Computational Particle Fluid Dynamics Simulation on Combustion Characteristics of Blended Fuels of Coal, Biomass, and Oil Sludge in a 130 t h − 1 Circulating Fluidized Bed Boiler

: In this study, the co-combustion of coal and biomass, and the tri-combustion of coal, biomass, and oil sludge in a 130 t h − 1 circulating fluidized bed (CFB) boiler are investigated via the computational particle fluid dynamics (CPFD) approach. Furthermore, the effect of biomass feeding position is also comprehensively evaluated. The results show that for the co-combustion of coal and biomass, the O 2 mole fraction at the furnace outlet rises from 0.0541 to 0.0640 as the biomass blending ratio enhances from 40% to 100%, while the CO 2 mole fraction reduces from 0.1357 to 0.1267. The mole fraction of NO x and SO 2 at the furnace outlet decreases from 4.5867 × 10 − 5 to 3.9096 × 10 − 5 and 2.8253 × 10 − 4 to 4.6635 × 10 − 5 , respectively. For the tri-combustion of three fuels, the average NO x mole fraction initially grows quickly and then declines gradually, ranging from 4.1173 × 10 − 5 to 4.2556 × 10 − 5 . The mole fraction of SO 2 at the furnace outlet increases from 3.5176 × 10 − 4 to 4.7043 × 10 − 4 when the ratio of oil sludge rises from 10% to 20%. The uniformity of temperature and gas components distribution is “new inlet > secondary air inlet > feed inlet”. As for the three inlet positions, the mole fractions of NO x at the furnace outlet are between 3.9096 × 10 − 5 and 5.1537 × 10 − 5 , while those for SO 2 are between 2.5978 × 10 − 4 and 2.5278 × 10 − 4 .


Introduction
The consumption of fossil energy continues to increase with the sustained economic growth and increased industrial demand [1,2].The extensive utilization of fossil energy leads to problems such as environmental pollution and the greenhouse effect, which have become key issues that need to be solved urgently [3,4].China is a large coal-consuming country.To reduce the impact of excessive coal utilization on the environment and alleviate the pressure brought on by the energy crisis, utilizing renewable energy to partially replace coal has become an important direction of current research [5].Among the many renewable energy sources, biomass, as environmentally friendly, renewable, and widely sourced, has received widespread attention in recent years [6,7].
Currently, the predominant method of utilizing biomass is thermochemical conversion (including pyrolysis, gasification, and combustion), in which direct combustion accounts for 97% of the total biomass energy generation [8][9][10].However, the direct combustion of biomass results in many problems such as ash deposition, slagging, and rusting, which Energies 2024, 17, 149 2 of 17 could be effectively alleviated by the co-combustion of biomass and coal [11].Many researchers [12][13][14] have carried out investigation on the co-combustion characteristics of coal and biomass.Yang et al. [13] discussed the combustion and NO x emission characteristics during the co-combustion of biomass and coal.The results showed that co-combustion coal with biomass could effectively reduce pollutant emissions, but it also caused a decrement in the temperature in the furnace.Zhou et al. [12] applied the double Euler method to conduct a numerical simulation on a circulating fluidized bed (CFB) boiler, and verified the accuracy of the selected model.According to numerous research findings [15,16], they found that the co-combustion of coal and biomass could reduce the emissions of gas phase pollutants like NO x from the fluidized bed.The existing research mainly focuses on power plant boilers.However, the effect of blending biomass on the combustion characteristics of CFB boiler remains to be further discussed.
In the production process of oil fields, it is inevitable to produce oil sludge, which consists of water, oil, and sediment, among other constituents [17].Oil sludge is rich in petroleum hydrocarbons, which will cause serious pollution to the environment and even endanger human health.In addition, oilfield sludge has been identified as hazardous waste by most countries and must be disposed of [18].Among various disposal technologies for oil sludge, incineration has received extensive attention because it could achieve harmless disposal and utilize the heat energy in sludge [19].However, oil sludge has a high water content and is difficult to ignite.Therefore, oil sludge is usually blended with other fuels for incineration [18].To fully utilize the oil sludge resource, some coal could be partially substituted by CFB boilers to co-dispose oil sludge.In addition, the cost of treating oil sludge could be avoided and the secondary pollution brought on by its disposal could be efficiently managed by the current flue gas treatment system.Oil sludge could be properly and effectively disposed of, and biomass resources might be fully utilized via the tri-combustion of coal, biomass, and oil sludge in a CFB boiler.
The Euler-Euler approach, as a continuum model for the interaction force of a fluid and a solid, was adopted in the majority of earlier investigations [20,21].However, it is unable to account for movement process such as particle collisions and the impact of particle size distribution, which are fundamentally distinct from the process itself.As a way to solve this problem, the computational particle fluid dynamics (CPFD) method is gradually becoming the center of attention of researchers [22].Compared with traditional computational fluid dynamics (CFD), the CPFD method could model particles with diverse particle size distributions as well as the background of a massive number of particles in the engineering industry, which provides an efficient technological method for simulating CFB boilers employed in industrial settings [23].Recently, some researchers [24][25][26][27][28] have applied the CPFD method to conduct various numerical simulations.Their main focus was on the simulation of the reaction process in coal gasification, combustion process, and other related concerns.Unfortunately, few studies have been performed on numerical simulations of coal and biomass co-combustion using the CPFD method.In addition, to our knowledge, no research has employed the CPFD method to study the tri-combustion characteristics of coal, biomass, and oil sludge.Given that the CFB boiler is key equipment in the oil field production process, it faces tremendous pressure to reduce CO 2 emissions and protect the environment.Furthermore, the traditional CFD method has great limitations in simulating the combustion characteristics in the CFB boiler.Therefore, the study on applying the CPFD method to simulate the combustion characteristics of blended fuels of coal, biomass, and oil sludge in CFB boilers is an urgent need.
To evaluate the combustion characteristics of blended fuels of coal, biomass, and oil sludge in a 130 t h −1 CFB boiler, the CPFD numerical simulation approach is employed in this work.The co-combustion of coal and biomass, tri-combustion of coal, biomass, and oil sludge, and the effect of biomass feeding position are explored in depth.This study could serve as guidance for the combustion of blended fuels of coal, biomass, and oil sludge in CFB boilers.

