OPTIMIZATION OF COMBUSTION OF PALM KERNEL SHELL IN A GRATE FURNACE FOR SUPERHEATED STEAM GENERATION

Abstract

This study aimed at optimizing the combustion of palm kernel shell (PKS) in a grate furnace for superheated steam generation. PKS were collected from a local palm oil processing mill in Ogbomoso, Nigeria. D-optimal design under the Mixture Methodology was employed to optimize the mixed ratio of three selected additives, Aluminum oxide (Al2O3), Magnesium oxide (MgO) and Calcium oxide (CaO) and PKS. Ash yield of the mixture was analyzed using ASTM standard method. Higher Heating Value (HHV) of the mixtures was determined using GallenKamp Bomb Calorimeter. The proximate and ultimate analyses of the optimum mixture of PKS and selected additives were determined using ASTM 3174-76 method. The mineral elements of the ash yield were characterized using Atomic Absorption Spectrophotometer (AAS). The influence of mixed ratio of additives and PKS on the yield of ash and HHV were analyzed statistically using Design-Expert Software version 6.08. The results obtained from proximate and ultimate analyses were used to determine oxygen needed for combustion of additive fortified PKS. Boiler components were sized following backward calculation approach and a computer codes was developed using C- Sharp programming language. Evaluation of the developed furnace based on optimized PKS additive mixture and ratio of primary to secondary air setting for maximum combustion efficiency and minimum emission were carried out. Temperature and emission were measured using five K-type thermocouples and air quality meter respectively, which were placed at different locations in the grate furnace (above the grate, core of the furnace, steam collection header, above steam header and exhaust port). Evaluation of the furnace economic cost of fuels in terms of power produced was also done. Results of the optimum condition suggested by the model for the process variable such as Al2O3, MgO, CaO, PKS and particle size was 2.5 %, 0.0 %, 5.0 %, 92.5 % and 5.50 mm respectively with lowest ash yield (0.31%) and HHV (20.64 kJ/g). The R square of the model equation for ash yield and HHV are 0.7951 and 0.7344 respectively. The computer codes developed with Graphics User Interphase (GUI) using C-Sharp automatically sizes the furnace components with two dimensional working drawings. In order to generate superheated steam for the production of 5 kW of electricity, 5.5 kW turbine, 3.6 m superheater, 3.2 m riser, furnace of 1.432 m height and 0.45 m3 volume were required. The primary to secondary air supplied of 40:60 recorded highest combustion efficiency (64.5%) and minimum level of CO (285 ppm) at 6% O2. For a fuel feed rate of 17.3 kg/hr. and primary to secondary air ratio 40:60, maximum temperature, steam pressure and mass flow rate recorded for 5 kW power rating were 220 o C, 0.38 MPa, 0.002 kg/s respectively. The power generated and estimated cost based on boiler fired with fuel oil and PKS additives mixture indicated that PKS mixture can produce 42 % of power of what fuel oil produces given the same amount of fuel.