1.1 General Background

In gas–solid fluidization theory, solid particles become fluidized when an ascending gas imposes a high enough drag force to overcome the downward force of gravity. Fluidized bed reactor (FBR) is an apparatus developed based on the fluidization theory, which has been applied to many kinds of multiphase chemical reactions for energy conversion, chemical, petroleum, metallurgy, and nuclear industries [1, 2]. Fluidized bed reactors have been operated commercially since the 1920s, with the advent of the Winkler coal gasifier in Germany [3]. The first attempt to burn solid fuels in a fluidized bed combustor (FBC) was made in the Soviet Union in the 1950s to develop boilers for industry and district heating. After that, the FBC technology experienced extensive development again from the 1970s [4,5,6].

During a typical process in FBC, fuel and desulfurization adsorbents, typically limestone, are injected into the lower part of the combustor to be quickly dispersed with the high-temperature bed material fluidized by the combustion gases. Fluidization gas, usually air, is blown into the bed through the air distributor from the wind box at the bottom of the furnace [5]. The fuel is immediately heated due to the high heat transfer coefficient, followed by drying, ignition, volatiles combustion, and char combustion. The limestone is decomposed and absorbs the SO2 generated from fuel combustion. With the increase of the fluidizing gas velocity, a bed will experience different fluidization states: fixed bed, bubbling fluidized bed, turbulent fluidized bed, fast bed, and pneumatic conveying.

Generally, large-scale FB reactors employ circulating fluidized bed (CFB) to achieve a satisfactory combustion performance, sulphur removal, and operation reliability [6]. In a CFB combustor, the bottom of the furnace is usually a bubbling or turbulent fluidized bed, while the upper part is normally a fast bed or pneumatic conveying. The small particles (bed material, char, fine ash, and desulfurization absorbent) can be carried out of the furnace by gas flow. They are then captured by a cyclone separator and recirculated to the furnace. This ensures higher combustion and desulfurization efficiencies. The CFB combustion technology shows many advantages: flexible fuel applicability, stable temperature control, flexible load control, low NOx formation, efficient desulfurization in the boiler, high combustion efficiency, and so on [4, 7,8,9,10,11,12].

Over the last several decades, fluidized combustion has attracted widespread attention, and a large number of investigations have been done, including hydrodynamic and mixing characteristics [5, 13,14,15,16,17], combustion behavior [18,19,20,21,22,23,24], heat and mass transfer enhancement [25,26,27,28,29,30,31], pollutant emission controls [5, 32,33,34,35,36,37], scale-up [6, 11, 12, 38, 39], and so on. It has been used successfully to burn a variety of solid fuels, including high-sulfur petroleum coke, different ranks of coal, and solid wastes [40,41,42,43]. In recent years, integrating CFB technology with oxy-fuel combustion (including pressurized oxy-fuel combustion) [44,45,46,47,48,49,50,51,52,53,54,55,56] or chemical looping combustion (CLC) [57,58,59,60,61,62] for carbon capture has become highlighted research topics due to the demand for tackling global warming.

Gas–solid mixing is always an important topic in fluidized bed combustors since it determines heat and mass transfer, combustion behavior, combustion efficiency, and pollutant emissions [11, 24, 63]. The mass transfer in a fluidized bed combustor is a complicated phenomenon. After many years of development, it is generally accepted that oxygen in the bubble phase must pass through inert bed material to reach the fuel surface in the emulsion phase [64], which is different from the direct contact between fuel and oxygen in pulverized coal combustor (Fig. 1.1). The dense phase of the fluidized bed reactor presents an anoxic atmosphere on the whole, thus the char combustion in the dense bed is mainly controlled by oxygen diffusion. In the upper part of the reactor, namely in the transport zone, a core annular structure with a dilute phase surrounding the fuel particle is usually proposed, and the air supply, e.g., the secondary air, needs to reach the fuel in the center of the furnace. Generally, increasing bed voidage and disturbances can enhance the mass transfer, provided that the gas and fuel are evenly distributed in the bed [65, 66].

Fig. 1.1
A diagram depicts emulsion phase, bubble phase and splash phase. Splash phase is inside the wind box and has fuel in it.

