Optimization of utilization of Eucalyptus Grandis tree species in tea factories in Kenya

Abstract

Recent upward increase in global fuel oil prices has led a number of Tea factories to convert their boilers from fuel oil to wood fired. The conversion to wood fuel may also be necessitated by the need to reduce carbon foot print of the factories. The combustion characteristics of fuel oil are well documented and consistent within certain limits. However, the wood combustion characteristics vary widely depending on the species and moisture content. For a specific species, if the wood is fed in the boiler at moisture content higher than the equilibrium moisture content, there is; poor combustion, wastage, poor boiler efficiency and poor steam quality.

To optimize wood utilization in the Tea factories, information concerning the used tree species, the air drying procedures and the wood equilibrium moisture content ( in the prevailing environment) are required. The information is necessary for prior planning of the tree harvesting, air drying, estimation of the boiler efficiency, determination of the steam - wood ratio and hence the required quality of wood for specified steam requirements. This would lead to efficient use of the wood. In this study, the current harvesting and air drying process of selected tea factories were examined. In all those surveyed, the harvesting programme emphasized mainly to ensure availability of the wood but not to have equilibrium moisture content nor high calorific value at the time of feeding the wood into the boiler.

The calorific value and moisture content of air- drying wood from Eucalyptus grandis tree species were determined and the equilibrium moisture content and the . corresponding calorific value in the prevailing ambient meteorological condition were also obtained. The findings shows that the stem drying duration was 135 days of air drying and attained an equilibrium moisture content of 26.71 % with calorific value of 19. 70MJ/kg. The bark attained the equilibrium moisture content of25.45 % after 90 days of air drying with a calorific value of 19.10 MJ/kg.

The branch attained equilibrium moisture content of 24.77 % after 45 days of air drying with calorific value of 18.01 MJ/kg. Overall, the equilibrium moisture content was found to be on average 25.64 % and the corresponding calorific value was 18.94 MJ/kg. The boiler efficiency was also estimated at different value of moisture content. It had a minimum of 42.28 % at moisture content of 128.70 % and a maximum of 77.20 % at equilibrium moisture content of 26.71 %. The steam - wood ratio at equilibrium moisture content was estimated as 7.63 kg of steam per kg of wood. This information is crucial in assisting factories to plan their harvesting and air drying procedures to ensure that wood is fed into the boiler at Equilibrium moisture content and hence maximum calorific value. The quantity of wood required for a certain steam requirements can also be estimated and the boiler efficiency monitored for optimum utilization of the wood.