Maintaining flame stability is a major challenge for oxyfuel combustion, as reported in early pilot-scale experimental studies [ 9, 10]. This challenge stems from the lower adiabatic flame temperature, lower flame propagation speed, and delayed ignition of coal particles in a CO₂-diluted environment.

The lower flame transportation velocities in an O₂/CO₂ environment are evident. Chen et al. [ 11] investigated the laminar flame speed using the CHEMKIN flame speed calculator model. The adiabatic flame temperature and flame propagation speed of CH₄ in an O₂/CO₂ atmosphere were both lower than those in an O₂/N₂ atmosphere. In addition, this difference increased with increasing CO₂ concentration. The lower flame propagation speed of pulverized-coal combustion in CO₂ was reported by Kiga et al. [ 12] using a microgravity experimental facility.

Two types of ignition mechanism have been observed for pulverized-coal combustion: homogeneous ignition and heterogeneous ignition [ 13]. Chen et al. [ 11] used a closed homogeneous CHEMKIN reactor model with the GRI-Mech reaction mechanism to investigate homogeneous ignition, where methane is used as a substitute for the volatiles, ignition delay is significantly longer in the CO₂-diluent gas than in the N₂, and the final combustion temperature is about 400 K lower in the CO₂-diluent gas. Shaddix and Molina [ 14] observed similar delays in the ignition of high-volatile bituminous coal particles when burning in an O₂/CO₂ atmosphere compared with an O₂/N₂ atmosphere. Huang et al. [ 15] and Huang [ 16] investigated the ignition characteristics of differently ranked Chinese coals during oxyfuel combustion using a flat-flame supported entrained flow reactor. The results, as shown in Figure 4, indicate that the ignition delay time is inversely proportional to Y_{O_2,S}^n , where n is 0.15¯0.2. The presence of high-concentration CO₂ leads to increased ignition delay time and decreased ignition stability for high-volatile coal. In contrast, the presence of high-concentration CO₂ leads to decreased ignition delay time and increased ignition stability for low-volatile coal. The effects of CO₂ on the heterogeneous ignition temperature of two coal chars were studied using a wire-mesh reactor [ 17].

Lower laminar burning velocity and narrower flammability limits may cause combustion destabilization in oxyfuel combustion [ 11]. This opens up opportunities for advanced oxy-coal burner design including the following variables: ① flue gas recycle ratio; ② oxygen concentrations in the primary, secondary, and tertiary streams; ③ the distribution of gas volumes in the different streams and their momentum; and ④ the preheat temperature of different streams.

A series of swirl oxyfuel burners was designed and applied to 0.4 MW_th, 3 MW_th, and 35 MW_th oxyfuel combustion boilers [ 18, 19]. The effects of the blockage ratio of bluff body, swirl number, oxygen injection tube number and size, oxygen concentrations in the primary stream, and recycle ratio were investigated. To achieve a stable pulverized-coal flame under both air-fuel and oxyfuel combustion, the following design rules were discovered and adopted: The oxygen is premixed with flue gas before its injection to the burner as primary stream or secondary stream; the oxygen partial pressure of the primary stream is maintained below 18% to ensure safety while oxygen is supplied from the secondary stream; the tertiary stream is switched off to ensure a similar momentum ratio of the primary stream and secondary stream; and the momentum of the primary stream is kept the same to ensure its pneumatic conveyance capabilities. The performance of the 3 MW_th swirl burner was extensively validated at the 3 MW_th pilot facility, and showed that stable combustion was achieved, and a burnout rate of 98% was obtained under oxyfuel combustion, in comparison to 95% under air combustion. Figure 5 shows the predicted temperature profiles in the 3 MW furnace. Generally, the profile is very similar under air combustion and oxyfuel combustion, although the temperature peak of the latter decreases by about 200 K. These rules for combustion system design were also adopted for the design of a combustion system for a 200 MW_e oxyfuel utility boiler, both tangential fired and wall-fired.

