**4.3 Surface morphology and pore-size distribution**

The morphology of coal, waste fuel, unburned carbon and bottom ash grains was studied using scanning electron microscopy with the secondary-electron beam method. The surface structure of coal and waste fuel was determinated and the texture of a typical grain of unburned carbon and bottom ash is shown in Fig. 12, 13. A general view (with magnification of 50x) and a surface detail (with magnification of 1500x) are shown for each material studied.

Fig. 12. SEM micrographs of unburned carbon particle

The surface texture shown in Fig. 12., 13. indicates that the porosity of unburned carbon collected at waste fuel co-combustion with coal is much better developed than that of unburned carbon when pure coal without waste fuel was combusted. But some caution is needed in such conclusions due to somewhat low representativity of one studied grain towards the average unburned carbon sample. Therefore pore-size distribution and specific surface area measurements were conducted in order to prevent misinterpretation when comparing adsorption properties of unburned carbon collected during pure coal combustion and during co-combustion of coal and waste fuel. Specific surface area of unburned carbon collected at pure coal combustion was 194 m2/g, whereas during co-combustion of the same

The morphology of coal, waste fuel, unburned carbon and bottom ash grains was studied using scanning electron microscopy with the secondary-electron beam method. The surface structure of coal and waste fuel was determinated and the texture of a typical grain of unburned carbon and bottom ash is shown in Fig. 12, 13. A general view (with magnification of 50x) and a surface detail (with magnification of 1500x) are shown for each

A) general view (magn. 50x) B) surface detail (magn. 1500x)

The surface texture shown in Fig. 12., 13. indicates that the porosity of unburned carbon collected at waste fuel co-combustion with coal is much better developed than that of unburned carbon when pure coal without waste fuel was combusted. But some caution is needed in such conclusions due to somewhat low representativity of one studied grain towards the average unburned carbon sample. Therefore pore-size distribution and specific surface area measurements were conducted in order to prevent misinterpretation when comparing adsorption properties of unburned carbon collected during pure coal combustion and during co-combustion of coal and waste fuel. Specific surface area of unburned carbon collected at pure coal combustion was 194 m2/g, whereas during co-combustion of the same

**4.3 Surface morphology and pore-size distribution** 

Fig. 12. SEM micrographs of unburned carbon particle

Fig. 13. SEM micrographs of bottom ash

material studied.

coal with waste fuel the specific surface area of unburned carbon reached 297 m2/g, which is significantly higher value. This work was focused on the comparison of minor and trace elements behaviour during the co-combustion of coal and waste alternative fuels with the previous results regarding the combustion of the same pure coal in the same power station but without the added waste fuel. Elemental behaviour exactly in the combustion chamber did not change noticeably when waste alternative fuel was co-combusted with the coal. Even the most abundant elements in waste alternative fuel (related to coal) - Zn, Cl and Br - showed nearly the same behaviour. This observation can be explained through similar high volatility of these elements both in the coal and in the waste materials. (Bartoňová at al., 2009).


Table 6. Ash contents and concentrations of elements in coal (C), unburned carbon (UC), bottom ash (BA), fly ash (FA) and waste fuel (WF)

Co-Combustion of Coal and Alternative Fuels 79

output flow was fly ash). Mass flows of input and output materials (BA – bottom ash, FA – fly ash, E,s – solid emission particles) and volume of gaseous emissions (VE,g) are summarized in Table 8. The ash and water contents in these materials are given in Table 8. as well. Mass flows relate to undried samples. Proximate and ultimate analyses of input and

Emissions from combustion unit were analysed and CO, NOX and SO2 were determined in flue gas, while As, Se, Cd, Hg and Pb were determined in solid particles captured on the filter in flue gas stream. The results of emissions analysis are given in Table 9. In the boiler mantle there are four holes into the combustion chamber. Using these holes as progressive sliping thermocouples to measure temperatures in the fluidized bed at different levels through the holes, where the proble was plugged, three samples of gaseous emissions and ash were collected directly from the fluidized bed. The sliping probe measured temperatures in the fluidized bed at the inlets. All combustion regimes were sampled from storage tanks of fuel and all four sections of the electrostatic precipitator. Furthermore, there was continuous measurement of emissions of NOX, CO, SO2 in the flue gases (see Fig. 5.). The balance of fuel and combustible waste, the mass flow, moisture content and ash, as well as the mass flow bed ash (BA) and fly ash (FA), the volume of gaseous emissions (VE, g), the

output materials are given in Table 7.

Fig. 14. Simplified diagram of the combustion facility

**5.2 Analyses of emissions** 

Comparison of elemental contents in bottom ash and fly ash was performed to describe further behaviour of elements when leaving the combustion chamber. It was established that when waste fuel was co-combusted with coal, a slight shift towards the higher enrichment of most elements in fly ash (vs. bottom ash) was observed. This trend is the most significant in case of Zn, Cl and Br which are the very elements that were the most abundant in waste fuel (when compared to coal). Therefore it can be concluded that the elements showing high concentrations in waste fuel tend to concentrate in fly ash. Specific surface area of unburned carbon collected at the test where waste fuel was co-combusted with the coal (297 m2/g) was significantly higher that that of unburned carbon from the combustion test without waste materials (194 m2/g). Comparison of pore-size distribution curves obtained for both unburned carbons revealed that unburned carbon collected during coal and wastes combustion contains larger amount of small pores, whereas macropores are more abundant in the unburned carbon form coal combustion without the waste alternative fuel. The unburned carbon collected at the co-combustion of the coal and wastes is undoubtedly of better adsorption properties.
