240321 SHV Brief Gemeenten en NVG Adviescommissie bijlage 1 -Science Nanoparticle emissions from residential wood combustation 2019

Renewable and Sustainable Energy Reviews 103 (2019) 515–528

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Nanoparticle emissions from residential wood combustion: A critical T literature review, characterization, and recommendations ⁎ Rebecca Trojanowskia,b, Vasilis Fthenakisa, a Center for Life Cycle Analysis, Earth and Environmental Engineering, Columbia University, New York, NY, United States b Sustainable Energy Technologies, Brookhaven National Laboratory, Upton, NY, United States

A R T I C LE I N FO A B S T R A C T

Keywords: The increased use of wood as a heating fuel in residential and small commercial buildings has increased concern Biomass about potential environmental and safety impacts, specifically particulate matter (PM) emissions in the nan- Wood stoves ometer range. Larger particles (> 2.5 µm) can be effectively removed from exhaust streams by emission control Nanoparticle devices. However, nanoparticles (NP), due to their size, are more difficult to capture in exhaust flue gases. In Emissions addition, NPs have a higher surface to volume ratio, allowing them to absorb organic compounds, causing them Regulations to be more reactive than their larger counterparts. This review focuses on the NPs produced from residential wood heating devices. Current emission regulations do not reflect the NP count or type produced from re- sidential wood combustion, although most published studies show that a significant portion of the particles produced during combustion is in the nano-size range. Fuel type, device type and combustion periods have all shown to impact, at various degrees, the NPs produced. Contrary to common expectations, it appears that modern units may generate a higher count of NPs, although emitting less particulate mass than older units. This investigation supports arguments of needed particle type and count regulations in addition to the current mass based emission regulations.

In addition to a critical review and analysis, recommendations are made regarding future testing, monitoring and environmental impact studies that address the significance of NP emissions.

1. Introduction United States (US) accounts for more than one-third of primary PM2.5 emissions and the US Environmental Protection Agency (EPA) lists the The increased use of wood as a heating fuel in residential buildings three largest sources of PM2.5 as utility fuel, RWC and prescribed has increased concern about potential environmental health and safety burning (land management tool to maintain forest health) [6]. Speci- impacts. The increased use is partially due to the historically low cost of fically in New York State (NYS), residential wood heating contributes wood fuel, but also because the technology offers a renewable alter- 275% more PM2.5 than all industrial, commercial and institutional (ICI) native to fossil fuel heating. In addition, single room heaters using ei- heating emissions combined, 550% more PM2.5 than the electricity ther pellets or cordwood can help offset significant amounts of home generation sector, and 35% more PM2.5 than the transportation sector; heating oil or propane without the larger investment of replacing a overall, contributing more than 90% of the PM2.5 emissions in NYS even boiler. As the use of biomass is encouraged as a rewewable fuel source though it only provides less than 2% of NYS overall heating needs [7].

and considered to be an important player in the transition to renewable Similar trends are reported by the Danish Ecological Council; the fuel sources, it suffers from higher particulate matter (PM) emissions in number of particles per unit volume of wood burning exhaust in Den- comparison to oil or natural gas fired systems. The World Health mark, were found to be than 230 times greater than those from diesel Organization estimates that more than 50% of the world population trucks [5]. uses biomass fuels for cooking and heating [1] and these PM emissions from residential wood-smoke have corresponded to declines in local air 2. Background quality in Europe [2–4]. As of 2015, residential wood combustion (RWC) devices were re- Especially of concern are emissions from wood combustion of sub- sponsible for more than 45% of PM2.5 (particles with an aerodynamic micron particles, i.e., nanoparticles (NPs) (< 300 nm) The absorbed diameter < 2.5 µm) in Europe [5]. Similarly, biomass burning in the compounds in some cases may be potentially carcinogenic and induce a

⁎ Corresponding author. E-mail address: vmf5@columbia.edu (V. Fthenakis).

https://doi.org/10.1016/j.rser.2019.01.007 Received 22 March 2018; Received in revised form 10 December 2018; Accepted 1 January 2019 Available online 15 January 2019 1364-0321/ © 2019 Elsevier Ltd. All rights reserved.

