Air separation units for coal power plants

Jun 22 2011


For over twenty years, Air Liquide has been a world leader in the development of air separation units (ASU) for the Integrated Gasification Combined Cycle (IGCC). For almost ten years, the Group has contributed to the development oxy-combustion coal power plants with CO2 capture, and it continues to improve and customize its air separation process for coal-fired power plants. This article summarizes what has recently been achieved and what could be expected in the next decade from these ASU developments in order to significantly improve the oxy-combustion and IGCC processes. By Jean-Pierre Tranier, Nicolas Perrin, Richard Dubettier, Air Liquide

There are three ways to capture CO2 from coal power plants:

Post-combustion capture is essentially N2/CO2 separation, and it does not require an oxygen production plant.

With oxy-combustion, oxygen is used in the form of a high-purity oxidant stream. This enables combustion in a nitrogen-depleted atmosphere. This process results in the production of a flue gas that is highly concentrated in CO2, thus simplifying the CO2 capture process. An oxygen production plant is necessary.

The third way to capture CO2 from a coal power plant is by IGCC with pre-combustion capture. This method also requires an oxygen production plant. During pre-combustion, coal is partially oxidized in a gasifier, and a syngas containing mainly CO, CO2, and H2 undergoes a shift in which CO is converted into CO2 and H2 in the presence of water. Finally, the CO2 is captured (typically by absorption) and the H2 is burnt in a gas turbine.

How to produce oxygen onsite

A commercial-scale coal-fired oxy-combustion power plant requires thousands of tons of oxygen each day. Currently, cryogenic distillation is the only commercially viable technology that will produce such large quantities of O2. Other air separation technologies like pressure swing adsorption, vacuum swing adsorption, and polymeric membranes cannot economically produce such quantities. Ceramic membranes (oxygen ion transport membranes) are not yet commercially available for large-scale oxygen production, therefore making it difficult to compare them to cryogenic distillation, both in terms of investment and performance.

Cryogenic ASU performances have improved tremendously over the last forty years. It is estimated that power consumption has been cut almost in half, while distillation column productivity (i.e., flow per square meter) has multiplied threefold. The technology should continue to advance over the next decade, specifically through targeted improvements in oxy-combustion and IGCC plants.

Coal oxy-combustion special requirements

ASU oxy-coal combustion is chiefly characterized by three elements: size (typically over 8 000 metric tons per day for industrial-scale plants); low pressure (between 1.1 and 1.7 bar absolute); and potentially low oxygen purity. Low oxygen purity would mean a value in the range of 85-98% O2 content compared to the typical 99.5-99.8% O2 content of high-purity units. Using low-purity O2 enables significant ASU power consumption savings.

The cycles for the production of low-purity oxygen at 95% were developed in the early 1990s, primarily for two applications: gasification (including IGCC) and oxygen enrichment of blast furnace vent streams. These applications required the design of plants that demonstrated specific separation energy(1) around 200 kilowatt-hour per metric ton (kWh/t) of pure O2.

The cycles developed in the 1990s were not fully adapted for oxy-combustion. Instead, they were optimized to produce relatively high-pressure oxygen (5-80 bar absolute) and in some cases to perform co-production of nitrogen. In the past five years, however, programs have been launched to optimize ASUs for oxy-combustion by adapting the process cycle to the specific requirements of oxy-combustion (i.e., low oxygen pressure and no nitrogen requirement) rather than fully redesigning the ASU. Additionally, this type of adaptation has incorporated technology improvements made in ASUs produced since the 1990s. Thus far, the energy requirement of the ASU has been improved from 200kWh/t to less than 160 kWh/t with heat integration, a process that consists of transferring heat from the ASU compressor(s) to the steam cycle.

Heat integration achieves reduction of two types of energy losses: those associated with compression and those associated with the heating of boiler feed water. This transfer of heat can be direct (feed water preheating) or indirect (oxygen preheating, coal drying, or the heating of any fluid in the oxy-combustion cycle). Several studies have been performed on heat integration, and one conclusion has been that the benefits of the process are very dependent on the overall design of the plant: ambient conditions, efficiency of the steam cycle, cooling system (dry versus wet), coal type (water and sulfur content), etc. In some cases, a reduction of more than 10% in the ASU’s power consumption can be achieved.

The next steps for optimization

Other potential ASU optimizations have been identified and are currently under development to achieve separation energy around 140 kWh/t with heat integration by 2015. With trending cryogenic improvements, ASUs can be expected to reach 120 kWh/t with heat integration by 2020. This would mean a 40% reduction in power consumption from the state-of-the-art ASU of the past decade.

Figure 2 shows the magnitude of these improvements. This trend is expected to continue in the future, since the overall energy spent during separation is still significantly greater than the theoretically required separation energy (50 kWh/t).

When considering the oxy-fuel plant overall, it is possible today to design a plant with an HHV efficiency loss in the 5-7 percentage points range. By 2015, this range should reach 4-6 percentage points. By 2020, it could reach 3-5 percentage points.

