By Richard L. Axelbaum
Jens Professor of Environmental Engineering Science,
Washington University in St. Louis
Research Assistant Professor, Washington University in St. Louis
Associate Professor, Xi’an Jiaotong University
Coal provides enormous benefits to society and continues to be a major energy source for power generation because of its large reserves, ease of transportation and storage, and low price. Coal-fired power generation also is one of the largest contributors to CO2 emissions. One promising technology for CO2 mitigation is oxy-combustion. However, first-generation oxy-combustion technologies, which operate under atmospheric pressure, suffer from a significant penalty in net generating efficiency—over 10 percentage points—primarily due to the auxiliary energy consumption from the air separation unit (ASU), flue gas recirculation (FGR), and gas processing unit (GPU). A promising new technology is pressurized oxy-combustion (POC), which can increase the plant efficiency by recovering the latent heat in the flue gas moisture and coupling it back into the steam cycle. An advanced POC technology is currently being developed at Washington University in St. Louis (WUSTL), Missouri, U.S.A. This technology can achieve an increase of more than six percentage points in net generating efficiency over the first-generation oxy-combustion process and is paving the way for low-cost carbon capture.1
BACKGROUND ON PRESSURIZED OXY-COMBUSTION (POC)
In the simplest sense, pressurized oxy-combustion is oxy-combustion occurring within a pressurized vessel with a condensing heat exchanger downstream. There are several justifications for this added complexity. First, the large white plumes emitting from the stacks of a power plant, as seen from Figure 1, represent a large amount of moisture. When this moisture condenses in the atmosphere, the heat being released has no benefit to the power cycle. Thought of another way, the amount of energy needed to evaporate water to form this plume is quite large and this energy is lost to the power plant. POC harnesses this energy.
Since carbon capture and storage (CCS) requires pressurization of CO2, there is no change to net loss of efficiency to pressurizing the combustion process. When the flue gases are at an elevated pressure, the condensation temperature for the moisture can be high enough that the latent heat can be captured and fed into the steam cycle, increasing the efficiency of the process. Although a pressure of 10–16 bar is sufficient to recover most of the latent heat, pressures up to 80 bar have been proposed.2,3
The basic flow sheet for POC is similar to that of atmospheric pressure oxy-combustion, which includes the ASU, FGR, and GPU, but it also includes a condensing heat exchanger to capture the latent heat of the moisture. The ASU supplies oxygen at around 95% purity to the boiler. FGR is a widely accepted method in oxy-combustion for moderating combustion temperature and boiler wall heat flux, and FGR may also be employed as carrier gas to deliver pulverized coal to the boiler. Even though coal water slurry feeding has been proposed for pressurized coal delivery,4 dry feeding of pulverized coal yields higher boiler efficiency and has been widely used in gasifiers at pressures up to 40 bar.5 After the warm flue gas leaves the boiler(s), particulates are removed using a filter. The warm particle-free gas then enters a condensing heat exchanger designed to recover the latent heat of the flue gas moisture and feed it into the steam cycle. Pressurization allows for simultaneous removal of SOx and NOx and the capture of latent heat in one device. The CO2 is further compressed and purified in the GPU before it is pipeline ready.
BENEFITS OF PRESSURIZED OXY-COMBUSTION
The primary benefits of POC include:
- capturing the latent heat of condensation and utilizing it to increase cycle efficiency;
- reducing the efficiency penalty associated with using high-moisture fuels, since latent heat is recovered, thereby making low-rank coal more valuable;
- simplifying the capture of SOx and NOx because pressure allows for co-capture of these pollutants in a simple water wash column;
- greatly reducing gas volume, thereby reducing the size and cost of equipment;
- avoiding air ingress, thereby reducing the GPU purification cost, and
- increasing the optical thickness in the boiler, which allows for optimization of radiant heat transfer and reduced flue gas recycle.
The first few benefits are explained in more detail below.
