Simulation Study of an Oxy-Biomass-Based Boiler for Nearly Zero Emission Using Aspen Plus

: Bioenergy integrated CO 2 capture is considered to be one of the viable options to reduce the carbon footprint in the atmosphere, as well as to lower dependability on the usage of fossil fuels. The present simulation-based study comprises the oxy bio-CCS technique with the objective of bringing about cleaner thermal energy production with nearly zero emissions, CO 2 capture and puriﬁcation, and with the ability to remove NO x and SO 2 from the ﬂue gas and to generate valuable byproducts, i.e., HNO 3 and H 2 SO 4 . In the present work, a simulation on utilization of biomass resources by applying the oxy combustion technique was carried out, and CO 2 sequestration through pressurized reactive distillation column (PRDC) was integrated into the boiler. Based on our proposed laboratory scale bio-CCS plant with oxy combustion technique, the designed thermal load was kept at 20 kW th using maize stalk as primary fuel. With the objective of achieving cleaner production with near zero emissions, CO 2 rich ﬂue gas and moisture generated during oxy combustion were hauled in PRDC for NO x and SO 2 absorption and CO 2 puriﬁcation. The oxy combustion technique is unique due to its characteristic low output of NO sourced by fuel inherent nitrogen. The respective mechanisms of fuel inherent nitrogen conversion to NO x , and later, the conversion of NO x and SO 2 to HNO 3 and H 2 SO 4 respectively, involve complex chemistry with the involvement of N–S intermediate species. Based on the ﬂue gas composition generated by oxy biomass combustion, the focus was given to the fuel NO x , whereby di ﬀ erent rates of NO formation from fuel inherent nitrogen were studied to investigate the optimum rates of conversion of NO x during conversion reactions. The rate of conversion of NO x and SO 2 were studied under ﬁxed temperature and pressure. The factors a ﬀ ecting the rate of conversion were optimized through sensitivity analysi ê s to get the best possible operational parameters. These variable factors include ratios of liquid to gas feed ﬂow, vapor-liquid holdups and bottom recycling. The results obtained through optimizing the various factors of the proposed system have shown great potential in terms of maximizing productivity. Around 88.91% of the 20 kW th boiler’s e ﬃ ciency was obtained. The rate of conversion of NO x and SO 2 were recorded at 98.05% and 87.42% respectively under parameters of 30 ◦ C temperature, 3 MPa pressure, 10% feed stream holdup, liquid / gaseous feed stream ratio of 0.04 and a recycling rate of the bottom product of 20%. During the simulation process, production of around four kilograms per hour of CO 2 with 94.13% purity was achieved. organic material left behind in the ash portion of the fuel, and the subsequent formation of CH 4 and CO, contribute to the increase in losses which lowers the thermal e ﬃ ciency of the boiler. During actual boiler operation, the ﬂue gas heat losses calculated are based on the ﬂowrate, temperature and composition of ﬂue gas; Dias et al., 2004, among others, studied conventional combustion in a biomass boiler and obtained thermal e ﬃ ciencies of up to 77% [7]. Under an oxy atmosphere in the present study, the thermal e ﬃ ciency was shown to be 88.91%, which is comparatively under oxy conditions compared to the referenced conventional condition. on variations of the mole fraction values of NO and SO 2 , which are declining with an increasing holdup ratio (%). NO reduction is more prominent at a lower rate of holdup, whereas SO 2 saw a linear decline along the holdup ratio. It may be concluded that the conversion of SO 2 to H 2 SO 4 could be enhanced with gradual holdup increments. The sudden change in the mole fraction of NO and SO 2 at lower holdup rates was observed by Mujtaba et al., 1998 [65], where a relative easy distillation was observed at the start of the batch process at lower holdup ratios.


Introduction
Based on a proposed laboratory scale biomass boiler with 20 kW th capacity, a simulation study with an objective of optimizing the oxy biomass with CO 2 -enriched flue gas processing system (oxy

Methodology
A proposed 20 kW th biomass boiler with oxy combustion and CCS techniques was analyzed using the Aspen plus simulation program. The objectives of this simulation were to determine the best possible operational parameters for the integrated sensitivity analysis, where the maximum efficiencies for oxy biomass thermal process and the CO 2 purification process could be attained by the least effort and with nearly zero emissions. The process industry-designed Aspen plus simulation software includes a wide range of tools such as components, physical property databases and process models. The whole simulation process was divided into two parts in the present work: oxy biomass combustion with CO 2 -enriched flue gas recirculation, and purification of the produced CO 2 -enriched flue gas through conversion of NO x and SO 2 .
For the first step of the simulation, the Peng Robinson equation of state physical property [45] was used to simulate the operations of the thermal boiler. The advantage of using Peng Robinson property method is that its equations of state give better agreement between the experimental and simulation results, and it can be used in both the gaseous and liquid phases. In this equation, the pressure term of the Van der Waals equation was modified. The empirical form of this equation is mentioned in Equation (1): where b = 0.0778 RT c P c ; a = a(T c α); a(T c ) = 0.45724 re P c ; a 0.5 = 1 + ak 1 − T 0.5 re ; k = 0.37464 + 1.54226ω − 0.26992ω 2 ; and T re = T T c . Equation (1) can be re-written as: where A = ap R 2 T 2 ; B = bp RT ; and Z = pv RT . The ELECNRTL as one of the electrolyte property method was used to calculate and predict the best possible process results for simulating the PRDC process in order to purify the CO 2 rich stream of flue gas. Furthermore, the Steam-TA property method was used to better predict the water/steam properties. During the simulation study of the oxy biomass with CCS techniques, the most suitable process models available in Aspen plus were assigned to specify the operational functions of process units, which were interconnected through material, and energy flow to build the process flowsheet. The properties of maize stalk biomass were used in the present simulation study, whose proximate and ultimate analyses data, as described in Table 1, were obtained using laboratory instruments i.e., TRGF-8000 (Hebi Tianrun Electronic Technology Company, HeBi, China) and EA-3000 Euro Vector EA-IRMS (EUROVECTOR Srl, Italy) respectively. A pressurized reactive distillation column (PRDC) was used to model the conversion of SO 2 and NO x into H 2 SO 4 and HNO 3 along with purification of CO 2 during the present simulation study. In this model, the following three main factors were investigated for their effects on the process operations: These variable factors have shown great influence on the overall process of PRDC in terms of achieving nearly zero emission along with the purification of CO 2 for CCS.