In oxyfuel combustion, high concentrations of gases participating in radiative heat transfer (i.e., CO₂, H₂O) can result in significantly different emissivity within the boiler, influencing the radiative transfer and the absorption of heat within the system.

A few modified weighted-sum-of-grey-gases (WSGG) models were proposed in recent years to consider the wide range of H₂O to CO₂ ratios for oxyfuel combustion. Based on the up-to-date HITEMP 2010 database, we obtained new parameters of a WSGG model for oxyfuel combustion by combining the features of a full-spectrum k-distribution (FSK) model with a WSGG model, in which the weighting factor and absorption coefficient were directly obtained from the k-distribution, and the fitting formula of the absorption coefficient was improved [ 20]. The new parameters of this WSGG model were extensively validated by comparing the radiative source terms and radiative heat fluxes predicted with the line-by-line (LBL) model integration of the HITEMP 2010 database for a one-dimensional slab system. Figure 6 shows the predicted emissivity of different parameters in WSGG models. The results show that the new parameters can significantly improve prediction accuracy in oxyfuel combustion.

Different researchers have observed conflicting heat-transfer results, and have considered that these should be optimized to ensure efficient operation. However, for a retrofit case where in-furnace heat transfer is matched, and for a given flue gas oxygen concentration, the oxyfuel case results in a decreased furnace exit-gas temperature, for which the match of convective heat transfer in the duct of the flue gas would be questionable. Andersson et al. [ 24] found that the recycle ratio is an appropriate tuning parameter to achieve similar boiler performance for air and oxyfuel combustion, which is a goal if existing boilers are to be changed from air to oxyfuel operation. They found that the temperature, and thereby the total radiation intensity of the oxyfuel flames, increases with decreasing flue gas recycle rate. In addition, the total radiation intensities are similar during oxyfuel and air-fuel operation as long as the temperature distributions are similar. Smart et al. [ 25] studied convective heat-transfer coefficients in oxy and air coal combustion. They found that an acceptable operational range of the flue gas recycle ratio exists in which the convective heat transfer in oxy-coal combustion can be approximately matched to that of air combustion. Thus, an existing coal-fired power plant can be retrofitted to function as an oxy-coal plant without replacing the boiler or heat exchangers.

Figure 7 shows a summary of the relative heat transfer of oxyfuel to air-firing, based on data from our simulation work of a 200 MW_e tangential-fired furnace, and data from experiments and simulations on a large pilot- or full-scale furnace. It is evident that relative heat transfer is slightly dependent on oxygen concentration, as revealed by the predicted results in the study by Black et al. [ 26], and the experiment in the Callide 30 MW_e [ 27] and the Schwarze Pumpe 30 MW_th furnaces [ 28]. In particular, experimental data from the pilot-scale facility shows that the relative heat-transfer ratio increases by several percent in a wide scope of oxygen concentration, that is, from 25% to 30% by volume. In this study, similar heat transfer can be achieved at 28.5% and 27.1% oxygen concentration for dry and wet recycle, respectively. However, the required oxygen concentration is approximately 26% in design calculations of boiler performance, which are based on thermochemistry equilibrium assumptions, regardless of the influence of the relatively slow CO reaction. It is critical to keep in mind the difference between numerical simulation and design calculations of boiler performance, which should be validated with experimental data from future large-scale demonstrations. The optimum oxygen concentration should depend on a comprehensive consideration of the combustion properties and heat-transfer properties.

Under oxyfuel conditions, a typical operation would involve starting up under air firing, and then gradually switching to oxyfuel mode by recycling the flue gas into the boiler [ 31]. However, the process of smoothly switching modes from air firing to oxyfuel is still considered to be a challenge [ 11]. Very little research has been conducted on appropriate switching strategies or on valuable operational highlights such as controlling recycle flue gas, oxygen, and air streams. Related research that is reported in the literature can be divided into two types: pilot experiments and process simulations. On the experimental front, researchers have operated several oxy-combustion pilot plants as a means to obtain practical operational experience for commercial demonstration [ 32, 33]. These operational results provide a benchmark for understanding the mode-switching process in an oxy-combustion power plant. However, deeper insight into the dynamic characteristics of this novel technology is still constrained, due to a lack of sufficient experimental measurements. Applying dynamic process simulation is a powerful way to identify the comprehensive characteristics of an oxy-combustion power plant.

