The riddle of the sand5 September 2002
Alkalis in some of the more difficult biomass fuels can cause bed agglomeration problems in fluidised bed boilers. The answer is to use diabase - a basaltic rock which is a byproduct of the rock-crushing industry - instead of conventional sand as the bed material.
In 1996 a fluidised bed boiler, previously successfully operated on sludge, was installed at the Schauman Wood Oy factory in Kuopio, Finland. At Kuopio, plywood residue was one of the main fuels. Performance deteriorated and the bubbling bed sand had to be changed frequently - as often as every three days - to keep the boiler operating.
The boiler bed cross-section and air system were converted to be compatible with the new fuel, but this was not sufficient for proper bed operation. The focus of the investigation shifted to the bed material itself, and research was done at Kvaerner's own pilot plant.
The problem centres on quartz mineral, which constitutes 20-90 per cent of ordinary sand. The quartz mineral reacts with the alkalis in the ash of plywood residue, resulting in molten allkali silicate with a low melting point. The result is agglomeration of the bed particles (ie, coalescence without melting).
The basic mechanisms are:
1. Release of ash forming compounds and encrusting of the fluidised bed material.
• Inorganic compounds, which have been organically bound, are released during combustion, forming ash.
• A part of the ash reacts with the bed material forming a eutectic compound.
• The compound encrusts the bed particles.
2. Activation of the crust
• Encrusting creates a reactive glue.
• The glue joins particles to each other, resulting in neck growth between bed particles.
• Fluidisation is interrupted, creating more local hot spots and thus more areas that are not fluidising.
The solution turns out to be to use diabase, a byproduct of rock crushing, which does not contain quartz, as the bed material.
Diabase, which is inexpensive and widely available, was tested at Kuopio, along with some other alternatives, and found to perform very well. With diabase there is no agglomeration, even with more than 50 per cent plywood residue in the fuel. Boiler availability is high and bed material consumption has decreased dramatically.
Diabase is also being used in a new Kvaerner 74 MWt power boiler recently installed at the Pellos Plywood Mill in Ristiina, another Schauman Wood facility.
Pellos is the biggest plywood factory in Europe and one third of the boiler fuel is plywood residue.
At Ristiina it was possible to design the boiler right from the outset for the intended fuels, with several improvements over Kuopio. It has a hydro beam floor, allowing evacuation of coarse bed material over the whole cross section, with material being returned to the boiler bed after screening.
The Ristiina boiler, completed in May 2002, is proving omnivorous, with a fuel mix of bark, plywood residue and grinding dust, plus the possibility of using peat and forest residue.
Yet another Schauman Wood plant, the Savonlinna Plywood Mill, will also employ diabase in its new Kvaerner boiler, scheduled for completion at the end of 2003.
Some of this boiler output will be used for district heating, so the fuel mix is different, in fact less demanding, than that at Kuopio and Ristiina. Wood fuel, forest residue or peat will be used during high load periods. It will also burn the plywood mills byproducts: bark; plywood residue; and grinding dust.
With the elimination of the adverse effects of soluble alkalines, the diabase bed lends itself to other demanding biomass fuels such as chipboard, pulp mill screening rejects, tree leaves, straw, and olive waste.