The OCAC concept has been validated by operation of FB boilers on lab-scale, semi-industrial scale and full industrial scale [1,2,3,4,5]. Based on the current progress in the development of OCAC technology, it has been proven that the OCAC is a feasible technology showing obvious benefits in the applications on a CFB boiler [6]. However, there are still challenges remaining for more perspectives.

7.1 Fundamentals Regarding Transport Phenomena and Chemical Reactions

7.1.1 Heat and Mass Transfer and Reactions

Although the OCAC concept has been proven to be effective in various thermal conversion processes, many basic principles are not clear, such as: (1) the effect of OC on the fluidization behaviour, heat and mass transfer, including in the dense bed, splash zone and transport zone, especially the transient character of bubbles, clusters and wall layers; (2) the influence of different redox atmospheres on the reaction characteristics of OC, including oxygen transfer mode, intermediate reaction processes, reaction rate, and the ratio of oxygen buffering; (3) the influence of different reaction conditions on particle temperature and microstructure, etc. Besides, it is difficult to analyze quantitatively the influence of various factors on the reaction process in a complex environment through experimental research; (4) the properties and existence of reducing/oxidizing regions should be further investigated. Therefore, multi-scale simulation studies including quantum chemical calculations, molecular-scale simulations, particle-scale models, and industrial-scale simulations, etc. can help to reveal the exact mechanism affected by different factors, which is rarely reported.

7.1.2 Problems Associated with Alkali Metals and S

The absorption of alkali metals by an oxygen carrier may potentially be beneficial to alleviate a series of ash-related problems, such as bed agglomeration, ash deposition and corrosion on the heating surface and the formation and emission of PM in the flue gas, improve the efficiency and safety of the system. This is because the alkali metal contents not only diffuse into the OC bed particles but also forms compounds with high melting point [7]. For example, the ilmenite ore can absorb K and form a stable potassium-titanium compound [8,9,10]. However, the absorption mechanism of alkali metals and alkaline-earth metals (such as K, Ca and Na) by different oxygen carriers is still unclear and needs further study, which is of great significance for the optimization and selection of bed materials.

7.2 Pollutant Transformation Routes and Control Strategies

7.2.1 NOx and SOx Formation and Emission Mechanism

At present, the effect of the OC addition on NOx emission is still divided, based on the results from bubbling beds and CFB boilers, and the influence of OCAC technology on NOx formation and transformation is still unclear and needs to be investigated. In some studies, the presence of OC is found to bring several advantages to reduce NOx in the FB system [3]. OC reduces the hot spots and volatile flames and so reducing NO formation. In addition, the active components, such as iron, in the OC shows a catalytic effect on the reduction of NOx [11]. On the other hand, it also brings negative effects. The presence of OC consumes more reducing gases, thus, promotes NOx formation. The oxidative OC may react with nitrogen-containing gas components, such as NH3 and HCN, generating more NO [12, 13]. In general, the influence of OC on NOx formation and reduction mechanisms needs to be further studied.

In the application of OCAC technology, more uniformly distributed oxygen and temperature can promote the reactivity of a desulfurizing agent [14]. Besides, appropriate OCs, such as ilmenite ore, absorbs S [10, 15], which further reduces the emission of SOx and alleviates the corrosion risk. However, studies on the absorption characteristics of OC over S along with the pathways of S migration and transformation are not clear. Furthermore, the SO3 emissions related to OCAC have not yet been reported, and this should be well studied since SO3 is corrosive to the heating surfaces at low temperatures.

7.2.2 The Emissions of Cl and Dioxin

Studies on chloride emissions in oxygen- carrier-aided air combustion have not been reported yet. However, for fuels with high Cl content, such as MSW, chloride emissions impose great risks of corrosion and ash deposition, and the Cl emission from the OCAC process needs to be paid special attention. In addition, Cl is an important source for the highly toxic dioxins. The dioxins are formed through precursors in the furnace and this formation is closely related to the temperature, residence time, and gas turbulence in the furnace, and the implementation of OCAC may potentially affect these factors which will subsequently have a significance in the convection pass outside the range of OCs. Therefore, the formation and emissions of dioxins should be considered during application of OCAC when burning fuels with high chlorine content such as MSW.