CPFD Methods
In this study, the combustion of blended fuels of coal, biomass, and oil sludge in a CFB boiler is simulated utilizing Barracuda 17.4, employing the CPFD approach, which applies the multiphase particle in-cell (MP-PIC) method to calculate the coupling of continuous fluid and high-density particles in three-dimensional space.

Governing Equations
The gas phase is processed via the Eulerian method.Based on the Navier-Stokes equation, the turbulence adopts the large eddy simulation (LES) method [29].For the particle phase, the Lagrangian method is employed to describe it.During simulation calculations, the solid and the gas phases are coupled.The main governing equations are listed in Table 1 [30][31][32][33].

. Chemical Reaction Models
To simplify the calculation, a one-step reaction kinetic equation is employed to describe the combustion process of volatile gases released from three fuels [34][35][36][37].The combustion reaction models of relevant components are shown in Table 2. Furthermore, it is assumed that the coke consists solely of carbon, with no other elements present, and that the N and S in the fuels are completely volatilized.The composition and content of the coal volatiles are calculated based on the prediction model proposed by Maffei et al. [38].Meanwhile, it is assumed that the composition and content of the oil sludge volatiles are consistent with those of coal.Additionally, the content of gaseous pollutants is determined through ultimate analysis.The composition and content of biomass volatiles are obtained from experimental data provided by Kong et al. [39][40][41].According to previous studies [42,43], thermal NO x and fast NO x in CFB boilers could be ignored due to the furnace temperature typically being below 1000 • C, and the anoxic combustion occurs in the dense phase region.A suitably simplified NO x mechanism is adopted to describe NO x generation, assuming that all N and S in the coal and biomass are contained in volatiles.The relevant NO x models are listed in Table 3 [24][25][26]37].The computational geometry for this study is a CFB boiler with a capacity of 130 t h −1 .As depicted in Figure 1, the boiler system in investigation is engineered with a solitary drum structure, underpinned by a natural circulation mechanism.It incorporates an adiabatic cyclone for gas-solid separation to reduce heat loss.The convection heating surface is meticulously arrayed within the convection flue of shaft to optimize thermal exchange.Pertaining to its performance characteristics, the boiler is rated with a saturated steam pressure of 9.81 MPa and a saturated steam temperature is 310 • C.Moreover, it operates with a thermal efficiency that exceeds 90%. Te symmetrical half of the boiler is applied as the computing domain to simplify and speed up the calculations.