Simulation Model
The simulation model is based on two interconnected processes: oxy biomass combustion with flue gas recirculation, and CO 2 purification unit processing, including the conversion of SO 2 and NO x , and the generation of by-products in the form of H 2 SO 4 , HNO 3 and CO 2 purification. A synchronized process flow according to the proposed laboratory scale plant process is illustrated in Figure 1. In the oxy biomass combustion part of the understudied simulation system, three reactor models, namely RYield, RGibbs and RStoic, were used respectively to simulate the conversion of non-conventional maize stalk biomass fuel into conventional form, the combustion by minimizing the Gibbs free energy, and the conversion of fuel inherent nitrogen into NO x at the pre-specified temperature under an oxy atmosphere. RYield model generates the elemental building blocks of non-conventional components. The RGibbs model produces the combustion products and the RStoic model converts the fuel inherent nitrogen into NO x .
Energies 2019, 12,1949 5 of 21 The simulation model is based on two interconnected processes: oxy biomass combustion with flue gas recirculation, and CO2 purification unit processing, including the conversion of SO2 and NOx, and the generation of by-products in the form of H2SO4, HNO3 and CO2 purification. A synchronized process flow according to the proposed laboratory scale plant process is illustrated in Figure 1. In the oxy biomass combustion part of the understudied simulation system, three reactor models, namely RYield, RGibbs and RStoic, were used respectively to simulate the conversion of non-conventional maize stalk biomass fuel into conventional form, the combustion by minimizing the Gibbs free energy, and the conversion of fuel inherent nitrogen into NOx at the pre-specified temperature under an oxy atmosphere. RYield model generates the elemental building blocks of non-conventional components. The RGibbs model produces the combustion products and the RStoic model converts the fuel inherent nitrogen into NOx. The combustion product achieved using the RGibbs model is a mixture of flue gases containing CO2, CO, H2O, SO2, N2, NO, NO2, and excess O2, with ash as solid residue. In modeling the oxy biomass combustion, NO produced by the model does not actually represent the exact NOx formation. To accurately determine the fuel nitrogen conversion into NOx, the RStoic model was introduced into the flue gas flow channel based on our previous experimental work [46]. The detailed reaction mechanism occurring in the RStoic reaction model involves the conversion of fuel nitrogen into fuel NOx or elemental nitrogen, as illustrated in Section 2.3.1. The enthalpy of combustion taken by the flue gas is transferred to water for heating in the boiler through a heat exchanger. Ash removal was modelled by integrating the solid separator process model. The flue gas exiting the RStoic reactor model was set at 1000 °C, which was reduced to 180 °C after passing through the boiler. The thermal energy of the flue gas at 180 °C was used in the economizer for heat recovery by the introduced water. The relative low temperature flue gas stream was allowed to heat up the oxidizers by air preheater, where the temperature of the oxidizers was raised to 80 °C. The flue gas produced during the combustion process contains moisture, which should be minimized to a certain level in order to not impede the process efficiency. The exhaust flue gas stream was connected to the condenser followed by flash for the removal of moisture in the flue gas. The condensate stream containing 0.99 mole fraction of water after flushing from the flue gas stream was used as a liquid feed for the CO2 purification unit for the absorption of NOx and SO2 to yield the acidic condensates. The cooled dry flue gas stream was then split into two segments; one was allowed to return to the combustion reactor RGibbs after mixing with the pure O2 stream (considering an excess O2 factor of not more than 4% in the flue gas exiting the boiler), while the second (Stream 9) was set as the feed source for the CO2 purification unit.
The CO2-enriched flue gas was compressed by the compressor process model. The resultant flue The combustion product achieved using the RGibbs model is a mixture of flue gases containing CO 2 , CO, H 2 O, SO 2 , N 2 , NO, NO 2 , and excess O 2 , with ash as solid residue. In modeling the oxy biomass combustion, NO produced by the model does not actually represent the exact NO x formation. To accurately determine the fuel nitrogen conversion into NO x , the RStoic model was introduced into the flue gas flow channel based on our previous experimental work [46]. The detailed reaction mechanism occurring in the RStoic reaction model involves the conversion of fuel nitrogen into fuel NO x or elemental nitrogen, as illustrated in Section 2.3.1. The enthalpy of combustion taken by the flue gas is transferred to water for heating in the boiler through a heat exchanger. Ash removal was modelled by integrating the solid separator process model. The flue gas exiting the RStoic reactor model was set at 1000 • C, which was reduced to 180 • C after passing through the boiler. The thermal energy of the flue gas at 180 • C was used in the economizer for heat recovery by the introduced water. The relative low temperature flue gas stream was allowed to heat up the oxidizers by air preheater, where the temperature of the oxidizers was raised to 80 • C. The flue gas produced during the combustion process contains moisture, which should be minimized to a certain level in order to not impede the process efficiency. The exhaust flue gas stream was connected to the condenser followed by flash for the removal of moisture in the flue gas. The condensate stream containing 0.99 mole fraction of water after flushing from the flue gas stream was used as a liquid feed for the CO 2 purification unit for the absorption of NO x and SO 2 to yield the acidic condensates. The cooled dry flue gas stream was Energies 2019, 12,1949 6 of 21 then split into two segments; one was allowed to return to the combustion reactor RGibbs after mixing with the pure O 2 stream (considering an excess O 2 factor of not more than 4% in the flue gas exiting the boiler), while the second (Stream 9) was set as the feed source for the CO 2 purification unit.
The CO 2 -enriched flue gas was compressed by the compressor process model. The resultant flue gas was cooled through the cooler model prior to entering the PRDC model. A RadFrac column was used to perform the functions of the pressurized reactive distillation column (PRDC), where the gaseous feed stream (11) entered through the bottom end, and the liquid feed stream (L5) was injected through the top stage of the column. In the RadFrac model, kinetic and equilibrium reaction models must be synchronized in the Reaction module of the Aspen plus. The detailed NO x and SO 2 absorption mechanisms involving the formation of complex species is described in Section 2.3.2. The pre-defined kinetic parameters of NO x and SO 2 absorption reactions were integrated into the Reaction module to study their respective conversion into acids. Similar to the CO 2 -enriched flue gas stream, the condensate stream was used as the liquid feed for the reactive distillation column. Its pressure and temperature values were synchronized according to flue gas stream, and absorption studies were carried out by varying the liquid to gas ratio in order to determine the best possible values for CO 2 purification. During the PRDC process, NO x and SO 2 underwent the pre-specified reaction scheme and was converted to HNO 3 and H 2 SO 4 respectively. The acidic forms were obtained as liquid condensate from the bottom stage of the reactive column, whose fractional compositions were adjusted by varying the recycle ratio to get the best possible values of NO x and SO 2 . The resulting CO 2 -rich and effluent-lean vapor stream leaving the reactive column was compressed through the compressor model to obtain CO 2 liquefaction, whereby liquefied CO 2 is available for transportation and storage under saline aquifers or for further use. The operational parameters and their ranges in the present simulation process are summarized in Table 2.

