Co-Combustion of Blends of Coal and Underutilised Biomass Residues for Environmental Friendly Electrical Energy Production

: This study investigated the co-combustion of the blends of coal and biomass residues from poplar sawdust, rice husk, pine nut shells, and sunﬂower residues for ecofriendly energy production. Proximate and ultimate analyses and caloriﬁc values of the coal and biomass residues were also carried out to evaluate the properties of the coal and biomass residues. The volatile matter in coal was reported as 43.38 wt% and ranged from 56.76 wt% to 80.95 wt% in the biomass residues. The ultimate analysis reported the carbon and sulfur content of coal as 68.7 wt% and 5.5 wt%, respectively. The coal and biomass blends were prepared using different ratios on the thermal basis of coal and biomass given as 100:0, 90:10, 80:20, 70:30, 60:40, and 50:50 by weight percentage. The consequent stoichiometric air requirements for all the blends were also calculated. The results revealed that the combustion of 60:40 of coal and sunﬂower residue blend was the most efﬁcient blend, resulting in less emission of NO x , SO x , and CO 2 in the ﬂue gas compared to the combustion of pure coal. The study revealed a great perspective of the selected biomass residues to blend with coal for environmentally friendly and sustainable energy production.


Introduction
The global economy and energy consumption have been rapidly increasing over the last century due to the increase in industrialization, which increases dependence on fossil energy sources and causes serious environmental pollution [1][2][3][4]. Energy is vital to the

Feedstock Selection and Sample Preparation
The biomass samples were collected from the residue stocks of pine nut shells, sunflower leftovers, rice husk, and poplar sawdust. The coal samples were obtained from coal mines located in Lakhra district, Sindh, Pakistan. Both the coal and biomass samples were crushed separately using a jaw crusher and were grounded in a disc pulverizer to get sieved particles passing through the size-60 mesh number.

Analysis of Coal and Biomass
The proximate analyses of the coal and the biomass samples were carried out to determine the moisture, volatile matter, ash content, and fixed carbon following the ASTM standard methods ASTM D3175-07, ASTM 3174-04, and ASTM D-3172-13, respectively [30][31][32]. Ultimate analysis was carried out to find the elemental chemical constituents such as carbon, hydrogen, nitrogen, and sulfur in the samples used, while the oxygen content was measured by calculating the difference. The carbon and sulfur contents in the samples were monitored using a carbon/sulfur analyzer (Model no Leco SC-144 DR), and the nitrogen content was measured using Flash EA 1112 Series elemental analyzer from CE Instruments, Thermo-Quest. The gross calorific values (GCVs) of the samples were determined using a bomb calorimeter following the ASTM method ASTM-D 5865-13 [33].

Blending of Coal and Biomass on a Thermal Basis
To study the co-combustion of coal and biomass samples, the blends were prepared using different biomass blending ratios on a thermal basis of coal:biomass (wt%) given as 100:0, 90:10, 80:20, 70:30, 60:40, and 50:50, as mentioned in Table 1. The co-combustion was studied in a total oxidative environment. This reaction was continuous; thus, less conversion occurred due to less residence time. For complete burning, an excess ratio of air/fuel was provided throughout the co-combustion. Stoichiometric air was also calculated for the co-combustion of coal and biomass, as shown in Table 1.

Combustion and Flue Gas Emissions
The co-combustion of samples was studied in a tube furnace. The flue gases from the combustion of coal and different coal-biomass blends that were prepared by mixing in accordance with the thermal basis of parent coal and biomasses were analyzed. The tube furnace was connected to an air compressor and had a rotameter to control and maintain the specified flow rate of air during the combustion. A portable flue gas analyzer (model Teledyne PEM 9002) was used to monitor the produced NO x , CO 2 , O 2 , and SO 2 . A precisely measured weight of 1 g sample of each blend was placed in the tube furnace at an initial temperature of 100 • C. The probe of the flue gas analyzer was inserted into the exit of the tube furnace. The readings of NO x , CO 2 , O 2 , SO 2 , and CO emissions were noted down after the temperature intervals of 50 • C and continued until the temperature reached 950 • C for each experimental run. After completion of the co-combustion, the blend residue was left in the tube furnace for 1 h to allow the complete conversion of biomass to ash. The ash was then analyzed to determine the contents of carbon and sulfur using the carbon/sulfur analyzer. The ultimate analysis of ash was used to estimate the unburned carbon in the ash and calculate the combustion efficiency of the blends.

