Optimum diameter of a circulating fluidised bed combustor with negative wall heat flux

Abstract

The focus of the world is the reduction of greenhouse gases like carbon dioxide, which contribute to the global warming currently experienced. Since most of the carbon dioxide emitted into the atmosphere is from fossil fuel combustion, alternative energy source were developed and others are currently under study to see if they can be good alternatives. One of these alternative sources of energy is the combustion of wood instead of coal. Wood has an advantage for being a neutral carbon fuel source, and that currently installed infrastructure used to combust coal can be retrofitted to combust wood or a mixture of wood and coal in an attempt to reduce the carbon dioxide emissions. In this study the effect a change in diameter of a combustor has on irreversibilities in a 7 m circulating fluidised bed combustor with a negative wall heat flux, firing a mixture of air and solid pitch pine wood, was investigated. An analytical expression was derived that predicts the entropy generation rate, thereby the irreversibilities, of a combustor with a negative wall flux as a function of the combustor diameter. A numerical code was used to compute the molar fractions of combustion products needed as an input in the analytical expression. In the numerical code the combustion process was modelled by a non-premixed combustion model, with a P1 radiation model. Simulations were run using a steady Reynolds averaged Navier Stokes model. The analytical expression predicted the optimum diameter that results in minimum irreversibilities to be 0.32 m for a rich mixture with an Air-Fuel mass ratio of 6, and an incoming air temperature of 400 K.