Reaction Mechanism
The reaction mechanism involved in the RStoic reactor model used for the conversion of fuel nitrogen to NO x or elemental nitrogen and absorption of NO x and SO 2 in RadFrac reactive distillation column model of the Aspen plus simulation are described in the following sections:

Fuel N Conversion
Fuel-NO x is formed from oxidation of fuel nitrogen within the fuel matrix [47]. It may evolve as volatile nitrogen or may remain in the char [48]. NO formed from fuel inherent nitrogen can be greatly influenced by its distribution in the volatiles and char of fuel matrix [49]. Fuel NO x formation through char nitrogen undergoes heterogeneous reactions due to the interaction of formed NO x with the active carbon sites at the surface of char leading to its reduction to elemental nitrogen. Volatile nitrogen after the formation of intermediate species in the form of HCN and NH 3 follows various possible routes, due to reactions with H, O, OH radicals to become oxidized, yielding NO x , or it may be reduced to elemental nitrogen. The volatile nitrogen in biomass converts directly to NH 3 which may react with  (3)). It worth noting that more NH 3 is produced than HCN in biomass combustion [50].
The liberated NH 3 and formed HCN from the fuel nitrogen undergo various reaction processes which may result in the formation of NO through oxidation, or may reduce to N 2 . NO x formation from fuel nitrogen depends on various factors such as residence time, temperature and char particle size. The fate of fuel nitrogen contributing to the formation of NO or N 2 is illustrated in Figure 2 (as part of our previous work) [46]; the conversion rate to NO for a maize stalk biomass fuel was considered in the present simulation study.
Energies 2019, 12,1949 7 of 21 part of our previous work) [46]; the conversion rate to NO for a maize stalk biomass fuel was considered in the present simulation study.

NO and SO2 Absorption
The overall reaction mechanism involving the conversion of NOx and SO2 into several byproducts is comprised of a series of binary phase reactions, where the gaseous phase chemistry involves the conversion of NO to NO2 and SO2 to SO3.

NO and SO 2 Absorption
The overall reaction mechanism involving the conversion of NO x and SO 2 into several by-products is comprised of a series of binary phase reactions, where the gaseous phase chemistry involves the conversion of NO to NO 2 and SO 2 to SO 3  Hydroxylamine (NH 2 OH). The reaction scheme is shown in Figure 3. During the present simulation, the main reactions were integrated with the aim of optimizing the maximum absorption of NO x and SO 2 in binary phases. The gas phase chemistry involves the oxidation and hydrolysis of oxides of nitrogen and sulfur. Oxidation of NO into NO 2 i.e. Reaction (4) is the governing reaction in the gas phase; it is strongly influenced by pressure.
NO oxidation is favored by low temperatures and high pressures [51]. The reaction is more prominent at elevated pressure, even for shorter reaction times. Maximum conversion to NO 2 can be achieved within a shorter residence time due to its greater solubility in water [52]. The rate of the reaction is first order with respect to O 2 and second order with respect to NO [51,53].
The formed NO 2 is in equilibrium with its dimer N 2 O 4 (Reaction 5). The dimer formation favors by low temperatures [54].  During the present simulation, the main reactions were integrated with the aim of optimizing the maximum absorption of NOx and SO2 in binary phases. The gas phase chemistry involves the oxidation and hydrolysis of oxides of nitrogen and sulfur. Oxidation of NO into NO2 i.e. Reaction (4) is the governing reaction in the gas phase; it is strongly influenced by pressure.
NO oxidation is favored by low temperatures and high pressures [51]. The reaction is more prominent at elevated pressure, even for shorter reaction times. Maximum conversion to NO2 can be achieved within a shorter residence time due to its greater solubility in water [52]. The rate of the reaction is first order with respect to O2 and second order with respect to NO [51,53].
The formed NO2 is in equilibrium with its dimer N2O4 (Reaction 5). The dimer formation favors by low temperatures [54].
The hydrolysis of N2O4 is the main reaction. N2O4 may be absorbed directly in the liquid phase, yielding nitric acid directly (Reaction 6) or through the intermediate formation of nitrous acid (Reaction 7).
Miller et al., [55] combined these reactions (6) and (7) into a single reaction (8); The interaction between SO2 and NO2 has been studied extensively by Jaffe et al. [56] and Armitage et al. [57]. Both publications studied experimentally the catalytic oxidation of SO2 in the presence of Miller et al. [55] combined these reactions (6) and (7) into a single reaction (8); The interaction between SO 2 and NO 2 has been studied extensively by Jaffe et al. [56] and Armitage et al. [57]. Both publications studied experimentally the catalytic oxidation of SO 2 in the presence of NO 2 . The oxidation of SO 2 is significant at temperatures higher than 900 • C; thus, at lower temperatures in the PRDC, direct oxidation was not included in the reaction scheme. Rather, NO 2 , instead of the available O 2 , reacts as an oxidizer in the conversion of SO 2 into SO 3 (Reaction 9), and subsequently produces H 2 SO 4 on hydrolysis according to reaction (10).
A detailed kinetic scheme of the involved reactions is illustrated in Table 3. 10 , -

Thermal Performance
Thermal efficiency is a direct indication of fuel resource conservation and emission reduction. In the present work, an indirect method to determine the boiler efficiency when ASME (American Society of Mechanical Engineers) standards are used was included in the first part of simulation. All the system and process losses were first taken into consideration, and the difference between the cumulative losses and the energy input was taken as the boiler efficiency. The losses include dry flue gas exiting the boiler (L DFG ), moisture produced due to oxidation of H 2 plus evaporation of moisture content of the fuel (L H 2 O ), unburnt combustibles in the form of CO (L CO ), unburnt losses and fly and bottom ashes (L ASH ), and radiation and convection losses (L R ). These losses were calculated using a set of empirical equations, as shown in Equations (11)- (14); where where Energies 2019, 12,1949 10 of 21 The efficiency of the boiler was determined using Equation (15).