R. Trojanowski, V. Fthenakis Renewable and Sustainable Energy Reviews 103 (2019) 515–528

more pronounced pro-inflammatory response than larger particles of their size, are more difficult to capture in exhaust flue gases. Ad- the same material [8–11]. Depending on the combustion source, small ditionally, commercial biomass heating systems (office buildings and amounts of sulfates, nitrates, and metals may also be present along with those used for district heating) are more efficient and equipped with adsorbed volatile organic carbon (OC) [12]. Perhaps the combination of emission control devices mentioned above. However, residential units metals present, organic compounds, persistent free radicals and/or low often do not have these PM removal systems1 and so they are major solubility or degradability are the likely source of observed health im- sources of PM in the air. Therefore, this review paper focuses on the NPs pacts of airborne PM such as oxidative stress [9,13,14]. A review by produced from residential wood heating devices. In addition, the paper Bøman et al. discussed exposure effects such as asthma, respiratory offers suggestions for future studies from an environmental perspective symptoms, mortality, and declined lung function had a stronger cor- as the use of wood combustion devices continues to increase. relation in areas where residential wood smoke combustion was a major source of PM [4]. Other reports show that NPs can become suspended in 3.

Residential wood combustion systems wash water, make their way into organisms and eventually cause ser- ious health effects to humans through the food chain [15]. Residential systems may be classified as primary heaters or sec- Many studies have looked at the health effects associated with wood ondary heaters. Primary heaters include biomass boilers or furnaces combustion NPs, but to date little is known about the environmental which serve to heat an entire home and may also include domestic hot fate of these NPs and their eventual pathway to humans. One review water. Secondary heaters, also commonly referred to as ‘room heaters’, indicates that 99% of the total number concentration of particles in the are used as a supplemental heat source, particularly for one room, and ambient atmospheric environment are made up of NPs 300 nm and in some cases may only be used for ambience. This category typically smaller [8]. However, there is no systematic description of anthro- includes wood stoves, masonry stoves, and fireplaces and fireplace in- pogenic NPs and their occurrence, fate, and effects on the environment serts. [16]. Several studies showed that ultrafine NPs emissions (< 100 nm) Wood heaters may be manually or automatically fed.

Manual sys- may affect the climate and this is dependent on the chemical properties tems are typically fed in batches with fuel types such as cordwood (logs of the aerosol [3,17,18] and their residence times [16,19]. or split wood), cribwood (dimensional lumber), and briquettes. It is known that black and brown carbon has a significant role in Automatically fed systems transport wood chips and pellets via an altering the climate [20,21]. Nienow et al. [12] report that approxi- auger to the combustion chamber; they may be top fed, horizontally fed mately 15% of all atmospheric particles between the sizes of 1 and or bottom fed [7]. Systems which are automatically fed or include 1000 nm in diameter are soot, which are mostly elemental carbon (EC), controls, require electricity to operate the fuel screw, glow-plug ignitor and typically are non-toxic to cells but are known to absorb sunlight and fans, whereas manual systems may not require electricity. Tradi- and contribute to global warming. Black carbon NPs are of particular tional stoves that lack automatic controls do not need an electric source concern since, in addition to absorbing light, they are inefficient light and simply rely on the operator to ignite the fuel source and control air scatters as they are much smaller than the 400–700 visible light spectra dampers. Advanced units may include fans, lambda (oxygen), tem- [19].

RWC not only contributes to photochemical smog [22,23] but has perature sensors, and automatically controlled dampers that require an been reported to be a significant source of black and OC [24]. A recent electric source to operate. Some wood heater designs also promote the study shows that 56% of the black carbon soot in Europe is caused from use of duel fuel i.e. a manually fed cordwood and automatically fed RWC and with a projected increase in its use to meet renewable energy pellets goals, the contribution of black carbon soot is expected to grow up- wards to 70% by 2030 [5]. The accumulation of NPs in the environment may not only induce 3.1. Primary heaters climate change but also affect atmospheric visibility decreasing the sun's light intensity, and have an effect on the environmental hydroxyl One type of primary wood heating appliance is the wood-fired hy- radical concentration, ultimately depleting the ozone. Smita et al. [25] dronic heater, a technology that has been evolving rapidly in the U.S. summarized a scenario in South Asia where 70% of the brown clouds and Europe over the past decade. Hydronic heaters heat water, or are made up of soot from biomass combustion in cook stoves. They otherworking fluid, and then pump the fluid to the home via baseboard correlated the brown clouds to glacial melting, reduced sunlight and or traditional cast iron radiators.

