Thermodynamic analysis of some volatile species at elevated pressure and temperature during combustion of Pakistani Lakhra and Thar lignite chars

Volatile species (e.g. Na-,K-,Cl-, and S) are released during thermal conversion of coal causing eventual problems such as fouling, slagging and corrosion, especially in gas turbines, on super heaters and in the colder part of heat recovery systems. Thermodynamic study of the released inorganic compounds is supportive before eventually launching experimental work. Therefore, thermodynamic equilibrium was calculated using FactSage 5.2, simulating combustion conditions at elevated pressure (5, 10 and 15 bar) and elevated temperature (1000, 1400 and 1600°C). Their releasing order of various volatile species was estimated by plotting mole fraction of each species at temperature 1000°C and pressure 1 bar. These calculations predict that SO2 is the most stable species for combustion. At all pressure the lowest amount of NaCl and KCl volatilized for LKH and THR lignite char. Evolution of NaCl, KCl and HCl depends on the amount of Cl present in both lignite chars.


INTRODUCTION
Pakistan is abundant natural coal reserves in Sindh, Punjab, Baluchistan, Northwest frontier province and has 33.0 trillion tons world third largest resources in southeastern part of the country that is, Thar (Final USAID-Pakistan circular Debt Report-2013).Generally, the electricity generation from coal is economical and cost competitive.Worldwide, approximately 41% energy is producing through coal but Pakistan despite of huge coal resources only 0.1% of its power is produces through coal and severe energy crises is raising every year (Usmani, 2014) but maximum efficiency may be achieved by reforms the existing and installation of additional power plants (The News, 2013) Pakistani Lakhra and Thar lignite's due to low ash, high moisture, volatility and Sulphur need some special techniques for their utilization in combustion, gasification and power generation.The efficiency of power generation may reduce by high moisture moreover the presence of ash is responsible for slugging and fouling problem in conventional boiler.Unfortunately during combustion at high temperature and pressure, highly volatile species (Na, K, Cl and S) released and penetrate in hot gas filtration unit and gas turbine blades.The power plant efficiency may be achieved by hot gas cleaning (Blacing et al., 2011) Therefore the comprehensive knowledge of released volatile species during combustion at elevated temperature and pressure is required prior to carry out experiment by utilization of Pakistani lignite chars.
Thermodynamic equilibrium calculations is the basis for obtaining such information and its method is capable of identifying species that are stable only at high temperature especially those employed in commercial combustors.The results are useful guidance for predicting the trends occurring during Combustion.
Therefore the present study simulates the combustion at high pressure (5 to 15) bar at 1000°C and Temperature (1000 to 1600°C) at 1 bar pressure and also predicts the release of volatile species for Pakistani Lakhra and Thar lignite chars using equilibrium calculations by Fact sage 5.2.

MATERIALS AND METHODS
Two kinds of Pakistani THR and LKH lignite from Fuel Research Center, PCSIR, Karachi, Pakistan, are mainly used.Their proximate and ultimate analysis is given in Table 1.Char was made by following procedure.Firstly, the 60 or 76.06 g by weight of raw THR or LKH coal was loaded twice with some ceramic beads in fixed bed inserted in an electric furnace with sufficient heat and it underwent a carbonization in nitrogen atmosphere under temperature 750°C and ambient pressure with heating rate 15°C/min and flow rate 664.675L/h for one hour.Then these chars removed from ceramic beads were respectively pulverized with particle size 0-0.154mm (< 100 mesh).The composition of the above chars and ashes are listed respectively in Table 1 and 2. The chemical composition of Pakistani chars is included in Table 3.

Calculation procedure
Thermodynamic equilibrium calculations were performed using Fact sage 5.2.The initial model composition is based on the data of elementary and chemical composition relate to 1 g of lignite char.The thirteen elements C, H, N, S, O, Cl, Al, Ca, Fe, K, Mg, Na and Si were included in the computation, as listed in Table 1 and 3.The results were calculated by plotting mole fraction of each species vs Pressure (5 to 15 bar) at 1000°C and Temperature (1000 to 1600°C) at 1 bar pressure for combustion process as shown in Figure 1 to 10 and calculated mole fractions are listed in Table 4  and 5.

Definition of conditions
To simulate the condition of combustion of LKH lignite Char excess air, O2=1.6733 g or (167.33%) and N2= 5.5085 g or (550.85%) and for THR lignite char excess air, O2=1.5411 g or (154.11%) and N2= 5.0733 g or (507.33%) was added to system.

