In the course of the climate change and increased focus on CO
Impedance spectra of interdigital electrode (IDE) structures are taken before and after mounting in wood-driven SHS to get information about the particles in the exhaust stream. It appears that the capacitive parts of the impedance spectra at a fixed frequency are appropriate for a fast signal evaluation. The good correlation with established offline measuring methods is discussed and the capability of thermal regeneration is demonstrated. The offline measurements of this work shall give the experimental basis for the development of online measurements in order to control the particle emissions of wood-driven SHS.
The utilization of biomass for the generation of electricity and heat is
becoming more important due to an increased focus on CO
This growing number of wood-driven SHS is accompanied by an additional load of emitted air pollutants. One of them that is worth focusing on is the fraction of particulate matter (PM), because it is considerably higher than that of comparable oil or gas burners. According to the German Federal Environmental Agency, it already exceeds the PM emission caused by engines (cars, trucks, motorcycles, etc.) in 2003 (GFEA, 2007). The importance of that problem is also underlined by the constitution of the promoting cluster (“emission reduction in biomass-driven SHS”) by the German Federal Environment Foundation (GFEF, 2014).
Fine dust is categorized by its aerodynamic particle diameter in the classes
PM
In addition, volatile organic compounds (VOC), ammonia, carbon monoxide and oxides of nitrogen build up during combustion. Besides their direct toxic impact, they also contribute to the secondary organic aerosols (SOA) as well as to the inorganic fraction by conversion processes in the atmosphere (Nussbaumer, 2010).
The health risk of fine dust is primarily caused by its small particle sizes.
While the human body has defense mechanisms against larger particles, fine
dust is able to penetrate the lungs via the respiratory system. PM
Due to the large surface area of the pulmonary tissue, particles can also migrate to the blood and lead to serious diseases of the cardiovascular system (e.g., heart attack), especially for long time exposures. Long-term studies show that there is a correlation between elevated mortality rate, particularly by respiratory system diseases or lung cancer, and the fine dust pollution (LUA, 2005).
Klippel and Nussbaumer (2007a) showed that health effects of fine dust depend not only on its particle sizes but are also mainly influenced by its chemical composition. Accordingly, the weakest health effects are caused by dust that mostly consists of mineral salts, such as those occurring in well-adjusted combustion systems. Compared to that, the cell toxicity of organic dusts from incomplete combustion is approximately 100 times higher. Besides the soot fraction (BC), they include mostly condensable organic compounds (COC). Additionally, a large number of different polycyclic aromatic hydrocarbons (PAH) can be found that are well known for their high carcinogenic impact. An overview of the health effects of fine dust is given by Nussbaumer (2012).
The German 1st Federal Immission Protection Directive (1st BImSchV) regulates
the operation and inspection of small- and medium-sized heating systems
(GFRG, 2010). The need for a reduction in pollutants is reflected in stricter
emission limits in the latest version of 22 March 2010. Table 1 shows the
two-stage lowering of the maximum concentrations of CO and dust allowed.
There are some interim arrangements for existing systems. If single-room
heating systems do not fulfill the requirements of maximum allowed emission
levels of 0.15 g m
Emission limits for small heating systems according to the 1st BImSchV (GFRG, 2010).
Currently, the 1st BImSchV does not define limits for the emission of
nitrogen oxides (NO
Wood consists for the most part of cellulose and hemicellulose (60–80 % of the dry mass). These polysaccharides give tensile strength to the wood by its fibrillae. The phenolic macro-molecule lignin contributes about 30 % of the dry mass. It exhibits molecule masses in the range of 5000 to 10 000 u and, in contrast to the weak parts of the plant, it gives compression strength to the wood. In addition, there are some extractives like resins or minerals. The exact composition differs depending on the type of wood and individual growing conditions.