A dynamic model of an oxyfuel combustion system was developed, based on a 3 MW_th oxyfuel pilot system and using Aspen Dynamics. The performance of this model was evaluated in detail by comparing its numeric results with experimental data, with a focus on both steady-state and dynamic behavior [ 34]. In addition, the transition between air combustion and oxyfuel combustion was optimized through dynamic simulation [ 35]. This transition was implemented by adjusting the air intake valves, recycled flue gas valves, exhaust valves, and oxygen intake valves; by monitoring oxygen concentration at the outlet of the furnace chamber and in the primary/secondary gas flow; and by monitoring the primary/secondary gas flow rates. Figure 8 illustrates the variation in primary gas flow rate and oxygen concentration. In order to maintain the stability of radiative heat transfer in the boiler, the inlet oxygen concentration was controlled so that it increased slowly during the transition, eventually remaining at 26%. Fluctuation of the oxygen concentration at the outlet of the chamber was regulated at 2%¯5% (eventually stabilizing around 3%) to ensure safe and economical combustion. In addition, the fluctuation of the primary gas-flow rate was small enough, around 18% with an oxygen partial pressure, to permit safe and reliable pulverized-coal transportation. The transition process took 30 min to complete, and the mode-switching strategy studied in the model was successfully applied at the 3 MW_th test facility.

In addition to the issue of the mode-switching process, the issue of air-leakage monitoring and control at oxyfuel combustion plants must be resolved. CO₂ concentration decreases by around 6% when air leakage is at 8% [ 36]. This decrease is significant when assessing the influence of recirculation in the flue gas species content, since CO₂ purity is one of the challenges for CO₂ capture in oxyfuel combustion. At the 3 MW_th test facility, a positive furnace-pressure operating condition was investigated to determine its effect on reducing air leakage in the overall system. As shown in Figure 9, the CO₂ concentration gradually increased when the furnace pressure was set at 60 Pa. This obvious increase in CO₂ concentration indicates that a positive furnace-pressure operating condition is preferable in oxyfuel combustion. Figure 9 shows that the CO₂ concentration of the flue gas can be raised to 80% (vol, dry) in both model and test results, by optimum regulation.

Mineral impurities in coal may cause ash deposition, erosion, slagging, and fouling on the surface of heat exchangers, which may lead to tube explosion, heat transfer, and boiler-efficiency loss. Therefore, mineral behavior during oxyfuel combustion has attracted extensive research in recent years.

High CO₂ concentration under oxyfuel conditions will aggravate the deposition of ash [ 37]; the migration and transformation of various minerals are also different under these conditions [ 38]. Early studies based on the X-ray diffraction analysis of total ash show only a weak indication of mineral carbonation under oxyfuel combustion conditions. Recent studies focus more on the behavior of iron- and calcium-related minerals, and all these studies indicate enhanced deposition and slagging potential under oxyfuel conditions. Compared to a traditional coal-combustion atmosphere, weight loss due to pyrite decomposition increased in an oxyfuel atmosphere, and a high CO₂ concentration resulted in a shortened decomposition process and a slight extension of the oxidation process. The melting endothermic of pyrite was higher in an oxyfuel atmosphere (30.4 J.g⁻¹) than in air combustion (5.0 J.g⁻¹). The decomposition of calcite was clearly delayed in the oxyfuel atmosphere, and TG-DSC curves indicated a decomposition temperature of up to 867 °C in the oxyfuel atmosphere, compared to 697 °C in air combustion. The melting endothermic of calcite was higher, up to 32.4 J.g⁻¹ in the oxyfuel atmosphere, compared to 15.3 J.g⁻¹ in air combustion. The melting temperatures of minerals were lower in the oxyfuel atmosphere compared to those in air, which resulted in easier mineral melting, and caused serious ash deposits in the oxyfuel combustion.