Meshing and Validation
In this study, the Cartesian meshing technique is applied to generate a high-quality grid.To reduce the difficulty of grid generation and computational cost, the complex structures are appropriately simplified.To capture the fine structure, the mesh of the fine structure is refined.Figure 2 illustrates the meshing.

Meshing and Validation
In this study, the Cartesian meshing technique is applied to generate a high-quality grid.To reduce the difficulty of grid generation and computational cost, the complex structures are appropriately simplified.To capture the fine structure, the mesh of the fine structure is refined.Figure 2 illustrates the meshing.

Meshing and Validation
In this study, the Cartesian meshing technique is applied to generate a high-quality grid.To reduce the difficulty of grid generation and computational cost, the complex structures are appropriately simplified.To capture the fine structure, the mesh of the fine structure is refined.Figure 2 illustrates the meshing.To confirm the effect of the grid on the calculation results, the average temperatures of the cross-section of height direction alone the furnace are compared under the 491,000, 689,000, and 885,000 grids in this study.As shown in Figure 3, the results indicate that the average temperature varies significantly when 491,000 grids are employed.The cross-section average temperature tends to remain stable as the grids number rises from 689,000 to 885,000, demonstrating that the computation accuracy is comparable.Therefore, 689,000 grids are chosen for ensuring simulation accuracy and simultaneously minimizing computational expense and time.To confirm the effect of the grid on the calculation results, the average temperatures of the cross-section of height direction alone the furnace are compared under the 491,000, 689,000, and 885,000 grids in this study.As shown in Figure 3, the results indicate that the average temperature varies significantly when 491,000 grids are employed.The cross-section average temperature tends to remain stable as the grids number rises from 689,000 to 885,000, demonstrating that the computation accuracy is comparable.Therefore, 689,000 grids are chosen for ensuring simulation accuracy and simultaneously minimizing computational expense and time.

Model Validation
To verify the accuracy of the model, the simulation results are compared with the measured data.As shown in Table 4, the numerical simulation results are in good agreement with the measured data.The relative errors of bed temperature, bed pressure, and furnace outlet flue gas temperature are of less than 1.13%, 2.69%, and 0.84%, respectively.Therefore, the simulation results could be considered credible.

Model Validation
To verify the accuracy of the model, the simulation results are compared with the measured data.As shown in Table 4, the numerical simulation results are in good agreement with the measured data.The relative errors of bed temperature, bed pressure, and furnace outlet flue gas temperature are of less than 1.13%, 2.69%, and 0.84%, respectively.Therefore, the simulation results could be considered credible.In this study, coal, biomass, and oil sludge are selected for analysis (collected from Karamay, Xinjiang Province, China).These three fuels have low calorific values of 19.28, 16.12, and 7.99 MJ/kg, respectively.The results of the ultimate and proximate analyses of the three fuels are shown in Table 5.The main components of the bed material are sand and ash.The bed material is viewed in the simulation as a single, inert entity.The maximum particle size of bed material should not be more than 3 mm, and the mass proportion of less than 100 mm should not be more than 25%.The biomass particle size complies with the following standards: less than 5 mm in diameter, less than 10 mm in cross-sectional dimension, and less than 30 mm in length.Less than 5% of them have a mass percentage of less than 10 mm in diameter and length.The particle size distribution of coal/oil sludge is shown in Figure 4.
Energies 2024, 17, x FOR PEER REVIEW 7 of 18 30 mm in length.Less than 5% of them have a mass percentage of less than 10 mm in diameter and length.The particle size distribution of coal/oil sludge is shown in Figure 4.

Simulation Conditions
Before the simulation, the state of fluid state and its particle makeup are specified in the computational domain.The flow of gas is parallel to the surface.Based on industry data, the gas parameters are additionally provided.A specific number is chosen for the boundary pressure at the top outflow of cyclone.The thermal wall is employed to characterize the remaining constraints.The surface of the cyclone separator and the conical portion of the furnace bottom are believed to be adiabatic walls.There is a radiative emissivity of 0.7 for the wall.The simulation lasts 30 s with the standard scenario of using coal as fuel.Table 6 provides a summary of the main simulation parameters and base case operational circumstances.

