**2. Literature Review**

This short literature review is devoted to pollutant emissions from oxy-combustion of biomass fuels in fluidized-bed systems. The only exception is the study by Gao and co-workers [12] that was carried out in a TG-MS system with algae *Chlorella vulgaris*, a promising biofuel for "green" electricity generation.

Tan and co-workers [13] conducted oxy-combustion co-firing tests with different coals and wood pellets in the CanmetENERGY 800 kWth CFB combustor (406 mm ID). The fraction of cofired biomass was in the range 20–50%wt. The combustion characteristics, pollutant and trace metals emissions were measured at O2 concentration in the combustion gas between 24 and 25%vol. The authors concluded that the addition of wood pellets did not have a significant influence on combustion conditions. Emissions of NO were in a narrow range of 14–20 ng/J of heat input. All tests were carried out with the addition of limestone at the Ca/S ratio of 3. Emissions of SO2 varied from 35 to 95 ng/J of heat input. Concentrations of CO were stable and below 200 ppm when the O2 concentration in the

flue gas exceeded 3.5%. In all tests, the NO and SO2 emission rates were below limits set by Canada and the UE.

Duan and co-workers [5] investigated NO emissions during oxy-fuel co-firing of biomass (rice husk, wood chips and dry wood flour) and bituminous coal in a 10 kWth CFB combustor (65 mm ID, 2.3 m height). The ratio of biomass in the fuel blend was in the range 0–100%wt. All tests were carried out at the same conditions—bed temperature 850 ◦C, excess oxygen 5% and primary O2 fraction 0.7. In the case of single fuel combustion, NO emissions were in the range 56–195 mg/m3 <sup>n</sup> and they were greater for biomass fuels with the highest value observed for rice husk and the lowest for wood chips. Oxy-fuel combustion produced less NO than combustion in air. The conversion of fuel-N to NO increased with increasing H/N mass ratio in tested fuel. In the case of co-firing biomass and coal, NO emissions increased with increasing fraction of biomass in the fuel blend for all biomass fuel tested under air and oxy-fuel conditions.

Lupiáñez and co-workers [14] studied oxy-combustion of anthracite and corn stover blends in a laboratory-scale bubbling fluidized bed combustor (203 mm ID, 2.5 m height) with thermal input around 30 kW. Combustion tests at bed temperatures 850–900 ◦C in air and 30/70% mixtures of O2 and CO2 were carried out with 80/20 blends (on energy basis) of coal and biomass. In two tests, limestone was added for the in situ SO2 capture at the Ca/S molar ratio of 2.5. Measurements of the flue gas composition revealed that emissions of SO2 (corrected to 6% O2 and normalized to mg/MJ) were in the range from 105 (oxy-combustion at 870 ◦C) to 906 mg/MJ (air combustion at 860 ◦C). These emissions were related to the chlorine content in corn stover; the higher the Cl content in corn stover the lower the SO2 emission. The desulphurization efficiency exceeding 80% was achieved with limestone addition. Emissions of NO were not influence by the Cl in corn stover and they were in the range from 68–172 mg/MJ. According to the authors, the expected reduction in NO emissions during oxy-firing was not observed, owing to the higher excess of O2. Finally, it was concluded that the influence of coal/biomass ratio on NO emissions was negligible. The catalytic activity of limestone and higher O2 excess were responsible for the larger conversion of fuel-N to NO during oxy-combustion tests.

In the subsequent studies [15,16] Lupiáñez and co-workers investigated the influence of limestone on gaseous emissions during oxy-combustion of lignite and corn stover in the fluidized-bed combustor described in the previous paper. The author founded that the SO2 capture was influence by the limestone type and fragmentation of its particles. Emissions of SO2 decreases with the Ca/S ratio and bed temperature. On the other hand, emissions of NO increased with Ca/S ratio and the presence of calcined limestone in the bed. They were more influenced by the excess of O2 and CO concentration in the bed than by the biomass share in the blend or chlorine content in the fuel.