Conversion Performance
The abatement efficiency of the effluents, i.e., NO and SO 2 , is a direct indication of the CO 2 purification, since these effluents settle in the distillation unit in acidic forms and are separated as the acidic condensate product of the reactive column, whereas the purified CO 2 stream leaves as vapor product of the reactive column. During simulation, the NO x and SO 2 removal efficiencies were calculated using Equations (16) and (17) respectively;

Results and Discussion
Various parameters of the present simulation study have been evaluated regarding the subsequent combustion of maize stalk biomass under an oxy atmosphere with flue gas recirculation and the treatment of flue gas channel drawn out for pressurized reactive distillation column (PRDC). The results of the oxy biomass thermal system are comprised of thermal performance and evaluation of losses incurred during the combustion process. In addition, the heating and cooling curves of the boiler are evaluated and presented. As an important part of the present study is to optimize the extent of CO 2 purification through the subsequent conversion of NO x and SO 2 into their respective by-products through PRDC, the variable factors relevant to the specific operation of distillation column were analyzed and the respective results are described in detail. The ratio of liquid to gaseous feed stream, rate of bottom product recycling, ratio of holdup of the feed streams at the distillation column are important factors affecting the operation of the PRDC. These factors can play key roles in meeting the objectives of achieving oxy biomass-CCS. The results are described below:

Thermal Efficiency
The losses occurring in the boiler vary widely due to boiler design and operational modes. Biomass fuels have their distinguished primary characteristics in terms of elemental or proximate analysis. In the present scenario, maize stalk comprises a higher ratio of H 2 O and H 2 in its fuel matrix. The inherent moisture evaporates during the combustion process and accompanies the flue gas. Hydrogen contained in the biomass fuel matrix is excessively oxidized, forming H 2 O, which is added into the already evaporated moisture content in the flue gas. Considering Equation (13) regarding losses due to H 2 O formation, it gives a clear overview of an inverse relationship between the concerned losses and the calorific value of the biomass. Alternatively, the thermal performance of the boiler is adversely influenced by the higher moisture contents of the flue gas. The major losses in the present study are those due to moisture formation in the flue gas, accounting for 74.12% (1.64 kW th ) of the total thermal losses. The detailed losses are plotted in Figure 4.
On the other hand, the radiation and convection losses rely mainly upon the design and structure of thermal boiler. In the present study, it was kept at constant value. Other incorporated losses include dry flue gas leaving the boiler through a vent fan, which accounts for 7.17% (0.16 kW th ). The unburnt organic material left behind in the ash portion of the fuel, and the subsequent formation of CH 4 and CO, contribute to the increase in losses which lowers the thermal efficiency of the boiler. During actual boiler operation, the flue gas heat losses calculated are based on the flowrate, temperature and composition of flue gas; Dias et al., 2004, among others, studied conventional combustion in a biomass boiler and obtained thermal efficiencies of up to 77% [7]. Under an oxy atmosphere in the present study, the thermal efficiency was shown to be 88.91%, which is comparatively under oxy conditions compared to the referenced conventional condition.
The losses occurring in the boiler vary widely due to boiler design and operational modes. Biomass fuels have their distinguished primary characteristics in terms of elemental or proximate analysis. In the present scenario, maize stalk comprises a higher ratio of H2O and H2 in its fuel matrix. The inherent moisture evaporates during the combustion process and accompanies the flue gas. Hydrogen contained in the biomass fuel matrix is excessively oxidized, forming H2O, which is added into the already evaporated moisture content in the flue gas. Considering Equation (13) regarding losses due to H2O formation, it gives a clear overview of an inverse relationship between the concerned losses and the calorific value of the biomass. Alternatively, the thermal performance of the boiler is adversely influenced by the higher moisture contents of the flue gas. The major losses in the present study are those due to moisture formation in the flue gas, accounting for 74.12% (1.64 kWth) of the total thermal losses. The detailed losses are plotted in Figure 4.  On the other hand, the radiation and convection losses rely mainly upon the design and structure of thermal boiler. In the present study, it was kept at constant value. Other incorporated losses include dry flue gas leaving the boiler through a vent fan, which accounts for 7.17% (0.16 kWth). The unburnt organic material left behind in the ash portion of the fuel, and the subsequent formation of CH4 and CO, contribute to the increase in losses which lowers the thermal efficiency of the boiler. During actual boiler operation, the flue gas heat losses calculated are based on the flowrate, temperature and composition of flue gas; Dias et al., 2004, among others, studied conventional combustion in a biomass boiler and obtained thermal efficiencies of up to 77% [7]. Under an oxy atmosphere in the present study, the thermal efficiency was shown to be 88.91%, which is comparatively under oxy conditions compared to the referenced conventional condition.
In order to address the extent of GHGs and pollutant reduction in the boiler, it is important to have a comparative overview of oxy and conventional combustion techniques. The main difference lies in the oxidizer atmosphere, where the nitrogen of the air acts as a diluent during the conventional combustion process. In the case of oxy the combustion technique, nitrogen is replaced by recycled flue gas, whereas pure oxygen is required as the main oxidizer. The pure oxygen (up to 95%) is usually obtained through a cryogenic air separation process which splits air into its components. To make CO2 capture viable, it will be necessary to enhance its formation, which is carried out by recycling the produced flue gas as a result of combustion under an oxy atmosphere. The average flue gas composition of both the combustion systems is illustrated in Table 4. In order to address the extent of GHGs and pollutant reduction in the boiler, it is important to have a comparative overview of oxy and conventional combustion techniques. The main difference lies in the oxidizer atmosphere, where the nitrogen of the air acts as a diluent during the conventional combustion process. In the case of oxy the combustion technique, nitrogen is replaced by recycled flue gas, whereas pure oxygen is required as the main oxidizer. The pure oxygen (up to 95%) is usually obtained through a cryogenic air separation process which splits air into its components. To make CO 2 capture viable, it will be necessary to enhance its formation, which is carried out by recycling the produced flue gas as a result of combustion under an oxy atmosphere. The average flue gas composition of both the combustion systems is illustrated in Table 4: It can be inferred through Table 4 that the resultant flue gas during conventional combustion is enriched mainly with N 2 , whereas CO 2 is prevalent in cases of oxy combustion. The corresponding SO 2 and NO x results can be justified with similar results to those observed by Shao et al., 2013 [60], where adequate NO x and SO 2 emission reductions were observed by changing the conventional air to oxy combustion atmospheres. Li et al., 2013 [61], considering the internal flue gas recirculation in a co-firing boiler, observed the same phenomenon of lower NO x and SO 2 emissions in oxy atmosphere compared to the corresponding conventional one. Through flue gas recycling, NO x can be highly reduced due to char nitrogen reacting with the produced NO x [62].
Usually, CO 2 formation in conventional air combustion accounts for around 16% of the total flue gas; the rest of the flue gas is composed of N 2 . This order is different in case of oxy air combustion, where the exhaust stream is mainly composed of 80-95% CO 2 . CO 2 , having different thermal and chemical characteristics and being denser than nitrogen in the air, impacts on the overall process. Consequently, heat accumulation and reclamation due to recycling is advantageous for the efficiency of the process.