Simulation Conditions
Before the simulation, the state of fluid state and its particle makeup are specified in the computational domain.The flow of gas is parallel to the surface.Based on industry data, the gas parameters are additionally provided.A specific number is chosen for the boundary pressure at the top outflow of cyclone.The thermal wall is employed to characterize the remaining constraints.The surface of the cyclone separator and the conical portion of the furnace bottom are believed to be adiabatic walls.There is a radiative emissivity of 0.7 for the wall.The simulation lasts 30 s with the standard scenario of using coal as fuel.Table 6 provides a summary of the main simulation parameters and base case operational circumstances.

Co-Combustion of Coal and Biomass
To investigate the effect of blending ratio on co-combustion characteristics of coal and biomass.The blending ratios of biomass are selected as 40%, 50%, 60%, 80%, and 100%, respectively.To ensure constant boiler output, the total amount of fuel is calculated based on the weighted average low calorific value of the mixed fuel.The air amount calculation process is as follows: Firstly, the theoretical air amount is calculated according to the ultimate analysis of the three fuels.Then, according to the field operation data, the excess air coefficient is selected to be 1.28, so that the actual air amount could be calculated.Finally, according to the air amount ratio in the actual operation process, the air amount of primary air, secondary air and return air is obtained.Table 7 lists the simulation parameters for different cases.Taking case 2 as an example, Figure 5 illustrates the variation in flow pattern with time from 0 to 30 s. Within 0-10 s, the particles gradually fluidize, and the bed rises, with some particles entering the cyclone separator to participate in circulation.After 18 s, the flow pattern of the particles gradually stabilizes, which is similar to the results of Shen et al. [36].To reduce the impact of particle migration on the results, 20-30 s are selected for time average statistical analysis.
Taking case 2 as an example, Figure 5 illustrates the variation in flow pattern with time from 0 to 30 s. Within 0-10 s, the particles gradually fluidize, and the bed rises, with some particles entering the cyclone separator to participate in circulation.After 18 s, the flow pattern of the particles gradually stabilizes, which is similar to the results of Shen et al. [36].To reduce the impact of particle migration on the results, 20-30 s are selected for time average statistical analysis.

Combustion Characteristics
The distribution of the average temperature of the furnace in various cases is shown in Figure 6.It can be seen from the figure that the furnace temperature distribution is basically the same under different biomass blending ratios.Because of the combustion of fuel and the circulation of a large amount of high-temperature materials from the cyclone separator, the furnace temperature rises sharply.The furnace temperature achieves its maximum at approximately 0.5 m from the bottom.The introduction of a large amount of low-temperature air at the secondary air inlet reduces the furnace temperature.The watercooled walls arranged along the height of the furnace absorb a large amount of heat, causing the temperature of the furnace to gradually drop.As the biomass blending ratio increases, the furnace temperature gradually decreases, which may be related to the lower calorific value of the material and the larger particle size.The furnace outlet flue temperature decreases from 1122.2 to 1114.2K as the biomass blending ratio goes from 40% to 100%.

Combustion Characteristics
The distribution of the average temperature of the furnace in various cases is shown in Figure 6.It can be seen from the figure that the furnace temperature distribution is basically the same under different biomass blending ratios.Because of the combustion of fuel and the circulation of a large amount of high-temperature materials from the cyclone separator, the furnace temperature rises sharply.The furnace temperature achieves its maximum at approximately 0.5 m from the bottom.The introduction of a large amount of low-temperature air at the secondary air inlet reduces the furnace temperature.The water-cooled walls arranged along the height of the furnace absorb a large amount of heat, causing the temperature of the furnace to gradually drop.As the biomass blending ratio increases, the furnace temperature gradually decreases, which may be related to the lower calorific value of the material and the larger particle size.The furnace outlet flue temperature decreases from 1122.2 to 1114.2K as the biomass blending ratio goes from 40% to 100%.
Energies 2024, 17, 149 9 of 17 cooled walls arranged along the height of the furnace absorb a large amount of heat, caus ing the temperature of the furnace to gradually drop.As the biomass blending ratio in creases, the furnace temperature gradually decreases, which may be related to the lowe calorific value of the material and the larger particle size.The furnace outlet flue temper ature decreases from 1122.2 to 1114.2K as the biomass blending ratio goes from 40% to 100%.