High Efficiency Through Latent Heat Recovery From Flue Gas
The primary motivation to use POC, rather than atmospheric oxy-combustion, is to utilize the recovered latent heat from the flue gas, which compensates for the parasitic energy consumption of carbon capture. The temperature at which phase change occurs is strongly dependent on operating pressure. For example, at atmospheric pressure the flue gas moisture condenses at 50–55°C. At a pressure of 80 bar, condensation occurs at 150–200°C.6 The significant increase in condensation temperature makes it feasible to utilize the latent heat. Both direct-contact and non-contact heat exchangers could be used for latent heat recovery.
The latent heat that is captured can be utilized in the steam cycle by heating boiler feed water. This approach leads to replacement of roughly half of the steam extraction from the turbines. Less extraction allows more steam flow through the turbine and, thus, an increase in gross power.6
Integrated Emissions Removal
Higher pressure enables integrated emissions control, which can replace traditional and expensive emission control equipment such as the Selected Catalytic Reduction (SCR) for NOx and Flue Gas Desulfurization (FGD) for SOx.
Earlier studies have demonstrated that when flue gases are compressed in the presence of water, conversion of gaseous pollutants to weak sulfuric and nitric acids is enhanced by chemical interactions between S- and N-containing species. This occurs at elevated pressure, but not atmospheric pressure. While the precise chemical reaction mechanism that occurs under pressure is still a subject of study, the process is loosely referred to as the “lead chamber” process, which is a well-known process for manufacturing sulfuric acid. Test results have shown that almost all the SOx and about 80% of NOx is removed at 15 bar. An extra column operating at about 30 bar may also be employed for a more complete removal of NOx.7 The key requirements for the process are that the NOx/SOx ratio is greater than about 0.5, the pressure is greater than 15 bar, and the process occurs in the presence of liquid water.8, 9
This process of emissions capture can be combined with the process of flue gas moisture condensation and latent heat recovery in a single counter-flowing water wash column, as illustrated in Figure 2. Wet flue gas at a temperature greater than the acid-gas dew point (≥300°C) flows into the gas-liquid reactor column from the bottom. The flue gas flows against a stream of cooling water, thereby reducing the flue gas temperature. When gas temperature decreases to the dew point, condensation of the flue gas moisture occurs, releasing the latent heat, which is captured in the cooling water. Dew point increases with pressure, and thus the temperature of the water leaving the column increases with pressure. At 16 bar the value is about 167°C, which is sufficiently high to allow the heat to be used for boiler feed water heating.
When applied in a POC system, this approach includes the following benefits:
- Unlike the protocol for atmospheric pressure oxy-combustion systems, the flue gas need not be compressed because it is already at elevated pressure; thus, the challenges of avoiding corrosion when compressing a sour gas is eliminated.
- The capture of flue gas latent heat occurs along with SOx and NOx, which is more economical as compared to separate capture systems.
- Acid gas condensation occurs in a single device, reducing the chance of corrosion in other parts of the system.
- Because no cooling is necessary before the flue gas enters the DCC, the overall efficiency of the process is maximized.
STAGED, PRESSURIZED OXY-COMBUSTION
An extension of POC, the novel staged, pressurized oxy-combustion (SPOC) process, can reduce the efficiency penalty for carbon capture in coal-fired power plants by over half. The SPOC process incorporates a unique boiler configuration to enable combustion of pulverized coal at elevated pressure (approx. 15 bar) with minimal flue gas recycle.
The SPOC process is depicted in Figure 3. A key feature, as compared with the traditional oxy-combustion processes, is that two or more pressurized boilers are connected in series on the gas side. In addition to allowing for reduced FGR, the use of multiple boiler modules also provides added flexibility in plant design and operation under variable loads. Although four boilers, or stages, are shown in the figure, fewer stages may be employed by increasing the amount of flue gas recycle. The optimum operating pressure of the SPOC boilers is around 15 bar.10 Coal is fed to the centerline at the top of each boiler, and burns as it flows through each of the respective boilers. The products of the upstream boiler, including any excess oxygen, are passed to the following stage, wherein more coal is introduced. The process repeats until nearly all of the oxygen is consumed in the final stage. The temperature of the products is further reduced in a convective heat exchanger, followed by ash removal. The flue gas is then cooled in a direct contact cooler (DCC), where moisture is condensed, the latent heat is captured, and SOx and NOx are removed. The majority of flue gas then goes to the GPU where it is further purified to meet the stringent specifications for storage or EOR.