Heating/Cooling Curves of Heat Exchangers
The heating rates of the heat exchanger are a function of the varying temperature with the passage of time. The heating curve obtained through the operation of the heat exchanger is considered a measure of heat exchanger's effectiveness. Due to the steady state simulation, the time factor was not included in the present study. However, heat duties (kW) of the boiler heat exchanger model were calculated, corresponding to the change in temperatures of both the cold and hot streams. The curves obtained by heat transfer in the heat exchanger with gradual variations in the hot and cold side temperatures is illustrated in Figure 5.
SO2 and NOx results can be justified with similar results to those observed by Shao et al., 2013 [60], where adequate NOx and SO2 emission reductions were observed by changing the conventional air to oxy combustion atmospheres. Li et al., 2013 [61], considering the internal flue gas recirculation in a co-firing boiler, observed the same phenomenon of lower NOx and SO2 emissions in oxy atmosphere compared to the corresponding conventional one. Through flue gas recycling, NOx can be highly reduced due to char nitrogen reacting with the produced NOx [62].
Usually, CO2 formation in conventional air combustion accounts for around 16% of the total flue gas; the rest of the flue gas is composed of N2. This order is different in case of oxy air combustion, where the exhaust stream is mainly composed of 80-95% CO2. CO2, having different thermal and chemical characteristics and being denser than nitrogen in the air, impacts on the overall process. Consequently, heat accumulation and reclamation due to recycling is advantageous for the efficiency of the process.

Heating/Cooling Curves of Heat Exchangers
The heating rates of the heat exchanger are a function of the varying temperature with the passage of time. The heating curve obtained through the operation of the heat exchanger is considered a measure of heat exchanger's effectiveness. Due to the steady state simulation, the time factor was not included in the present study. However, heat duties (kW) of the boiler heat exchanger model were calculated, corresponding to the change in temperatures of both the cold and hot streams. The curves obtained by heat transfer in the heat exchanger with gradual variations in the hot and cold side temperatures is illustrated in Figure 5. Heat given by hot stream through a step change in temperature is transferred to heat the cool stream. Flue gases exiting RStoic model were adjusted at 1000 °C; they were then allowed to cool to below 200 °C in the boiler model. Heat duty generated during cooling process of the flue gas in the Heat given by hot stream through a step change in temperature is transferred to heat the cool stream. Flue gases exiting RStoic model were adjusted at 1000 • C; they were then allowed to cool to below 200 • C in the boiler model. Heat duty generated during cooling process of the flue gas in the heat exchanger was used to raise the temperature of the cool water stream from ambient temperatures to around 60 • C.

Pressurized Reactive Distillation Column (PRDC)
In order to achieve the maximum conversion rates of SO 2 and NO x for the purification of CO 2 and consequent formation of by-products (H 2 SO 4 and HNO 3 etc.) during the pressurized reactive distillation column (PRDC) process, the effects of the various parameters such as fuel inherent nitrogen conversion rate and the variable factors affecting PRDC process have been analyzed. The results obtained through sensitivity analysis of the involved factors were in agreement with the relevant results obtained by referenced researchers.

Effect of Conversion Rate of Fuel Inherent Nitrogen on Rate of Conversion
In the case of oxy combustion, where nitrogen in air is replaced by a pure oxygen stream, the NO x formation is sourced by fuel nitrogen only. The conversion mechanism of fuel nitrogen into NO x or elemental nitrogen (N 2 ) depends on many factors, such as the fuel characteristics, devolatilization conditions and combustion mode on the selectivity of oxidation toward NO x formation or reduction towards elemental nitrogen [63]. One of the factors is the surrounding atmosphere of the burning fuel particles, which dictates the fate of fuel inherent nitrogen [47]. Accordingly, the formation of intermediate species such as NH 3 and HCN does not quantify the ratio of NO x produced to elemental nitrogen in flue gas. Analyzing the fuel inherent nitrogen conversion into NO x and extending it further to PRDC gives an good idea of achieving the enhanced conversion efficiency at comparatively higher concentrations of NO x in the flue gas.
As explained in the simulation model, the mechanism of nitrogen distribution into NO x and N 2 was integrated into Aspen by varying the conversion rates based on our similar experimental work [47]. During the present simulation study, various ratios of NO to elemental N 2 were assigned to quantify the subsequent conversion of NO into HNO 3 in the reactive distillation column. The results, as illustrated in Figure 6, show that through enhancing the Fuel-N conversion rate to NO accelerated the absorption of NO into PRDC, a similarly higher ratio of HNO 3 as a by-product of the PRDC process is obtained. Although higher NO x formation is not considered favorable due to its atmospheric deterioration characteristics, in order to achieve the purification of CO 2 simultaneously with HNO 3 production as the bottom product, higher concentrations do not adversely affect the conversion in the PRDC process.
heat exchanger was used to raise the temperature of the cool water stream from ambient temperatures to around 60 °C.

Pressurized Reactive Distillation Column (PRDC)
In order to achieve the maximum conversion rates of SO2 and NOx for the purification of CO2 and consequent formation of by-products (H2SO4 and HNO3 etc.) during the pressurized reactive distillation column (PRDC) process, the effects of the various parameters such as fuel inherent nitrogen conversion rate and the variable factors affecting PRDC process have been analyzed. The results obtained through sensitivity analysis of the involved factors were in agreement with the relevant results obtained by referenced researchers.

Effect of Conversion Rate of Fuel Inherent Nitrogen on Rate of Conversion
In the case of oxy combustion, where nitrogen in air is replaced by a pure oxygen stream, the NOx formation is sourced by fuel nitrogen only. The conversion mechanism of fuel nitrogen into NOx or elemental nitrogen (N2) depends on many factors, such as the fuel characteristics, devolatilization conditions and combustion mode on the selectivity of oxidation toward NOx formation or reduction towards elemental nitrogen [63]. One of the factors is the surrounding atmosphere of the burning fuel particles, which dictates the fate of fuel inherent nitrogen [47]. Accordingly, the formation of intermediate species such as NH3 and HCN does not quantify the ratio of NOx produced to elemental nitrogen in flue gas. Analyzing the fuel inherent nitrogen conversion into NOx and extending it further to PRDC gives an good idea of achieving the enhanced conversion efficiency at comparatively higher concentrations of NOx in the flue gas. As explained in the simulation model, the mechanism of nitrogen distribution into NOx and N2 was integrated into Aspen by varying the conversion rates based on our similar experimental work [47]. During the present simulation study, various ratios of NO to elemental N2 were assigned to quantify the subsequent conversion of NO into HNO3 in the reactive distillation column. The results, as illustrated in Figure 6, show that through enhancing the Fuel-N conversion rate to NO accelerated the absorption of NO into PRDC, a similarly higher ratio of HNO3 as a by-product of the PRDC process is obtained. Although higher NOx formation is not considered favorable due to its atmospheric deterioration characteristics, in order to achieve the purification of CO2 simultaneously with HNO3 production as the bottom product, higher concentrations do not adversely affect the conversion in the PRDC process.

