Performance Modelling of Co-Fired Palm Kernel Shell -Pulverized Coal Blend in Steam Power Plant

: This study investigates the performance of co-firing palm kernel shell (PKS) and pulverized coal in a steam power cycle. The process simulation software Aspen Hysys was used in the simulation. The study analyzed the various emitted combustion product gases. The cycle efficiency and Power output for each fuel sample blend was carried out. This provided the basis for the performance comparison of each fuel sample. The Sample D with mix ratio 80 (coal)-20 (PKS) came out the best fuel blend having a CO 2 emission of 1844.198854 Kgmol/h, an SO 2 emission of 8.830612928 kgmol/h, a cycle efficiency of 0.41 and an estimated power of 104, 076 kW. It was concluded that coal blended with palm kernel shell at a mix ratio of 80-20 has a suitable heating value for steam generation which promotes a cleaner coal usage with a significant reduction in the emission levels of greenhouse gases.

Access to energy specifically electricity is vital for economic and social development. Coal remains an essential fuel for global energy systems and the release of toxic products of combustion into the air is the major challenge in its usage. Coal power plant are some of the largest single point source emitters; a typical 1,000 MW coal-fired power plant emits over 6 million tons of CO2 per year (IEA, 2006). Experts agree that one of the viable options which can be explored to reduce the greenhouse gas emissions is the use of renewable sources such as biomass (Alie (2004), Assadi (2009), Diaz-Somoama (2003, Budianto (2015)). Biomass co-firing entails a process of supplementing base fuel with dissimilar fuel. It is a low-cost option for efficiently and cleanly converting biomass to electricity by adding biomass as a partial substitute fuel in high efficiency coal boilers. The primary reason for co-firing coal with biomass is as a means of reducing the potential environmental impacts associated with the combustion of fossil fuels. It is important to be conversant with the types of cofiring techniques available in order to be on the right course as regards the choice of co-firing technique suited for the study. Direct co-firing is a more 'economic' and the preferred option as both fuels can be used in the boiler at the same time. Zulfiqar et al (2005) found that it is possible to reach a 10% co-firing ratio without any modification to the plant. According to Baxter (2005) in his review on opportunities for affordable renewable energy, he claims that co-firing installation cost ranges between $50/KW and $300/KW depending on the capacity of biomass. Low cost is achievable because biomass co-firing uses an existing coal infrastructure with minimal changes. As of 2011, co-firing biomass contributed 55GW in Europe and North America. This is expected to grow to 270GW in 2030, which will represent a 10% in renewable energy contribution to the total world energy consumption (Irena, 2012). Indirect co-firing systems are usually complex and expensive solutions as the biomass is converted to gaseous fuel before firing in the same boiler with coal but they reduce problems related with corrosion, fouling and slagging caused by inorganic compounds present in the secondary fuel. Parallel co-firing might not be economically feasible as separate boilers are required for each fuel. There are some studies in regards cofiring of coal and biomass materials aimed at promoting a cleaner coal usage. Boylan (1996Boylan ( ), (2009 (2018) carried out A Techno-Economic study of the reduction of greenhouse gas emissions through biomass co-firing. However, studies on cofiring are scanty in Nigeria, hence, the objective of this study was to evaluate the performance of co-firing palm kernel shell (PKS) and pulverized coal in a steam power cycle in Nigeria using the simulation software Aspen Hysys

MATERIALS AND METHODS
The seven fuel pellets produced in the study by Ighodaro et al (2020) were subjected to Rankye cycle simulation using the commercial software Aspen Hysys version 11. As shown in Figure 1, the palm kernel shell and pulverized coal is first mixed before passing through a Let-down Valve where it is depressurized before going into the Gibbs combustor where it mixes with air in the combustion process. The combusted gases enter the Heat Recovery Steam Generator (HRSG) where high quality steam is generated. The generated steam enters the turbine where work is extracted by the expanding steam. The steam passes through the condenser where heat is rejected, the condensate is then channeled to the pump to continue to cycle. The fuel, air and combusted exhaust were modelled using Peng-Robinson Equation of State. Stream was modelled as pure water using property correlations consistent with the ASME steam tables. For every state point on the steam cycle, at least three state properties need to be defined such as pressure, vapour fraction and fluid package for other state properties i.e. molar flow, molar entropy, molar enthalpy, density, temperature, mass flow e.t.c.
to be computed. The selected fluid package could either be Basis I or Basis II for either of ASME Steam model or Peng-Robinson Equation of state model respectively depending on the type of fluid.

RESULTS AND DISCUSSION
The seven fuel pellets produced in the study by Ighodaro et al. (2020) is presented in table 1. The property of each of the fuel samples used in the simulation is presented in Table 2     The accuracy of this simulation carried out on the Aspen Hysys software is validated by using analog Programming. This is because when a simulation case is set up, there are 1 of 3 possible results. The results could have Syntactic errors -case didn't run. The results could have Semantic errorscase ran but didn't give the right results. The results could be completely devoid of errors as this was the case because the analytical method using the Steady Flow Energy Equation which is valid for all the seven tested fuel samples simply affirmed the simulation results obtained.  Table 4 From table 4, fuel sample with the highest power output might not be environmentally friendly. Considering this criterion, sample D (80-20) is the best fuel sample having a power output of 104,076 kW.
Conclusion: Biomass co-firing with coal is believed to be able to enhance the domestic energy security through renewable energy utilization as well as improve the utilization of power plants The products of combustion, cycle efficiency and estimated power generation per fuel sample were key indices taken into consideration in choosing the most suitable fuel at the end of the performance simulation on Aspen Hysys. Sample D with mix ratio 80-20 came out the best fuel blend as it promotes a cleaner coal usage with a significant reduction in the emission levels of greenhouse gases.