Higher Net Generating Efficiency
The SPOC process has several other benefits to increase efficiency and reduce capital and operating costs: (1) FGR is minimized, which decreases flue gas volume, equipment size, and parasitic pumping loads, and increases boiler efficiency; (2) a high gas temperature is maintained to maximize the overall amount of radiative heat transfer, as compared to convective, thereby minimizing the amount of heat transfer surface area and reducing boiler exergy losses; and (3) moisture condensation and emissions removal are combined in a compact DCC device to remove SOx and NOx while recovering latent heat, which minimizes equipment size and cost.
As shown in Table 1(a), the net generating efficiency of the SPOC process can be over six percentage points greater than that of first-generation atmospheric pressure oxy-combustion. The penalty associated with carbon capture on net generating efficiency can be as low as three percentage points, as compared with the traditional air-fired power plant. The improvement in net generating efficiency over the reference atmospheric pressure case is due to a number of factors, but most of the savings are related to the SPOC process.
The capture of latent heat in the DCC is a major contributor to the increase in net generating efficiency of the SPOC process over atmospheric pressure oxy-combustion, adding 10% more heat to the steam cycle. The increase in efficiency and reduction in equipment costs translate into substantial reduction in the added cost of electricity (COE) associated with carbon capture, thereby showing potential to meet the U.S. Department of Energy’s target of less than 35% increase in COE.13 Électricité de France independently evaluated the SPOC process and an alternative pressurized oxy-combustion process and compared them with atmospheric pressure oxy-combustion and air-fired combustion (Table 1(b)).14 The goal was to understand the advantages and disadvantages of the different approaches to pressurized oxy-combustion from an energetic and exergetic standpoint. This study showed that the increase in radiative heat transfer relative to convective heat transfer in the SPOC process increases the overall exergy transfer from the flue gas to the steam cycle, which increases the plant efficiency. In addition, since the SPOC process required less FGR, the auxiliary load was lower, further increasing the plant efficiency.
Controllable Radiation Heat Transfer
Pressurization of the combustion process enables operation at higher combustion temperature and, thus, reduced FGR, to a degree that is not possible at atmospheric pressure. This is because at sufficient pressure, radiation heat transfer is dramatically altered since the combustion gas, which contains char and ash particles, becomes optically dense. Recognizing this, the team has developed, through CFD-aided design and fundamental studies, a unique approach to pressurized boiler design that can provide control of wall heat flux under very high flame temperatures.15,16 This approach is called “radiative trapping” as it utilizes the optically dense medium to trap the radiative energy emitted by the high-temperature flame within the reactor core (Figure 4) and control the heat transfer to the boiler tube surfaces. The SPOC boilers have additional design features to ensure that there is no flame impingement on the water wall. The ash deposition rate is substantially lower than that of a traditional air-blown boiler, and ash fouling and slagging are minimized.
WUSTL has recently completed construction of a lab-scale (approx. 100-kWth) pressurized combustion facility, shown in Figure 5. The facility will be utilized to demonstrate the staged oxy-combustion approach and obtain key experimental data to validate the computational fluid dynamics results.
Pressurized oxy-combustion, with the potential to capture over 90% of the CO2 at high efficiency and affordable costs, is poised to transform coal-based power generation. Through the recovery of flue gas moisture latent heat and the minimization of flue gas recycling, staged, pressurized oxy-combustion is able to achieve a net generating efficiency of 36.7% (HHV, supercritical conditions) with only about 3% penalty on net generating efficiency. Further improvements are anticipated as advances are made in ASUs and CO2 purification technology, and in the development of modular boilers for pressurized coal combustion.
