**8. Conclusion**

Since 1996 29 large fluidized bed boilers with desulphurization ability during combustion process have been launched in the Czech Republic. The differences in design and various concepts of these units have helped collect a lot of valuable data and gain a great deal of experience. The opportunity had not existed before these units were constructed.

Because the boilers for co-combustion of coal and alternative fuels in the Czech Republic were developed from know-how of foreign suppliers, it was not possible to get familiarized

5. Combination of low flow rate and high temperature causes the particles, which are coated with fuel ash, to contact each other and form weak physical bonds or to agglomerate. The formation of these weak bonds or agglomeration is due to the surface of the particles having a low eutectic point or ash softening temperature. This low value is caused by the high alkali content, specifically sodium and potassium compounds, formed during combustion of the boiler fuel. The agglomerated particles, subjected to high temperatures, then begin to sinter or stick together through bond densification

6. Agglomeration begins when part of the fuel ash melts and causes adhesion of bed particles. Beginning of agglomeration in the fluidized bed is often indicated by occurrence of temperature differences in the bed and the presence of large fluctuation of bed pressure. When the feeding of the fuel continues it eventually leads to a de-

7. To rate the propensity of fuels against fouling, the alkali index has been developed. This index relates the mass of alkali metal oxides K2O + Na2O produced with ash to the GJ of energy generated thermally and may be used for biomass feedstock. Above 0,17 kg alkali/GJ fouling is likely and above 0,34 kg/GJ fouling is virtually certain to occur. 8. Ash deposition from biomass fuels which contain certain chemicals can also create corrosion and erosion of metals. Two most abundant inorganic elements are Si and K that form silicates with a low melting point. The combustion leads to the condensation of molten silicates which are likely to cause fouling and corrosion. Analyses showed that corrosive reactions occur between chemical compounds in the ash particles and the

elements in the metal on un-cooled samples at gas temperatures near 650 oC. 9. Solutions for the problems resulting from the low melting points of the ash are: use of additives, use of alternative bed materials in the case of fluidized bed combustion and

10. There are three routes of formation of NOX during coal combustion, namely: thermal, prompt and fuel - NOx. Biomass has high contents of volatile matter and low contents of fixed carbon, so that the effect of char on formation of NOX and N2O may be significant. However, the catalytic effect of the ash could be important for residues

11. Concentrations of heavy metals are in compliance with environmental directive of EU2000/76/EG (including cancerous harmful components and benzopyren +Cd+Co+Cr+As). Combustion of alternative fuels and coal has no significant influence to

Since 1996 29 large fluidized bed boilers with desulphurization ability during combustion process have been launched in the Czech Republic. The differences in design and various concepts of these units have helped collect a lot of valuable data and gain a great deal of

Because the boilers for co-combustion of coal and alternative fuels in the Czech Republic were developed from know-how of foreign suppliers, it was not possible to get familiarized

experience. The opportunity had not existed before these units were constructed.

ashes on their melting points is well demonstrated.

thereby forming a strong physical and chemical bond.

fluidization of the whole bed.

blending of biofuels with coals, lignite.

which have high CaO contents.

leaching and Ph factor.

**8. Conclusion** 

comparable to those of sewage sludge, wood, peat and coal. The K2O content of the fuel

with the technical parameters until they started operation. The first operation hours of the most of these boilers were affected by the typical characteristics of Czech coal. Highly abrasive ash matter, high humidity, clay impurity of the fuel and higher content of other elements in the raw fuel (stone, wood, metal) made it necessary to modify fuel feed channels, crushers, separating plants and fuel intake to fluid channels. Many times before, these problems resulted in total unit reconstruction or even replacement. Frequent fuel supply discharges led to reduction of durability of heavy linings of the combustion chamber, especially cyclone bricking and chutes under the cyclones. Some problems were caused by ash extraction from fluidized layer, its cooling down, granulometrics finishing and further manipulation. Other problems occured in sintering fluidized particles when combustion temperatures were well below 900 oC. In spite of this mass sintering happens in various parts of the boiler. Last but not least, there is a trend to reduce desulphurization costs if the molar ratio Ca/S is in the range of / from 2.5 to 3, which means higher operation costs compared to wet tailings.

A quite new area of fluidized bed boilers is the combined combustion of coal and alternative fuels or the co-combustion of assorted fuels from renewable sources. Despite some slowdown in the expansion of activities in energetics, there are further projects in the area of applied research focused on operation process optimization, efficiency improvements and operation costs minimization. These are the areas where the information obtained from measurement results in various boiler types can be used.
