Clean and lean

Maryland-based LPP combustion LLC has developed an innovative patented technology for lean, premixed, prevaporised (LPP) combustion of fuels. Hence, these fuels burn cleanly in gas-fired power turbines and other combustion devices, producing environmentally-friendly power and providing substantial fuel flexibility to power generators. A gas turbine utilising LPP combustion technology to burn biofuels creates a “dispatchable” (on-demand) renewable power generator with low criteria pollutant emissions and no “net” carbon emissions.

Conventional liquid fuel combustion

Traditionally, spray diffusion burners (Figure 1A) have been employed in gas turbines that operate on liquid fuels, including petroleum-based fuels such as naphtha, kerosene and diesel and for renewable fuels such as ethanol and biodiesel. However, this diffusion mode of operation produces high emission levels of NO_x, CO and particulate matter. The current technology for burning liquid fuels in gas turbines is to use water and/or steam injection with conventional spray diffusion burners. Emissions levels for a typical “state-of-the-art” gas turbine, such as a GE 7FA burning fuel oil #2 in diffusion mode with water/steam injection, are 42ppm NO_x and 20ppm CO. Water/steam injection has a dilution and cooling effect, lowering the combustion temperature and thus lowering NOx emissions. However, water/steam injection is likely to increase CO emissions as a result of local quenching effects. Thus, the “wet” diffusion type of combustion system for liquid fuels must trade off NO_x for CO emissions and still results in high levels of particulate matter.

Figure 1: A. Conventional liquid fuel spray diffusion flame (left) – B. Typical lean, premixed natural gas flame (middle) and C. Biodiesel LPP Gas™ flame (right).

In recent years, increasingly stringent emissions standards have made lean, premixed combustion more desirable in power generation and industrial applications than ever before, since this combustion mode provides low NO_x and CO emissions without water addition. Lean, premixed combustion of natural gas avoids the problems associated with diffusion combustion and water addition (Figure 1B). As a result, lean, premixed combustion has become the foundation for modern dry low emissions (DLE) gas turbine combustion systems. When operated on natural gas, these systems provide NO_x and CO emissions of ≤25ppm without the need for water addition.

However, DLE systems cannot currently operate in premixed mode on liquid fuels because of autoignition and flashback within the premixing section. Autoignition of the fuel/air mixture can occur before the main combustion zone, when the ignition delay time of the fuel/air mixture is shorter than the mean residence time of the fuel in the premixer. It is more likely to occur with the higher-order hydrocarbon fuels (eg fuel oils and biodiesel), which have shorter ignition delay times compared to natural gas. These short ignition delay times have proven difficult to overcome when burning in lean, premixed mode.

The LPP combustion process

A patented fuel vaporisation and conditioning process was developed and tested to achieve low emissions (NO_x, CO and PM) comparable to those of natural gas while operating on liquid fuels, without the need for water or steam addition. In this approach, liquid fuel is vaporised in an inert environment to create a fuel vapour/inert gas mixture, called LPP Gas™, which has combustion properties similar to those of natural gas. Premature ignition (autoignition) of the LPP Gas™ is controlled by the level of inert gas added during the vaporisation process. An extra advantage of the fuel vaporisation and conditioning process is the ability to achieve fuel-interchangeability of a natural gas-fired combustor with liquid fuels. The fuel switching change-over from natural gas to LPP Gas™ is done on the fly and does not require the turbine to be shut down or run at reduced load. Tests conducted in both atmospheric- and high-pressure test rigs utilising commercial DLE burners (designed for natural gas) found operation to be similar to that achieved when burning natural gas (Figure 1C).

Figure 2: The LPP Combustion process

Figure 2 shows a simplified process diagram for the LPP System. Liquid fuels were supplied to the LPP fuel conditioning skid using a fuel pump. An inert gas (nitrogen) and heat were provided to the LPP skid to vaporise and condition the liquid fuel. Although nitrogen was used for this application, other inert diluents such as exhaust gas or CO₂ could also be used. For testing purposes, the heat was applied to the skid using electrical heaters. For commercial systems, combinations of electrical, thermal and waste heat could also be used to provide energy for fuel heating and vaporisation. To maximise system efficiency for commercial application, waste heat utilisation is preferred to supply heat to the LPP skid. Once the liquid fuel is vaporised and conditioned in the LPP System, the resulting LPP Gas™ can be used as a substitute for natural gas and in potentially any combustion device originally designed for such a gas. The resulting emissions from burning LPP Gas™ are similar to those for natural gas including NO_x, CO and particulate matter. Since both biodiesel and ethanol contain little or no sulphur, natural gas SO_x emission levels are also achieved. The same clean blue flame typical of natural gas is achieved when burning LPP Gas™ derived from liquid fuels.

The LPP Combustion System changes the nature of the fuel by adding an inert gas during the vaporisation process thus preventing autoignition during the relevant timescales for fuel transport, mixing and burning with air. The vaporisation of liquid fuel takes place away from the combustion device in a separate skid-based fuel conditioning device under temperature conditions much less severe than in the combustor. This reduces burner maintenance compared to traditional spray diffusion methods.