Pu and co-workers [17] studied the influence of oxygen concentration in the oxidizing medium (O2/CO2 mixture with O2 concentrations in the range 21–40%) on NO emissions in a lab-scale bubbling fluidized-bed combustor at 850–950 ◦C. Combustion tests were carried out with 2 g samples of anthracite and anthracite blended with 10–30%wt. of pine powder. At 900 ◦C, the conversion rate of fuel-N to NO increased with increasing O2 concentration from 40.4% to 42.6% in the case of anthracite and from 35.1% to 41.2% in the case of 20% pine powder blend. Tests with 35% O2/65% CO2 at different temperatures revealed a slight increase in NO emissions with increasing temperature in the case of anthracite. The opposite trend was observed for the blend containing 20% pine powder. The total NO emission was reduced from 12.48 mg at 850 ◦C to 11.46 mg at 950 ◦C which corresponded to a 3.4% decrease in the fuel-N conversion to NO. The influence of pine powder fraction in the blend was studied at 950 ◦C with 35% O2 in the O2/CO2 mixture. It was found that the addition of pine powder reduced the NO emission and fuel-N conversion to NO. The fuel-N conversion decreased from 42.9% in the case of anthracite to 37.8% in the case of blend containing 30% pine powder.

Wang and co-workers [18] investigated nitrogenous gas (NO, N2O, HCN) emissions from co-combustion of coal and biomass (corn straw and wheat straw) under oxy-fuel

conditions with 50% oxygen concentration in the oxidizing medium. Tests were carried out in a 100 kW CFB combustor with blends containing 30% biomass and in atmospheres containing O2/CO2 and O2/recycled flue gas mixtures (RFG) at temperatures 800–900 ◦C. As expected, the NO and N2O emissions increased with increasing excess oxygen in the oxidizing medium. Tests with different fractions of corn straw in the fuel (10, 20 and 30%) revealed an increase in emission factors of NO, N2O, HCN by 241, 238 and 249%, respectively. The results of tests with coal and wheat straw at 800 and 900 ◦C showed that the NO emission factor increased slightly with increasing temperature. On the other hand, the emission factors of N2O and HCN were lower at 900 ◦C than those at 800 ◦C.

Sher and co-workers [19] studied the influence of the oxidizing medium (air, O2/N2/CO2 mixtures) on gaseous emissions and temperature profile during combustion of miscanthus, straw pellets and wood pellets in a 20 kWth fluidized-bed combustor. The authors observed a significant decrease in CO emissions for all three fuels when O2 concentration in the oxyfuel medium exceeded 25%. Emissions of NO decreased with increasing O2 concentration in the oxy-fuel medium and at 30% O2 they were like those for combustion in air. It was found that NO concentrations in the flue gas are related to the nitrogen content in the fuel tested.

Varol and co-workers [8] investigated oxy-fuel combustion of high-sulfur lignite and wood pellet blends (up to 60%) in a laboratory-scale (100 mm ID, 5.1 m height) CFB combustor. The main objective of their study was to determine the effect of biomass share on NOx, SO2 and CO emissions. The results showed that increasing biomass share in the fuel blend had a negligible influence on NOx emissions. Emissions of CO decreased slightly with an increase in biomass share. Concentrations of SO2 in the flue gas were related to the sulfur content in the fuel and they decreased with increasing biomass share in the blend.

Nguyen and co-workers [20] studied SO2, NO and CO emissions during co-combustion of lignite and wood pellets (50–100%wt. share in the fuel) in an oxy-fuel 100 kWth test facility. All tests were carried out with 25% oxygen in the oxidizing medium. An increase in biomass share caused a decrease in NO, SO2 and CO concentrations from 19.2 mg/MJ (corrected to 6% O2 in the flue gas) to 16.1 mg/MJ, 92.8 mg/MJ to 25.0 mg/MJ, and 7.5 mg/MJ to 5.5 mg/MJ, respectively. The authors concluded that oxy-combustion of pure biomass can lead to negative CO2 emissions of, approximately, −647 g/kWth.

Gao and co-workers [12] investigated pollutant formation during air and oxy-fuel combustion of microalgae *Chlorella vulgaris* and its blends with Chinese lignite in a TG-MS (thermogravimetry coupled with mass spectrometry) system. The presence of microalgae in the fuel blend during combustion in air resulted in lower emissions of CO2, CO and NO2, but in enhanced formation of NO, COS and SO2. Similar trend was observed in the case of oxy-fuel combustion of microalgae/lignite blends.

The main conclusions of the conducted review are summarized in Table 1.




#### **Table 1.** *Cont.*


**Table 1.** *Cont.*

\* BFB—bubbling fluidized-bed combustor, \*\* TG-MS—thermogravimetric analyzer with mass spectrometer.