Gas Emission
Figure 7 shows the gas component distribution along the height direction of the furnace under different cases.It can be seen from the figure that the average mole fraction of O 2 first decreases rapidly along the height direction of the furnace, indicating that the fuel particles burn rapidly, increase at the secondary air inlet, and then gradually decrease overall.Table 8 illustrates the temperature and main gas mole fractions of furnace outlet under cases 1 to 5.An upward adjustment of the biomass blending ratio from 40% to 100% correlates with a rise in the mole fraction of O 2 at the furnace outlet, ascending from 0.0541 to 0.0640.Concurrently, there is an observed decrement in the mole fraction of CO 2 , which diminishes from 0.1357 to 0.1267.Upon introduction of the heterogeneous fuel into the furnace, an intense combustion reaction is initiated, precipitating a localized deficit of O 2 and, consequently, the incomplete oxidation of a fraction of the fuel.The NO x distribution changes along the furnace height direction in different cases are almost the same, increasing rapidly at first and then gradually decreasing.N-containing volatiles release a large amount of NO x through violent combustion; so, the NO x mole fraction increases rapidly.Subsequently, NO x is reduced to N 2 by coke and CO, and the mole fraction gradually decreases.As the biomass blending ratio increases from 40% to 100%, the NO x mole fraction at the furnace outlet decreases from 4.5867 × 10 −5 to 3.9096 × 10 −5 .The SO 2 mole fraction drops from 2.8253 × 10 −4 to 4.6635 × 10 −5 , which corresponds to the ultimate analysis results of coal and biomass (see Table 5).amount of NOx through violent combustion; so, the NOx mole fraction increases rapidly.
Subsequently, NOx is reduced to N2 by coke and CO, and the mole fraction gradually decreases.As the biomass blending ratio increases from 40% to 100%, the NOx mole fraction at the furnace outlet decreases from 4.5867 × 10 −5 to 3.9096 × 10 −5 .The SO2 mole fraction drops from 2.8253 × 10 −4 to 4.6635 × 10 −5 , which corresponds to the ultimate analysis results of coal and biomass (see Table 5).

Tri-Combustion of Coal, Biomass, and Oil Sludge
Five distinct blending ratios are selected to examine the tri-combustion characteristics of coal, biomass, and oil sludge.As mentioned before, the total fuel feeding amount is determined by the low calorific value of the three fuels.The excess air coefficient is 1.28.The simulation parameters for several cases are listed in Table 9.The distribution of the average temperature of the furnace section along the furnace height direction under various cases is shown in Figure 8. Similar to the co-combustion of coal and biomass, the furnace temperature change trends in different calculation examples are essentially the same.The furnace temperature initially increases rapidly, decreases at the secondary air inlet, then increases slightly, and finally decreases gradually along the height direction of the furnace.The increase in the blending ratio of sludge and biomass reduces the average temperature of the furnace.The furnace outlet flue gas temperature ranges from 1117.9 to 1120. 4  The distribution of the average temperature of the furnace section along the furnace height direction under various cases is shown in Figure 8. Similar to the co-combustion of coal and biomass, the furnace temperature change trends in different calculation examples are essentially the same.The furnace temperature initially increases rapidly, decreases at the secondary air inlet, then increases slightly, and finally decreases gradually along the height direction of the furnace.The increase in the blending ratio of sludge and biomass reduces the average temperature of the furnace.The furnace outlet flue gas temperature ranges from 1117.9 to 1120.4K under different cases.

Gas Emission
The distribution of the average mole fraction of gas components along the furnace height direction for different cases is illustrated in Figure 9.It is obvious that the average mole fraction of O 2 decreases rapidly initially, increases at the secondary air inlet, and then decreases slowly, which indicates that the fuel particles combust rapidly at the bottom of the furnace.Table 10 shows the temperature and main gas mole fractions of furnace outlet under cases 6 to 10.An augmented biomass proportion is associated with a diminished requisite for O 2 .As the biomass blending ratio is escalated from 35% to 50%, there is a discernible increment in the molar fraction of O 2 at the furnace outlet, which intensifies from 0.0606 to 0.0667.The average mole fraction of CO 2 shows an opposite trend to that of O 2 .When the blended fuel enters the dense phase zone, it burns violently, resulting in a local lack of oxygen and incomplete combustion of part of the fuel.Therefore, optimization of the secondary air could be considered to ensure complete combustion of the blended fuels.
The changes in the average mole fraction of NO x along the furnace height direction are almost the same in different cases, with an initial rapid increase and then a gradual decrease.The violent combustion of volatiles releases a large amount of NO x , causing its mole fraction to increase rapidly.Subsequently, NO x is reduced to N 2 by coke and CO.The NO x mole fraction at the furnace outlet is between 4.1173 × 10 −5 and 4.2556 × 10 −5 .The distribution of SO 2 along the furnace height is tightly related to the blending ratio of different fuels.In general, the rise in the coal and oil sludge blending ratio promotes the increase in the average mole fraction of SO 2 , which corresponds to the ultimate analysis results (see Table 3).It is worth noting that when the oil sludge ratio increases from 10% to 20%, the SO 2 mole fraction at the furnace outlet increases from 3.3041 × 10 −4 to 4.7043 × 10 −4 .Therefore, attention should be paid to the removal of SO 2 when blending oil sludge.
The NOx mole fraction at the furnace outlet is between 4.1173 × 10 and 4.2556 × 10 .The distribution of SO2 along the furnace height is tightly related to the blending ratio of different fuels.In general, the rise in the coal and oil sludge blending ratio promotes the increase in the average mole fraction of SO2, which corresponds to the ultimate analysis results (see Table 3).It is worth noting that when the oil sludge ratio increases from 10% to 20%, the SO2 mole fraction at the furnace outlet increases from 3.3041 × 10 −4 to 4.7043 × 10 −4 .Therefore, attention should be paid to the removal of SO2 when blending oil sludge.