- Gopan, A., Kumfer, B.M., Phillips, J., Thimsen, D., Smith, R., & Axelbaum, R.L. (2014). Process design and performance analysis of a Staged, Pressurized Oxy-Combustion (SPOC) power plant for carbon capture. Applied Energy, 125, 179–188.
- Hong, J., Field, R., Gazzino, M., & Ghoniem, A. (2009). Performance of the pressurized oxy-fuel combustion power cycle with increasing operating pressures. Paper presented at the 34th International Technical Conference on Clean Coal & Fuel Systems, 31 May–4 June, Clearwater, FL, U.S.
- Fassbender, G. (2001). Power system with enhanced thermodynamic efficiency and pollution control, Patent US 619600 B1. Washington, DC: U.S. Patent and Trademark Office.
- Hong, J.S., Chaudhry, G., Brisson, J.G., Field, R., Gazzino, M., & Ghoniem, A.F. (2009). Analysis of oxy-fuel combustion power cycle utilizing a pressurized coal combustor. Energy, 34(9), 1332–1340.
- U.S. Department of Energy National Energy Technology Laboratory (U.S. DOE NETL). (n.d.). Entrained flow gasifiers, www.netl.doe.gov/research/coal/energy-systems/gasification/gasifipedia/shell
- Zheng, L.G. (Ed.). (2011). Oxy-fuel combustion for power generation and carbon dioxide (CO2) capture. Woodhead Publishing Series in Energy. Cambridge: Elsevier.
- White, V., Wright, A., Tappe, S., & Yan, J. (2013). The Air Products Vattenfall oxyfuel CO2 compression and purification pilot plant at Schwarze Pumpe. Energy Procedia, 37, 1490–1499.
- Normann, F., Jansson, E., Petersson, T., & Andersson, K. (2013). Nitrogen and sulphur chemistry in pressurised flue gas systems: A comparison of modelling and experiments. International Journal of Greenhouse Gas Control, 12, 26–34.
- Murciano, L.T., White, V., Petrocelli, F., & Chadwick, D. (2011). Sour compression process for the removal of SOx and NOx from oxyfuel-derived CO2. Energy Procedia, 4, 908–916.
- Gopan, A., Kumfer, B.M., Axelbaum, R.L. (2015). Effect of operating pressure and fuel moisture on net plant efficiency of a staged, pressurized oxy-combustion power plant. International Journal of Greenhouse Gas Control, 39, 390–396.
- U.S. DOE NETL. (2010). Cost and performance baseline for fossil energy plants. Vol. 1, rev. 2: Bituminous coal and natural gas to electricity, DOE/NETL-2010/1397.
- U.S. DOE NETL. (2012). Advancing oxy-combustion technology for bituminous coal power plants: An R&D guide, DOE/NETL- 2010/1405.
- Axelbaum, R. (2014, 1 August). Staged, pressurized oxy-combustion for carbon capture: Development and scale-up. Presentation at 2014 NETL CO2 Capture Technology Meeting, Pittsburgh, PA, U.S.
- Hagi, H., Nemer, M., Le Moullec, Y., & Bouallou, C. (2014). Towards second generation oxy-pulverized coal power plants: energy penalty reduction potential of pressurized oxy-combustion systems. Energy Procedia, 63, 431–439.
- Xia, F., Yang, Z., Adeosun, A., Gopan, A., Kumfer, B.M., & Axelbaum, R.L. (2016). Pressurized oxy-combustion with low flue gas recycle: Computational fluid dynamic simulations of radiant boilers. Fuel, 181, 1170–1178.
- Xia, F., Yang, Z., Adeosun, A., Kumfer, B.M., & Axelbaum, R.L. (2016). Control of radiative heat transfer in high-temperature environments via radiative trapping—Part I: Theoretical analysis applied to pressurized oxy-combustion. Fuel, 172, 81–88.
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