be purchased directly by the client, thus avoiding multiple mark-ups. The use of a single EPC contractor for the overall project can simplify the coordination of construction and reduce the overall project risk.
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4.2.4. Operation and maintenance (O&M) contracts Air Products specialises in safe and efficient plant operation and maintenance, and so it is often called upon by clients to operate and maintain their plants according to a specified required performance, for example operating efficiency and product availability. Guarantees and associated bonuses and penalties may also be provided. This relieves the client of the need to operate and maintain a plant which might be quite different from others on his facility and unfamiliar to his operators. 5. Conclusions • Oxygen supply requirements for oxyfuel coal combustion are unique and enable the use of process cycles with particularly low power consumption. • Air Products has developed a state-of-the-art scalable ASU concept that uses an efficient and cost-effective three column cycle optimised to provide a low specific power for oxyfuel coal combustion applications whether or not it is integrated with the power generation cycle. • A number of processes were evaluated as part of the reference plant development, and it was found that although it is possible to achieve a lower specific separation power than the triple column process proposed by Air Products, this would require significant machinery development and modification of well-proven boiler equipment. • In comparing specific power consumption figures, care is needed to ensure that they are comparable and attainable within the combination of specifications for a particular project, such as operating range and level of integration. • The reference plant concept uses multiple individual process units where necessary but allows single train cold boxes to be built to supply up to around 7000 tonnes/day oxygen within the
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size of existing proven equipment and 10,000 tonnes/day oxygen with modest scale-up. Single train machinery is possible up to about 8000 tonnes/day. • The plant is capable of rapid load change to meet the requirements of the power generation system to modulate its output as necessary to meet the requirements of the electricity distribution network. • It is possible to configure the ASU to effectively store energy by swapping electrical load from one time period to another. To design the system appropriately, the expected average loading of the ASU and power plant must be considered and the benefits of increased stored energy weighed against additional ASU costs. • Heat integration of the ASU with the boiler system can significantly lower the effective energy consumption of the oxygen plant, however the usefulness of the heat from the ASU depends on its temperature level and the configuration of the power generation cycle. References Allam, R.A., Spilsbury, C.G., 1992. A study of the extraction of CO2 from the flue gas of a 500 MW pulverised coal fired boiler. Energy Convers. Manage. 33 (5–8), 373–378. Allam, R.A., Higginbotham, P., White, V., 2003. The use of oxygen in CO2 efficient power generation. In: International Congress of Refrigeration. Beysel, G., 2009. Enhanced cryogenic air separation – a proven process applied to oxyfuel. In: 1st Oxyfuel Combustion Conference, Cottbus, 8th Sep. Darde, A., Prabhakar, R., Tranier, J.-P., Perrin, N., 2009. Air separation and flue gas purification units for oxy-coal combustion systems. Energy Procedia 1, 527–534. Dillon, D.J., White V., Allam, R.J., Wall, R.A., Gibbins, J., 2005. Oxy combustion processes for CO2 capture from power plant. IEA GHG report 2005/9. Tranier, J.-P., Dubettier, R., Perrin, N., 2009. Air separation unit for oxy-coal combustion systems. In: 1st Oxyfuel Combustion Conference, Cottbus, 8th Sep. White, V., Armstrong, P., Fogash, K., 2009. Oxygen supply for oxyfuel CO2 capture. In: 1st Oxyfuel Combustion Conference, Cottbus, 8th Sep.