Figure 10 shows a recent study [ 39] of the mineral behavior of a sub-bituminous ZD coal. In this study, all ash is generated in a high-temperature (1700 K) drop-tube furnace. The ash is characterized by a computer-controlled scanning electron microscope (CCSEM). Figure 10 clearly shows that low-melting-point carbonates (Ca, Fe, Mg) are generated under oxyfuel conditions, which indicates enhanced slagging. At the same time, more silicates and aluminosilicates are generated, which have medium melting points and indicate enhanced coalescence of minerals due to high carbon monoxide yield during char oxidation.

The generation and elimination of sulfur trioxide (SO₃) is another critical issue for oxyfuel combustion, as SO₃ can cause corrosion in the combustion chamber, flue gas duct, coal mill, and so on. Due to the enrichment effect of flue gas recycle, the SO₃ concentration in oxyfuel combustion is usually 3¯5 times greater than in air firing, which increases the acid dew point of flue gas from around 400 K to 430 K.

The conversion of sulfur dioxide to sulfur trioxide has been well studied for air firing. Sulfur dioxide can be converted through high-temperature gaseous oxidation or mineral catalyst conversion. If the selected catalyst reduction (SCR) method is used to control nitrogen oxides, most of the SO₃ is generated via the SCR catalyst. Normally, 1%¯5% of SO₂ is converted to SO₃ in both air-firing and oxyfuel combustion. That is, the higher SO₃ concentration in oxyfuel combustion is due to the higher SO₂ concentration. No evidence exists of a higher conversion rate under oxyfuel conditions. Our recent study even shows that the conversion ratio of SO₂ to SO₃ by an SCR catalyst is significantly decreased in a high-CO₂ environment (Figure 11), due to competition between CO₂ and SO₂ at the active absorption site on the SCR catalyst. Therefore, for a compatible air combustion/oxyfuel combustion power plant design that includes SCR, which is necessary to meet strict environmental legislation, SO₃ corrosion may be more severe than for a power plant design that does not include SCR.

Modern wet flue gas desulfurization (WFGD) can effectively remove SO₂ at efficiencies of up to 95%¯99%. Therefore, for coal with a sulfur yield higher than 1%, the flue gas should recycle after WFGD in order to decrease SO₂/SO₃ concentration, and thus the potential for corrosion. Another practice that is usually adopted in order to inhibit corrosion is to reduce the moisture content of the flue gas, using flue gas condenser (FGC), which cools and dries the flue gas to temperatures around 300 K. However, the use of WFGD and FGC results in relatively high investment and operation costs; therefore, alternatives are required. In-furnace injection of a calcium-based SO₂ sorbent and the injection of sodium- based or calcium-based SO₃ sorbents before the flue gas pre-heater can be a cheaper choice.

An air separation unit (ASU) provides the oxygen for combustion in an oxyfuel power plant. The current state of technological development necessitates the application of a cryogenic distillation unit. Preliminary calculations indicate that this operation alone requires about 60% of the power consumption for carbon capture, and reduces the overall efficiency of the power plant by 7%¯9% [ 41]. Further studies have been conducted to reduce ASU energy consumption by investigating the use of high-temperature looping technology for oxygen production as an alternative to conventional cryogenic distillation methods. A new application of perovskite-type oxides as oxygen carriers provides pure O₂ or O₂/CO₂ gas streams for oxyfuel combustion [ 42]. This oxygen production process consists of two main steps, as shown in Figure 12: ① oxygen adsorption; and ② oxygen desorption.

