Alternatives: Oxy-fnel combustion
The primary difficulty with carbon capture is the need to selectively extract CO, from large volumes of relatively inert gases that also include large quantities of reactive pollutants (e.g., SO,). The extraction of CO, becomes far more difficult if the treated exhaust stream includes particulates, sulfates, and nitrates, which might be the case if the fossil fuel used is coal. Hence, for a coal-fired power plant, extensive gas cleanup would be necessary in order to limit the contamination (and potential rapid degradation) of the solvents. A preferred approach to the problem would be to work with a relatively clean fuel, such as natural gas, which is primarily methane, with product streams that are also gaseous.
In the conventional gas turbine, the largest inert diluent is the atmospheric nitrogen. Nitrogen removal prior to combustion would leave carbon dioxide and water would make up the bulk of the remaining constituents, greatly reducing the volumes of gas to be treated. In fact, if the exhaust gases consisted only of a mixture of CO, and H.O, there would be no need for chemical treatment to extract CO, at all, since the water can be easily condensed, isolating the CO,. Eliminating the solvent for carbon capture greatly simplifies the concept of capture, not to mention making it easier to permit since a relatively toxic solvent can be a challenge to permit. Condensing the water vapor from the exhaust through a heat exchanger will also yield a relatively high purity CO, stream.
If the fuel and oxygen are mixed in a stoichiometric mixture, however, the combustion process results in extreme temperatures capable of distressing hot-gas-path components. The challenge is to find a way of moderating the peak combustion tempera true. An obvious choice would be to recycle one or more of the exhaust products. Har ing CO, and H,0 in abundance in the exhaust makes them an obvious choice to moderate temperatures, as well as the serving the duty as the working fluid. Recycling part of the exhaust gases will moderate the combustor temperatures, but still leave the basic design unaffected— isolation of CO, by condensing water from the exhaust.
There are two novel designs exploring this concept. One approach uses H.O (steam) as the recycled working fluid to moderate temperature conditions. Another approach recycles CO, to achieve a similar goal (Ratlii, 2018). In each case, the energy released is recovered in a power turbine (or a heat exchanger if a boiler is used to raise steam). With the emphasis on efficiency, the focus here will be on energy recovery using a rotating apparatus such as a power turbine.
Both cycles make use of a uniquely designed expansion turbine element to recover power. Gas temperatures will have to be limited to a maximum value based on the materials of construction. The most critically stressed element is the first stage blade, where temperatures, pressures, and stresses are greatest. Another common requirement for both systems is the use of an ah' separation unit (ASU) to provide high purity oxygen as the oxidizer for the fuel. This step eliminates the need to separate CO, from inert nitrogen and argon in the exhaust gases, although it creates additional parasitic loads for the entire process.
4.2.1 Охл’-fuel H.O cycle
The Oxy-Fuel (Clean Energy Systems, n.d.) cycle shares many features with a conventional gas turbine combined cycle, including an expansion turbine, a heat recovery section, and similar plant auxiliaries. The major differences are the lack of a compressor section (which nominally could absorb 60 to 70% of the power turbine’s output) and the inclusion of the air separation plant (ASU) for the supply of the oxygen for combustion (Anderson, 2008).
The working fluid in the power turbine is a mixture of H,0 and CO„ with steam being the dominant working fluid. Because of the high concentration of steam (over 90%), it may function more like a steam turbine even though the original design basis was a gas turbine.
Several features of the turbine design are significant departures from conventional Brayton cycle. One that stands out is the performance improvement. A recent demonstration (based on a heavily modified W251 В12 gas turbine) is reported to be capable of 150 MWe output, despite the original turbine design limit from the manufacturer of 43 MWe (Clean Energy Systems, n.d.). Removal of the compressor yielded a three-fold increase in unit rating. This boost in output is roughly equivalent of taking a sub-scale D-Class turbine and modifying it to achieve F-Class performance.
As an alternative to water (steam) as moderator/working fluid, carbon dioxide could be used as well. Carbon dioxide is compressed and recycled to blend with the fuel (and oxygen) (Breaking ground for a groundbreaker: the first Allam Cycle power plant, 2016). Like the Oxy-H,0, water is continuously extracted in order to prevent its buildup somewhere in the cycle. The information provides an estimate of the power turbine output for the noted mass flow, and a calculated turbine inlet temperature. The key point is that both the Oxy-CO, and Oxy-H,0 are capable of producing impressive amounts of power, while in the conventional gas turbine application 60% or more that power would be consumed by the compressor.
