Co-firing tests with sawdust and coal have been carried out at FORTUM's Naantali-3 CHP power plant (315 MWfuel). The Naantali-3 plant is a tangentially-fired pulverised-coal unit with a Sulzer once-through boiler that produces 79 MW electricity, 124 MW district heat and 70 MW steam. Naantali-3 is equipped with roller coal mills (Loesche), modern low-NO,burners (IVO RI-JET), over-fire air (OFA), electrostatic precipitator (ESP) and flue-gas desulphurization plant (FGD). Coal and sawdust were blended in the coal yard, and the mixture fed into the boiler through coal mills. Tests were carried out for three months during the April 1999 to April 2000 period with pine sawdust (50-65% moisture as received). During the tests, sawdust proportions of 2.5-8% (from the fuel input) were examined. The co-firing tests were successful in many ways, but the behaviour of the coal mills caused some problems, and therefore the simultaneous feed will not be the solution in a long-term use. Fortum has developed a new concept for co-firing coal and biofuels in large pulverised-coal fired boilers. The experiences gained from the Naantali co-firing tests and a good knowledge of low-NOT burning and combustion behaviour of different fuels at Fortum, was used when the new co-firing concept was developed. This concept consists of a separate biofuel grinding system and bio- or bio-coal-burners. By using this system, it is possible to utilize many kinds of biofuels in PC-boilers as well as increase the share of biofuels, compared to the simultaneous feed of biofuel and coal. (C) 2003 Published by Elsevier Science Ltd.