Proximate and Ultimate Analysis
The characteristics of feedstock coal and biomass samples were assessed by determining the calorific value and performing proximate and ultimate analyses, as shown in Table 2. The proximate analysis indicated that the moisture content and amount of fixed carbon in coal were higher than in the biomass residues. The percentage of the volatile matter was higher in the biomass samples as compared to coal, which indicated that the biomass residues were more combustible than pure coal. The results of the ash content in the biomass and coal samples were random. The ultimate analysis is helpful in estimating the air requirements for the combustion and co-combustion of the fuels. Carbon and sulfur were the main targets in this study because sulfur causes slagging and corrosion, as well as the as SO x emissions during the combustion. The GCV of coal was reported to be higher than that of the biomass residues, which was attributed to the higher carbon content in coal, and it could be of great advantage in energy production by increasing the GCV of the blended fuels.

Flue Gas Emissions
The flue gases released from the co-combustion of samples were balanced at a rate of 6% O 2 by applying the correlation stated by Munir et al. reported elsewhere and given in Equation (1) [16].
The co-firing of biomass and coal reduces the CO 2 and SO 2 emissions, may reduce the NO x emissions, and represents a near-term, low-risk, low-cost, and sustainable energy development. Different gases are emitted in flue gases such as NO x , SO x , CO, CO 2 , and O 2 , which indicate the consumption of O 2 in the oxidation reactions during the combustion, which resulted in the breakdown of carbon bonds and production of carbon oxides. The sulfur content in coal and biomass reacts with O 2 to form sulfur oxides, mostly SO 2 , while the nitrogen content mostly reacts with O 2 to form NO x (NO, NO 2 ). During the initial stage of combustion, the O 2 levels were maximum but started to drop at 350 • C, which increased the concentration of CO 2 , as shown in Figure 1a. of combustion, the O2 levels were maximum but started to drop at 350 °C, which increased the concentration of CO2, as shown in Figure 1a. At 450 °C, the CO emissions level was found to be at its peak with a value of 31,836 (±5%) ppm. When CO emissions were high, the CO2 emissions were low. This could be attributed to the devolatilization of coal. After this step, the concentration of CO started to decrease, and that of CO2 increased, which could be due to the start of char oxidation. Coal and biomass residues were co-combusted up to 950 °C. Therefore, fuel NOx emissions were released, but there were no thermal NOx emissions generated because the thermal NOx is generated at a temperature of approximately 1400 °C and above [34,35]. The maximum level of NOx emissions was reported as 34 ppm (±5%) at 700 °C. The sulfur content was higher in coal, as shown in Table 2, which resulted in 7452 (±5%) ppm emissions of SO2 from the combustion of coal. At 950 °C, the CO2 emissions were maximum, because the volatile matter of blends was fully combusted but fixed carbon was still burning.
The emissions analysis from the co-combustion of coal and poplar sawdust are shown in Figures 1-3. The co-combustion of coal and poplar sawdust blend (90:10) produced a maximum CO concentration of 42,571 (±5%) ppm at 650 °C. When the CO level At 450 • C, the CO emissions level was found to be at its peak with a value of 31,836 (±5%) ppm. When CO emissions were high, the CO 2 emissions were low. This could be attributed to the devolatilization of coal. After this step, the concentration of CO started to decrease, and that of CO 2 increased, which could be due to the start of char oxidation. Coal and biomass residues were co-combusted up to 950 • C. Therefore, fuel NO x emissions were released, but there were no thermal NO x emissions generated because the thermal NO x is generated at a temperature of approximately 1400 • C and above [34,35]. The maximum level of NO x emissions was reported as 34 ppm (±5%) at 700 • C. The sulfur content was higher in coal, as shown in Table 2, which resulted in 7452 (±5%) ppm emissions of SO 2 from the combustion of coal. At 950 • C, the CO 2 emissions were maximum, because the volatile matter of blends was fully combusted but fixed carbon was still burning.
The emissions analysis from the co-combustion of coal and poplar sawdust are shown in Figures 1-3. The co-combustion of coal and poplar sawdust blend (90:10) produced a maximum CO concentration of 42,571 (±5%) ppm at 650 • C. When the CO level was high, the CO 2 level was low, but the O 2 level was high. When the CO level started to decrease, the CO 2 level started to increase. This gradual change in CO, CO 2 , and O 2 can be observed in Figure 1b. The maximum NO x emission observed was 138 (±5%) ppm at 650 • C, whereas the maximum measured value of SO 2 was 5033 (±5%) ppm at 650 • C.
Sustainability 2021, 13, x FOR PEER REVIEW 6 of 21 was high, the CO2 level was low, but the O2 level was high. When the CO level started to decrease, the CO2 level started to increase. This gradual change in CO, CO2, and O2 can be observed in Figure 1b. The maximum NOx emission observed was 138 (±5%) ppm at 650 °C, whereas the maximum measured value of SO2 was 5033 (±5%) ppm at 650 °C.

Figure 2.
