**1. Introduction**

Various human activities related to the use of fossil fuels such as electricity generation, agriculture, and transport release large quantities of CO2 into the atmosphere. Carbon dioxide as a greenhouse gas (GHG) is a major contributor to global warming. In the transition to a low-carbon economy, i.e., in the next few decades, the world will still be heavily dependent on fossil fuels as primary energy sources despite unprecedented progress in solar and wind energy generation. Therefore, the utilization of advanced systems for CO2 capture, utilization, and sequestration (CCUS) becomes vital to abate emissions and lessen the impact of CO2 emissions on climate change. Coupling CCUS with renewables should help to reach the 2 ◦C temperature rise limit recommended by the Paris Agreement.

The contribution of biomass to global primary energy consumption is the largest among all sources of renewable energy [1]. Biomass is carbon neutral because, unlike fossil fuels, it is a part of the natural carbon cycle. Carbon dioxide released during the combustion of biomass is absorbed by plants during their life cycles.

The basic characteristics of biomass as a fuel include moisture content, ash yield and composition, carbon content, and higher heating value (HHV). Vassilev and co-workers [2] reviewed the chemical composition of many biomass crops and found large variability

**Citation:** Kosowska-Golachowska, M.; Luckos, A.; Czakiert, T. Composition of Flue Gases during Oxy-Combustion of Energy Crops in a Circulating Fluidized Bed. *Energies* **2022**, *15*, 6889. https://doi.org/ 10.3390/en15196889

Academic Editor: Albert Ratner

Received: 23 August 2022 Accepted: 16 September 2022 Published: 20 September 2022

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in moisture content, ash yield, and types of inorganic matter (minerals) in raw materials. However, the results of proximate and ultimate analyses when reported on a daf (dry, ash-free) basis were in a narrow range.

Among various biomass sources, dedicated energy crops (non-food crops, mostly perennial grasses, and fast-growing trees) are grown solely to provide renewable feedstock for the generation of electricity and heat and the production of biofuels [3]. These high biomass yield crops can be cultivated on marginal or reclaimed land; they are non-invasive and have low water and fertilizer needs. Herbaceous energy crops, e.g., miscanthus and switchgrass, can store approximately two times more CO2 per 1 hectare of cultivation than woody crops such as poplar and willow.

Among CCUS technologies, oxy-fuel combustion is a promising process because it uses almost pure oxygen instead of air and generates a high-purity stream of CO2 (95% or more) that can be stored or utilized to produce carbon-based fuels or chemicals. Biomass firing or co-firing in oxy-fuel systems integrated with CO2 capture can be considered a carbon-negative technology.

Fluidized bed boilers, both stationary and circulating, are the best choice for firing low-quality coals, refuse-derived fuels, and biomass [4–6]. The well-known benefits of fluidized-bed combustion technology include high fuel flexibility and turndown ratio, low emissions of NOx, and high availability. Today, circulating fluidized-bed (CFB) boilers firing 100% biomass fuels are available commercially. Units at Konin (55 MWe) and Połaniec (205 MWe) power plants in Poland [7], and the 299 MWe Tees unit in the UK (the largest bio CFB in the world) [8], can be mentioned as examples.

Oxy-fuel CFB boilers are better than their pulverized-fuel (PF) counterparts. They do not require sophisticated burners and fuel preparation and feeding systems. Because the combustion temperature in oxy-CFB systems is lower than in PF units, oxy-CFB boilers can meet current NOx and SOx emission limits without additional de-NOx and de-SOx equipment. The combustion temperature can be controlled by cooling a part of the circulating particles in an external heat exchanger and recirculating them back to the combustion chamber [9]. This allows for using higher initial O2 concentrations in smaller streams of recirculated flue gas, which results in units with reduced dimensions, higher thermal efficiency, and lower capital costs. Energy losses in oxy-fuel systems caused by air separation and CO2 purification systems working at high pressures can be reduced in oxy-CFB units operating at elevated pressure. The latent heat in flue gases can be recovered and air infiltration can be eliminated in pressurized systems, which can improve the purity of CO2 and reduce the cost of its capture [10,11].

Carbon dioxide utilization and storage require a gas stream with a concentration of CO2 of 95% or more. The presence of impurities such as N2, O2, NOx, SO2, and other N- and S-containing gases affects the design, energy consumption, and costs of the CO2 processing unit (CPU). Knowledge of pollutant formation and their concentration is, therefore, important for the design, performance, and cost of the CPU [12].

Several publications on the combustion of biomass in oxy-CFB systems are available in the open literature. A few of them provide some information on emissions of NOx, SO2, and other pollutants. The main observations drawn from these studies are presented in Table 1.

In this study, combustion tests of three dedicated energy crops, namely miscanthus (*Miscanthus gigantheus*—a perennial gras with bamboo-like stems), Virginia mallow (*Sida hermaphrodita*—a perennial forb native to the eastern U.S.), and basket willow (*Salix viminalis*—a multi-stemmed shrub) were conducted in the bench-scale oxy-CFB reactor at 850 ◦C. On-line measurements of NO, NO2, N2O, NH3, HCN, SO2, and CO concentrations were taken during air- and oxy-fuel combustion. The influence of the chemical composition of oxidizing gas and biomass fuels on the emissions of above-mentioned pollutants was assessed. The results were compared to the emissions from wheat straw (agricultural biomass), Scots pine (woody biomass), and bituminous coal (reference fuel).


**Table 1.** Pollutant emissions from oxy-CFB combustion—a summary.

While there are several publications on NOx emissions from the oxy-combustion of coal, relevant publications on the oxy-combustion of biomass are scarce [20]. There are no studies in the open literature on pollutant emissions during oxy-CFB combustion of energy crops such as *Miscanthus giganteus* and *Sida hermaphrodita*. This paper fills a gap in this field. This manuscript provides a comparative analysis of pollutant emissions during the combustion of energy crops and other fuels such as woody- and agro-biomass and bituminous coal. So, in this respect, it is similar to our previous work [12]. The test apparatus and methodology were the same to make sure the results can be compared.

#### **2. Materials and Methods**

#### *2.1. Fuel Tested*

Three different energy crops were selected for this study, *Miscanthus giganteus*, *Sida hermaphrodita*, and *Salix viminalis*, all originating from plantations in Poland. Agricultural biomass (wheat straw), woody biomass (Scots pine), and Polish bituminous coal were used as the reference fuels; the results of their combustion tests were already presented in reference [12]. Samples of tested fuels were milled and sieved to less than 0.2 mm for proximate and ultimate analyses. The basic characteristics of these fuels are summarized in Table 2. The ash yields for *Miscanthus giganteus* and *Sida hermaphrodita* are significantly higher than that for *Salix viminalis*. The energy crops tested in this study are characterized by a high content of volatile matter (VM). Their higher heating values (HHV) are in the range of 17.5 to 18.2 MJ/kg, typical for biomass fuels. Contents of sulphur and nitrogen in these crops are lower than in agricultural biomass (wheat straw) and coal. An additional advantage of these energy crops is a very low content of chlorine (less than 0.1%).

**Table 2.** Characteristics of tested fuels.


Note: db—dry basis, daf—dry, ash free, \* by difference to 100%.