Effect of Biomass Inlet Position
To study the effect of biomass inlet position on combustion characteristics, in addition to feeding biomass from the feed inlet, two other biomass inlet positions are selected, namely the secondary air inlet and the new inlet (1 m above the secondary air inlet).

Furnace Temperature Distributions
Figure 10 shows the gas phase temperature distribution of the entire CFB boiler under different biomass inlet positions.It can be seen from the figure that the overall temperature trend in the furnace is similar under different inlet positions.Compared with feeding biomass fuel through the feed inlet and secondary air inlet, the temperature distribution of the biomass furnace fed from the new inlet is more uniform, which indicates that feeding biomass from the new inlet is beneficial to promoting the combustion of the blended fuel.

Effect of Biomass Inlet Position
To study the effect of biomass inlet position on combustion characteristics, in addition to feeding biomass from the feed inlet, two other biomass inlet positions are selected, namely the secondary air inlet and the new inlet (1 m above the secondary air inlet).

Furnace Temperature Distributions
Figure 10 shows the gas phase temperature distribution of the entire CFB boiler under different biomass inlet positions.It can be seen from the figure that the overall temperature trend in the furnace is similar under different inlet positions.Compared with feeding biomass fuel through the feed inlet and secondary air inlet, the temperature distribution of the biomass furnace fed from the new inlet is more uniform, which indicates that feeding biomass from the new inlet is beneficial to promoting the combustion of the blended fuel.

Gas Emission
Figure 11 shows the distribution of different gas components in the CFB boiler under different biomass inlet positions.It can be seen from the figure that since the total air amount remains unchanged, the distribution of O2 and CO2 in the furnace is almost the same under different biomass inlet positions.The uniformity of O2 and CO2 distribution under the three biomass inlet positions is that the new inlet has the best uniformity, followed by the secondary air inlet and the feed inlet has the worst.This phenomenon corresponds to the distribution of the temperature field (see Figure 10).Upon the introduction of biomass particulates via the feed inlet, secondary air inlet, and new inlet, the mole fractions of NOx at the furnace outlet are 3.9096 × 10 −5 , 4.1022 × 10 −5 , and 5.1537 × 10 −5 , respectively, while the mole fractions of SO2 are 2.5978 × 10 −4 , 2.5738 × 10 −4 , and 2.5278 × 10 −4 , respectively.As the reaction proceeds, the formation of NOx is limited by the increase in CO content.The increase in coke particles is also conducive to the reduction of NOx, and the content of NOx is in a dynamic fluctuation.The mole fraction of NOx at the furnace outlet increases after the biomass is fed through the new inlet, which may be related to the shortening of the reaction range in the reduction zone.Since the total amount of fuel input and air volume remain unchanged, the concentration of SO2 changes slightly.