Emission profiles from the combustion of (a) 80% coal and 20% poplar sawdust, (b) 70% coal and 30% poplar sawdust. The 80% coal and 20% poplar sawdust blend produced a maximum CO level of 22,920 (±5%) ppm at 550 °C. The maximum NOx level was reported as 95 (±5%) ppm at 750 °C, and the maximum SO2 level was reported as 3948 (±5%) ppm at 750 °C ( Figure 2a). When 70% coal and 30% blend of poplar sawdust was co-combusted, a sudden increase in the concentration of CO was observed. At 450 °C, the CO concentration reached its maximum value of 35,846 (±5%) ppm, and it gradually decreased with an increase in temperature. When the CO level was high, the CO2 and O2 levels were 1.3% and 19.6%, respectively. These readings revealed the devolatilization and char oxidation of the coal and biomass blends, as shown in Figure 2b. At 700 °C, the maximum NOx level was reported as 77 (±5%) ppm, whereas that of SO2 was reported as 1247 (±5%) ppm at 950 °C. The 80% coal and 20% poplar sawdust blend produced a maximum CO level of 22,920 (±5%) ppm at 550 • C. The maximum NO x level was reported as 95 (±5%) ppm at 750 • C, and the maximum SO 2 level was reported as 3948 (±5%) ppm at 750 • C (Figure 2a). When 70% coal and 30% blend of poplar sawdust was co-combusted, a sudden increase in the concentration of CO was observed. At 450 • C, the CO concentration reached its maximum value of 35,846 (±5%) ppm, and it gradually decreased with an increase in temperature. When the CO level was high, the CO 2 and O 2 levels were 1.3% and 19.6%, respectively. These readings revealed the devolatilization and char oxidation of the coal and biomass blends, as shown in Figure 2b. At 700 • C, the maximum NO x level was reported as 77 (±5%) ppm, whereas that of SO 2 was reported as 1247 (±5%) ppm at 950 • C. Moreover, when 60% coal and 40% poplar sawdust blend was studied, the level of CO suddenly increased to 44,500 (±5%) ppm at 550 °C. At this level, the percentage of O2 decreased from 20.9% to 18.1%, and a slight increase in the CO2 level was observed. The maximum NOx level was 47 (±5%) ppm at 700 °C, whereas the maximum SO2 was 871 (±5%) ppm, as shown in Figure 3a. When the 50% coal and 50% poplar sawdust blend was co-combusted, the maximum CO emission was reported as 19,788 (±5%) ppm at 450 °C, the O2 level was 18.3%, and CO2 was 2.6%. The maximum NOx concentration was 89 (±5%) ppm at 650 °C and the maximum SO2 concentration was found to be 66 (±5%) ppm at 950 °C ( Figure 3b).
The emissions analysis from the co-combustion of coal and rice husk are shown in Figures 4-6. The co-combustion of 90:10 coal and rice husk biomass revealed a maximum CO level of 10,620 (±5%) ppm at 400 °C. When the CO level was high, the CO2 level was low, but the O2 level was high. When CO emission level started to decrease, the CO2 level started to increase. This gradual change in CO, CO2, and O2 can be seen in Figure 4b. Moreover, when 60% coal and 40% poplar sawdust blend was studied, the level of CO suddenly increased to 44,500 (±5%) ppm at 550 • C. At this level, the percentage of O 2 decreased from 20.9% to 18.1%, and a slight increase in the CO 2 level was observed. The maximum NO x level was 47 (±5%) ppm at 700 • C, whereas the maximum SO 2 was 871 (±5%) ppm, as shown in Figure 3a. When the 50% coal and 50% poplar sawdust blend was co-combusted, the maximum CO emission was reported as 19,788 (±5%) ppm at 450 • C, the O 2 level was 18.3%, and CO 2 was 2.6%. The maximum NO x concentration was 89 (±5%) ppm at 650 • C and the maximum SO 2 concentration was found to be 66 (±5%) ppm at 950 • C (Figure 3b).