Effect of Liquid/Gaseous Feed Ratio on Rate of Conversion
Large scale usage of water in the industries has created shortages of drinking water. Based on this fact, the moisture content leaving the boiler with flue gas after condensing and flashing out from the gas stream was used as a liquid feed for the present simulation study. Keeping in view the optimum level of liquid required to achieve maximum reactive distillation, a series of liquid feed to gaseous feed ratios was established, and its effects on NO and SO 2 reduction were observed, as shown in Figure 7. From Figure 7, the mole fraction of NO and SO 2 are shown to decline with increases in the ratio of liquid to gaseous feed rate. Although the mole fractions of NO and SO 2 are decreasing with the liquid to gaseous feed ratio, the decrease in the mole fraction of SO 2 is more prominent, showing a linear decrease, whereas NO fraction decreases sharply in the lower liquid to gas feed ratio due to a faster absorption reaction compared to the corresponding higher ratios.
the gas stream was used as a liquid feed for the present simulation study. Keeping in view the optimum level of liquid required to achieve maximum reactive distillation, a series of liquid feed to gaseous feed ratios was established, and its effects on NO and SO2 reduction were observed, as shown in Figure 7. From Figure 7, the mole fraction of NO and SO2 are shown to decline with increases in the ratio of liquid to gaseous feed rate. Although the mole fractions of NO and SO2 are decreasing with the liquid to gaseous feed ratio, the decrease in the mole fraction of SO2 is more prominent, showing a linear decrease, whereas NO fraction decreases sharply in the lower liquid to gas feed ratio due to a faster absorption reaction compared to the corresponding higher ratios. The maximum removal efficiencies of NO and SO2 have been achieved at higher liquid to gaseous feed ratios due to the availability of more moisture in the liquid phase, as shown in Figure  8. A higher moisture content gives rise to more surface area for absorption of the acidic gases; however, on an industrial scale, limiting the level of moisture required for maximum reactive distillation is more economical and favorable, as keeping the liquid to gaseous feed ratio at its optimum level is necessary to achieve maximum CO2 purification. On the other hand, through focusing on the bottom (liquid) products, satisfactory results validating the conversion of NO and SO2 into their respective acidic forms were achieved. The mole fractions of H2SO4 and HNO3 in the bottom products were plotted with an increasing trend of liquid to gaseous feed ratio, as displayed in Figure 9, in which a linear decline in the molar concentrations The maximum removal efficiencies of NO and SO 2 have been achieved at higher liquid to gaseous feed ratios due to the availability of more moisture in the liquid phase, as shown in Figure 8. A higher moisture content gives rise to more surface area for absorption of the acidic gases; however, on an industrial scale, limiting the level of moisture required for maximum reactive distillation is more economical and favorable, as keeping the liquid to gaseous feed ratio at its optimum level is necessary to achieve maximum CO 2 purification. gaseous feed ratios was established, and its effects on NO and SO2 reduction were observed, as shown in Figure 7. From Figure 7, the mole fraction of NO and SO2 are shown to decline with increases in the ratio of liquid to gaseous feed rate. Although the mole fractions of NO and SO2 are decreasing with the liquid to gaseous feed ratio, the decrease in the mole fraction of SO2 is more prominent, showing a linear decrease, whereas NO fraction decreases sharply in the lower liquid to gas feed ratio due to a faster absorption reaction compared to the corresponding higher ratios. The maximum removal efficiencies of NO and SO2 have been achieved at higher liquid to gaseous feed ratios due to the availability of more moisture in the liquid phase, as shown in Figure  8. A higher moisture content gives rise to more surface area for absorption of the acidic gases; however, on an industrial scale, limiting the level of moisture required for maximum reactive distillation is more economical and favorable, as keeping the liquid to gaseous feed ratio at its optimum level is necessary to achieve maximum CO2 purification. On the other hand, through focusing on the bottom (liquid) products, satisfactory results validating the conversion of NO and SO2 into their respective acidic forms were achieved. The mole fractions of H2SO4 and HNO3 in the bottom products were plotted with an increasing trend of liquid to gaseous feed ratio, as displayed in Figure 9, in which a linear decline in the molar concentrations On the other hand, through focusing on the bottom (liquid) products, satisfactory results validating the conversion of NO and SO 2 into their respective acidic forms were achieved. The mole fractions of H 2 SO 4 and HNO 3 in the bottom products were plotted with an increasing trend of liquid to gaseous feed ratio, as displayed in Figure 9, in which a linear decline in the molar concentrations of HNO 3 and H 2 SO 4 occurs, which, by contrast, is a direct indication of abatement of NO x and SO 2 from the flue gas stream, thus allowing CO 2 -enriched stream to be purified.
Energies 2019, 12,1949 15 of 21 of HNO3 and H2SO4 occurs, which, by contrast, is a direct indication of abatement of NOx and SO2 from the flue gas stream, thus allowing CO2-enriched stream to be purified.

Effect of Column Holdup on Rate of Conversion
Liquid holdup is one of the most important hydrodynamic parameters in characterizing the vapor/liquid flow configuration in the structured distillation columns [64]. The column performance is influenced by holding up a certain percentage of the total feed streams by keeping the recycle ratio, liquid to gaseous feed ratio, column pressure and temperature at constant levels. Holdup is the ratio of feed volume kept in the bed compared to the total volume of column occupied by the packing material. It is influenced by the feed flow rate, viscosity and density of the working medium. The effect of holdup on the column performance could be positive or negative. In the present study, liquid feed holdup showed effects on the column performance in terms of reduction of NOx and SO2. The vapor holdup did not show any effect on the column performance. The obtained results of liquid feed holdup show the effects of increasing rate of holdup on NOx and SO2 removal efficiency from the flue gas. Figure 10 illustrates the holdup rate profile on variations of the mole fraction values of NO and SO2, which are declining with an increasing holdup ratio (%). NO reduction is more prominent at a lower rate of holdup, whereas SO2 saw a linear decline along the holdup ratio. It may be concluded that the conversion of SO2 to H2SO4 could be enhanced with gradual holdup increments. The sudden change in the mole fraction of NO and SO2 at lower holdup rates was observed by Mujtaba et al., 1998 [65], where a relative easy distillation was observed at the start of the batch process at lower holdup ratios.