Hydronic systems are most common in extreme weather conditions which have impacted local agricultural Europe and the Northeast US. Older wood-fired hydronic technology production. The authors also discussed the NPs reaction with hydroxyl involved outdoor-installed units with large integral water volumes and radicals in the environment and their immediate binding due to the updraft combustion (commonly referred to as outdoor wood boilers, very reactive nature of both. Typically, hydroxyl radicals degrade pol- OWBs). These units operate with air flow controls that set the firing rate lutants due to their strong oxidation nature, however with the NP re- to maintain a target boiler water temperature. These units control action, the free radicals are no longer available and an increase in combustion by just opening and closing a damper operated by an pollutants and greenhouse gases are observed. aquastat [7]. During periods of low-heat demand, these units go into a In addition to NPs affecting the atmospheric chemistry and inducing “slumber” or smoldering combustion mode, (described in Table 1), climate changes, NPs can eventually deposit onto soils and bodies of which can create high emissions. Also, these units have very large water causing secondary contamination and other environmental ef- combustion chambers allowing them to be easily overfilled potentially fects [19,26].

Hata et al. found wood combustion NPs (in comparison to causing a hundred-pound charge of wood in a boiler with a nominal larger particles) had the largest fraction of particle-bound water soluble output capacity in the range of 100,000 Btu/h to smolder. OC which is known to contribute to cloud condensation nuclei gen- Hydronic heaters may be classified as single-stage or two-stage ga- eration [27] but may also interact with the natural OC in water; dis- sification boilers. Single-stage units only involve one chamber for solved OC can carry trace metals and other contaminants [28] that are combustion and an air damper or two. These units are typically less otherwise insoluble and immobile causing carcinogenic effects on or- efficient as unburned gases leave the flue. Two-stage gasification sys- ganisms [22]. tems involve two chambers; one chamber to burn most of the biomass Larger particles (greater than 2.5 µm) typically derive from the in- organic fraction of the fuel in the form of ash, or may be agglomerations 1 Some stoves may be sold with catalyst; however, this practice is more of fine and ultrafine particles, but they can be successful removed from common in the US than in the EU.

Recently a prototype of a residential boiler exhaust streams by emission control devices [23] such as electrostatic with a mini ESP was introduced in the EU but this is not a common practice yet precipitators (ESP), cyclones, and baghouses. Nanoparticles, due to [84].

516

R. Trojanowski, V. Fthenakis Renewable and Sustainable Energy Reviews 103 (2019) 515–528

Table 1 Common Operation Conditions for Residential Wood Combustion Devices. Operating Condition Characteristics

Start-up or ramp up Fuel was just ignited and the unit ramping up to its full output, fan speeds (if applicable) and/or dampers are fully open; CO concentrations tend to be high as well as PM with low to moderate O2 values. Steady state operation Unit is running at full output, max fan speed (if applicable), fully open dampers, high burn rate of fuel, low CO, low PM, low to moderate O2 values. Intermediate or modulation or partial load Unit is running at less than full output because heat demand has been met; fan speeds are ramped down (if applicable), partially closed dampers; low to moderate burn rate of fuel, slowed combustion. Slumber Unit has overheated or met heat demand and can no longer modulate to slow combustion and reduce output. Dampers at this point may be fully closed or barely open to maintain a fire; in the case for a manual system, fans (if applicable) are off; CO concentration tend to be very high but can become low towards end of slumber, combustion is very low or stopped. Burn-out (i.e., end-phase) Fuel is almost consumed; high O2 values; CO values can range from low to high depending on the unit.

Fig. 1. Schematic of hydronic wood heater. a) single-stage unit b) two-stage gasification [29] (Courtesy of John Siegenthaler P.E.).

comparison to traditional OWBs. These new technologies are two-stage combustion designs with oxygen and/or temperature sensors and an advanced control system; they operate with full or partial external thermal storage, include multiple air dampers and fans to modulate and/ or stage combustion, and are intended for use either indoors or outdoors. These units contain approximately one-tenth the water vo- lume of older units internally but are installed with large external thermal storage. Fig. 3 below shows an example of an advanced wood- fired boiler. The concept is that these units will serve primarily to heat the storage tank while running at optimal, full-output condition. Heat from the storage will then be used to heat the home. With the in- tegration of air supply controls, temperature, and or thermal storage, the efficiency and emissions (mass based) performance is significantly Fig. 2. Hydronic heater integrated with heating system. Shown here is the unit improved due to the optimization of the air supply to fuel ratio [7]. integrated with a forced hot air furnace, but other integrations may include Thermal storage allows boilers to reduce cycling which in turn may radiant floors, radiators, or baseboards [30]. reduce emissions from start-up and shut down periods.

