Summary and conclusions

Afbrænding af lettere forurenet træaffald i fyringsanlæg på fx møbelfabrikker

The object of this project is to propose new regulations for the burning of minor contaminated wood residues with a content of glue higher than 1%. According to the present regulations the burning of waste wood with a content of glue higher than 1% is imposed with a duty. Moreover the Danish Environmental Protection Agency has settled that the burning of waste wood must follow the EU Directive on Waste Incineration.

Minor contaminated wood residues are pieces of wood, e.g. saw dust, shavings, wood chips, sanding dust, wood pieces and board residues containing glue, enamel, enamel paint, residuals of foil and laminated materials, wood preservatives and other substances which through burning do not cause significant increase in the emission of substances, which are harmful to the environment. Because of the content of nitrogen containing glues in especially boards, the increased emission of NO_x is excluded from above.

We suggest that this definition is supplemented by a positive and negative list, i.e. a list of materials and substances that either may or must not be burned in smaller boilers.

It is suggested that the positive and negative lists are open for extensions due to new knowledge about existing or introduction of new substances and materials.

Our survey of Danish boilers firing minor contaminated wood residues is based on contact to a number of manufactures of boilers and companies representing the Danish wood and furniture industries firing i.e. chips from chip and MDF boards.

We have sent letters to boiler manufactures with a number of questions, and we have sent letters to wood and furniture industries with questions in six categories:

The survey has been coordinated with parallel surveys conducted by the local authorities of Ikast and The Confederation of Danish Industries.

Most of the boilers have a capacity between 0.5 and 1.5 MW input effect, and more the 50% of the boilers has been installed before 1990.

Approximately 30% of the boilers are equipped with an Automatic Measuring System (AMS) for oxygen. Only few newer boilers are equipped with oxygen control for combustion optimisation.

Approximately 19% of the boilers are equipped with a AMS for carbon-monoxide (CO).

Only 10% of the plants have installed a bag filter as a supplement to a multi-cyclone. The plants with bag filter are all installed after 1990.

We have examined the regulations of the burning of wood waste in Germany, Austria, The Netherlands and Sweden. Sweden has no special regulations. With only smaller differences in emission limits of dust, CO and NO_x these countries treats minor contaminated wood residues like virgin wood, although there is certain requirements to plant structure, operation, etc. and higher emission limits for NO_x.

There are a number of measurements of emissions from Danish boilers fired with wood waste. The data is not very well documented. It is difficult to provide measuring results, because the total number of measurements is few.

The measurements show a confused picture of the emissions with single very high values for CO and NO_x. In spite the fact that chips of chipboard and residues of boards is fired, more measurements clearly show that the emission of NO_x not necessarily is significantly higher than for virgin wood.

The limitation of emissions can be achieved in two different ways – either by end-of-pipe solutions (filters, cyclones, etc.) – or by better combustion conditions. A combination of the two gives the most optimal result. We have evaluated the prospects for reduction of the emissions of particles, CO and NO_x and other unwanted combustion products.

Reduction of particle emission

Earlier particle removal has been done by multi-cyclones, but recently bag filters have become standard also at smaller boiler plants down to 1 MW in order to comply with the emission limit of 40 mg/m³. Bag filters are normally very efficient, and very low emission values can be reached – normally less than 10 mg/m³ and often below 1 mg/m³.

Bag filters are significantly more expensive than multi-cyclones, both in investment and operation costs, since the bags have to be exchanged at intervals of few years. The existing technology for particle removal is evaluated to be optimal for the proposed limit values for particle emission.

Reduction of NO_x-emission

The only realistic technology for NOx removal at smaller wood fired plants is flue gas recirculation. At existing plants flue gas recirculation is normally not a realistic solution, because the plants must be designed for the larger flue gas flow in the recirculation loop.

Reduction in CO-emission

The emission of CO and other unwanted organic combustion products is closely connected to the combustion conditions i.e. the structure of the fuel, plant design and running conditions.

Requirements on the fuel

It is important that the fuel is reasonably homogenous, and does not contain larger pieces of wood, that burns slowly. The fuel must neither be to fine with too much sawdust or sanding dust. The optimum is a mixture fine and coarse particles giving a fixed structure allowing the primary air to be distributed uniformly through the fuel layer.

Requirements on plant design and control

It is extremely important, that the stoker and combustion chamber is designed for burning of the used type of wood – including its water content and structure.

It is recommended that all plants – both new and existing – must/ought to be equipped with a measuring system for oxygen and automatic control of the combustion air flow, in order to achieve a constant O₂-koncentration in the flue gas. Moreover continuous fuel feeding ought to be standard on new plants and also at existing plants, if retrofit is possible. On/Off control of the feed will have significant negative influence on the combustion quality, but with oxygen measurement and control the effect will be less. On/Off control ought to be accepted on plants, where it is not possible to retrofit continuous control.

The combustion air must be supplied at locations (zones) where there is a demand for it. Bad air distribution could cause high emissions of CO, etc. and bad burnout even though the total air flow is correct.

Experiences with video monitoring on waste incineration plants with moving grate is very positive and the systems are used at almost all Danish incineration plants and on at least two district heating plants. A camera placed above the grate will show many “combustion errors” giving both the owner and boiler manufacturer important knowledge about the cause of possible problems and the possibilities to solve them and to optimise the plant design and control.

We propose the following emission limits in the unit of mg/m³(n,t) at 10 % O₂

There are small differences between regulations concerning new and existing plants.

The ash from burning of minor contaminated wood residues will have a composition like ash from burning of virgin wood. The ash from plants comprising the proposed definition of minor contaminated wood residues can be disposed according to the regulations for ash from burning of virgin wood in the Danish statutory order of ash from biomass, “Bioaskebekendtgørelsen”, Ref. 6.

Reduction of particle emission

Reduction of NOx-emission

Reduction in CO-emission

Requirements on the fuel

Requirements on plant design and control

Reduction of NO_x-emission