Design of zeolite boiler in thermochemical energy storage and transport system utilizing unused heat from sugar mill

In Tanegashima, an isolated island in Japan, a 1500 ton/day sugar mill produces raw sugar and residual bagasse simultaneously. The bagasse is burned to generate steam that drives power turbines, but the bagasse boiler burns more bagasse than that is required. Accordingly, the process has a high temperature flue gas and a sizeable amount of unused heat at around 200 °C is exhausted from the sugar mill. Conversely, many other factories on the island burn imported oil in package boilers to generate process steam at temperatures up to 120 °C, all year around. To resolve this spatial and seasonal mismatch, we employ a thermochemical energy storage and transport system that uses a zeolite steam adsorption and desorption cycle. We introduce a basic design of a heat discharging device (a “zeolite boiler”) that features a moving bed with an indirect heat exchanger. The zeolite boiler's performance is predicted by an in house designed simulation model that includes empirical equilibrium adsorption kinetics and a heat transfer model of the exotherm for zeolite 13X adsorbing water vapor. The effects of the mass flow rates of zeolite and injected steam are computed and suggest that optimal conditions to produce 800 kg/h of steam could decrease fuel consumption by the existing oil-fired package boiler of 23.2%. We then design an advanced zeolite boiler to recover the zeolite's sensible heat, which would otherwise be wasted. Employing this economizing process yielded a total decrease in fuel consumption of 29.6%.