7.2.3 Emissions of Mercury, Heavy Metals and PM

The emission of mercury, heavy metals, PM and other pollutants in connection to OCs have not been investigated yet. The OCAC process improves the distribution of oxygen and temperature in the combustion system, the active OC particles may react with different forms of mercury and heavy metals to change their physico-chemical properties. Thereafter, the evolution process and emission rules of mercury and heavy metals might differ. Under OCAC operation, the flue gas has lower quantities of gaseous combustibles, which means that the flue gas may have lower carbon content, such as soot in fly ash. This not only affects the adsorption of mercury and heavy metals by fly ash, but also affects PM emissions related to soot. In addition, using OC as bed materials has the potential to adsorb alkali metals, which may be beneficial to reduce the formation and emission of PM related to alkali metals.

7.3 More Testing and Application of OCAC

7.3.1 More OC and Fuel Testing

More fuel types (such as more types of coal, solid waste, hazardous waste, etc.), OC types (including natural ores, synthetic oxygen carriers, and industrial waste residues) and addition ratios of OC need to be tested. Currently, most of the research has focused on biomass fuels and only limited types of OCs are considered. The selection of different types of fuel and OC will have a significant impact on the OCAC process. For example, the ash content of biomass fuels is usually small, and the OC can be added periodically. However, during combustion of high-ash fuel (such as coal), the bed material of the CFB boiler needs to be discharged frequently, so adding OC, maintaining the concentration of OC in the combustor, and recovering OC from the bottom ashes are all topics that need further study. In addition, since the use of OC implies a cost, the management of OC is very important for the application of OCAC technology to industrial CFB boilers [6]. At present, the study of OC regeneration and recycling on an industrial scale test facility is lacking, which is the next step to be carried out urgently. Such matters can accumulate the basic data of OCAC technology to cope with the application scenarios of multiple fuels and processes.

In addition, it should be noted that if this technology is applied to existing boilers, in principle, the introduced OC should avoid changing the original fluid-dynamic state of the boiler, that is, the mass distribution and particle size distribution of OC should match the flow characteristics of the original inert bed material. Otherwise, if the OCAC technology is applied to a newly built boiler, it will have a more flexible design scheme. According to the fuel characteristics, boiler load, and OCs type, the size distribution of a bed material can be flexibly adjusted to adapt different combustion requirements. For example, a higher proportion of fine OC particles can promote the oxidation of combustible gases in the dilute upper zone to improve combustion efficiency, which is especially meaningful for burning high-volatile fuels. This can also increase the heat-transfer coefficient of superheater and reheater resulting in the increase of boiler load.

7.3.2 More Applications of OCAC Technology

The existing research on OCAC technology focuses on the combustion and gasification in a fluidized bed reactor, while the application of OCAC in other processes has not been reported. However, OCAC technology may also potentially have applications in other conversion processes with poor mixing and reactivity. For example, OCAC technology can be applied to gasification-combustion coupling processes in two CFB reactor to achieve efficient, low-NOx combustion. It can also be used in the process of pyrolysis/gasification to produce activated carbon and combustible gas in a polygeneration process. In addition, OCAC technology can also be be tried in reactor configurations, such as fixed bed, rotary kiln incinerator, grate furnace, and other furnaces to improve the oxygen distribution and to achieve a better heat and mass transfer.

7.4 Techno-economic Analysis of OCAC Technology

Economic efficiency is a most important factor that determines whether a new technology can be commercially applied. Specific techno-economic analysis of the OCAC technology is important for the commercial application. However, no such studies have been reported yet, to the best of our knowledge. The profit (P) from the application of OCAC technology depends on economic benefit (B) and cost (C).