Effect of Column Holdup on Rate of Conversion
Liquid holdup is one of the most important hydrodynamic parameters in characterizing the vapor/liquid flow configuration in the structured distillation columns [64]. The column performance is influenced by holding up a certain percentage of the total feed streams by keeping the recycle ratio, liquid to gaseous feed ratio, column pressure and temperature at constant levels. Holdup is the ratio of feed volume kept in the bed compared to the total volume of column occupied by the packing material. It is influenced by the feed flow rate, viscosity and density of the working medium. The effect of holdup on the column performance could be positive or negative. In the present study, liquid feed holdup showed effects on the column performance in terms of reduction of NO x and SO 2 . The vapor holdup did not show any effect on the column performance. The obtained results of liquid feed holdup show the effects of increasing rate of holdup on NO x and SO 2 removal efficiency from the flue gas. Figure 10 illustrates the holdup rate profile on variations of the mole fraction values of NO and SO 2 , which are declining with an increasing holdup ratio (%). NO reduction is more prominent at a lower rate of holdup, whereas SO 2 saw a linear decline along the holdup ratio. It may be concluded that the conversion of SO 2 to H 2 SO 4 could be enhanced with gradual holdup increments. The sudden change in the mole fraction of NO and SO 2 at lower holdup rates was observed by Mujtaba et al., 1998 [65], where a relative easy distillation was observed at the start of the batch process at lower holdup ratios. of HNO3 and H2SO4 occurs, which, by contrast, is a direct indication of abatement of NOx and SO2 from the flue gas stream, thus allowing CO2-enriched stream to be purified.  Figure 9. Effect of liquid/gas feed ratio on liquid product composition.

Effect of Column Holdup on Rate of Conversion
Liquid holdup is one of the most important hydrodynamic parameters in characterizing the vapor/liquid flow configuration in the structured distillation columns [64]. The column performance is influenced by holding up a certain percentage of the total feed streams by keeping the recycle ratio, liquid to gaseous feed ratio, column pressure and temperature at constant levels. Holdup is the ratio of feed volume kept in the bed compared to the total volume of column occupied by the packing material. It is influenced by the feed flow rate, viscosity and density of the working medium. The effect of holdup on the column performance could be positive or negative. In the present study, liquid feed holdup showed effects on the column performance in terms of reduction of NOx and SO2. The vapor holdup did not show any effect on the column performance. The obtained results of liquid feed holdup show the effects of increasing rate of holdup on NOx and SO2 removal efficiency from the flue gas. Figure 10 illustrates the holdup rate profile on variations of the mole fraction values of NO and SO2, which are declining with an increasing holdup ratio (%). NO reduction is more prominent at a lower rate of holdup, whereas SO2 saw a linear decline along the holdup ratio. It may be concluded that the conversion of SO2 to H2SO4 could be enhanced with gradual holdup increments. The sudden change in the mole fraction of NO and SO2 at lower holdup rates was observed by Mujtaba et al., 1998 [65], where a relative easy distillation was observed at the start of the batch process at lower holdup ratios.  The column performance in terms of NO x and SO 2 conversion efficiencies (%) is presented in Figure 11, where NO at a much higher level of removal efficiency is affected a little as compared to SO 2 . The rate of liquid feed holdup is seen to be more favorable to the conversion of NO to HNO 3 at lower rates, till a certain holdup rate i.e. 20% occurs, at which time there is no prominent effect on NO conversion. SO 2 conversion to H 2 SO 4 is enhanced at an average rate of 0.02% on every 6% increment of holdup, attaining a maximum efficiency of 75% at 40% holdup. SO 2 removal efficiency is enhanced at a faster rate than NO under the similar trend of liquid feed holdup ratio. Meanwhile, there is no visible negative effect on the NO 2 removal performance by holdup ratio. Iloeje et al. 2015 [66] observed there inverse relationship between NO x concentration in flue gas stream and holdup volume per stage of a single column absorber. The present simulation result shows good agreement with the modeling studies conducted by Iloeje et al., in such a way that lower NO x concentrations are observed at higher holdup rates. In contrast to NO x behavior, the SO x concentration shows the opposite trend. The reason for this behavior depends on the selectivity of the conversion rate of NO x over SO 2 , such as NO after oxidation to NO 2 acting as the oxidizing agent for the oxidation of SO 2 to form SO 3 , as shown in reaction (4) and reaction (9). Therefore, the NO x concentration is reduced first among the holdup rates, which results in an increase of SO 2 concentration (as shown in Figure 10); however, the conversion rate of SO 2 to H 2 SO 4 becomes constant at 85% after 20% of liquid holdup is attained ( Figure 11).
Energies 2019, 12, 1949 16 of 21 The column performance in terms of NOx and SO2 conversion efficiencies (%) is presented in Figure 11, where NO at a much higher level of removal efficiency is affected a little as compared to SO2. The rate of liquid feed holdup is seen to be more favorable to the conversion of NO to HNO3 at lower rates, till a certain holdup rate i.e. 20% occurs, at which time there is no prominent effect on NO conversion. SO2 conversion to H2SO4 is enhanced at an average rate of 0.02% on every 6% increment of holdup, attaining a maximum efficiency of 75% at 40% holdup. SO2 removal efficiency is enhanced at a faster rate than NO under the similar trend of liquid feed holdup ratio. Meanwhile, there is no visible negative effect on the NO2 removal performance by holdup ratio. Iloeje et al. 2015 [66] observed there inverse relationship between NOx concentration in flue gas stream and holdup volume per stage of a single column absorber. The present simulation result shows good agreement with the modeling studies conducted by Iloeje et al., in such a way that lower NOx concentrations are observed at higher holdup rates. In contrast to NOx behavior, the SOx concentration shows the opposite trend. The reason for this behavior depends on the selectivity of the conversion rate of NOx over SO2, such as NO after oxidation to NO2 acting as the oxidizing agent for the oxidation of SO2 to form SO3, as shown in reaction (4) and reaction (9). Therefore, the NOx concentration is reduced first among the holdup rates, which results in an increase of SO2 concentration (as shown in Figure 10); however, the conversion rate of SO2 to H2SO4 becomes constant at 85% after 20% of liquid holdup is attained ( Figure 11). It can be concluded that based on the reaction mechanism involved during the PRDC process, a mild liquid holdup ratio on one side may enhance the conversion rate of NOx into HNO3, while on the other hand, it may lower the conversion rate of SO2 into H2SO4. In order to balance the formation of both by-products, a value of 10% for holdup was taken for the present simulation study.

Effect of Recycling Rate of Column's Bottom on Rate of Conversion
The bottom recycling in the pressurized reactive distillation column (PRDC) influences the performance of the PRDC process by altering the composition of liquid stream in the reactive distillation column. In the present scenario, the rate of bottom recycling tends to raise the mole fraction of NOx and SO2 in the reactive column's gaseous (top) product, as illustrated in Figure 12. It shows the adverse impact of bottom recycling on CO2 purification, as it will reduce the conversion of effluents, i.e., NOx and SO2 to HNO3 and H2SO4, and consequently, will enhance the mole fraction of these effluents in the gaseous product stream. It can be concluded that based on the reaction mechanism involved during the PRDC process, a mild liquid holdup ratio on one side may enhance the conversion rate of NO x into HNO 3 , while on the other hand, it may lower the conversion rate of SO 2 into H 2 SO 4 . In order to balance the formation of both by-products, a value of 10% for holdup was taken for the present simulation study.

