Electricity + Control June 2017

PRESSURE + LEVEL MEASUREMENT

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number of combined turbine trips. Even though sufficient steam is mostly available to operate both turbines at maximum capacity, turbine trips cannot be eliminated, due to instances of low off-gas productions as seen in Figure 7. In Table 3 the time percentages are displayed for both T 1 and T 2 , for all of the different steam flow scenarios, where power co-gener- ation occurs at maximum capacity and for when the turbines are in trip. The results show, as expected, that when the flaring percentage reduces, turbines will operate more frequently at maximum capac- ity with less time in a tripped state. The reported values in Tables 2 3 do not show significant changes when the flaring percentage is lowered from 10,0% to 5,0%. The conclusion to be drawn is not that there exists a flare percentage where after improved flaring control does not increase energy utilisation, but that the combined boiler houses’ capacity is the limiting factor. This correlates with what was seen in Figures 7 and 8. Table 3: Time percentages for turbines operating at full capacity or be- ing in trip. Time % for Maximum Operating Capacity Time % for Turbine in Trip Taking this information into account, it will make sense to simulate scenarios where the combined boiler houses’ capacity is increased to further investigate the effect on power co-generation for this engineering plant. With an increase in steam production capabilities additional tur- bines will have to be added for the simulation model or the operating limits of T 1 and T 2 should be increased. Conclusion An engineering works that produces and utilises fluctuating burn- able off-gases during normal plant operations was investigated. The residual off-gases that are not used for plant process purposes are used to generate steam, in boiler houses, and the remaining off- gases are flared to atmosphere, nullifying the energy potential. Steam flow productions are necessary for a variety of plant processes and these usages may fluctuate over time. Once all plant steam demands are met, excess available steam is utilised for power co-generation through the use of steam turbines. References [1] E. Vine (2008). Breaking down the silos: the integration of energy efficiency, renewable energy, demand response and climate change. Energy Efficiency 1, pp 49 – 63. Flare % T1 T3 T1 T3 46,9 40,0 30,0 20,0 10,0 5,1 21,7 38,4 63,4 78,0 82,7 84,4 20,0 15,2 14,0 17,0 15,1 18,0 37,0 52,4 60,2 62,0 7,7 8,2 7,0 8,2 9,7 6,4 3,7 5,0

350 300 250 200 150 100 50 0 Steam flow (ton/h) Steam flow (ton/h)

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Time (h)

Figure 7: Total potential steam productions flared to atmosphere.

As mentioned, the boiler houses consist of a maximum steamproduc- tion capacity of 260 ton/h. Furthermore, for line pressure control to ensure that air does not enter the off-gas pipelines, which are open to atmosphere, a certain quantity must always be flared. It is therefore never possible to utilise all the energy potential from the off-gases.

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Time (h)

Figure 8: Potential steam productions flared to atmosphere that could have been realised.

Influence of regulating off-gas flaring percentage It was mentioned earlier that management was under the impres- sion that off-gas flaring was regulated to be 10% of the volume flow, whereas in the previous section it was reported that on a volume basis 46,9% of the off-gases were flared. Five different off-gas flaring scenarios are chosen that range from 40,0% down to an optimistic 5,0%. For each scenario the potential steam that could have been avail- able are calculated and simulations are performed by means of the mathematical model. The power co-generations are given in Table 2 .

Table 2: Turbine simulation results for various off-gas flaring percentages.

Max Potential Power Generation [MW]

Combined Power Generation [MW]

Flare %

Trips

46,9 40,0 30,0 20,0 10,0

21,4 25,2 30,1 33,4 35,0 35,5

20,5 23,1 27,0 29,7 30,8 31,4

18 25 20 15 13 11

5,0

From the results in Table 2 it can be seen that if off-gas flaring was controlled, significant increases in power co-generation could poten- tially have taken place. Allowing for only 10,0% of flaring, the potential power co-generation could increase with up to 50,0% with a reduced

June ‘17 Electricity+Control

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