Similar to hydronic heaters, furnaces also are designed to burn and the second to burn the off-gases not fully combusted in the first cordwood, wood chips or pellets to heat an entire home and typically chamber; this typically involves the need for a fan or blower to push the are controlled by a home's thermostat. However, the heat from a fur- gases into the second chamber. Burning the exhaust gases allows for a nace is distributed as a gas (hot air) through ducts rather than as a more complete combustion and ultimately a gain in efficiency. liquid in pipes which is pumped through a home. Fig. 4 below shows Differences between the two systems are shown in Fig. 1 below. In both the difference between a hydronic and forced air OWB. Typically wood scenarios below, hot water would leave the system and be piped to the fired furnaces are located indoors. Johansson et al. [31] describe in home for heating and/or domestic hot water use. Fig. 2 shows how the detail common wood boiler designs for up-draught combustion while a system may be integrated with the home's heating system. NYSERDA report details down-draught combustion devices [7]. Down- The newest technologies for wood boilers are characterized by their draught boilers pull air in and use it to fully combust unburned gases low mass (small water volume) and smaller combustion chambers in below the active flaming area.

Up-draught boilers pull air in and heat it

517

R. Trojanowski, V. Fthenakis Renewable and Sustainable Energy Reviews 103 (2019) 515–528

Fig. 3. Advanced wood-fired boiler with thermal storage [29] (Courtesy of John Siegenthaler P.E.).

above the active flaming area. Modern/advanced boilers typically em- catalyst by closing a damper and redirecting the fuel gas path based on ploy down-draught combustion while conventional boilers and wood a stove temperature. stoves use up-draught combustion 3.3. Fire places and masonry heaters 3.2. Secondary heaters A third category includes units that may be used also as secondary Unlike a boiler or furnace, a wood stove is not controlled by a heaters but are not regulated by the US EPA. This category includes central thermostat to meet the home's heating demand. Wood stoves fireplaces and masonry heaters. Fireplaces generally have no controls only heat a small local area and are considered to free standing and associated with their design and simply exhaust flue gases up a chimney enclosed, unlike an insert or open fireplace. Older stove technology due by natural convection. Fireplaces are not regulated in terms of their (cordwood) typically uses an updraft burn which introduces primary air emissions as they are classified as a recreational burn, not a primary into the combustion chamber at the bottom of the stove and under the source of heat [34]. fuel injection causing the air and flames to naturally draft up.

These Masonry heaters similarly to fireplaces have no control mechanisms, stoves include a manual damper for the user to adjust to their desired are not a free standing device and typically built into the home's heat output. Some newer stoves introduced advanced controls to adjust blueprint. These units operate at a single high burn rate only, rapidly the dampers automatically (based on oxygen, temperature and/or heating the brick mass and thereby avoiding some emissions typically thermostat setting) as well as gasification and secondary combustion associated with low burn rate operations. They are high mass units with techniques which push combustion gases into chambers below the fuel heat exchanger channels for the hot flue gas to pass through the ma- further burning the gases, thereby improving both efficiency and sonry bed before exhausting to the outdoors. Thus the masonry material emission performance. stores heat and release the heat stored over a prolonged period of time. Stoves may also be categorized as catalytic or non-catalytic. Catalytic stoves employ a ceramic honeycomb design coated with a 4. Fuel types rare-earth metal catalyst placed in the path of the flue gas to enable combustion of the volatile gases at lower temperatures [7]. Therefore, Wood heater designs range from single fuel systems (e.g., cordwood, the construction and air supply vary between the two.

Often in older cribwood, pellets, wood chips, and briquettes (also called biobricks)), to technology especially, the user of the stove is responsible to ‘engage’ the multi-fuel systems which have been designed to burn pellets, corn

Fig. 4. Outdoor wood boilers (OWBs) a. hydronic [32] b. forced hot air [33].