The emissions analysis from the co-combustion of coal and rice husk are shown in Figures 4-6. The co-combustion of 90:10 coal and rice husk biomass revealed a maximum CO level of 10,620 (±5%) ppm at 400 • C. When the CO level was high, the CO 2 level was low, but the O 2 level was high. When CO emission level started to decrease, the CO 2 level started to increase. This gradual change in CO, CO 2 , and O 2 can be seen in Figure 4b. Sustainability 2021, 13, x FOR PEER REVIEW 8 of 21 The maximum NOx emission level was reported as 70 (±5%) ppm at 750 °C, while the maximum concentration of SO2 was reported as 1117 (±5%) ppm at 750 °C. Furthermore, for the 80% of coal and 20% rice husk co-combustion, a sudden increase in the CO emission level was observed, and at 600 °C, it reached a maximum value of 13,244 (±5%) ppm and gradually decreased with an increase in the temperature. At 750 °C, the maximum NOx level was reported as 32 (±5%) ppm, whereas the emission of SO2 was reported as 611 (±5%) ppm, as shown in Figure 5a. The maximum NO x emission level was reported as 70 (±5%) ppm at 750 • C, while the maximum concentration of SO 2 was reported as 1117 (±5%) ppm at 750 • C. Furthermore, for the 80% of coal and 20% rice husk co-combustion, a sudden increase in the CO emission level was observed, and at 600 • C, it reached a maximum value of 13,244 (±5%) ppm and gradually decreased with an increase in the temperature. At 750 • C, the maximum NO x level was reported as 32 (±5%) ppm, whereas the emission of SO 2 was reported as 611 (±5%) ppm, as shown in Figure 5a. Sustainability 2021, 13, x FOR PEER REVIEW 9 of 21 Figure 5. Emission profiles from the combustion of (a) 80% coal and 20% rice husk, (b) 70% coal and 30% rice husk.
When 60% coal and 40% rice husk blend was combusted, the amount of CO emissions was 11,480 (±5%) ppm at 400 °C. At this level, the amount of O2 decreased from 20.9% to 15.5%, and a slight increase in the CO2 level was observed, as shown in Figure 6a. Moreover, the maximum amount of NOx was 90 (±5%) ppm at 650 °C, whereas the maximum SO2 emissions were 409 (±5%) ppm at 750 °C. When the 50% coal and 50% rice husk blend was combusted, the CO emissions peaked at 19,240 (±5%) ppm at 500 °C. In this case, the maximum SO2 released was 928 (±5%) ppm at 750 °C, whereas the maximum NOx was 928 (±5%) ppm at 750 °C, as shown in Figure 6b. The co-combustion of 70% coal and 30% rice husk generated 9030 (±5%) ppm of CO at 400 • C. The maximum amount of NO x produced was reported as 51 (±5%) ppm at 750 • C, whereas the measured SO 2 was 472 (±5%) ppm at 950 • C, as shown in Figure 5b.
When 60% coal and 40% rice husk blend was combusted, the amount of CO emissions was 11,480 (±5%) ppm at 400 • C. At this level, the amount of O 2 decreased from 20.9% to 15.5%, and a slight increase in the CO 2 level was observed, as shown in Figure 6a. Moreover, the maximum amount of NO x was 90 (±5%) ppm at 650 • C, whereas the maximum SO 2 emissions were 409 (±5%) ppm at 750 • C. When the 50% coal and 50% rice husk blend was combusted, the CO emissions peaked at 19,240 (±5%) ppm at 500 • C. In this case, the maximum SO 2 released was 928 (±5%) ppm at 750 • C, whereas the maximum NO x was 928 (±5%) ppm at 750 • C, as shown in Figure 6b. Sustainability 2021, 13, x FOR PEER REVIEW 10 of 21 Figure 6. Gaseous profiles from the combustion of (a) 60% coal and 40% rice husk, (b) 50% coal and 50% rice husk.
The emissions analysis from the co-combustion of coal and sunflower are shown in Figures 7-9. When 90% coal and 10% sunflower residue blend was used, the maximum amount of CO emission was reported as 9773 (±5%) ppm at 400 °C with O2 was 18.2%, and CO2 was 2.6%. The maximum amount of NOx was reported as 22 (±5%) ppm at 800 °C, whereas the maximum SO2 emissions were 942 (±5%) ppm at 700 °C, as shown in Figure  7b.
The use of 80% coal and 20% sunflower leftover exhibited a sudden increase in CO emissions to 13,860 (±5%) ppm at 450 °C, which was then gradually decreased by an increase in the temperature. When the CO emissions level was high, CO2 was only 4.3% and O2 was 16.6%, which revealed the devolatilization and char oxidation reaction of the coal and biomass blend, as shown in Figure 8a. The emissions analysis from the co-combustion of coal and sunflower are shown in Figures 7-9. When 90% coal and 10% sunflower residue blend was used, the maximum amount of CO emission was reported as 9773 (±5%) ppm at 400 • C with O 2 was 18.2%, and CO 2 was 2.6%. The maximum amount of NO x was reported as 22 (±5%) ppm at 800 • C, whereas the maximum SO 2 emissions were 942 (±5%) ppm at 700 • C, as shown in Figure 7b.
