**1. Introduction**

Utilization of fossil fuels, mainly coal, for power generation is associated with emissions of large quantities of carbon dioxide—a greenhouse gas responsible for global warming and climate change. As the world will still depend on coal for several decades to come, it is necessary to develop technologies such as carbon capture and storage (CCS) to significantly reduce emissions of CO2 and avoid negative consequences of global warming. Among CCS technologies, oxy-fuel combustion emerges as a technically feasible and cost-effective option for the reduction of CO2 emissions from coal-fired power plants [1–3]. When combined with co-firing of biomass—a renewable carbon-neutral fuel—it can offer negative CO2 emissions [4–6].

In an oxy-fuel combustion system, fuel is burned in a mixture of oxygen and recycled flue gas. As molecular nitrogen is eliminated from the oxidizing medium, the flue gas consists mainly of CO2 and water vapor. Partial recirculation of flue gas is necessary to control the combustion temperature in the boiler. Substituting recycled flue gas for N2 in the oxidizing medium leads to a smaller exhaust gas stream, higher boiler efficiency and lower NOx emissions [3].

Nowadays, pulverized coal (PC) combustion is the dominant technology of heat and power generation in utility and industrial sectors. However, circulating fluidized-bed (CFB) combustion gains popularity, particularly for the utilization of low quality, high-ash coals,

**Citation:** Kosowska-Golachowska, M.; Luckos, A.; Kijo-Kleczkowska, A. Pollutant Emissions during Oxy-Fuel Combustion of Biomass in a Bench Scale CFB Combustor. *Energies* **2022**, *15*, 706. https://doi.org/10.3390/ en15030706

Academic Editor: Maria Founti

Received: 5 December 2021 Accepted: 14 January 2022 Published: 19 January 2022

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and discards. Today, high-efficiency, utility CFB units with supercritical steam parameters and electric outputs up to 600 MW are available commercially. Fluidized bed combustion is also considered the best available co-firing technology [4,7–9]. Co-firing of coal and biomass in an oxy-CFB boiler has been demonstrated at the 30 MWth scale at Fundación Ciudad de la Energía (CIUDEN) in Spain [10,11].

In the case of oxy-combustion, CFB boilers have a few advantages over PC counterparts. The combustion temperature in oxy-CFB systems can be controlled by cooling (in an external heat exchanger) and recycling a part of circulating solid particles. This reduces the stream of recirculated flue gas and makes an oxy-CFB boiler smaller, more efficient, and cheaper. Due to lower combustion temperature, oxy-CFB units can meet NOx and SOx emission limits without additional de-NOx and de-SOx systems. Moreover, the oxy-CFB technology does not require sophisticated burner and fuel feeding systems.

A significant progress has been made in the case of oxy-combustion in PC boilers. However, to date, only a few studies have been published on combustion or co-combustion of biomass and coal in oxy-fuel fluidized-bed combustors. Some few among them are devoted to the formation and emissions of pollutants such as NOx, SO2 and CO.

Carbon dioxide storage/sequestration requires a gas stream with 95% or more CO2 purity. To achieve that a CO2 processing unit (CPU) is employed for drying, cleaning and compression of the exhaust gas. The presence of impurities affects the design and energy consumption in the CPU. The impurities of interest are O2, N2 and Ar supplied by the ASU and related to the purity of oxygen produced and excess O2 used in the oxycombustion system. They also include NOx, SO2 and other N- and S-containing gases such as HCN, NH3, COS and H2S. Therefore, the knowledge on pollutant formation and their concentration in the flue gas is of crucial importance for the proper design of the CPU and assessment of its performance and cost.

In this study, combustion tests of three kinds of biomass (agricultural, woody and energy crop) and a reference coal were carried out in the laboratory-scale CFB reactor at 850 ◦C. Concentrations of NOx (NO, NO2, N2O and their precursors, such as NH3 and HCN), SO2 and CO were measured online during conventional and oxy-fuel combustion. The main objective was to assess the influence of oxidizing atmosphere and composition of tested biomass on the formation mechanism and emissions of pollutants. A comparison with emissions from bituminous coal can be useful to determine emission levels in the case of co-firing coal and biomass.

The study revealed that oxidation of nitrogen species released with volatile matter was responsible for high emissions of NOx during combustion of biomass fuels in air and mixtures of O2 and CO2. Emissions of SO2, N2O and CO for the combustion of biomass in all atmospheres were lower than those for the combustion of reference coal. Oxy-combustion of biomass in O2/CO2 mixtures at 30% and 40% O2 caused a decrease in emissions of N2O and CO while NO and SO2 emissions increased.