518

R. Trojanowski, V. Fthenakis Renewable and Sustainable Energy Reviews 103 (2019) 515–528

husks, wood chips, and olive pits. In addition to the system design itself, Pellets are commonly used in automatic feed systems. They are a fuel type may also impact emissions, efficiency, and performance. Fuels more consistent fuel type due to their uniform size, shape, and moisture that contain bark for instance are known to contain many contaminants content. They are delivered to the end-user either by delivery trucks as trees will push the “unwanted” matter to their leaves or bark. In (similar to oil) or in 18 kg plastic bags. High quality pellets include summary Jensen found mineral compounds are up to 10 times more those made of sawmill waste; grasses, if used, often have higher ash abundant in bark than in wood, with bark ash contents up to 60%wt, content, chlorine, and sulfur levels which can cause the combustion predominantly calcium, silica, and phosphorus [35]. Additionally, chamber to corrode over time, making them less desirable unless ad- phenolic acids in bark are mostly found in outer bark cells. Low mo- ditives are used [43]. lecular weight extractives are often very abundant in bark such as al- Pellets are not to be produced from recycled material such as treated cohols, resin acids, alkaloids, and terpenes.

There can also be small or painted lumber as the emissions from burning materials of this type amounts of essential oils (volatile low molecular-weight terpenes) (e.g. would produce toxic emissions or large variations in burns. Required incense cedar, ponderosa pine, and eucalypts, among many others) certification programs exist in most European countries (ENplus) and [36]. Moisture content of biomass fuels are also another key factor in Canada (CANplus) to protect pellet consumers and ensure the pro- the ignition time and devolatilization rate [37], as well as the overall curement of a high quality pellet [44]. Unlike Europe, pellets fuel in the combustion performance [38,39]. US are only recommended but not mandated to meet a certification. The elements that may be found in a solid biomass fuel, and the The Pellet Fuel Institute (PFI) recently launched a third-party accred- emission from their combustion, also depend on where the tree was itation standards program to provide specifications for residential and grown. Specifically, trees living in areas with toxic air pollution or commercial-grade pellet fuel to ensure the consumer would purchase ground water contaminants may have toxic metals accumulated in the high quality pellets [45]. The Northeast States for Coordinated Air Use bark and leaves [40].

In the sections below various common fuel types Management (NESCAUM) sampled pellet fuels from 100 difference are described in some detail. companies and, despite US standards, [7], some pellet fuels did not meet standards. In some cases, the pellets did not meet the specified 4.1. Cordwood moisture content, showed seasonal variability, exceeded standard for chloride, and many samples had higher than normal values for Arsenic Cordwood or as more commonly referred firewood, is un-treated, (As), Cadmium (Cd), and Mecurgy (Hg). with or without bark and air-dried wood. Cordwood can vary in terms of species (fuel density) and pieces of the same species further may vary 4.3. Wood chips in moisture content, the amount of bark covering the surface, weight, and size. To complicate this fuel further, combustion of cordwood is Biomass fuels with particle sizes typically smaller than 5 cm × 5 cm subject to natural chimney draught, cannot be ‘turned on or off’ like × 5 cm are commonly classified as wood chips. Wood chips have 2–3 automatic fuels can with instantaneous ignition, and is subject to var- times lower bulk density than pellet fuel [39], they may contain bark ious lighting techniques. and their moisture content is typically higher than that of pellets.

To minimize this variability and increase best combustion practices, Higher quality or lower moisture wood chips typically are chipped stove or boiler manufacturers typically recommend their systems be from seasoned cordwood by a fuel grade chipper with set screens to fired with either two main types of cordwood: deciduous or hardwoods achieve a uniform desired size. As biomass tends to re-absorb moisture and coniferous or evergreens or softwoods. Examples of hardwoods if left uncovered, proper care is necessary to maintain lower moisture include oak (red or white), maple, or beech and softwoods include pine, content chips after chipping. In some cases solar heat (hot air for spruce or Douglas Fir. Traditionally, round logs are cut into four to six drying) has been employed to produce high quality [46–48]. pieces to improve the surface area and combustion performance. Lower quality, higher moisture content chips may be chipped by a Manufacturers typically recommend seasoned (air-dried) cordwood for general agricultural chipper directly after removing a tree (allowing for use in their devices with moisture ranges on a dry basis of 16–20%, no seasoning of logs and in some cases leaves, grass and dirt may also be however some units have claims to the ability to burn green wood included) which tends produce a shredded material, producing longer (moisture levels above 35%).

In addition, piece size, amount, and pieces of wood, twigs, dust, and a variety of non-uniform sizes. Wood loading procedures are listed in manufacturer's written instructions and chips produced in this type of condition would have a much lower device manuals. There are currently no known cordwood fuel specifi- heating value than those of uniform size, lower moisture, and free of cations to ensure consumer's fuel meet the recommendations of man- dust. Similar to cordwood there are no known specifications to ensure ufacturers in either the US or Europe, however a technical committee in consumer's fuel meet the recommendations of manufacturers in either Europe is currently drafting standards for cordwood (CEN/TC 335) the US or Europe, however a technical committee in Europe is currently [41]. drafting standards for chips as well (CEN/TC 335) [41].

