How modern gas turbines achieve excellent environmental performance at high load

January 19th, 2018, Published in Articles: EE Publishers, Articles: Energize, Featured: EE Publishers, Featured: Energize

This article explains how the gas turbines used at the Dedisa and Avon peaking power plants – which were highlighted in the June 2016 edition of Energize (“Privately owned peaking power station nears completion”) – achieve their environmental performance with lower NOx emissions even when operating at peak load.

Dedisa and Avon peaking power stations both use Ansaldo Energia AE94.2 single-cycle gas turbines. Dedisa, which is situated near Port Elisabeth in zone 13 of the Coega IDZ, and which started commercial operations on 20 October 2015, has two of them, while Avon, near Kwa-Dukuza, between Durban and Richards Bay, started commercial operations on 20 July 2016.

License agreements

Ansaldo Energia (AE) starting building steam turbines and generators under license for GE in 1949. The company also built steam turbines and generators under license for ABB and in 1991 started building gas turbines under license for Siemens and GE. The license agreement between AE and Siemens ended in May 2014 when Shanghai Electric Group bought 40% of AE. It was agreed that AE would retain the rights to continue making future developments to the gas turbines. Presently, AE manufactures gas turbines in the range from 70 to 280 MW.

Emissions issues

Air emissions continuously monitored in each stack are NOx, SOx, particulate matter (PM) and CO. The NOx emissions are the only emissions that really play a role. In general the amount and nature of air emissions depends on factors such as the fuel (the nitrogen chemically bounded in the diesel fuel), the type and design of the combustion unit (in this case Silo type external combustors are used), operating practices (amount of hours of operation to supply peaking demand whereby short duration maximum output is more important than efficiency), emission control measures (e.g. primary combustion control, in this case usage of lean premix combustion for fuel gas and water injection when using liquid fuel), and the overall system efficiency.

The gas turbines installed at Dedisa and Avon peaking plants were designed in or about 2013. Emission limit values for new plant in that year were set according to the World Bank’s environmental, health and safety guidelines [5].

A new 2017 design, firing liquid fuel such as light distillate oil No. 1 GT, or heavier distillate oil (No. 2 GT), must comply to ASTM D2880; or diesel fuel in accordance with SANS 342 CF1, as shown in Table 1.

PM SO2 NOx O2
50 ppm Use of 1% of S or less 150 (~74 ppm) 15%

Table 1: Emission guidelines (in mg/Nm3 or as indicated) in non-degraded areas for combustion turbines using distillate/light fuel oil and unit size ≥50 MWth.

NOx emissions

The Dedisa and Avon power plants provide power at peak demand which is a relatively short period each day. As a result, maximum output is seen to be of more importance than efficiency (despite diesel fuel being expensive) and because they use silo combustors, these plants inject demineralised water into the burners to reduce NOx emissions. This increases the output, but lowers the efficiency.  Since the turbines operate for a relative short duration each per day (five hours per weekday to meet peak demand), the quantity of water required is fairly low.

In these gas turbines, burning liquid fuel, the hybrid burners allow water to be injected into the burners directly after the diesel fuel. This is a practice that Siemens also uses, however Siemens also uses water injection via the liquid fuel as an emulsion using a water and oil premixing skid. Based on the comments of the article in Energize, it is assumed that AE uses a water-oil emulsion injection in the AE94.2s at Dedisa and Avon.

When silo-type combustors are used, as is the case for the AE 94.2, water injection is more feasible because these combustors are not so affected by water quenching. The silos are larger than other types of combustors and therefore water does not reach the flame tube wall so easily. Furthermore, the silo combustors use ceramic tiles. This decreases the air cooling required for the combustor flame tube walls and more air is available for combustion.

The silo low-NOx combustor design for the AE 94.2 uses eight burners [7]. The NOx emissions originate primarily from burning of the N2 in the air and depend to a lesser extent on the fuel composition with regard to Nitrogen.

To achieve lower NOx emissions (other NO emission compounds are negligible), with fuel gas consisting of a lean air/fuel ratio with premixing of air and fuel together, water or steam is injected into the combustion chamber. NOx emissions from nitrogen in the fuel cannot be lowered with water injection. In most cases when gas is used as a fuel, dry low NOx (DLN) combustors are used, and for fuel gas the burners and combustors lower the NOx emissions in this way.

