**6.2 The boiler efficiency and its operational reliability**

Based on the combusted coal and the sewage sludge of given ratio the approx. 0.3 % drop in efficiency of the boiler was observed - Table 17. The content of the combustible carbon in the combustion products corresponds with the fine hard coal combustion.


Table 17. The boiler efficiency ηk and the combustible matter content in the ash, where CLP indicates the combustible matter content in the bedding ash and CUP indicates the combustible matter content in the ash

If we focus on the operational efficiency of the boiler under the condition of the additional combustion of the sludge it it is advisable to monitor unwanted states like rust formation caused by high and low temperataure and silting the heat transfer surfaces and abration. In the case of boilers with the fluid bed and the additive desulphurisation, the marks of the chlorine

Co-Combustion of Coal and Alternative Fuels 87

desulphurisation. This reduction in SO2 content in the waste gases is obtainable only under the condition of additional combustion of the sewage sludge. The additive gets into the sewage sludge during the process of the sludge hygienisation by the lime dosing at the sewage plant. Furthermore it is hydrated on Ca(OH)2 by the sludge humidity. The lime hydrate is rid of CaO while entering the fluidized bed boiler. The CaO then reacts with the SO2 and the CaSO4. The amount of the additives in the sludge lowers the lime stone consumption as the primary source (Szeliga 2008). The analysis of heavy metals and microelements in the combusted fuels (the energy coal, the coal sludge, the sewage sludge) were carried out in the laboratories of The Technical University of Ostrava. The evaluation was made for the single coal combustion and then for the mixture of coal and sludge. The redistribution of heavy metals and microelements during the additional combustion of the sewage sludge to the combustion hard residues and the emissions are a matter of further research. The combustion test proves there are further opportunities for additional fuel combustion in the fluidized bed boilers. The advantages of this kind of sewage sludge usage are mainly in the reliable decomposition and oxidation of the organic harmful elements and significant sludge volume reduction. Another suitable way of using the sludge is to reduce its humidity, which improves fuel efficiency, transport and manipulation. The disadvantage of the thermic usage of the sewage sludge is higher concentration of heavy metals and microelements entering the combustion equipment. Co-combustion of coal and sewage is possible only if there is appropriate content of heavy me tals in the sewage sludge entering the combustion process. The monitoring and

analyses of heavy metals in the sewage sludge are nessesary.

The most important findings from the research can be summarized as follows:

volatiles consist mainly of combustibles – CO, H2 and CxHY.

1. Stability of combustion depends on two factors: a) regular and uniform feed regulation of the fuel mixture, b) perfect homogenization of the fuel mixture. Otherwise, pulsation

2. Experience with the combustion of sewage sludge showed that the highly volatile matter contents significantly affect the overall combustion process. Care must be taken to achieve complete combustion of the volatiles to ensure higher combustion efficiency

and low emissions of CO, hydrocarbons and PAH (polyaromatic hydrocarbons). 3. During devolatilization the biomass undergoes a thermal decomposition with subsequent release of the volatiles and formation of tar and char. The results show that the quantities of char and gas formed depend on the type of carbonised material. Furthermore, increasing the pyrolysis temperature leads to a decrease in the quantity of char formed and an increase in the quantity of volatiles. Analyses of the compositions of the volatiles from straw and stover as well as from wood chips and sewage sludge show that CO, H2, CO2 and CH4 are the main gaseous components. High moisture contents have been found to increase the devolatilisation time. For dry residues, in addition to the expected immediate ignition and the highly volatile matter contents, the

4. The composition of the ashes from sewage sludge, coal, peat and wood influences melting point. It is known that the Na2O contents of the residues are low and

**7. Findings** 

in the furnace can occur.

rust pop up even if the chlorine content in the fuel is low. The ratio Cl/SO2 has an impact on the high-temperature chlorine rust intensity. As for the chlorine content in the fuel sludge, it is evident that it does not exceed the volume found in the hard coal. The HCl concentration in waste gases influence the low-temperature rust intensity. In this particular case the rust rust in the recuperative air heater needs to be considered as well. Another characteristic of the operational efficiency of the combustion equipment is the silting of the heat transfer surfaces. It is important to notice the research on the thermoplastic characteristics of he ash. These characteristics are demonstrated by the following temperatures: tA – softening point , tB – melting point a tC – pour point. The lowest temperature for the mixture of the sludge and coal is approx. 1220 °C for the half-reductive atmosphere.