The use of 80% coal and 20% sunflower leftover exhibited a sudden increase in CO emissions to 13,860 (±5%) ppm at 450 • C, which was then gradually decreased by an increase in the temperature. When the CO emissions level was high, CO 2 was only 4.3% and O 2 was 16.6%, which revealed the devolatilization and char oxidation reaction of the coal and biomass blend, as shown in Figure 8a. At 750 °C, the maximum amount of NOx was 36 (±5%) ppm, whereas that of SO2 was 775 (±5%) ppm at 900 °C. Furthermore, when 70% coal and 30% sunflower residue were combusted, the amount of CO suddenly increased to 15,618 (±5%) ppm at 400 °C. At this level, the amount of O2 decreased from 20.9% to 19.8%, and a minor increase in CO2 was observed, as shown in Figure 8b. The maximum amount of NOx was 18 (±5%) ppm at 700 °C, whereas zero SO2 emissions were detected in this case. At 750 • C, the maximum amount of NO x was 36 (±5%) ppm, whereas that of SO 2 was 775 (±5%) ppm at 900 • C. Furthermore, when 70% coal and 30% sunflower residue were combusted, the amount of CO suddenly increased to 15,618 (±5%) ppm at 400 • C. At this level, the amount of O 2 decreased from 20.9% to 19.8%, and a minor increase in CO 2 was observed, as shown in Figure 8b. The maximum amount of NO x was 18 (±5%) ppm at 700 • C, whereas zero SO 2 emissions were detected in this case. The co-combustion of 60% coal and 40% sunflower resdues produced a maximum CO level of 8638 (±5%) ppm at 400 °C. When the CO level was high, the CO2 level was low, but the O2 level was high as well. When the CO level started to decrease, the CO2 level started to increase, and this gradual change in CO, CO2, and O2 can be seen in Figure  9a. The maximum NOx emissions were 48 (±5%) ppm at 700 °C, and the maximum amount of SO2 was 28 (±5%) ppm at 950 °C. When the 50% coal and 50% sunflower leftover blend was combusted, the peak value of CO was 10,643 (±5%) ppm at 350 °C. In this case, no SO2 was observed, whereas the maximum amount of NOx was 50 (±5%) ppm at 600 °C, as shown in Figure 9b.
The emission analyses of the co-combustion of coal and pine nut shells are given in Figures 10-12. A blend of 90% coal and 10% pine nut shells produced a maximum CO level of 16,865 (±5%) ppm at 400 °C. The co-combustion of 60% coal and 40% sunflower resdues produced a maximum CO level of 8638 (±5%) ppm at 400 • C. When the CO level was high, the CO 2 level was low, but the O 2 level was high as well. When the CO level started to decrease, the CO 2 level started to increase, and this gradual change in CO, CO 2 , and O 2 can be seen in Figure 9a. The maximum NO x emissions were 48 (±5%) ppm at 700 • C, and the maximum amount of SO 2 was 28 (±5%) ppm at 950 • C. When the 50% coal and 50% sunflower leftover blend was combusted, the peak value of CO was 10,643 (±5%) ppm at 350 • C. In this case, no SO 2 was observed, whereas the maximum amount of NO x was 50 (±5%) ppm at 600 • C, as shown in Figure 9b.
The emission analyses of the co-combustion of coal and pine nut shells are given in Figures 10-12. A blend of 90% coal and 10% pine nut shells produced a maximum CO level of 16,865 (±5%) ppm at 400 • C. Meanwhile, when the CO and O2 levels were high, the CO2 level was low. As the CO level was decreasing, the CO2 level started to increase, and this gradual change in CO, CO2, and O2 can be seen in Figure 10b. The maximum amount of emitted NOx was 817 ppm at 850 °C, whereas that of SO2 was 1962 (±5%) ppm at 650 °C.
When the 80% of coal and 20% PNS blend was combusted, the amount of CO increased suddenly to 11,729 (±5%) ppm at 400 °C, and it gradually decreased with an increase in the temperature. When the CO level was high, the CO2 level was only 4.3% and O2 was 16.6%, as shown in Figure 11a. These findings revealed the devolatilization and char oxidation reaction of the coal and biomass blend. At 700 °C, the maximum amount of NOx was 28 (±5%) ppm, while that of SO2 was 1237 (±5%) ppm. Meanwhile, when the CO and O 2 levels were high, the CO 2 level was low. As the CO level was decreasing, the CO 2 level started to increase, and this gradual change in CO, CO 2 , and O 2 can be seen in Figure 10b. The maximum amount of emitted NO x was 817 ppm at 850 • C, whereas that of SO 2 was 1962 (±5%) ppm at 650 • C.
When the 80% of coal and 20% PNS blend was combusted, the amount of CO increased suddenly to 11,729 (±5%) ppm at 400 • C, and it gradually decreased with an increase in the temperature. When the CO level was high, the CO 2 level was only 4.3% and O 2 was 16.6%, as shown in Figure 11a. These findings revealed the devolatilization and char oxidation reaction of the coal and biomass blend. At 700 • C, the maximum amount of NO x was 28 (±5%) ppm, while that of SO 2 was 1237 (±5%) ppm. The co-combustion of 70% coal and 30% PNS generated 10,102 (±5%) ppm of CO at 550 °C, as shown in Figure 11b. The maximum amount of NOx was 28 (±5%) ppm at 750 °C, while the amount of SO2 was 561 (±5%) ppm at the exit (950 °C).
