**6.1 The combustion test description**

The combustion test with the mechanically drained digested sewage sludge (the water proportion in the sludge was approx. 63 %) was carried out at circulating fluidized bed power station in Třinec with an output of 130 MWt - Table 4. The mixture of hard energy coal and the coal sludge of average efficiency Qi r = 19 MJ.kg-1, water ratio Wr = 7,5 %, ash content Ar = 30 % was combusted at the fluid boiler. During the combustion test the fuel was distributed to the boiler in the ratio: 11 %weight-sewage sludge from the Central Sewage Plant of Ostrava, 28 %weight-energy coal and 61 %weight-coal sludge. During the additional combustion of the sludge the mixture characteristics changed as follows: heating value Qi r = 17 MJ.kg-1, water ratio wr = 14,5 %, ash content Ar = 28 %. Based on the fact that the total heating value of the fuel mixture thus dropped by cca 2 MJ/kg during the additional combustion, the volume of the mixture must be enlarged by approx. 0,65 kg.s-1 to maintain the constant boiler output. However the total coal consumption does not raise and this fact is important. The description of the combusted fuel is illustrated in Table 14., 15., and 16. The glory-angle of the mixture was rapidly changed for the worse, compared to the hard coal. The chain feeders of the crude fuel worked reliably and had no failures. Thanks to the

Co-Combustion of Coal and Alternative Fuels 85

During the combustion test the sludge moisture was approx. 65 % compared to the hard coal moisture of 7, 5 %. The higher moisture makes the temperature drop behind the crasher

belt T13

feeder vibrating

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

Energy coal and coal shed 89.1 – 90.5 0.25 6.5

Energy coal, coal shed and sludge 88.8 – 90.2 0.3 5.8

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

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

indicates the combustible matter content in the bedding ash and CUP indicates the

Fuel **ηk CLP CUP**

crusher

CCC1

chain feeder

rotary seal

hammer crusher

belt T14

boiler-room

pneumatically distribute

% % %

CFB

and it results in sealing the crasher with the mixture of the wet mud.

belt T12

Fig. 16. The scheme of the distribution of the fuel to the CFB boiler

combustion products corresponds with the fine hard coal combustion.

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

fossil fuel

container deepmined

belt T11

combustible matter content in the ash


mixture passing through the chain feeder the big pieces of the sludge were crushed. The combustion test showed that 15 % of the sludge content in the mixture was the cut-off amount able to pass the swing-hammer crasher. The moisture was of a fundamental importance concerning the allowable amount of the sludge in the mixture.

Table 14. The fuel characteristics in crude and waterless form


Table 15. Fuel combustible composition


Table 16. The silicate analysis of the energy coal, sewage sludge and coal sludge (RTGfluorescence method)

mixture passing through the chain feeder the big pieces of the sludge were crushed. The combustion test showed that 15 % of the sludge content in the mixture was the cut-off amount able to pass the swing-hammer crasher. The moisture was of a fundamental

**Crude form sample Energy coal Sewage sludge Coal sludge** 

Water ratio Wr [%] 10.23 62.36 6.22

Ash content Ar [%] 24.77 19.52 31.78

**Waterless sample Energy coal Sewage sludge Coal sludge**  carbon Cd [%] 59.88 22.50 56.34 hydrogen Hd [%] 3.92 3.48 3.36 sulphur Sd [%] 0.35 0.63 0.22 nitrogen Nd [%] 1.30 2.40 1.21 oxygen Od [%] 6.96 19.14 4.99 ash Ad [%] 27.59 51.85 33.88

carbon Ch [%] 82.70 46.72 85.21 hydrogen Hh [%] 5.42 7.22 5.08 sulphur Sh [%] 0.48 1.31 0.33 nitrogen Nh [%] 1.79 4.99 1.84 oxygen Oh [%] 9.61 39.75 7.55

> **MnO [%]**

Energy coal 16.02 0.28 7.2 1.96 0.04 <0.83 0.92 <2.02 0.86 1.6 0.22

sludge 13.78 0.25 2.49 10.24 0.11 <0.83 13.73 <2.02 0.38 3.35 6.44

Coal sludge 22.46 0.35 9.48 3.26 0.05 <0.83 1.07 <2.02 1.28 1.87 0.24

Table 16. The silicate analysis of the energy coal, sewage sludge and coal sludge (RTG-

**MgO [%]** 

**CaO [%]**  **Na2O [%]** 

**K2O [%]** 

**SO3 [%]**  **P2O5 [%]** 

r [kJ·kg-1] 18,854 1,476 18,962

**Energy coal Sewage sludge Coal sludge** 

importance concerning the allowable amount of the sludge in the mixture.

Table 14. The fuel characteristics in crude and waterless form

Table 15. Fuel combustible composition

**TiO2 [%]**  **Al2O3 [%]** 

**Fe2O3 [%]** 

**[%]**

**Sample SiO2**

fluorescence method)

Sewage

Heating value Qi

During the combustion test the sludge moisture was approx. 65 % compared to the hard coal moisture of 7, 5 %. The higher moisture makes the temperature drop behind the crasher and it results in sealing the crasher with the mixture of the wet mud.

Fig. 16. The scheme of the distribution of the fuel to the CFB boiler