Besides these constituents, forming the dry mass, a considerable amount of water is present in wood. The wood moisture, which is defined as the ratio of water mass to the wood dry mass, can measure up to 150 % for fresh chopped trunks. During drying it is reduced, depending on the ambient conditions. Wood moisture is an important parameter for combustion, because it reduces the lower heating value (LHV) by its evaporation during combustion.
According to Nussbaumer (2010), the combustion of
wood can be generally divided into three phases. In the first phase, the wood
is dried, causing the residual wood moisture to evaporate. Afterwards,
gasification and pyrolysis begin, at which point volatile hydrocarbons are
released into the gas phase and carbonaceous organic compounds (cellulose,
hemicellulose and lignin) start to decompose. Coke, volatile compounds (CO,
H
During combustion with a restricted oxygen supply, particles form from
incompletely burned components. At high temperatures (
In contrast to the organic fraction, the emission of mineral components can also occur in complete combustion. They form either by evaporation and subsequent condensation in the cooling phase or are carried away from the ashes by the exhaust stream. They mainly consist of chlorides, sulfates, carbonites and oxides of the alkaline metal and alkaline earth metals (Oser et al., 2003).
In contrast to other kinds of combustion systems, the temperatures in
wood-driven SHS are low enough that the formation of NO
A consequence of varying fuel properties (wood moisture, loading of the
combustion chamber, etc.) and the complex relationships between the operating
parameters is a discontinuous combustion with partly high emissions. Hence,
the accepted opinion in research and in the chimney sweeper trade is that the
typical mode of operation of wood-driven SHS is far away from the optimum.
Thus, in general, the actual emissions significantly exceed the values
measured under norm conditions. This supports the assumption that the
fraction of wood-fired heating as a source of fine dust is considerably
underestimated (Nussbaumer, 2010). Studies in a German residential area show
that wood-fired heating is responsible for up to 57 % of the ambient
PM
Therefore, it is reasonable to adjust the available variables like primary air, secondary air and fuel supply to the current combustion conditions to achieve minimum emissions. To evaluate these conditions, a control system needs comprehensive sensor technology for the direct measurement of all relevant emission quantities. Similar approaches for small-scale biomass furnaces were recently investigated by Bischof et al. (2013).
In contrast to large combustion systems or to the control of combustion
engines (lambda probe), continuously working sensor systems are not common in
wood-driven SHS. If continuous systems are present anyway, it is mostly about
temperature-sensitive elements for power management. These can either
mechanically control (bimetal, thermostat) the inlet air, causing bad
combustion conditions, or regulate the fuel amount, e.g., in modern pellet
combustion systems. Besides a measurement of the boiler temperature in
water-bearing systems, there is the possibility of placing thermo-elements
directly in the combustion chamber. In some rare cases, lambda probes are
used to gauge the needed amount of inlet air. In contrast to combustion
engines, which usually work in a narrow range around the stoichiometric
mixture (0.9
The sensor systems currently in use with regard to SHS refer to the measurements observing emission limits that are regulated by law. In contrast to continuously working sensor systems, these kinds of snap sample measurements are not applicable for electronic combustion control. They are only suitable for the detection of leakages or maladjustments. These 15 min measurements are performed by chimney sweepers and serve for observation of the limits of dust and CO. Additionally, the temperature, oxygen content and pressure difference is taken to benchmark the operating status.
Measurement methods regarding PM include mainly gravimetric ones, but optical
(scattered light, photoemission) and electrostatic methods are in use, too.
The gravimetric methods feature, e.g., filter cartridges that are weighed
after sampling or are part of a resonance circuit. The gaseous measurands
(CO, O
The instruments that are compliant for measurements according to 1st BImSchV exhibit a measurement uncertainty of 30 to 50 % concerning the dust content. Furthermore, the costs of EUR 5000 to EUR 20 000 per instrument are a high investment for the user.
Ways of conversion of the hydrocarbons to organic dust components according to Nussbaumer (2010) (Weiss et al., 2014).