4.2. Pellets 4.4. Briquettes

Pellet fuel or more commonly referred to as just pellets, are solid Biomass briquettes similar to pellets, use biomass sawdust or fiber biofuels made of compressed organic matter or biomass. Biomass type feedstock (straw, paper, agricultural waste, leaves, and so on), and feedstock for pellet production includes wood shavings, wood chips, use a densification process making them uniform in size and easy to grasses, straws, lumber mill scrap, and wastes from energy crops. store and transport but are pressed into a rectangular shape resembling Typically pellets use a sawdust type feedstock with little to no bark so a traditional red building brick or larger disk or solid or hollow log the use of a hammer mill is common as a first step to treat the raw shape of 7–25 cm in length and 10 in 30 cm in diameter. If the fines are material. The raw material may be dried if exposed to outdoor weather heated to a temperature which breaks down the biomass lignin under elements or if the raw material was green. During the processing state high pressure no binders for the briquettes are needed [49] but in some high temperatures cause the lignin to break down and melt which will cases, if the fibrous material is not heated or processed under high later solidify, holding the pellet together.

In some pellet plants a small pressure, a binder material is used such as molasses, starch, or even cow amount of cornstarch or flours may be added to act as additional “glue”. dung and cactus [50]. Finally, the processed material is pushed through a pellet die at high Depending on the type of raw material, the first steps of production pressures where the pellet is cut to the desired length (typically on the are sieving, drying, or crushing, followed with preheating, densifica- order of 0.6–3.8 cm [42]). tion, cooling, and packing the briquettes [49]. Studies show higher

519

R. Trojanowski, V. Fthenakis Renewable and Sustainable Energy Reviews 103 (2019) 515–528

Manual and automatic feed heaters (specifically boilers) rely on different mechanisms to meet the heat demand. Automatic feed boilers can modulate down to a lower burn if they sense they are approaching the heat limit and then go into a burn-out period to be restarted again by an electric ignition when there is a call for heat again. Manually fed boilers on the other hand, cannot restart automatically and so are forced to ‘slumber’ their fuel bed by shutting air dampers and slowing the rate of fuel combustion. In some advanced technologies the boilers have the ability to modulate down slowly and maintain a 30% of nominal output without going into a full ‘slumber’. Once there is a call for heat again, these boilers will open the air dampers and adjust the fans to achieve their full output once again. Gaegauf et al. [56] reported the particle size distribution of batch-wise fired appliances (wood stoves) varied significantly during a burn cycle, while wood log and Fig. 5. Particle pollution of different heat sources compared with diesel truck- continuous fed boilers showed a fairly constant particle size distribu- data from Denmark adopted from [5]. tion. Beyond the difference in size distribution of the emitted particles, differences in the particle's properties exist which will be discussed in detail below.

These differences are likely to influence the biological quality briquettes are produced under higher processing pressures and effects induced by wood smoke particles [10]. temperatures, longer dwell times, and increased binder contents Studies have shown combustion conditions and the resulting parti- [51,52]. Briquettes may be used in automatic or manually fed systems; cles strongly depend on fuel type, combustion technology, and type of the typically automatic systems only include primary heaters or com- operation [3,10,20,21,56–59]. One study in particular detailed the mercial grade systems. emissions of pellets in a pellet boiler and pellet stove, wood chips in a wood chip boiler, and two different cordwood species in a cordwood 5. Combustion conditions influencing nanoparticle emissions boiler and cordwood stove. The study found significant differences in terms of emissions factor (mg/MJ), even in the cases when spruce or Wood-heating appliances have highly variable combustion condi- beech cordwood was used in the same stove run at steady state; 67 and tions which cause large variations in the physicochemical character- 93 mg/MJ, respectively [60]. Additionally, the study found the period istics of the emitted particles [21].

