Measurement and evaluation of gaseous and PM emissions from wood fired stoves into indoor locations

Introduction:

The aim of the study was to measure and evaluate the influence of the operation of small combustion systems in the form of wood stoves on indoor air quality. The focus here was on the reloading process and a distinction was made between normal and rapid opening of the fireplace door and the reloading of excessive quantities of wood. Fine particulate matter (PM10, PM2.5 and PM1), ultrafine particulate matter (UFP), black carbon (BC), carbon monoxide (CO) and nitrogen oxides (NO_x: NO + NO₂) were measured.

The measurements were mainly carried out in a model room in the technical centre of the Institute of Combustion and Power Plant Technology (IFK) at the University of Stuttgart in order to ensure defined conditions. In addition, measurements were taken in a private household and in an office/sales room. There were numerous other sources (cooking, vacuuming cleaning, outside air pollution, etc.) in the private household and the office room, which superimposed the emissions from the respective stoves. The operator himself has the greatest influence on the air quality indoors and outdoors when operating individual wood stoves. Poor advice and ignorance when purchasing a wood stove and the associated fuel are already decisive factors that affect the quality of combustion and the resulting air pollutants.

Firewood that is too damp not only burns more poorly and therefore provides less heat, but also emits a large number of air pollutants that are harmful to health and the environment. In this study, it became clear that when the stove is operated in accordance with the regulations - apart from ultra-fine particles (UFP) - no to negligibly small amounts of air-polluting substances are released into the interior.

Results:

The tests in the model room showed that air pollutants are released into the interior when wood is added during stove operation in accordance with the operating instructions. This applies in particular to ultrafine particles (UFP). In addition, moderate quantities of carbon monoxide (CO) and nitrogen oxides (NO, NO₂) were also detected entering the interior from the wood stove. It can be assumed that particulate matter (PM10, PM2.5 and PM1) also enters the indoor location from the stove, but these cannot be distinguished from the emissions in the room itself, e.g. due to resuspension by the stove operator. Overlapping effects were observed above all in the previous investigations in a private household and an office room. Here, emissions from numerous sources dominated, such as cooking or vacuuming, outside air, laser printers and everyday activities in the living or working area, which had nothing to do with the emissions from the wood stove.

Black carbon (BC) emissions into the interior of the model room could not be detected, as proper operation requires the stove door to be opened as soon as there is only a basic ember in the stove. At this point, no incomplete combustion products such as BC are formed and therefore cannot enter the indoor location. Opening the stove door too quickly (‘tearing’) results in much higher (between 58 % and 88 %) pollutant emissions entering the interior due to greater suction effects than with normal or gentle opening. In the same way, excessive refilling quantities of wood have a negative effect on the indoor air quality. It takes longer to refill the stove, which means that the stove door is open for longer (approx. 1 second longer in this case). In addition, larger quantities of embers are produced, which in turn favour the formation of larger quantities of pollutants. Compared to normal opening of the stove door, between 9 % and 56 % more harmful gases (CO and NO_x) and up to 250 % more ultrafine particles (UFP) are released into the indoor air. In the case of UFP, the formation process could also take place in such a way that the gaseous precursor substances escape from the oven into the interior and the UFP only form in the indoor location.

With reference to existing guideline and reference values for indoor air quality of the Federal Environment Agency of Germany (UBA) in conjunction with the WHO (world health organisation), the air pollutant quantities measured and calculated in this work (with the exception of ultrafine particles) do not pose a demonstrable risk to human health according to the current state of knowledge. An increased hazard potential of ultrafine particles is probable, but their health assessment is still pending, as there are difficulties in clearly separating the effects from those of other air pollutants. In any case, there is a need for further research.