To use sensors for the control of SHS operating parameters, the measurement method has to work continuously and must withstand the rough conditions in combustion systems by its technical implementation. Low costs per unit are essential for an economical realization.
In the field of gas sensing, metal oxide gas sensors (MOX) are suitable for
this challenge. For example, tin oxide sensors for the measurement of CO have
already been developed for fire detection in lignite-fired power plants.
Despite the rough ambient conditions (particularly dust), they achieve
lifetimes of up to 7 years and have been applied successfully in many power
plants (Kohl et al., 2001). Also, the NO
Direct measurement of the dust emission is crucial for controlled operation, because it cannot be determined just by the knowledge of other measurands like the CO concentration due to the fact that there is no clear correlation (Klippel and Nussbaumer, 2007b). For this task, common quartz bulk oscillators (QCM) are not suitable, because the limited temperature stability of quartz does not allow thermal regeneration. But, other piezoelectric materials like langasite may also be candidates for high temperature application (Richter et al., 2008; Richter and Fritze, 2014). Surface acoustic wave devices are commonly used as sensors in various applications, especially for the detection of mass deposits on its surface. But, under rough and fluctuating conditions – like those present in SHS – they are hard to operate. Because temperature deviations of only 1 K can be enough to interfere with the pure sensor response, temperature compensation, e.g., by a dual-port SAW, is necessary (Mujahid and Dickert, 2014). Beyond that, the elastic and conductance properties can spoil the result by a factor of up to 2. Our own approach for a capacitive dust measurement procedure will be detailed in the following section.
Used IDE structure consisting of a ceramic substrate
(Al
An approach for dust measurements in wood-driven SHS was investigated by the
authors in a joint research project with partners from industry and a chimney
sweep academy. The basis of the measurement is a ceramic substrate
(Al
In contrast to the detection of filter damages in cars, where the relevant timescale is on the order of hours or days, a sensor for the control of wood combustion has to operate on scales of less than a minute. Quick reaction and the detection of small amounts of dust are crucial for the application. Therefore, a capacitive approach is deployed. Dust deposited on and between the electrodes gives rise to the capacitance of the device by its permeability even when no conduction path develops.
To design simple electronics for a capacitive online measurement, it is first
necessary to analyze the IDE structure and dust properties in detail.
Therefore, in contrast to the future application, in this proof of principle,
all measurements were conducted in the lab of the university, not online in
SHS. Thus, the fresh IDE structures were first investigated by impedance
spectroscopy (IS), subsequently loaded with dust in SHS and afterwards
characterized by IS again. Impedance spectra were taken using a Solartron
SI1260 in combination with a Chelsea dielectric interface that facilitates
the measurement of small currents and therefore extends the measuring range
to small capacitances even at low frequencies. The complex capacitance
The real part of
The measured base capacitance
The capacitance shows only small fluctuations between measurements from day to day (0.04 %) in the laboratory setup. Signal changes of 0.1 % can be clearly separated from the noise. The cross-sensitivity to humidity is quite low. The capacitance of the fresh IDE structure at 16.3 kHz changes by only 0.03 %, when the relative humidity is varied from 0 to 30 % at room temperature. Figure 3 shows examples of three IDE structures with different levels of soot loading from a wood-driven SHS. As can be seen, even hardly visible loadings lead to clearly measurable results (middle). The response will reach very high values for loads in excess of the percolation threshold (right).
Socket mounted IDE structures. Left: fresh; middle: light loaded
(response
Besides IDE, the substrate possesses a platinum resistance wire (PT10) that
is arranged in a U shape around the IDE in a sublayer and that serves as a
resistance thermometer and heating. It allows an exact temperature control as
well as thermal regeneration at temperatures up to 800
Single soot particle left between the platinum electrodes after a
first regeneration step. The particle was removed by a second regeneration
(electrode width 27.5
The dissipated heating power during temperature-controlled regeneration can serve as a further measurand. As indicated in Fig. 5, higher dust loadings require an elevated amount of heat. This optional analysis step may deliver additional information about the dust loading and will be the object of further investigations, as well as the detailed temperature dependence of the IDE structure.
