Two years ago, at a time of low electricity prices, deregulation, restructuring and general uncertainty in the electricity business, Swedish municipal-owned multi-utility Eskilstuna Energi and Miljö took a bold decision. It embarked on a project to build an innovative wood-fuelled CHP plant featuring relatively untried technology. Admittedly there was some financial support from the biomass-promoting Swedish National Energy Administration, which, conveniently, is based in Eskilstuna. But the nagging question “are we doing the right thing?” was impossible to answer with certainty, says Lars Andersson, MD of Eskilstuna Energi and Miljö.

Events since then, notably electricity prices, have however gone very much the project’s way, bearing out the well-worn adage that fortune does indeed favour the brave.

The Eskilstuna station was synchronised to the grid in December 2000 and 13 March 2001 saw its official inauguration, by Swedish prime minister Göran Persson, in the company of 400 guests from ten countries.

This was also a key milestone in the brief history of Powerformer, whose inventor Mats Leijon, now an academic, was present at the ceremony. Eskilstuna is the first application of this innovative high voltage generation technology at a thermal station. The other three known projects to date are hydro units, with the prototype, Porjus, handed over in 1998 and two others, Porsi and Höljebro, due to go commercial later in 2001.

Eskilstuna generates at 136 kV, supplying electricity to the 136 kV grid without an intermediary transformer. In June 2000, during an overexcitation test during commissioning, the plant set a world record for generator terminal voltage, 177 kV.

It just so happened that during the planning stages of the Eskilstuna plant original developers of Powerformer, ABB, were looking for a suitable site for their first non-hydro version. Lars Andersson recalls that “after signing lots of confidentiality forms” he and his staff were shown Powerformer, which allows direct coupling to the grid, eliminating the need for a step-up transformer and medium voltage switchgear, and increases overall efficiency. He is very pleased with his decision to opt for this technology, for which a contract was signed in 1998. Lifetime cost analyses show that Powerformer will deliver substantial cost savings at Eskilstuna. Powerformer-supplier Alstom sees Eskilstuna as a springboard for further orders around the world, and reports growing interest in the technology.

Results from the tests on Powerformer during the commissioning of Eskilstuna have been as expected or even better. Tests have included: measurement of no-load and short circuit characteristics; loss measurements; 3-phase sudden short -circuit tests; sudden ground-fault tests; ventilation measurements (including 120 probes to verify calculations); and vibration measurements (with 70 sensors distributed around the stator core, cables, frame and bearings).

The boiler

Eskilstuna uses a bubbling fluidised bed (BFB) boiler equipped with HYBEX grate, which was developed for difficult fuels. Supplied by Kvaerner Pulping, Power Division, the boiler technology is particularly suited to the rigours of biomass burning, being able to cope with fuels of low heating value and high moisture. It also has a fast response to load changes, high turndown ratio and low emissions.

An important contributor to the plant’s overall high efficiency and ability to deal with high moisture fuel is the flue gas condensing plant (see diagram, right) fitted downstream of the electrostatic precipitators.

The fuel, in the form of chippings, consists of timber cuttings and waste from the wood processing industry.

Steam turbine

The steam turbine is of the Alstom VAX type. There are two casings, one high-pressure module, type HP16, running at 10 727 r/min, which drives the Powerformer from one end via a gearbox (reducing the speed to 3000 r/min). The other is a low-pressure module, type LP22, driving the Powerformer from the other end. It operates at the same speed as the generator (3000 r/min).