Results

Five different burner configurations were investigated for each of the three steam generator operating modes (50%, 75% and 100% of load). All results are given for the

furnace exit (Fig. 1). Data are expressed in mg/m3 except for C02 that is given in percentage of mole fractions.

C02 and S02 emissions (Figs. 3, 7, 11 and 4, 8, 12, respectively) show expected trends. By replacing the fuel (heavy fuel oil) on one of the burners with natural gas the C02 and S02 emissions decreased. Further increase in natural gas contribution (two, three and finally all four burners fired by gas) continued similar trend. The main reason for this decrease is that natural gas contains less carbon than fuel oil and none of sulphur. One will question such a high S02 concentration in the case of all burners fired by heavy fuel oil but, it is the consequence of the composition of the chosen fuel oil that contains 3% of sulphur (Table 1). Nowadays, legislation will force the TPP Sisak to use fuel oil with lower sulphur content.

Nitrogen oxide (NOx) emissions are also reduced by further increase in natural gas contribution (Figs. 5, 9, 13) due to presence of nitrogen in the fuel oil. It is known that NOx formed by fuel NOx mechanism measures almost 80% of total NOx created in the oil flame. Calculated NOx emissions are higher than the actually measured in operating conditions. Therefore, data obtained by the NOx model used in the simulation should be viewed qualitatively i. e. trends should be considered and not the absolute values.

Results of soot emission show descending trend with the higher ratio of natural gas in total amount of the burned fuel (Figs. 6, 10, 14). That is in accordance with the measurements in operating conditions where it could be established that the soot and particles emissions are higher in the oil flames than in the gas flames. Relatively high soot emission in the case where all burners are fired by fuel oil could be most likely explained by the fact that the soot model does not include complex processes of pyrolysis, polymerization, nucleation etc., which are present in soot formation. The soot model used in the simulation is the approximate one and it includes only empirical expressions for soot formation and oxidation. Therefore, similarly as in the case of NOx emissions, the soot results should be taken qualitatively and not quantitatively.

50% of Furnace Load

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£

 

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Подпись: 4gas 3gas+1 oil 2gas+2oil 1gas+3oil 4oil Fig. 7. C02 emission (75% of load). 75% of Furnace Load

C02[%]

14

12 10 8 6 4 2 0

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4gas 3gas+1oil 2gas+2oil 1gas+3oil 4oi!

Fig. 8. S02 emission (75% of load).

 

NO [mg/m ]

1800

 

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nP>

 

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Fig. 10. Soot emission (75% of load).

100% of Furnace Load


COz [%]

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4gas 3gas+1oil 2gas+2oil 1gas+3oil 4oil

 

Fig. 11. C02 emission (100% of load).

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4gas 3gas+1oil 2gas+2oil 1gas+3oil 4oil

 

image219

Fig. 13. NO emission (100% of load).

 

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Results

Fig. 14. Soot emission (100% of load).

This paper focuses on the ecological aspect of using dual fuels. But it must not be forgotten that such combustion changes thermal and aerodynamic conditions in the furnace. Namely, regions of safe and efficient boiler operation must be established by considering heat fluxes transferred to the furnace walls and temperature/concentration fields in the furnace. So, the quality of combustion is investigated by analyzing local formation of soot and carbon monoxide as a product of incomplete combustion. That comprehensive analysis is not completely presented in this paper, but only some parts are given as an illustration, as shown on Fig. 15. Contours shown on this figure represent the wall heat flux (W/m2) for 100% of load. Strong asymmetry could be observed for the cases where two fuels are burned simultaneously. It is well known that gas flames have lower emissivity than oil flames. Therefore less heat is transferred to the furnace walls by the gas flames and gas temperature maximum (which is higher than in the oil flames) is shifted upwards, resulting in higher flue gas exit temperatures. Lower temperatures

released by oil flame can cause steam temperature, at the outlet of the end superheater, not to reach its nominal value. The problem could be solved by applying recirculation of flue gases in the case of oil combustion. Recirculated flue gases, which are taken from the convective section of the boiler (amount and temperature of which depends on boiler load and burner configuration), are introduced through the bottom of the furnace (cold funnel) which consequently lowers the maximal temperature in the furnace and shifts the temperature maximum towards the furnace exit. Hence, the differences in furnace flue gas exit temperatures for the oil, gas and their combinations become much smaller than without flue gas recirculation.

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2. Conclusions

In this paper the possibility of using dual fuels in power plants has been examined. The paper focused on the ecological aspect of using dual fuels, but the thermal and aerodynamic conditions in the furnace were also investigated in order to establish regions of safe and efficient boiler operation.

The analysis was conducted for the steam generator of the 210 MW Power Plant Sisak (Croatia) in which old burners were replaced by the new low-NOx burners capable of burning both heavy fuel oil and natural gas simultaneously in any proportion.

Five different burner configurations were investigated for each of the three steam generator operating modes (50%, 75% and 100% of load). In the basic configuration all four burners were fired by the heavy fuel oil, which gave the highest emissions of CO2, SO2, NOx and soot concentration. By replacing the fuel oil on some of the burners with the natural gas the pollutant emissions decreased. Therefore, dual fuel combustion could be seen as a way of increasing sustainability of power production. Not less important issue of using dual fuels is a possibility to set the best power plant operation strategy considering prices and availability of fuel supply.

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