Effect of Recycling Rate of Column's Bottom on Rate of Conversion
The bottom recycling in the pressurized reactive distillation column (PRDC) influences the performance of the PRDC process by altering the composition of liquid stream in the reactive distillation column. In the present scenario, the rate of bottom recycling tends to raise the mole fraction of NO x and SO 2 in the reactive column's gaseous (top) product, as illustrated in Figure 12. It shows the adverse impact of bottom recycling on CO 2 purification, as it will reduce the conversion of effluents, i.e., NO x and SO 2 to HNO 3 and H 2 SO 4 , and consequently, will enhance the mole fraction of these effluents in the gaseous product stream. Modeling work done by Iloeje et al. 2015 [66] supports the present trend of increased NOx concentrations along with enhancing the bottom recycle fraction.
From Figure 13, it is inferred that even though SO2 conversion is at a lower level than NO, its variation in rate of conversion is more influenced by enhancing the bottom recycle ratio than NO. The SO2 conversion rate is seen to reduce at an average rate of 1% on enhancing the recycling ratio of 0.1, whereas the NO conversion rate is reduced at a comparatively lower pace that is a 0.333% average decrease on enhancing the recycling ratio of 0.1. This further elaborates the reaction scheme implemented in the PRDC process, where the rate of H2SO4 yield was shown to be 1/3 times less than the corresponding yield of HNO3 by keeping bottom recycling rate at 0.2. At this rate of bottom recycling, around 99% of NO is converted to HNO3 and 86% of SO2 is converted to H2SO4. It is worth noting that NO2 in the aforementioned sensitivity studies has not shown any visible changes in its concentration; such behavior illustrates that the studied parameters did not have a prominent effect on the NO2 characteristics.

Conclusions
A simulation-based study of oxy biomass combustion using maize stalk as the primary fuel for the boiler with flue gas recirculation was conducted with the main objectives of determining the thermal performance of the boiler and the optimization of CO2-enriched flue gas cleaning up process Modeling work done by Iloeje et al. 2015 [66] supports the present trend of increased NO x concentrations along with enhancing the bottom recycle fraction.
From Figure 13, it is inferred that even though SO 2 conversion is at a lower level than NO, its variation in rate of conversion is more influenced by enhancing the bottom recycle ratio than NO. The SO 2 conversion rate is seen to reduce at an average rate of 1% on enhancing the recycling ratio of 0.1, whereas the NO conversion rate is reduced at a comparatively lower pace that is a 0.333% average decrease on enhancing the recycling ratio of 0.1. Modeling work done by Iloeje et al. 2015 [66] supports the present trend of increased NOx concentrations along with enhancing the bottom recycle fraction.
From Figure 13, it is inferred that even though SO2 conversion is at a lower level than NO, its variation in rate of conversion is more influenced by enhancing the bottom recycle ratio than NO. The SO2 conversion rate is seen to reduce at an average rate of 1% on enhancing the recycling ratio of 0.1, whereas the NO conversion rate is reduced at a comparatively lower pace that is a 0.333% average decrease on enhancing the recycling ratio of 0.1. This further elaborates the reaction scheme implemented in the PRDC process, where the rate of H2SO4 yield was shown to be 1/3 times less than the corresponding yield of HNO3 by keeping bottom recycling rate at 0.2. At this rate of bottom recycling, around 99% of NO is converted to HNO3 and 86% of SO2 is converted to H2SO4. It is worth noting that NO2 in the aforementioned sensitivity studies has not shown any visible changes in its concentration; such behavior illustrates that the studied parameters did not have a prominent effect on the NO2 characteristics.

Conclusions
A simulation-based study of oxy biomass combustion using maize stalk as the primary fuel for the boiler with flue gas recirculation was conducted with the main objectives of determining the thermal performance of the boiler and the optimization of CO2-enriched flue gas cleaning up process for CCS. The unique characteristics of the proposed system included fuel inherent nitrogen This further elaborates the reaction scheme implemented in the PRDC process, where the rate of H 2 SO 4 yield was shown to be 1/3 times less than the corresponding yield of HNO 3 by keeping bottom recycling rate at 0.2. At this rate of bottom recycling, around 99% of NO is converted to HNO 3 and 86% of SO 2 is converted to H 2 SO 4 . It is worth noting that NO 2 in the aforementioned sensitivity studies has not shown any visible changes in its concentration; such behavior illustrates that the studied parameters did not have a prominent effect on the NO 2 characteristics.

Conclusions
A simulation-based study of oxy biomass combustion using maize stalk as the primary fuel for the boiler with flue gas recirculation was conducted with the main objectives of determining the thermal performance of the boiler and the optimization of CO 2 -enriched flue gas cleaning up process for CCS. The unique characteristics of the proposed system included fuel inherent nitrogen conversion to NO x and the use of flushed out condensate water as the liquid feed for the CCS system, which can be considered as the novelty of the prescribed system. An indirect method of determining the thermal efficiency was adopted which resulted in 88.91% efficiency of the proposed 20 kW th biomass boiler. On further pursuance, NO concentration in flue gas enriched with CO 2 was treated through pressurized the reactive distillation column (PRDC) process, where up to 98.05% NO and 87.42% SO 2 were converted into valuable by-products including HNO 3 and H 2 SO 4 , respectively Sensitivity analysis through varying the factors of liquid to gas ratio, holdup ratio and bottom recycle ratio were performed. Around four kilograms per hour of CO 2 with 94.13% purity was achieved by optimizing the simulation system with variable parameters. The total gas stream leaving the reactive distillation column comprised 3.47% O 2 , 1.57% N 2 and 0.83% H 2 O along with 94.13% CO 2 . After separation of moisture through cooling and condensing the mix stream, the pure CO 2 can be available to transport for storage under saline aquifers or for use in enhanced oil recovery operations.
However, certain economic factors, such as retrofitting of the existing boilers to make them suitable for oxy combustion and the cryogenic production of pure oxygen through the air separation technique are being considered as the main issues for industrial applicability. Oxy combustion and flue gas processing techniques are among the possible mechanisms of cleaner production. The investigated simulation model based on the oxy biomass-CCS technique can play a potential role, as negative emission technology and can boost the likelihood of achieving low concentration targets [67]. Based on this simulation system, the arrangements of a proposed laboratory scale boiler, i.e., setting up CO 2 -enriched flue gas compressor and packed bed pressurized reactive distillation column, are in progress for future experimentations. The proposed laboratory scale system will not only validate the applicability of the oxy biomass-CCS technique, but will serve as a basis for attracting investment projects for plant scale operation.
Author Contributions: I.A.S. designed and carried out the simulation. I.A.S. and X.G. participated in the analysis of data. I.A.S. wrote the manuscript. X.G. and J.W. revised the manuscript. All authors have read and approved the manuscript.