The co-combustion of 60% coal and 40% PNS generated 14,766 (±5%) ppm of CO at 350 °C. At this stage, the amount of O2 decreased from 20.9% to 17%, and a slight increase in CO2 emissions was observed, as shown in Figure 12a. The maximum amount of NOx was 68 (±5%) ppm at 700 °C, whereas the maximum amount of SO2 was 371 (±5%) ppm at 900 °C. The co-combustion of 70% coal and 30% PNS generated 10,102 (±5%) ppm of CO at 550 • C, as shown in Figure 11b. The maximum amount of NO x was 28 (±5%) ppm at 750 • C, while the amount of SO 2 was 561 (±5%) ppm at the exit (950 • C).
The co-combustion of 60% coal and 40% PNS generated 14,766 (±5%) ppm of CO at 350 • C. At this stage, the amount of O 2 decreased from 20.9% to 17%, and a slight increase in CO 2 emissions was observed, as shown in Figure 12a. The maximum amount of NO x was 68 (±5%) ppm at 700 • C, whereas the maximum amount of SO 2 was 371 (±5%) ppm at 900 • C. Figure 11. Emission profiles from the combustion of (a) 80% coal and 20% pine nut shell, (b) 70% coal and 30% pine nut shell.
When 50% coal and 50% PNS blend was combusted, the CO emissions peaked at 22,923 (±5%) ppm at 700 °C. In this case, no SO2 was generated, and the maximum NOx was 16 ppm at 950 °C. Furthermore, O2 was less used; the minimum percentage of O2 was 17.2%, so less CO2 was produced, and the maximum CO was 3.7% (Figure 12b).
When coal is added to biomass, the volatilization rate is modified, the released heat is affected, and the combustion residue is reduced under the same final combustion temperature. Thus, the combustion efficiency is increased [36]. Guo et al. also showed that biomass and coal blends directly dominated by the heating temperature and the heat released by the biomass combustion, which increases the conversion rate of the coal [14]. Figure 11. Emission profiles from the combustion of (a) 80% coal and 20% pine nut shell, (b) 70% coal and 30% pine nut shell.
When 50% coal and 50% PNS blend was combusted, the CO emissions peaked at 22,923 (±5%) ppm at 700 • C. In this case, no SO 2 was generated, and the maximum NO x was 16 ppm at 950 • C. Furthermore, O 2 was less used; the minimum percentage of O 2 was 17.2%, so less CO 2 was produced, and the maximum CO was 3.7% (Figure 12b).
When coal is added to biomass, the volatilization rate is modified, the released heat is affected, and the combustion residue is reduced under the same final combustion temperature. Thus, the combustion efficiency is increased [36]. Guo et al. also showed that biomass and coal blends directly dominated by the heating temperature and the heat released by the biomass combustion, which increases the conversion rate of the coal [14].
Moreover, co-firing with biomass residues is the best approach among the studied alternatives for maximizing the savings of both fossil energy and greenhouse gases. Compared to the coal-fired base case, 20% co-firing with logging residues reduces the fossil energy consumption per kWh of electricity generated by 26% and decreases emissions of CO2 by nearly 20%. In general, the biomass from wood showed a slight advantage over the biomass from switch grass [37]. Figure 13 shows a comparison between the concentrations of emissions from different biomass blending ratios (BBRs) that can be used to determine the emitted flue gases. When 90% coal and 10% poplar sawdust blend was co-combusted, the concentrations of released CO, NOx, and SO2 were 42,571, 138, and 5033 ppm, respectively. The volumes of CO, NOx, and SO2 produced from the co-combustion of 80% coal and 20% poplar sawdust were 22,920, 95, and 3948 ppm, respectively. Moreover, the volumes of CO, NOx, and SO2 generated from the co-combustion of 70% coal and 30% poplar sawdust were 35,846, 77, and 1247 ppm, respectively. For 60% coal and 40% poplar sawdust, the volumes of CO, Figure 12. Emission profiles from the combustion of (a) 60% coal and 40% pine nut shell, (b) 50% coal and 50% pine nut shell.

