More than just low NOx

New combustion technology achieves ultra-low NOx levels and yields near 100% combustion efficiency with improved simple cycle efficiency, fuel flexibility and wide operating envelope.

LFI’s RVC combustor. Takes the main stabilisation mechanism, which is the recirculation zone, and places it in what is essentially a cavity. This cavity is either along the central axis of the combustor or wrapped around the combustor.

 

Today’s gas turbine owners and operators face several key issues. Not only are they required to reduce greenhouse gas footprint and comply with ever stricter emissions on e.g. NOx, SOx and CO, they are also under pressure to reduce water use, increase cycle efficiency and improve both fuel and operational flexibility.

 

In an effort to address these sometimes conflicting requirements, US-company Lean Flame Inc. has introduced a new combustor that will not only cut NOx but will also improve turbine performance. According to Lean Flame, the technology “leads to near zero NOx without selective catalytic reduction or water or steam injection”.

 

It also says that compared to the dry low NOx combustors currently available on the market, it is less expensive to manufacture due to fewer and simpler parts, and is easier to control.

 

With five US and 20 international patents issued, the technology appears to be a significant technological advance in the gas turbine market – one that the company’s President & COO is excited about. “We believe we are going to revolutionise the traditional gas turbine combustion space,” said Brian R. Morris. The new combustor offers:

 

• NOx levels of 2 ppm

• CO levels near zero

• Increased operability, turndown and availability

• Simple cycle efficiency improvements of 0.5-1 per cent

• Lifecycle improvements for 1st stage nozzles.

 

Lean Flame is a venture-backed company formed in 2006. Its formation was driven by the need for cleaner generation in the utility, independent power producer and oil and gas sectors.

 

Initially, Lean Flame will be addressing the after-market spare parts market for large Frame gas turbines, as well as medium and small turbines.

 

Morris said: “We are trying to address the 25 MW and upwards market. But we are also working with some development partners for small engines, so it will be applicable from the megawatt level to the very large industrial Frame engines.

 

“Every 4-6 years, there is a retrofit where the combustor has to be replaced; this is the after-market part of our business model. Where the OEM combustors do not meet emissions regulations and other performance characteristics, we would replace the OEM hardware with RVC combustors.”

 

Technology evolution

The Lean Flame technology is an improvement on Trap Vortex Combustion (TVC), which was first invented by the US Air Force Research Laboratory (AFRL). Together with GE, AFRL has been working on the technology for around two decades. One of the main drivers behind its development was the desire to reduce combustor size in aero engines, which in turn benefits fuel burn.

 

Morris noted: “It has been primarily for military applications, so we are not privy to details. But we have spoken to a number of AFRL and GE people and we know that they are getting some good results for the military applications. We made some improvements to the technology and there are some differences with regards to, for example, the geometries, mixing and overall technology functions.”

 

Lean Flame’s technology is known as Recirculating Vortex Combustion (RVC). Notably it enhances turbine performance due to better flame stability. “TVC is stable with its flame,” noted Morris, “but we believe we have taken flame stability characteristics to a new level.”

 

The novel design concept departs radically from conventional swirl or bluff body stabilized combustors that have been used for decades. Traditional swirl-stabilised nozzles consist of an axial or radial swirler that generates tangential swirl of the flow, which then expands in the combustion chamber and creates low velocity regions that can stabilise the flame.

 

Conventional combustors are lean pre-mix (i.e. fuel and air are highly mixed before entering the combustion chamber). Therefore as the combustor is operated under increasingly lean conditions to reduce NOx, it becomes susceptible to destabilisation of the main stabilisation mechanism i.e. the circulation zone (generated by the swirlers and bluff bodies in the flow), which serves to sustain the flame.

 

TVC technology achieves a more robust flame stabilisation mechanism by having combustion zones that are basically sheltered from the main flowfield.

 

TVC takes the main stabilisation mechanism, which is the recirculation zone, and places it in what is essentially a cavity. This cavity is either along the central axis of the combustor or wrapped around the combustor.

 

Dr. Donald W. Kendrick, Chief Technology Officer explained further. “You’re essentially locking the stabilisation mechanism, which is just a recirculation zone in a highly sheltered and protected environment, which is independent of the main combustion zone where wide swings in velocity, fuel heating value, cycling and pressures etc. are experienced.”

 

In the RVC, the stabilisation regions are in curvilinear cavities – similar to a doughnut wrapped around a cylinder.

 

“These offboard flame stabilisation regions in the cavity provide a mechanism that is far more robust and stable and is therefore able to provide a much wider turndown than a traditional swirl stabilised flame, which would blow out as you go leaner. Ours can extend the stability region to much leaner and richer conditions,” noted Dr. Kendrick.

According to Lean Flame, traditional swirl stabilised flames blow out at an equivalence ratio (fuel-air ratio divided by stoichiometric fuel-air ratio) of about 0.35. With its burners, the company says it can achieve equivalence ratio of 0.1 and below i.e. substantially leaner than traditional DLN technology.

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