The continuous combustion of glycerol in a fluidised bed

Abstract

It is difficult to burn a liquid fuel inside a fluidised bed. For the first time, liquid glycerol has been burned, when continuously injected into the bottom of an electrically heated bed of alumina particles (sieved to 355-425 μm), fluidised by air. The temperature in the bed was held at 700, 800 or 900 o C; usually (U/U mf) was 2.5. The bed's depth was varied, as also were (U/U mf) and the ratio of fuel to air supplied to the bed. Measurements were made of the concentrations of CH 4 , O 2 , CO and CO 2 , and also of the temperature, in the freeboard well above the bed. On entering the bed, the liquid glycerol, rapidly formed bubbles of vapour, which quickly decomposed thermally, yielding mostly CO and H 2. These gases then mixed with the other gases in the bed. It appears that the diffusive H 2 mainly burns between the fluidised particles. With the bed at 700-900 o C, no CO was detected far downstream of the bed, provided the equivalence ratio, θ, was below 0.7, i.e. with more than 43 % excess air. Under these fuel-lean conditions, all the carbon in the glycerol was oxidised to CO 2. However, in a more fuel-rich situation, with θ > 0.7, CO was detected well above the bed, particularly with a deeper bed, at a lower temperature and operating more fuel-rich. Thus, with the bed at 900 o C, CO was mostly oxidised inside the bed, but occasionally some CO burned on top of the bed. When a fuel-rich bed was below  850 o C, not all the CO burned in the bed. Achieving complete combustion inside a fluidised bed is partly a problem of mixing the products of glycerol's thermal decomposition with the fluidising air, which on entry exists mainly in bubbles. Consequently, increasing (U/U mf) promoted both mixing and combustion in a bed. In addition, in-bed combustion requires the bed to be sufficiently deep, hotter than  850 o C and θ to be less than a critical value. The effects of other variables are discussed. Nomenclature d b diameter of a bubble, m g acceleration due to gravity, 9.806 m s-2 G volumetric flowrate of glycerol vapour into a bed, ml s-1 T bed temperature of a bed, K or o C U superficial velocity of fluidising air through a hot bed, m s-1 U mf value of U at incipent fluidisation, m s-1 U rise rise velocity of a bubble, m s-1 V b volume of a bubble, m 3 x i mole fraction of species i in the gas phase ε mf voidage in a bed at minimum fluidisation θ equivalence ratio = (fuel supplied / fuel for a stoichiometric mix with the O 2 supplied) θ crit minimal value of θ for producing CO in the off-gases ψ x CO /(x CO + x CO2)

Recent Advances in Glycerol Catalytic Valorization: A Review

“…For instance, it was used to impregnate coal, in order to increase the heating value (up to 1.5 times), improving the combustion efficiency of the process, which can be used in coal-fired power plants, without any modification [47]. In addition, glycerol was used in fluidized bed feed with alumina sand for combustion [44,48,49] and combined with acetals [19].…”

Cenospheric and sand fluidized bed as an environment for waste rubber combustion-comparisons of decomposition dynamics and flue gas emission

“…Fluidized bed incinerators are characterised by high flexibility, i. e. the possibility of burning fuels in all physical states [1][2][3][4][5][6][7], homogeneous thermal conditions and proper mixing of the fuel-oxidant system. They are used to burn fuels such as coal [8][9][10], biomass [11,12] and to dispose of solid waste.…”