The four main combustion condi- of combustion, cold start or steady state had a significant impact in tions explored in studies typically include start-up (initial burn period), terms of emission production, for some cases ten times greater emis- steady state (efficient), intermediate (a modulating stage with an output sions output (mg/MJ) were observed. lower than the unit's maximum) and burn-out (final phase of the burn). Fig. 5 shows emission factors for various type of wood-heating ap- These periods are most often identified by the carbon monoxide (CO), pliances in comparison to gas and oil boilers, district heating and heat carbon dioxide (CO2), oxygen (O2), and mass of fuel values. The steady pumps. It is shown that older, conventional wood combustion tech- state phase is where the maximum efficiency is often achieved and PM nologies have the highest emission factors. The highest emissions seen emissions are reduced. Burn-out is often described as the charcoal in older units are due to smoldering conditions and limited heat-storing, phase. Some heaters have the ability to modulate down via control so combustion gases are less burnt than in modern units [31]. A review strategies such as those mentioned in the advanced boiler systems or in terms of wood smoke health perspectives by Bølling et al.

show that manually controlling the draught damper in a stove for instance al- smaller particles (20–125 nm) are more common amongst logwood, lowing for 30% of full output or a ‘low’ burn. The most common op- pellet and chip advanced boiler units while open fireplaces and con- erating conditions and their characteristics are listed in Table 1 above. ventional (old) stoves produce nanoparticles in the range of 50–600 nm One additional common operating characteristic, but certainly a less [10]. This suggests that as conventional units are displaced by modern, desirable, is incomplete combustion. Manufacturers often include in- more efficient and “cleaner” systems, although the total particulate structions to users which would prevent such practice. However, many emissions are reduced, the number of NPs may increase. However, this units will operate in an incomplete combustion mode if poor practices is not accurately represented by current regulatory gravimetric mea- are followed. Incomplete combustion is caused by small values of the sures as NPs contribute very little mass even when present in significant three most important ‘T's’ in combustion; time, temperature, and tur- numbers. bulence. Too short of a residence time to allow for proper mixing, in- adequate mixing of combustion and fuel in the combustion chamber 6.

Nanoparticle emissions during residential wood heating due to a lack of turbulence or oxygen, and low combustion tempera- tures or cold zones in the combustion chamber are examples for in- Current emission regulations from wood combustion devices in the complete combustion parameters [53,54]. Characteristics of incomplete US and EU are based on an overall mass concentration of particles and combustion are also an increase in volatile hydrocarbon emissions and therefore NP levels may not be fully reflected in these emission reg- CO. The constituents of fine particle emissions during incomplete ulations due to their low mass contribution. Further, as mentioned combustion are organic matter, elemental carbon, and fine ash [55]. above NPs are of significant interest due to their risk of exposure in Typically, biomass heating systems are able to operate at a high terms of health and the environment and so many groups have begun efficiency and low emission level in their steady state, full output mode. investigating (measuring, counting, and characterizing) the NPs asso- However, when inefficient systems are used, poor operating techniques ciated with RWC. Results from a study involving 14 different appliance are employed, or poor fuel choices are made, all causing for an in- types, from cordwood stoves to automatic feed boilers showed 95% complete combustion, higher PM emissions occur.

A Swiss study (particle number) of the particle emissions were smaller than 400 nm pointed out emissions could be up to 100 times higher if a stove was not [56]. Hata et al. found in laboratory studies that more than 30% (of operated properly [5]. In addition, if an advanced system's heating load cumulative mass) of the smoke particles from the burning of biomass is decreased, cordwood and automatic wood-fired boilers (pellet and fuel had a mass that fell within a range of < 100 nm and this con- chips) may have trouble modulating (controlling the burn rate/output tributed greatly to the total levels of toxic PAHs and water soluble OC through use of fans and or dampers), causing them to cycle frequently [22]. resulting to higher PM emissions and lower efficiency. Bølling et al. [10] described three typical classes of wood

520

R. Trojanowski, V. Fthenakis Renewable and Sustainable Energy Reviews 103 (2019) 515–528

combustion particles based on chemical composition and morphology; approximately 107 nm) [63]. They also found that an increase in spherical organic carbon particles, aggregated soot particles, and in- combustion in air supply impacted the particle size distribution; thus organic ash particles. Their review shows the smallest (aggregated soot more oxygen caused an increase in the concentration of particles with particles, 20–50 nm) stem from combustion in conventional stoves, smaller diameter. In addition, the study showed the particle bound open fireplaces, and boilers fueled with cordwood, wood chips and PAHs decreased as oxygen levels increased; organic carbon was found pellets during high temperature and incomplete combustion phases. to be 14% for burning conditions with increased O2 and 57% when Saarnio compliments Bølling et al.’s description of particle classes by there was an absence of O2 [63]. Particle bound PAHs were likely re- defining the main components of ultrafine particles as EC, OC, sulfates sulted from adsorption or condensation processes. (SO42-), and trace metals [3]. The following sections provide details on Kocbach et al. [64] collected particles from two wood combustion nanoparticle type, size, and or concentration found in their respective sources in Norway; one from a small farm heating from a wood stove studies.

