Upgrading of a traditional sugar cane mill to a modern mill and assessing the potential of energy saving during steady state and transient conditions—part II: models for a modified cogeneration unit

Abstract

It is known that there is a significant amount of thermal energy used for the sugar cane industry for the purpose of power production and for use in the sugar or ethanol processing in cane sugar industries. Likewise, it is understood that there are substantial amounts of waste heat that is not being recovered, in particular for traditional sugar mills. Regardless of this, energy conservation is given less consideration as compared to operational convenience due to the fact that sugar mills are self-sufficient in energy (heat and power). The identification of such potential heat loss areas (especially during transient conditions) suggests the sugar mills play a vital role in energy saving. In this study, a modified setup of the base case plant considered in part I of this paper is assessed for its energy potential and possible major heat losses during steady state and transient conditions where 2-h stoppage of the mill presses are considered to occur. For the modified setup, there are two major scenarios considered having two sub-scenarios each. The result of the assessment showed that the steady state assumption scenario of the modified plant (where bagasse drying is not considered) indicated a 20 % reduction in the losses considered which resulted in a 57 % power generation increase as compared to the steady state model of the base case plant. It is also possible to save excess bagasse by drying the bagasse for later use during unexpected stoppage. The carbon dioxide emission (amounting 29 t/day in case 2a of this study) that occurs during the use of fuel oil during such stoppages will thus be avoided. The simple economic analysis showed that it is only in case 2a where fuel oil cost is included in the operation cost that resulted in a negative NPV. Since the rest of the scenarios use bagasse as a fuel which is free, the NPV for all was positive. For the electricity price of 0.04 US $/kWh and discount rate of 15 %, the minimum payback period attained is about 3 years (case 1b) where the bagasse moisture content is 30 % whereas the maximum payback period is 6 years (case 1a) where there is no bagasse drying considered.