The 16 MWe pilot plant, due to enter service next year, thought to be the first of its kind in the world, is being built on the site of Enel’s existing Andrea Palladio coal fired power plant at Fusina, next to the Porto Marghera petrochemical facility, from where it will receive the hydrogen (produced as a byproduct of petrochemicals production). The hydrogen fuelled gas turbine will generate 12 MWe of the 16 MWe total. The gas turbine will be connected to an HRSG and the steam generated fed to the existing steam turbine of the adjacent Andrea Palladio coal fired plant, contributing 4 MWe of hydrogen generated output to that plant and reducing its coal burn. The resulting 16 MWe of hydrogen fuelled combined cycle capacity is estimated to have an overall electrical efficiency of 43%.

The hydrogen fuelled combined cycle plant is one of the projects being implemented under the Hydrogen Park initiative, a consortium formed in 2003 at the instigation of the Venice Industrial Union, with support, to the tune of about 4 million euros, from the Regione Veneto and the Ministry for the Environment. The consortium seeks to promote the development and application of hydrogen technologies in transport and electricity generation in the Porto Marghera area. In the future, the hydrogen needed to fuel the plant could also be obtained from the gasification of coal.

There are already gas turbines around the world operating on high levels of hydrogen. According to a paper by Robert Jones, Terry Raddings and Marcus Scholz of GE, presented at last year’s European Gasification Conference in Antwerp, which listed over 30 GE machines (B, E and F class) with high hydrogen experience, “high hydrogen fuel applications for gas turbines are well proven.” The GE hydrogen fleet leader would appear to be a Frame 6B unit at the Daesan petrochemical plant in Korea, which was installed in 1997 and is routinely running with hydrogen concentrations between 85% and 97%. There are also other initiatives around the world aiming at the development of high hydrogen gas turbine technology, notably those funded by the US DoE and programmes within the aviation industry.

However, as Massimo Balestri, Head of the Technical Support Unit at Enel’s R&D Centre, explains, the Fusina project is “about the construction of a CCGT” where the gas turbine “is to be fed with blends whose hydrogen content may reach up to 100%.” Distinctive features of the project include “low NOx emissions and partial condensation of the steam contained in the exhaust gas”, with combustion technology that is “innovative but also reliable”, achieving reduction in NOx emissions “by injecting steam in the combustion air flow and in the fuel gas.” Steam will be readily available from the HRSG.

The Fusina plant will employ a gas turbine of the GE10-1 type (single shaft, 11 compressor stages, 3 turbine stages), supplied by GE Oil & Gas – Nuovo Pignone, with a turbine inlet temperature of 1070°C, compression ratio 15.5, outlet temperature 482°C, exhaust flow rate 47.5 kg/s and speed of 11000 rpm.

The GT10 has a single can combustion chamber of the silo type, but Enel believes the technologies developed at Fusina will also be applicable to larger, multi-can, turbines.

Various versions of this combustion chamber, configured for diffusive flame, have been tested at Enel’s Sesta test facility in Tuscany during 2007 and 2008, in order to find systems suitable for operation on hydrogen and on blends of hydrogen and methane, with emissions levels compatible with local air quality regulations.

The approaches developed in Sesta are the result of joint studies between Enel Ricerca and GE Oil & Gas. They have proved to be effective for the reduction of NOx emissions as well as achieving low consumption of steam, thus limiting thermo-mechanical impacts on turbine blades. Very effective, particularly in terms of emissions abatement, says Massimo Balestri, was pre-mixing of steam and fuel.

Another feature of the Fusina project will be extensive monitoring of the gas turbine, along the lines of what Enel has been doing for some years on third generation CCGT plants, including on-line diagnostics designed to anticipate malfunctioning and possible damage.

The plan is to operate the Fusina plant on pure hydrogen, methane only and blends of methane and hydrogen, with natural gas as start-up and back-up fuel.

Pure hydrogen combustion clearly does not result in any carbon based pollutants, leaving NOx as the key issue, the main route for NOx production being the “Zeldovich”, or “thermal” mechanism (the rate controlling reaction being N2 + O ® NO + N, which is temperature dependent). The hydrogen flame temperature is relatively high, about 100-150 K higher than methane, increasing the challenge of NOx reduction for hydrogen fuelled turbines.

In its research to date Enel has been looking at two main options for NOx abatement: fuel dilution; and fuel staging. A couple of papers presented at the 9-13 June Asme Turbo Expo 2008 conference in Berlin (Stefano Cocchi, et al, paper GT2008-51271 and Peter Benovsky, et al, paper GT2008-51270) summarise some of the main results from the Sesta tests to date. As expected, NOx emissions with 100% hydrogen using the default GT10 burner in dry conditions, at around 700 ppmv, were about three times those with 100% methane. Also as expected, steam injection has proven to be effective in substantially reducing NOx emissions, with a steam to fuel mass ratio of about 2 needed to reduce NOx emissions to less than 200 ppmv (see graph below right). Efforts are now underway to further reduce the emissions. NOx levels of less than 50 ppmv are thought reachable with burner modifications, including improved mixing. CFD simulation has suggested that a doubling in the number of fuel nozzles should reduce NOx emissions by 40%, confirming the importance of air–fuel mixing in the formation of high temperature flame zones and consequently NOx production.

Hydrogen and Enel

Enel sees great potential in hydrogen as the energy carrier of the future and has decided to take an active role in developing the technology that would be needed in a transition to the hydrogen economy, including suitable combustion turbines. Enel is working on a programme aimed at both generating and utilising hydrogen, with priority currently given to the use of hydrogen for medium size power generation plants, although the utility is also looking at hydrogen production and storage.

Enel says the medium-term goals of its hydrogen research are:

• to acquire the necessary know-how of hydrogen combustion and emission control techniques in gas cycles;

• take the first steps in the arena of hydrogen generated electricity;

• to assess the medium-term feasibility of extracting hydrogen from coal in an ecological and economic way, with simultaneous production of electricity from IGCC systems with CO2 sequestration;

• to evaluate the possibility of using innovative, zero emission, high-efficiency hydrogen/ oxygen cycles in the long-term; and

• to develop technologies for the production of hydrogen from renewable sources, such as solar or wind energy, and to fully exploit the potential of hydrogen as a means of energy storage.