These efficiency losses are based on the comparison with an air-fired case without heat recovery below acid dew point, achieving 40% HHV efficiency for today and 2015, and 45% HHV efficiency for 2020. The efficiency loss is given as a range to take into account parameters such as coal sulphur and moisture content, steam cycle efficiency…

These estimates are based on ASU energy-efficiency improvements as well as predicted improvements in the overall oxy-combustion cycle. This includes recycle location; implementation of technologies such as coal drying; improvement in the steam cycle (double reheating or 700°C steam by 2020); and improvements in flue gas compression and the purification unit.

ASUs for IGCC

Given the gains already achieved in oxy-combustion, it is necessary to revisit the design of ASUs for IGCC developed in the 1990s. At that time, the main focus was on integration with gas turbines. Figure 3 shows the different options.

From the ASU design perspective, there are two solutions:

Low-pressure cycle, when no nitrogen is injected into the gas turbine. In this case the ASU design is very similar to the ASU for oxy-combustion. Similar gain could be achieved in separation energy, where the energy for compression of oxygen from atmospheric pressure to gasifier pressure remains constant. This cycle will be used when no integration is considered.

High-pressure cycle, when most of the nitrogen produced by the ASU is injected into the gas turbine. This cycle can be used when low or high partial integration or full integration is chosen. The advantage of this solution is a lower capital expenditure (CAPEX) and better separation energy, but it requires more additional compression energy to raise the pressure of the nitrogen. For an IGCC with CO2 capture, the high-pressure cycle may also be selected on the basis of the nitrogen requirements for diluting hydrogen before injection in the gas turbine.

Full integration is the lowest CAPEX option, as it does not require an independent air compressor to supply air to the ASU. However, it is not very convenient from an operational point of view. For this reason, partial integration is considered the best compromise from the point of view of CAPEX, efficiency and immediate operability. Low partial integration (with no air extraction from the gas turbine) would even be preferred to high partial integration for operability.

Given the improvements developed for oxy-combustion and the technological improvements of other ASUs built since the 1990s, significant gains can be achieved for ASUs for IGCC in the following areas.

Heat integration: semi-adiabatic compression of air (even if independent of the gas turbine) can be used to heat nitrogen before injection into the gas turbine and oxygen before injection into the gasifier and/or boiler feed water during preheating. Heat integration would also considerably reduce the ASU’s water-cooling requirements.

A new process cycle including, in particular, replacement of oxygen compression by additional air compression and oxygen pumps: thanks to progress made in the manufacturing of brazed aluminum heat exchangers, it is now possible to design an ASU with full internal oxygen compression by pump, for pressures within a range of 40-100 bar absolute. In the 1990s, most of the ASUs for IGCC had only partial internal oxygen compression (from atmospheric pressure to intermediate pressure 5-15 bar range) followed by external compression. This new process cycle could significantly reduce the plant’s CAPEX.

Reduction of the pressure drop when injecting diluent nitrogen in the gas turbine: this represents a major loss of energy in the current design of IGCC plants. According to various studies, this loss can be equivalent to an increase of 40-80 kWh/t of O2 produced. Reducing the pressure drop would require a redesign of the combustion chamber of the gas turbine.

Overall, the HHV efficiency of the IGCC could be increased by more than one percentage point through improvement in the ASU design and integration.

Energy storage and flexibility

Energy storage is another area with potential for development. At one point in the process, oxygen is produced in a liquid form. This liquid oxygen can be easily stored using cryogenic ASU technology. The idea is very simple. Liquid oxygen can be stored at off-peak hours (typically at night or when power from wind is available in great quantities) for later use during peak hours, potentially decreasing by 50% the ASU’s power requirements. For an oxy-combustion plant, more than 5% additional power could be dispatched to the network at peak hours.

Energy storage can also be used to store power from renewables (wind or solar). In terms of flexibility, Air Liquide has demonstrated that at least 5% per minute capacity change can be achieved in ASUs for IGCC. In other words, the ASU can be ramped down from 100% to 50%, or up from 50% to 100%, in less than ten minutes.

Conclusions

Air Liquide is committed to develop Air Separation Units in order to improve significantly the oxy-combustion and IGCC processes, making them attractive solutions for deriving electricity from coal while reducing CO2 emissions.

By optimizing the overall coal oxy-combustion system, it is possible to identify several key advantages of the solution: minimal efficiency loss associated with CO2 capture (3-5 percentage point penalty on HHV efficiency compared to no capture expected in 2020) together with near-zero emission, energy storage capability, quick load change and limited water requirements.

For the IGCC, even after the extensive efficiency improvements achieved in the 1990s, there is potential for further optimization that could increase HHV efficiency by more than one additional percentage point.

(1) Separation energy is defined as the power required to produce 1 metric ton of pure oxygen contained in a gaseous oxygen stream for a given oxygen purity at an atmospheric pressure (101325 Pa) under ISO conditions of 15°C and 60% relative humidity. Compressor driver efficiency (for electrical, steam, or gas turbines), heat for regeneration of driers, and power consumption of the cooling system are not considered in this definition.

Air Liquide


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