Series of measurements were performed with IDE structures loaded in different SHS (Buderus Blueline stove, Buderus Logano S231-25 wood gasifier) and with different kinds of wood-based fuels. In each case, the IDE structures are placed in the exhaust tube directly behind the SHS with the surface facing upstream in the direction of the combustion chamber. Because it is not possible to generate completely reproducible combustion situations (see Sect. 2), reference systems are necessary. The only feasible ones are the instruments according to 1st BImSchV and in accordance with VDI guideline 2066-1 from which two with different measurement principles are used. Due to the fact that they are not designed for continuous operation, they enforce 15 min measurements with corresponding high loading levels for the IDE sensor.
First results indicated that there is an acceptable correlation between the IDE sensor and references despite the fact that the capacitance signal is not volume weighted like the references (Weiss et al., 2014).
Dissipated heating power during thermal regeneration of a heavily loaded IDE structure (line) and a fresh IDE structure (dashed line).
Further results from a series with four different wood-based fuels using the
Buderus Blueline stove are shown in Fig. 6. The measurement numbers indicate
eight separate fires. To evaluate the relative accuracy of the IDE sensors,
each measurement is carried out using two of them in parallel. As can be
seen, the relative deviations are small, with only 6 % of the response on
average and some points so close to one another that they can hardly be
differentiated on this scale. A distinguishable correlation between IDE
sensor response and the reference values is achieved over all measurements,
despite various fuels. The remarkable deviations of the two reference
instruments (Fig. 6: nos. 5 to 8) may be caused by a saturation effect of
reference instrument 1 due to the high dust concentrations
(
Comparison of the response (
Measurement no. 8 exhibits an overestimation of the dust content by the IDE
sensor. Detailed inspection of the trends of temperature, dust, CO and O
The continuous in situ measurement of particles under firing conditions is an important task to reduce wood fire emissions, especially during the initial and ceasing states. This paper presents a first step in that direction, showing that cheap interdigital structures generate useful signals for an economical device. Further on, it could be shown that these devices can be used repeatedly in a cyclic manner. That opens the possibility of applying them in an online control system for wood-driven SHS.
At this early stage of research, the sensors already show a good correlation with reference instruments in offline measurements, although the data analysis of the IDE sensor presented here is not explicitly dedicated to dust loadings beyond the percolation threshold. But, such high loading levels were reached in the measurements due to the 15 min time interval according to 1st BImSchV, which is necessary for the reference instruments. Furthermore, a large optimization potential can be utilized by taking the actual exhaust stream velocity into consideration by measuring or controlling it in ways of geometrical flow dynamic manipulation. Also, the electrode structure can be further adapted to the main particle properties.
The influence of humidity on the capacitance of loaded sensors should be part of comprehensive studies. The varying chemical composition and morphology of the particles may lead to different effects due to the environmental dependency of the water permittivity. It can drop by a factor of approximately 8 in close vicinity to surfaces (Paul and Paddison, 2001). Comparative investigations with other methods like differential thermal analysis are expedient.
First tests of capacitive sensor readout with simpler electronics are conducted under lab conditions with promising results. As mentioned above, the dissipated heating power as another measurand as well as the thermal dependence of the permittivity will be part of further investigation.
The authors thank Bernd Vollmer and Hans-Eberhard Kopp of the North Rhine-Westphalia chimney sweep academy (Schornsteinfegerakademie Dülmen) as well as Ulrich Strohal and Jochen Fey of the Strohal enterprise (Strohal Anlagenbau, Staufenberg) for their technical support and preparation of the test combustion systems. Further gratitude goes to the German Federal Environment Foundation for supporting the project.Edited by: A. Schütze Reviewed by: two anonymous referees