Table 2: Typical CO2 emissions performance of new thermal power plants [5].

To avoid combustion instability and excessive CO emissions which would occur as the air/fuel reaches the lean flammability limit, below loads of between 40 and 60%, the lean burning premixed combustors switch to a diffusion-type combustion mode, whereby because of the higher flame temperature, higher NOx emissions take place [7]. When firing fuel gas, the lean diffusion burning at loads below 40% is done via pilot burners which stabilise the flame when at premix burning mode [3].

For firing liquid fuel, Siemens introduced HR3 burners with a DLN fuel oil premix firing feature in 1993. In this case fuel oil premix operates without water injection. This was quite unique at that time (1993) and is very interesting especially where water is an extremely valuable resource. It is also available for retrofit purposes [6].

While for fuel gas lean premixed combustion is a proven concept for NOreduction in gas turbine systems, lean premixed liquid fuel systems, also known as LPP (lean premixed prevaporised) systems are more complex, as the fuel has to be atomised, evaporated and premixed prior to combustion and there is a higher risk of auto-ignition and coking.

The hybrid burners, used in the two silo-type combustors, can burn fuel gas as well as liquid fuel. NOx emission reductions up to 40% can be obtained by injecting 50% parts of water to the fuel and even more when injecting a water fuel ratio of 1:1.

SOx emissions

The SOx emissions depend primarily on the sulphur in the fuel. Dedisa and Avon use diesel fuel with a low sulphur content (assume ≤ 50 ppm according to diesel fuel SANS 342 CF1), and adequate fuel and storage facilities have been erected, and the fuel and water is transported by road tankers to these sites. Demineralised water for injection at Dedisa and Avon is produced on site from the water delivered by road.

CO emissions

CO emissions depend very much on the fuel burning efficiency of the burner and with premix water injection generally has a greater impact on increased turbine maintenance than steam. Water or steam injection has the potential to increase CO and, to a lesser extent, HC emissions, especially at water-to-fuel ratios above 0,8.

PM emissions

PM emissions with liquid fuels are higher than from fuel gas. For light distillate as fuel guideline limit values are given by IFC (see Table 1).

CO2 emissions

Limiting values given by the World Bank IFC [5] for a gas turbine firing liquid fuel, primarily light distillate oil No. 1 GT, or heavier distillate oil No.2 GT, according to ASTM D2880 or diesel fuel according to SANS 342 CF1, are given in Table 2.

It is clear that with liquid fuel, the CO2 emissions are higher than when firing fuel gas. Coal-fired power stations have CO2 emissions from 1000 to about 1100 mg/kWh. Because of the cost of the fuel and water the liquid fuel-fired simple-cycle gas turbines are not suited for baseload operation.

References

[1] “Handbook 2013”, TurboMachinery International, 10/2012, Volume 53, Number 6.
[2] Die Gasturbine V94 zehn Jahre im Einsatz Von O.Smoch und B. Deblon (KWU Mulheim (Ruhr) VGB Kraftwerkstechnik 66 Heft 8 August 1986.
[3] Bernard Becker and Manfred Ziegner: “The new Siemens/KWU Model V64.3 Gas Turbine”, Reprint of translated German article in Motortechnische Zeitschrift 49, 1988.
[4] PowerGen2004 pamphlet
[5] IFC, World Bank Group: “Environmental, Health, and Safety Guidelines for thermal power plants”, Draft for second public consultation, May/June 2017.
[6] “Ready for the market: V94.3A Engine leads a wide Ranging Fleet”.  Turbomachinery International, March/April 2004.
[7] EPA-453/R-93-007: “Alternative Control Techniques Document — NOx Emissions from Stationary Gas Turbines”, Emission Standards Division, US Environmental Protection Agency, Office of Air and Radiation, Office of Air Quality Planning and Standards, January 1993.
[8] Gerhard Bohrenkämper, Dietmar Reiermann, Gerald Höhne and Dr. Ulrich Lingner: “Technology Evolution of the Proven Gas Turbine Models, V94.2 and V84.2 for New UNITS and Service Retrofits”, Siemens Power Generation, Germany.

Contact Hugo de Koningh, Consultant, Tel 083 630-0391, dkoningh@mweb.co.za