Fernando Preto

An examination of the exothermic nature of fluidized bed combustion (FBC) residues

Circulating fluidized bed combustion (CFBC) ashes from nine operational periods at the 183 MWe CFBC boiler at Point Aconi were examined for exothermic behaviour. Bed ashes and fly ashes were investigated using a Parr 1455 solution calorimeter. Limited tests were also carried out with additional samples from Point Aconi and from the 160 MWe TVA Bubbling Fluidized Bed Combustion boiler to evaluate the effects of particle size and aging on exothermic behaviour. For the Point Aconi ashes, heat release from the bed ash ranged from 11 to 52 J/g, and the maximum heat release rates ranged from 0.06 to 0.17 J/g/s. For the fly ash heat release varied from 114 to 187 J/g and the maximum heat release rates ranged from 0.8 to 1.9 J/g/s. In the fly ash samples, 50% or more of available CaO was converted to Ca(OH)2, while for the bed ash a third or less of the CaO was converted to Ca(OH)2. The exothermicity of the bed ash is directly proportional to the CaO content of the ash. However, this is not true for the fly ash. The exothermic behaviour of fresh FBC ash appeared to be greatly reduced by exposure in air over a 48-h period. Another conclusion of this work is that particle size effects the exothermic behaviour.

Agglomeration and Fouling in Petroleum Coke-Fired FBC Boilers

Experiments have been done subjecting ashes from industrial-scale FBC boilers to sulphating conditions in an oven for up to 105 days. These show that sulphation by itself causes agglomeration in the virtual absence of V, K, and Na, the elements normally associated with ash softening and classical fouling. In addition, it has been demonstrated that sulphation goes to completion over long periods of time and, at a specific level which differs from one ash to another, results in agglomeration. These experiments have also shown that there is a size range (75-300 μm) in which the agglomeration is worst, and particles that are smaller or larger either do not agglomerate or agglomerate more weakly. Added inert coal-derived ash decreases or prevent s the agglomeration. However, this ash does not appear to chemically combine with the sulphate, but acts by mechanically separating the sulphating particles. Finally, if alkali metals are present they can cause agglomeration at levels lower than those at which either the alkalis or sulphation separately cause agglomeration, i.e., they operate synergistically to cause fouling. Current work is being directed at examining these phenomena at higher temperatures (900°C and above).

Combustion of Wood Processing Residues in a Circulating Fluidized Bed

The combustion of wood processing residues was tested in the 0.8 MWth CANMET Circulating Fluidized Bed Combustor (CFBC) pilot plant. The specific residues tested were three different types of coniferous tree bark (i.e. from different locations to represent a range of possible fuels and fuel properties). Combustion conditions may be summarized as follows: fuel moisture levels 42–60%, fluidizing velocity 2.1–2.4 m/s; bed temperature 785–910 °C; maximum freeboard temperature 980–1070 °C and excess air levels 20–75%. The CFBC unit was able to burn the high moisture level fuels with no detrimental effect. In all trials the residues burned very well, with combustion efficiency greater than 99% based on overhead carbon loss. Emissions measurements were made of the following pollutant species CO, NOx , N2 O, SO2 , and dioxins and furans. The emissions levels were: 100–130 ppm NOx ; <1 ppm N2 O; 5–20 ppm SO2 and 400–1800 ppm CO. These emission levels are well below pollution guidelines for all major pollutants except CO. This however can be traced to the non-homogeneous nature of the coarse feed in the pilot plant. The problem can reasonably be addressed in a full-scale unit by a more stable feed system. A preliminary economic analysis of a new 25 MW FBC power plant firing these fuels was performed. Conservative inputs give a cost of 6 cents/kWh for the electricity produced and a economic wood haulage radius of 70 km.Copyright

Characterizations of Deposited Ash During Co-Firing of White Pine and Lignite in Fluidized Bed Combustor

Characterizations of ash deposits from co-firing/co-combusting of a woody biomass (i.e., white pine) and lignite coal were investigated in a fluidized-bed combustor using a custom designed air-cooled probe installed in the freeboard region of the reactor. Ash deposition behaviors on a heat transfer surface were comprehensively investigated and discussed under different conditions including fuel type, fuel blending ratios (20–80% biomass on a thermal basis), and moisture contents. For the combustion of 100% lignite, the compositions of the deposited ash were very similar to those of the fuel ash, while in the combustion of 100% white pine pellets or sawdust the deposited ash contained a much lower contents of CaO, SO3, K2O and P2O5 compared with the fuel ash, but the deposited ash was enriched with SiO2, Al2O3 and MgO. A small addition of white pine (20% on a heat input basis) to the coal led to the highest ash deposition rates likely due to the strong interaction of the CaO and MgO (from the biomass ash) with the alumina and silica (from the lignite ash) during the co-combustion process, evidenced by the detection of high concentrations of calcium/magnesium sulfates, aluminates and silicates in the ash deposits. Interestingly, co-firing of white pine pellets and lignite at a 50% blending ratio led to the lowest ash deposition rates. Ash deposition rates in combustion of fuels as received with a higher moisture content was found to be much lower than those of oven-dried fuels.