Comparison of Emission Concentrations from Different Biomass Blending Ratios
Moreover, co-firing with biomass residues is the best approach among the studied alternatives for maximizing the savings of both fossil energy and greenhouse gases. Compared to the coal-fired base case, 20% co-firing with logging residues reduces the fossil energy consumption per kWh of electricity generated by 26% and decreases emissions of CO 2 by nearly 20%. In general, the biomass from wood showed a slight advantage over the biomass from switch grass [37]. Figure 13 shows a comparison between the concentrations of emissions from different biomass blending ratios (BBRs) that can be used to determine the emitted flue gases. When 90% coal and 10% poplar sawdust blend was co-combusted, the concentrations of released CO, NO x , and SO 2 were 42,571, 138, and 5033 ppm, respectively. The volumes of CO, NO x , and SO 2 produced from the co-combustion of 80% coal and 20% poplar sawdust were 22,920, 95, and 3948 ppm, respectively. Moreover, the volumes of CO, NO x , and SO 2 generated from the co-combustion of 70% coal and 30% poplar sawdust were 35,846, 77, and 1247 ppm, respectively. For 60% coal and 40% poplar sawdust, the volumes of CO, NO x , and SO 2 were 44,500, 93, and 871 ppm, respectively. When the BBR was 50%, the amounts of CO, NO x , and SO 2 were 19,788, 89, and 66 ppm, respectively. In this study, a BBR of 50:50 was found to be the optimum BBR, because it produced 38% less CO and 99% less SO 2 than those of pure coal combustion. When 90% coal and 10% rice husk were co-combusted, 10,620, 70, and 1810 ppm of CO, NO x , and SO 2 were released, respectively. The amounts of CO, NO x , and SO 2 emitted from the co-combustion of 80% coal and 20% rice husk were 13,244, 32, and 611 ppm, respectively. Moreover, the amounts of CO, NO x , and SO 2 emitted from the co-combustion of 70% coal and 30% rice husk were 9030, 51, and 472 ppm, respectively. For 60% coal and 40% rice husk, the amounts of CO, NO x , and SO 2 were 11,480, 90, and 409 ppm, respectively. When the BBR was 50%, the amounts of CO, NO x , and SO 2 were 19,240, 71, and 928 ppm, respectively. The comparison between different BBRs is shown in Figure 13b. In this case, the BBR of 70:30 was determined to be the optimum, as it produced 72% lower CO and 90% lower SO 2 emissions than those of pure coal combustion. NOx, and SO2 were 44,500, 93, and 871 ppm, respectively. When the BBR was 50%, the amounts of CO, NOx, and SO2 were 19,788, 89, and 66 ppm, respectively. In this study, a BBR of 50:50 was found to be the optimum BBR, because it produced 38% less CO and 99% less SO2 than those of pure coal combustion. When 90% coal and 10% rice husk were co-combusted, 10,620, 70, and 1810 ppm of CO, NOx, and SO2 were released, respectively. The amounts of CO, NOx, and SO2 emitted from the co-combustion of 80% coal and 20% rice husk were 13,244, 32, and 611 ppm, respectively. Moreover, the amounts of CO, NOx, and SO2 emitted from the co-combustion of 70% coal and 30% rice husk were 9030, 51, and 472 ppm, respectively. For 60% coal and 40% rice husk, the amounts of CO, NOx, and SO2 were 11,480, 90, and 409 ppm, respectively. When the BBR was 50%, the amounts of CO, NOx, and SO2 were 19,240, 71, and 928 ppm, respectively. The comparison between different BBRs is shown in Figure 13b. In this case, the BBR of 70:30 was determined to be the optimum, as it produced 72% lower CO and 90% lower SO2 emissions than those of pure coal combustion. Kwong et al. also reported that the co-combustion of coal and rice husk caused a reduction in emissions by 10-30% [38]. The co-firing of coal and hydrothermally treated municipal waste has also been reported to reduce the emissions of a 70:30 blend [39]. Similarly, when 90% coal and 10% sunflower leftovers were co-combusted, the amounts of CO, NOx, and SO2 were reported as 9973, 22, and 942 ppm, respectively, as shown in Figure 13c. The amounts of CO, NOx, and SO2 released from the co-combustion of 80% coal and 20% sunflower leftovers were 13,860, 36, and 775 ppm, respectively. Moreover, the Kwong et al. also reported that the co-combustion of coal and rice husk caused a reduction in emissions by 10-30% [38]. The co-firing of coal and hydrothermally treated municipal waste has also been reported to reduce the emissions of a 70:30 blend [39]. Similarly, when 90% coal and 10% sunflower leftovers were co-combusted, the amounts of CO, NO x , and SO 2 were reported as 9973, 22, and 942 ppm, respectively, as shown in Figure 13c. The amounts of CO, NO x , and SO 2 released from the co-combustion of 80% coal and 20% sunflower leftovers were 13,860, 36, and 775 ppm, respectively. Moreover, the amounts of CO, NO x , and SO 2 released from the co-combustion of 70% coal and 30% sunflower leftovers were 15,168, 18, and 0 ppm, respectively. For 60% coal and 40% sunflower leftovers, the amounts of CO, NO x , and SO 2 were 8638, 48, and 28 ppm, respectively. When the BBR was 50%, the amounts of CO, NO x , and SO 2 were 10,643, 50, and 0 ppm, respectively. In this case, a BBR of 60:40 was determined to be the best choice, because the co-combustion of this blend generated 73% less CO and 99% less SO 2 than the combustion of 100% coal.

