Platinum Metals Rev., 1984, 28, (3), 115
Combustion in Wood-Burning Stoves
Platinum Catalyst Increases Thermal Efficiency and Greatly Reduces Pollution
The use of wood as a residential heating fuel has always been popular in the Scandinavian countries. More recently there has been a significant increase in the use of wood-burning stoves in many other parts of the world, especially in the United States of America. This trend was initiated by the escalating costs of fossil fuels and electricity and, to a certain extent, public concern about the reliability of future supplies of these fuels.
Heating with wood fuel can be less costly than heating with coal, oil or electricity, especially in rural and some suburban areas where wood is relatively inexpensive and plentiful. Furthermore, modern wood-burning stoves are at least as efficient as oil and gas fires. Tests have shown that radiant, damped-draught wood-burning stoves deliver from 50 to 70 per cent of the energy available in the wood to the surrounding living area.
Despite all its advantages the wood-burning stove does present some problems, not the least of which is smoke.
The main difficulty is that there is no economical method of continuously feeding a measured amount of wood into the stove to control the heat output. Large amounts of fuel are loaded into the stove and the combustion rate is controlled by the amount of air available. The unburned wood is heated to the point at which combustible volatile components are distilled off. These components comprise hydrocarbons (including polycyclic aromatic hydrocarbons), aldehydes, phenols and carbon monoxide and are often incompletely burned in the stove because there is insufficient air available for combustion. Even when sufficient air is available, temperatures are often too low to bring about any significant degree of burning. The nett result is that these volatile components can be released from the stove to either condense in the chimney or be emitted to the atmosphere. The condensed materials contribute to the danger of chimney fires while the smoke emissions can cause significant air pollution. Furthermore, the overall effect is a significant loss of potential heating value.
The application of new technology in the design of wood-burning stoves can now largely overcome the basic problems associated with their operation. The requirement is for a method of operation which enables smoke to be burned at the temperatures existing in a stove operating under damped conditions. These temperatures are typically in the range 200 to 400°C. The use of platinum group metal catalysts supported on ceramic honeycomb substrates is now a well established method of controlling emissions of unburned hydrocarbons and carbon monoxide from motor vehicle exhausts. The catalyst reduces the combustion temperature of the hydrocarbons and carbon monoxide so that they start oxidising at temperatures around 250°C. The catalytic reaction increases until the system reaches an equilibrium between the inlet gas temperature, the gas-flow rate and the amount of combustible material in the gas stream. The equilibrium temperature can be as high as about 800°C for an optimum sized catalyst system and at this temperature essentially complete oxidation of the combustibles in the gas stream proceeds very rapidly.
During the course of the past few years the application of catalytic oxidation to promote secondary combustion has led to the appearance on the market of a new generation of wood-burning stoves. One such product has recently been launched by Trolla Brug, a long established Norwegian company. Appropriately named the Pioneer, it combines the traditional appeal of a cast iron stove with the modern technology of catalytic secondary combustion.
The stove is a three-tier design comprising a primary combustion chamber, a secondary combustion chamber incorporating the catalyst and a third storey which acts solely as a heat exchanger. The catalyst, which was specifically developed by Johnson Matthey Chemicals Limited for wood-burning stove applications comprises platinum metal dispersed on a low cell density ceramic honeycomb support. The multi-storey design provides a large surface area for maximum heat exchange efficiency. The incorporation of the catalytic afterburner is claimed to result in a 30 per cent reduction in wood consumption for the same useful heat output compared to the non-catalytic version of the same stove. In addition smoke emission levels are greatly reduced and the potentially dangerous accumulation of inflammable condensates in the chimney avoided.