In the first step, air is used as a feed gas to saturate the perovskite oxygen carrier with O₂; in the second step, CO₂ is used as a sweep gas to desorb O₂ from the perovskite, producing an O₂-enriched CO₂ flue gas stream. The adsorption/desorption process of perovskite-type oxygen carriers is reversible and can be expressed as follows:

Several types of perovskite oxygen carriers have been investigated for oxygen production application [ 43¯ 45]. The major problem encountered with these perovskite materials is their relatively low oxygen desorption ability. Recently, Shen et al. [ 46] developed a novel perovskite-type oxygen carrier, Ba_1−xSr_xCo_0.8Fe_0.2O_3−δ, which had an excellent cyclic stable oxygen-production performance. However, current investigations into perovskite looping for oxygen-production technologies are still at the conceptual or laboratory scale.

As mentioned earlier, although oxyfuel technology is widely studied and applied, there are still drawbacks to be overcome. Firstly, an oxyfuel flame is less stable than a conventional air flame. Secondly, even though gross emission is substantially reduced in oxyfuel combustion, NO_x concentration in the exhaust gas is relatively high due to nitrogen leakage and accumulation in the process of flue gas recirculation. Thirdly, recirculation decreases the overall systematic efficiency.

MILD combustion, also known as flameless combustion, is a high-efficiency combustion technology characterized by near-isothermal distribution across the furnace, and by high reactant temperature but low temperature increment in the reaction zone [ 47, 48]. Typically, in order to establish MILD combustion, the reactant temperature must exceed the fuel auto-ignition temperature, and the reactants must be diluted past the flammability limit [ 49¯ 51]. Given these limitations, the combination of oxyfuel combustion and MILD combustion—or MILD-oxy combustion—is a novel concept in this field, and has these advantages:

(1)The high-temperature reactant and uniform temperature distribution of MILD combustion can improve oxyfuel flame stabilization.

(2)NO_x formation is highly suppressed, due to the intense oxygen dilution and low temperature increment in MILD combustion.

(3)Overall thermal efficiency is greatly promoted by utilizing hot recycled flue gas for MILD combustion.

(4)CO₂ dilution tends to eliminate the visible flame, and CO₂ participates in the reactions, rather than remaining inert.

Taking both MILD combustion and carbon capture into account, Li et al. [ 52] investigated the characteristics of the MILD-oxy combustion of different gaseous fuels in a recuperative furnace. They concluded that the replacement of N₂ by CO₂ can promote the establishment of MILD combustion, and that the region of the MILD combustion becomes wider as the reactant is diluted by high-concentration CO₂. MILD combustion occurs without preheating as long as a sufficiently high reactant dilution is achieved under fixed CO₂ and O₂ concentrations.

Coal has less reactivity and mobility than gaseous fuels, due to its solid nature, so it is usually more difficult to achieve MILD-oxy combustion of pulverized coal than of gaseous fuels. Stadler et al. [ 53] tested a novel approach for MILD-oxy combustion of pulverized coal by increasing the oxidizer (O₂/CO₂) injection velocity up to 100¯280 m.s⁻¹. They found that coal MILD-oxy combustion can be established without oxidizer preheating, that is, at ordinary temperatures, when the entrainment of burned gas caused by high-speed injection is sufficient strong. They also discussed the effect of gasification reactions on char reactivity, burnout, and species in the MILD-oxy combustion of pulverized coal. Saha et al. [ 54] investigated the MILD-oxy combustion of pulverized coal in a recuperative furnace. Their results showed that by means of the internal recirculation of hot burned gas in the recuperative furnace, the MILD-oxy combustion of both high and low rank coals can be successfully established without preheating, given an appropriate jet velocity, which is reported as 86.2 m.s⁻¹ in the study.

It is worth noting that an experimental investigation [ 55] on the MILD-oxy combustion of pulverized coal at ordinary temperatures was conducted at a 0.4 MW pilot-scale facility at HUST. MILD-oxy combustion of pulverized coal was successfully achieved (Figure 13) without highly preheating the oxidant. The integral system of flue gas recirculation was applied in this experiment, including ash separation and water vapor condensation. This result indicates the feasibility of an industrial application for this technology.