Comparison of Emission Concentrations from Different Biomass Blending Ratios
When Duki coal and bagasse blends were co-combusted, a BBR of 60:40 was found to be the optimum [25]. Finally, when 90% coal and 10% pine nut shells were co-fired, the amounts of CO, NO x , and SO 2 released were 16,865, 817, and 1962 ppm, respectively, as shown in Figure 13d. At a BBR of 80:20, the amounts of CO, NO x , and SO 2 were 11,729, 28, and 1237 ppm, respectively. The amounts of CO, NO x , and SO 2 released from the cocombustion of 70% coal and 30% pine nut shells were 10,102, 28, and 871 ppm, respectively. Moreover, the amounts of CO, NO x , and SO 2 released from the co-combustion of 60% coal and 40% pine nut shells were 14,766, 68, and 371 ppm, respectively. When the BBR was 50%, the amounts of CO, NO x , and SO 2 were 22,923, 16, and 0 ppm, respectively, and the comparison between different BBRs can be seen in Figure 13d. In this case, a BBR of 70:30 was determined as a good option, because it generated 68% less CO and 82% less SO 2 than those of pure coal. Thus, based on the above discussion, a BBR of 60-40% coal-sunflower leftover was the best among all tested BBRs, producing 73% less CO and 99% less SO 2 than those of pure coal. Guo et al. showed that the excessive amount of biomass and coal blends have advantages to the co-combustion process. They reported the optimum blending ratio of 30% for the composite biomass pellets (CBPs) and coal blends [18].

Combustion Efficiency
Combustion efficiency is an important factor that needs to be addressed. When 100% coal was combusted, the percentage of unburned carbon in the ash was 8.3%, whereas in the ash of the 50:50 coal-poplar sawdust, the unburned carbon was 7.11%, which means that this blend showed 14.33% better combustion efficiency than pure coal did. The percentage of unburned carbon in the ash of 70% coal and 30% rice husk was 5.18%, which indicated better combustion efficiency than that of pure coal by 37.57%. The unburned carbon of 60% coal and 40% poplar sawdust was 0.45%, which means that this blend showed 94.57% better combustion efficiency than pure coal did. The unburned carbon in the ash of 70% coal and 30% pine nut shells was 7.20%, which means that the combustion efficiency of this blend was 13.25% better than that of pure coal. Therefore, the combustion of coal and biomass blends was more efficient than the combustion of pure coal.
Moreover, based on the comparison of the combustion efficiencies of all the BBRs, it was established that the combustion efficiency of 60% coal and 40% sunflower leftovers was the highest, and this blend emitted less flue gas among all the blends studied. The blend of 40% coal and 60% straw produced the lowest levels of SO 2 and NO x emissions, but the total CO 2 emissions increased constantly when increasing the content of coal [36]. Yang et al. studied the behavior of NOx emissions for the co-combustion of biomass and pine sawdust that showed a good agreement with this study. They reported that the maximum reduction of NOx could be achieved with 50% biomass and 50% coal combustion [40]. Sahu et al. concluded that during the co-combustion of biomass and coal, the GHGs and flue gases, particularly NOx and Sox, are reduced by increasing the amount of biomass reasoned for the lower sulfur content in biomass feedstock compared to the sulfur content in the coal [40]. Su et al. studied the co-combustion of food waste with lignite coal and their blends using non-isothermal thermogravimetric analysis coupled with Fourier-transform infrared spectroscopy set on a mass basis as 1:6, 2:6, 3:6, 4:6, and 5:6 [41]. It was reported that CO 2 , SO 2 , and HCl were the main gaseous pollutants produced during the co-combustion.
The ratio of 4:6 was reported to be the optimum for the co-combustion of coal and food waste because the SO 2 and HCl emissions decreased at this ratio [41].

Conclusions
This study examined the effects of the co-combustion of coal with different underutilized biomass residues based on flue gas analysis and combustion efficiencies. The blend of 60% coal and 40% sunflower residue was found to be the optimum blend with 73% less CO and 99% less SO 2 emissions compared to the combustion of pure coal. Furthermore, the same blend exhibited the most efficient combustion. The biomass and coal blends reduced the flue gas emissions and their combustion was more efficient than that of coal. The study concluded that the blending of biomass with coal can be greatly beneficial in reducing the emissions of harmful gases, improving the combustion efficiencies and reducing the particulate matter emissions to the environment. Moreover, the inclusion of biomass for energy production by blending with coal can make the process more sustainable.

Conflicts of Interest:
The authors declare no conflict of interest.