Release of volatile species during high pressure combustion
The computed release of SO 2 , H 2 S, NaCl, KCl and HCl under pressurized combustion is listed in Table 4. On the basis of the calculated mole fraction the release of SO 2 gradually decreases with increasing the pressure from 5 to 15 bar, as shown in Figure 1, but higher than H 2 S due to higher combustion product while release of H 2 S gradually increases with increasing pressure during combustion, as indicated in Figure 2.Such changes are mainly as a consequence of the Le Chatelier-van't Hoff Law.Therefore, lower release of SO 2 to the gas phase occurs for THR Char and even a sharp decrease was observed at 15 bar.The release of H 2 S is showing little higher at 15 bar for LkH char.
The results of NaCl and KCl are depicted in Figures 3  and 4. The release of NaCl, KCl strongly decreases with increasing pressure from 5 to 15 bar for LKH and THR char.The sharp decrease of NaCl, KCl was observed in LKH char at 15 bar.
The release of HCl gradually increases with increasing pressure 5 to 15 bar for LKH and THR char as obviously indicated in Figure 5.The sharp increase in mole fraction of THR char was observed at 15 bar.The decrease and increase volatilization in LKH and THR Char may be lower content of Cl in LKH Char and little high content in THR Char but the system is in equilibrium and no effect on reaction will takes place because no of moles of the gas are same on each side of chemical equation of NaCl, KCl and HCl (Lechatelier Principle).8.0x10 -9 1.0x10 -8 1.2x10 -8 1.4x10 -8 1.6x10 (1) (2) (3)

Release of volatile species during high temperature combustion
The predicted results concerning the release of SO are listed in Table 5. Figure 6, consider the mole fraction as the base for release of SO 2 for two samples: It gradually increases with increasing temperature 1400 to1600°C for LKH and THR char may be the exothermic formation of SO 2 is shifting equilibrium right side for complete formation of SO 2 reaction so volatilization of SO 2 is observed higher at 1400°C (Lechatelier Principle).The predicted results of H 2 S are depicted in Figure 7.It strongly decreases with increasing temperature for both LkH and THR char may be decrease in volatilization of H 2 S is shifting the equilibrium to the left therefore strong decreased is observed for LKH char at 1600°C (Lechatelier Principle).
The release of NaCl and KCl is increasing with increases of temperature as clearly indicated in Figures 8  and 9.The high release of NaCl and KCl is observed at 1600°C for THR char and this may be due to the fact that in thermodynamic equilibrium system the influence of high temperature increases the partial pressure of the species so volatilization increases (Broström, 2010).The predicted results of HCl are shown in Figure 10.The release of HCl decreases with increasing temperature and this strong decrease HCl is observed for THR char at 1600°C may be because of the equilibrium shifts to the left, decreases the volatilization of HCl.(Chemical Equilibria)

Comparison of predicted combustion results at elevated pressure with high temperature
Thermodynamic equilibrium calculations were used to compare the predicted results of combustion both elevated pressure and temperature.The comparison of predicted combustion results at elevated pressure with high temperature as shown in Figure 11  temperature and pressure are predicted as follows: 1. Releasing order of species at elevated temperature: LKH char (SO 2 >H 2 S>NaCl>KCl<HCl) THC Char (SO 2 >H 2 S>NaCl>KCl<HCl) 2. Releasing order of species at elevated pressure: LKH Char (SO 2 >H 2 S>NaCl>KCl<HCl) THC Char (SO 2 >H 2 S>NaCl>KCl<HCl) On the basis of releasing order, the SO 2 release is higher at elevated temperature than elevated pressure.The release of SO 2 is highest for LKH char at 1400°C and lowest for THC char at 15 bar, the order of release at elevated temperature and elevated pressure is LKH Char >THC Char.For H 2 S the higher and lower release is observed for LKH char at 15 and 5 bar.The order of release is LKH char >THC char at elevated pressure and elevated temperature is THC char> LKH char.The NaCl release is higher for THR Char at 1600°C and lower for LKH char at 1000°C than elevated pressure.The order of release at elevated temperature and elevated pressure is THC char > LKH char.
The KCl release is higher for THR char at 1600°C and lower for LKH char at 1000°C than elevated pressure, the order of release is THR char >LKH char.The order of release at elevated temperature and elevated pressure is THC char > LKH Char.
The HCl release is higher for THR char at 15 bar and lower at 5 bar for LKH char than elevated temperature, the order of release is THR char > LKH char.The order of release at elevated temperature is LKH char > THR char Conclusion 1.The high volatilization of SO 2 for both at elevated temperature and pressure can be predicted that SO 2 is the most stable specie for combustion.2. The volatilization of SO 2 is predicted higher for LKH char (1400°C) at elevated temperature and lowers for THC char (15bar) at elevated pressure during combustion.3. The volatilization of H 2 S is predicted higher for LKH char (15 bar) at elevated pressure and lowers for LKH Char (1600°C) at elevated temperature.4. The release order of NaCl, KCl and HCl species at elevated pressure is not similar with elevated temperature.The predicted release of Nacl, KCl is higher for LKH and THC char at elevated temperature and while volatilization of HCl for LKH and THC char is higher at elevated pressure.5.The volatilization of Nacl, KCl and HCl is dependent on Cl content present in the LKh and THR Char.

Figure 11 .
Figure 11.Comparison of predicted equilibrium calculation results of combustion at elevated pressure and temperature.

Table 1 .
Proximate and ultimate analysis of Pakistani LKH and THR lignite and char.

Table 2 .
The ash composition and fusion temperature of Pakistan chars.

Table 5 .
Calculated mole fraction at Temperature 1000 to 1600°C and pressure 1 bar for combustion.