4.2.3 Other benefits
Oxy-Fuel systems are very nearly a closed loop system. The exhaust products (only two are expected) can be either condensed or compressed and recycled back to the combustor, where more fuel is added with flesh oxygen. This results in a cycle where there is minimal, or zero, release of pollutants such as NOj;. CO. and SO,. On paper, this is clearly a win-win, CO, control achieved without use of chemical solvents and near zero emissions of priority pollutants.
Added to this emission benefit is one more feature. An Oxy-Fuel system can also be a net water producing cycle. Hydrogen from the fuel will add to the total material balance and will have to be removed continuously. Thus, the Oxy-Fuel product streams (power, CO„ and water) all have some inherent value. Limited water access, which has become a problem for many new power projects, could be offset by the net production of water possible from an Oxy-Fuel system.
Commercializing the Oxy-Fuel design is likely to demand extensive financial support as well strong engineering capabilities, and the design features deviate so far from current Brayton cycle design trajectories that it could encourage new entrants into the power equipment (and environmental) market. This is not unlike the changes that occurred when the electronic market shifted from vacuum tubes to solid state devices creating an opening that established new industry leaders.
4.2.4 Alternatives that include coal
The world is still faced with several bedrock issues:
Not too many years ago, the direction seemed to be using coal (a low-cost fuel) with the combined cycle (a low capital cost technology choice), with the IGCC (sometimes referred to as “pre-combustion capture” or in situ capture) offering a unique technical solution. For many reasons, those plans failed, resulting in few operating IGCC units. Beyond the technical horizon was an unexplored approach combining the gasification technology with coal to produce a synthetic natural gas, and alternatively supplying this gas to a combined cycle fleet. This shifts the carbon extraction from the point of generation to the point of gas production. The carbon content delivered to the power generator is reduced because hydrogen has been added to the fuel, thus reducing the CO, emissions at that point (as noted in Figure 5, where both the efficiency and the fuel carbon content har e shifted favorably). Additionally, the byproduct gas streams at the point of syn-gas production can be more easily treated to extract additional CO, with effectively zero impact on the efficiency, performance or the cost of electricity at the point of generation. Potentially, only the cost of fuel may be affected, but alternatively, CO, has marketable value that could offset any fuel price changes, and while CH4 (methane) may be the primary product for the power market, the gasification process can be adapted to produce a range of hydrocarbon products from Cl to C6 and higher, where the longer chain hydrocarbons are of increased value, making the conversion process economically viable.
While a wide range of chemicals can be produced from the synthesis gases generated in a gasifier, perhaps one of the most appealing is the production of hydrogen via:
This expression is revealing since the product streams are now carbon dioxide, which can be easily separated, and hydrogen, which is representative of the ultimate low-carbon fuel. Hydrogen has applications that include power generation, transportation, and even energy storage. And the chemical separation of CO„ as already noted, is well established. Likewise, the energy released by this reaction can also dime the chemical recovery of CO, from a solvent extraction process.
4.2.5 Yet to be solved
The Oxy-Fuel offers great opportunity, but it comes along with many new challenges, none of them trir ial. Actual field demonstrations are likely to reveal many more issues than simple paper study, or even a Front-End Engineering Design (FEED). Only modest scale demonstrations, or short-term operating runs har e been achieved so far. For commercialization, however, there are more pressing challenges to address, including:
Ultimate challenge—The final fate of CO,
The final hurdle yet to be overcome is the ultimate disposal of CO, extracted from any of the processes considered. A hallmark of these newest design concepts is that they can eliminate (or at least minimize) many of the parasitic load problems associated with conventional carbon recovery plants. The handling of solvents is reduced, if not eliminated, but the final disposition of the CO, is still indeterminate. CO, for enhanced oil recovery offers economic benefits, but it may ultimately only recycle some of the carbon dioxide, and potentially increase emissions. Underground storage may come in several forms, but an alternative to EOR is storage in saline aquifers. Deep saline storage capacity for CO, has been estimated to be in excess of 1,000 Gt of CO, (Aydin, 2010). Compare this with annual releases in the range of 35+ from all industrial sources. In different terms, one research team estimated that a 0.7 km3 volume could store 80% of the CO, released from a 500 MW coal plant operating over 30 years (Jordan Eccles, 2009). Because of the physical dimensions of a large coal plant, that estimated storage volume would be roughly equivalent to the surface area of the facility.