Gas Emission
Figure 11 shows the distribution of different gas components in the CFB boiler under different biomass inlet positions.It can be seen from the figure that since the total air amount remains unchanged, the distribution of O 2 and CO 2 in the furnace is almost the same under different biomass inlet positions.The uniformity of O 2 and CO 2 distribution under the three biomass inlet positions is that the new inlet has the best uniformity, followed by the secondary air inlet and the feed inlet has the worst.This phenomenon corresponds to the distribution of the temperature field (see Figure 10).Upon the introduction of biomass particulates via the feed inlet, secondary air inlet, and new inlet, the mole fractions of NO x at the furnace outlet are 3.9096 × 10 −5 , 4.1022 × 10 −5 , and 5.1537 × 10 −5 , respectively, while the mole fractions of SO 2 are 2.5978 × 10 −4 , 2.5738 × 10 −4 , and 2.5278 × 10 −4 , respectively.As the reaction proceeds, the formation of NO x is limited by the increase in CO content.The increase in coke particles is also conducive to the reduction of NO x , and the content of NO x is in a dynamic fluctuation.The mole fraction of NO x at the furnace outlet increases after the biomass is fed through the new inlet, which may be related to the shortening of the reaction range in the reduction zone.Since the total amount of fuel input and air volume remain unchanged, the concentration of SO 2 changes slightly.

Gas Emission
Figure 11 shows the distribution of different gas components in the CFB boiler under different biomass inlet positions.It can be seen from the figure that since the total air amount remains unchanged, the distribution of O2 and CO2 in the furnace is almost the same under different biomass inlet positions.The uniformity of O2 and CO2 distribution under the three biomass inlet positions is that the new inlet has the best uniformity, followed by the secondary air inlet and the feed inlet has the worst.This phenomenon corresponds to the distribution of the temperature field (see Figure 10).Upon the introduction of biomass particulates via the feed inlet, secondary air inlet, and new inlet, the mole fractions of NOx at the furnace outlet are 3.9096 × 10 −5 , 4.1022 × 10 −5 , and 5.1537 × 10 −5 , respectively, while the mole fractions of SO2 are 2.5978 × 10 −4 , 2.5738 × 10 −4 , and 2.5278 × 10 −4 , respectively.As the reaction proceeds, the formation of NOx is limited by the increase in CO content.The increase in coke particles is also conducive to the reduction of NOx, and the content of NOx is in a dynamic fluctuation.The mole fraction of NOx at the furnace outlet increases after the biomass is fed through the new inlet, which may be related to the shortening of the reaction range in the reduction zone.Since the total amount of fuel input and air volume remain unchanged, the concentration of SO2 changes slightly.

Figure 3 .
Figure 3. Average temperature distribution versus the furnace height for various meshes.

Figure 3 .
Figure 3. Average temperature distribution versus the furnace height for various meshes.

Figure 4 .
Figure 4. Particle size distribution of coal/oil sludge.

Figure 4 .
Figure 4. Particle size distribution of coal/oil sludge.

Figure 5 .
Figure 5. Variation in flow pattern with time.

Figure 5 .
Figure 5. Variation in flow pattern with time.

Figure 6 .
Figure 6.Average temperature distribution along furnace height for various cases.

Figure 7 Figure 6 .
Figure 7 shows the gas component distribution along the height direction of the fur nace under different cases.It can be seen from the figure that the average mole fraction o O2 first decreases rapidly along the height direction of the furnace, indicating that the fue

Figure 7 .Figure 7 .
Figure 7. Gas mole fractions distribution along furnace height for various cases.

Figure 8 .Figure 8 .
Figure 8.Average temperature distribution along furnace height under different excess air ratios.

Figure 9 .
Figure 9. Gas mole fractions distribution along furnace height for various excess air ratios.

Figure 9 .
Figure 9. Gas mole fractions distribution along furnace height for various excess air ratios.

Figure 10 .
Figure 10.Temperature distribution for different biomass inlet positions.

Table 1 .
Main governing equations of gas and particle phase.

Table 3 .
NO x formation and reduction kinetic model.

Table 4 .
Comparison of simulation results with measured data.

Table 4 .
Comparison of simulation results with measured data.

Table 5 .
Ultimate and proximate analyses of coal, biomass, and oil sludge (wt %, as received basis).
a By difference.

Table 6 .
Operating conditions and simulation settings.

Table 7 .
Simulation parameters for different cases.

Table 8 .
Temperature and main gas mole fractions of furnace outlet under cases 1 to 5.

Table 9 .
Simulation parameters for various cases.
K under different cases.

Table 10 .
Temperature and main gas mole fractions of furnace outlet under cases 6 to 10.

Table 10 .
Temperature and main gas mole fractions of furnace outlet under cases 6 to 10.
Zhou and Chen Du were employed by PetroChina Xinjiang Oilfield Company.The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.