Heat and mass transfer as a function of the location of the char particle in the bed. uf, umf and uth are the fluidization velocity, minimum fluidization velocity and the velocity through the bubble, respectively

Full size image

It should be noted that there are still some problems related to poor mixing, especially in large-scale CFB combustors. In a typical CFB process, the fuel is fed from the wall of the furnace, while the primary air is injected over the cross-section through the distributor [5]. The dispersion of fuel in the bed as well as the uniformity of the air supply determines the fluidization conditions and the distribution of heat release in the bed. According to the experiment and simulation studies on both lab- and industrial-scale CFB combustors, it is found that the effective lateral fuel dispersion coefficient (Dsr) increases gradually with the increase of the boiler size until reaching a stable value [11, 63, 67,68,69,70]. The value of Dsr for fuel particles in industrial CFB boilers is of the order of 0.1 m2/s, which is affected by the fluidization velocity, the static bed height, the bed particle size, etc. [11, 63]. The fuel is usually introduced from the furnace wall in on or several feed points, sometimes air assisted. The fuel has to travel over the furnace cross section, a distance depending on boiler size. Therefore, the fuel distribution in the fluidized-bed combustor becomes more uneven when the boiler capacity is further increased to electric utility size. In addition to the lateral dispersion, light fuels, such as biomass and solid wastes, tend to be subject to more segregation and bad mixing of fuel and oxygen because of the rapid release of volatiles [71,72,73,74,75,76].

Poor gas–solid mixing may bring many operational problems, such as local defluidization, uneven temperature distribution, agglomeration, low burnout rate, large excess air ratio, high pollutant emission, and so on [5]. Such issues not only reduce the efficiency of the boiler but also cause safety and environmental problems. Even though technical measures have been introduced to improve the spatial mixing characteristics of boilers, such as optimizing the size distribution of fuel particles, increasing the number of fuel feed points, improving the air distributor performance to ensure even primary air supply, increasing secondary air momentum, etc. Here is still room for improvements.

In 2013, Thunman et al. [77] from Chalmers University of Technology proposed the innovative technology concept of oxygen-carrier-aided combustion (OCAC), which aims of improving the distribution of oxygen throughout the combustion chamber. In this concept, the conventional inert bed inventory (usually silica sand and ash) is partially or completely replaced by reactive particles, like metal oxides. Such so-called oxygen carriers can react with fuels via redox reactions. As a result, the redox reactions taking place on oxygen carriers can transfer oxygen from an oxidizing zone to a reducing region, which can improve the uniformity of the spatial oxygen distribution. The OCAC technology is a technical adjustment based on the traditional fluidized-bed reactor, which can be applied to various fuel conversion technologies, such as coal-fired large-scale CFB combustion, combustion/gasification technology for low-quality fuels (such as biomass and solid waste), oxy-CFB combustion for carbon capture and storage (CCS), and so on.

1.2 The Objectives of This Book

FBC technology with the advantages of flexible fuel applicability, stable temperature control, inherently low gas-pollutant emissions, etc. is an advanced fuel-conversion technology, applied in industry, and in heat and electric utilities. Theoretically, FBC has good gas–solid mixing and a resulting uniform temperature distribution. In the actual FBC operation, however, especially in large-scale reactors, the mixing of oxygen and fuel is still limited by the lateral solids dispersion coefficient (Dsr) and gas penetration. Such an issue not only reduces the combustion efficiency but also brings a series of problems related to the operation, such as excessive emissions and bed agglomeration.

OCAC technology, which is using oxygen carrier particles to partially or completely replace the traditional inert bed materials in fluidized-bed reactors. The oxygen carrier particles act as oxygen buffers to take up oxygen into their lattice from oxidizing regions and to subsequently react with volatiles in reducing areas, where CO2/CO and H2O/H2 are generated. This technology is aiming to improve combustion efficiency, enhance operational safety, and reduce pollutant emission by promoting the uniformity of the oxygen and temperature distributions in a fluidized-bed reactor. Benefiting from the oxygen buffering characteristic of oxygen carrier particles, OCAC can significantly improve the oxygen distribution throughout the combustion chamber, which can reduce CO and hydrocarbon emissions.

At present, the OCAC technology has already gone through a series of experimental studies at different scales and is considered feasible and economical. The main research institutions on the technology include the Chalmers University of Technology, CanmetENERGY-Ottawa and Vienna University of Technology, etc. Chalmers University of Technology’s research mainly focuses on the oxygen carrier aided air combustion, CanmetENERGY mainly investigates the OC aided oxy-fuel (natural gas, biomass, and coal) combustion, whereas Vienna University of Technology mainly investigates the OC aided gasification. The scientific concerns, including combustion and emission characteristics, ash-related problems, OC reactivation, and so on.

In this book, we intent to summarize the evolution and working principle of OCAC firstly, and then introduce the development status of OC aided solid fuel conversion technology critically from three aspects: OCAC without CO2 capture and with CO2 capture, and OC aided gasification. Some new concepts like OCAC coupled with staged conversion of fuel, OCAC with rotatory kiln, and multi-functional OCAC, are also proposed for the potential applications of OCAC technology. In addition, the future research directions as well as the possible extended strategies based on OCAC technology are outlooked. This is expected to promote a rapid development of OCAC technology in both fundamental research and practical applications.