Uses and types of catalysts for wood stoves

When burning wood, the combustion produces pollutants that have a negative effect on the environment and human health. The amount of emissions of these substances is mainly influenced by the type of fuel, the type of combustion equipment and also the condition of the combustion equipment (quality of installation, regular maintenance).

And that is why the emission limits of pollutants from small combustion plants are becoming increasingly strict. Equipment manufacturers have so far successfully met these emission limits mainly by means of primary measures, but in the future we can also expect the use of secondary measures (e.g. the installation of flue gas catalysts), which is why we bring you a classification of catalysts for reducing the emission flue gas of your wood-burning equipment.

Ceramic oxidation catalysts

The most economical option for biomass stoves.

Ceramic oxidation catalysts are usually made from a special type of porous ceramic material known as "cordierite", which is then coated with a catalytic material. Liquid cordierite is forced through a mold and then fired so that it dries and hardens. By using different molds, different sizes of catalysts can be extruded and the size of the channels (or cells) can also be changed.

Ceramic catalysts are best suited for medium or large volume requirements when a standard size cannot be used. Ceramic honeycombs are generally the most economical option for consumer appliances such as biomass stoves and pyrolytic self-cleaning ovens.

Cell size is a key variable contributing to catalyst performance. However, instead of directly referring to cell size, it is normal to quantify how many are per unit area, such as cells per square inch (cpsi) or cells per square centimeter.
The greater the number of cells per unit area, the greater the surface area and the greater the potential for the reduction of undesirable compounds. The disadvantage is that more cells means more walls between the cells and therefore more obstruction to the flow of flue gases.

When choosing the cell density, it is also important to consider the possibility of clogging with solids such as ash or food - this is less likely if the cells are larger than the solids.

To facilitate the installation of ceramic catalysts in appliances or stoves that are largely made of metal, careful gasket design is essential to avoid problems caused by different rates of thermal expansion. Gaskets are usually made of either vermiculite or braided wire.

Metallic oxidation catalysts

Ideal for controlling fumes from commercial furnaces.

Metallic oxidation catalysts resemble a honeycomb with many small channels on which a catalytic coating is applied. Metallic oxidation catalysts are made of extremely thin metal foil, typically only 0.05 mm thick. In comparison, the walls of a ceramic catalyst can be up to four times thicker. For this reason, metal catalysts offer less resistance to gas flow than the ceramic alternative.

Most metal catalysts used in biomass burning have between 25 and 40 cells per square inch (cpsi), while the most common cell densities used for cooking are between 50 and 100 cpsi.

Metal oxidation catalysts can be made in a variety of profiles, the most common being round, square and rectangular.

A big advantage of metal catalytic converters compared to the ceramic alternative is that they can be attached directly to other metal components without the need for any seals.

Metallic reduction catalysts

A more compact alternative where space is at a premium.

Metal reduction catalysts are often used to reduce NOx from large-scale biomass combustion processes.

The catalytic converter is made of a very thin (typically 0.05 mm) foil consisting of Fecralloy, an alloy of iron, chromium and aluminum. This is held together by vacuum brazing and often also by support rods in the case of very large diameters.

Metal catalysts have several advantages for this type of catalyst:

• They can be made up to a diameter of 1200 mm in one piece
• The pressure drop for a given size is lower than a ceramic catalyst
• Thanks to the high thermal conductivity of Fecralloy, local overheating can be prevented
• The catalytic coating is usually either a combination of vanadium oxide, titanium dioxide, and tungsten, or a zeolite.

Cell densities typically range from 25 cells per square inch (cpsi) up to 200 cpsi. The size of the cells is chosen based on the size of any solid particles in the flue gas so that they are not easily blocked.

Ceramic reduction catalysts

Industry standard for NOx reduction.

Ceramic reduction catalysts are often used to reduce NOx from large-scale biomass combustion processes.

There are two different approaches to the production of ceramic reduction catalysts: either the entire catalyst can be made from a mixture of materials that have catalytic properties, or the catalyst itself is made from a support material and then a catalytic coating is applied.

In the first type, the catalyst usually consists of vanadium oxide, titanium and tungsten. These homogeneous catalysts are ideal in situations where the flue gas contains abrasive particles, as the surface of the catalyst can be abraded without loss of activity.

In the second type, the catalyst usually consists of cordierite. This is then coated with either a vanadium or a zeolite catalyst.

Because applications of ceramic reduction catalysts are often large-scale, it is common practice to use multiple catalysts to produce a grid. Standard catalytic converter size is 150mm high, 150mm wide with a range of lengths.

Typically, either homogeneous catalysts or coated ceramic reduction catalysts are manufactured with cell densities ranging from 16 cells per square inch (cpsi) up to 200 cpsi. The optimum cell density is determined after considering all solids (eg ash) that are contained in the flue gas, as the cell size should be larger than the solids size to prevent clogging.

As standard, we offer catalysts for wood stoves to reduce flue gas emissions, thus joining the world trend and environmental protection.

Sources used for this article:

• https://vytapeni.tzb-info.cz/vytapime-pevnymi-palivy/10022-potencial-vyuziti-katalyzatoru-pri-spalovani-dreva-v-domacnostech
• https://whitebeam.net/products/