The B may come from: (1) The benefit from reduced power consumption of fan (B1). OCAC improves the efficiency of oxygen utilization and reduces the total amount of air, thus, reducing the energy consumption of the fan; (2) The benefit of higher combustion efficiency of boiler (B2). OCAC reduces the combustible gas concentration in the flue gas and the carbon content of fly ash; (3) The benefits of reducing the operating costs of flue gas purification units (B3). OCAC improves the in-furnace desulfurization efficiency and reduces the desulfurization cost. In addition, OCAC also may increase or reduce NOx and PM emissions, which will effect the costs of gas purification units; (4) The benefit of improving continuous operation time of boiler (B4). OCAC not only can improve the stability and safety of the combustion system, but also may reduce the ash accumulation and corrosion of heat exchangers, thereby potentially increasing the continuous working time of the equipment. The C may come from: (1) The cost of OC (C1). The use of OC instead of inert bed material will bring about changes in the cost of bed material, which requires specific analysis. For example, if using synthetic OC or natural ore OC increases the cost of bed material to varying degrees, but if using industrial waste slag (such as steel slag, etc.) it may even reduce the cost of bed material. C1 is influenced of the degree of recovery. (2) The cost of OC recovery (C2). OC recovery requires additional equipment (such as screening system and magnetic separation system), and the investment and operating costs of the OC recovery system will be increased. In addition, different application scenarios mean different types, reactivity, the regeneration frequency, and mixing ratios of OC, which all will affect the cost.

Generally, the P can be calculated by the following formula: P = B1 + B2 + B3 + B4 – (C1 + C1). All of these aspects need to be considered in the technical–economic analysis models. The quantitative discussion of the effect of the parameters involved should be based on process simulation to guide the low-cost application of OCAC technology.

7.5 Process Scale-Up and Optimization

7.5.1 Process Scale-Up

Most of the current research on OCAC technology is focused on small and medium-sized CFB boilers, and the application of OCAC on large-scale electric utility CFB boilers has not yet been involved. However, many technical details, such as furnace layout, gas–solid flow state, and heat-exchanger layout of large-scale CFB boilers are not the same as in small-scale CFB boilers. Therefore, the application of OCAC will not only bring about changes in temperature and oxygen distribution in the CFB system, but it also may affect the original flow state, heat-transfer characteristics, ash deposition and erosion characteristics of the furnace. All of these aspects need further research. In addition, the experience and evaluation of the long-term operation of OCAC technology are insufficient. For example, has long-term operation so far unexpected effects on system and equipment? How does OCAC affect the variable load operation of large boilers? What effect does the application of OCAC have on the erosion characteristics in the furnace? All such questions require more and longer-term industrial testing and detailed evaluation to answer.

7.5.2 Process Optimization

Based on the discussion and analysis of advantages and potential concerns, it is necessary to develop an economically optimal mode of operation. On the one hand, based on the relevant experience of experimental research and the sensitivity analysis of process simulation, a technically feasible and economically optimal operation scheme need to be formulated, including detailed operation parameters, such as the selection suitable OC species, blending ratios, flow regime, and outlet oxygen concentrations, etc.; on the other hand, minimize the cost increase due to the introduction and regeneration of OC to improve the economy of the whole process. This point can be divided into two parts: one is to develop efficient separation devices to recover and reuse OC from ash discharge, another is to strengthen the utilization of the discarded OC.

Although it is claimed that bed materials, such as natural OC ores, are not expensive, the cost is still higher than that of sand in conventional processes. Therefore, according to the physical and chemical characteristics of different types of oxygen-carrier, it is necessary to develop a specific OC recovery device for the reuse of OC to reduce the cost. For example, in addition to the magnetic separator developed according to the magnetic characteristics of ilmenite ore, one can also develop separators according to the density difference (flotation method) and the size difference (mechanical screening). However, it should be noted that due to attrition and aging, the OC's life is limited. Therefore, the cost can be further reduced by recycling and utilizing related elements in the discarded OC. Taking ilmenite ore as an example, the contents of Ti (due to the attrition of the iron-rich layer on the surface of OC), K and S (absorption from the fuel) in the regenerated bed material increases significantly, and the recovery and utilization of Ti and K from discarded OC in a controlled manner is a challenge. Furthermore, it is also conducive to the harmless treatment of OC.