and the other ambient sample collected from a residential neighbor- hood who uses wood as a major heating source, collecting all stages of 6.1. Cordwood stoves the burn cycle. To confirm the particles collected were indeed all from wood sources, the group took direct samples from a common single- Hosgood et al. [11] studied stoves from 15 Chinese homes which stage Norwegian stove with a high temperature combustion chamber used coal or wood. Using an aerosol monitor, portable diffuser charger (1000 °C) and another during an air-starved combustion condition with and electrometer, the group measured in real-time concentrations of a low temperature of 500 °C. The results from X-ray microanalysis NPs in the ambient air surrounding these homes. They also measured (XRMA) the study indicated Potassium (K) and sulfur (S) were the most total PM2.5 one to two meters from the main combustion source. Their abundant elements observed in the particles, while Silicon (Si) and findings suggested NP levels were not adequately accounted for in the Calcium (Ca) were present but slightly less. Kocbach et al. proposed the total mass records due to NPs insignificant mass contribution to PM2.5 S levels in the carbon aggregates were not directly from the source but measurements; this supports arguments of needed particle type and rather caused from a reaction with gases in ambient air.

While the mean count regulations in addition to the current mass based emission reg- diameters for primary combustion particles for both locations remained ulations. Overall the group concluded there was a clear association very similar, roughly 39 nm ± 11 and showed consistent number of between cooking and heating activities and elevated local NP levels particles (226–239), the morphology differed from Transmission Elec- [11]. tron Microscopy (TEM) analysis. The high temperature combustion Bologa et al. [61] tested an 8 kW stove with dry beech log wood and produced carbon aggregations while the low temperature combustion characterized the particle size distribution as well as the particle mass did not have aggregates but carbon dominated spherical particles with concentration. Their study showed the short start-up time of five to diameters between 50 and 600 nm [64]. The NPs produced during the seven minutes produced more particles than the steady state and burn- low-temperature combustion were similar to particle from smoldering out phases. Stack temperatures for the start-up, steady state and burn- forest fires. out phases were ~100 °C, 300–330 °C and 160–180 °C, respectively. A later study by Kochback et al.

[65] collected NPs from a common The group showed the start-up and steady state phases to have a mono- single-stage Norwegian wood stove in a laboratory at high-temperature modal particle size distribution of 180 nm while the burn-out phase had burn conditions, following Norwegian standards for testing wood stoves the highest concentration of nano-sized particles with a bi-modal dis- using a dilution tunnel. The group examined the NPs collected with tribution of 180–200 nm and 30–40 nm. High Resolution Transmission Electron Microscopy (HRTEM) to further Stack emissions were diluted from a welded construction stove and determine the microstructure of the NPs. Similar to an earlier study analyzed for their size distribution and total number concentration in [64], only carbon aggregates were found with the primary particles the Gaegauf et al. study [56]. The start-up period of the stoves showed having a mean geometric diameter was 31 ± 7 nm from TEM analysis. the largest particle size (i.e., average 200 nm), followed by the steady From HRTEM analysis the group found the wood smoke NPs had one state period with an average of approximately 175 nm and the smallest nuclei and a turbostractic structure; a concentric arrangement of layer particles (~ 30 nm) were found in the burn-out period.

Similar to the places with a two-dimensional graphitic crystal structure, lacking the study above, the lowest number concentration of particles less than ordered stacking of graphite [66]. In addition, the group found the NPs (0.8 µm) was seen in the steady state period and the highest in the burn- to have a total PAH count of 9745 ng/mg. out. Corsini et al. [67] studied an advanced cordwood stove with a Hays et al. studied the polycyclic aromatic hydrocarbons (PAHs) nominal output of 8 kW. The stove has state-of-the-art triple air supply associated with both Douglas Fir and White Oak at high (> 24%) and system with both primary and secondary air supply. They found the low (13%) moisture contents in a freestanding steel woodstove. Their ultra-fine particle (UFP) number concentration was 7.0 × 108 particles/ experiments were run in compliance with US test standards, measuring cm3 with an average GMD of 50 nm for beech logs while the UFP only a steady state condition, and used an Electrical Low Pressure number concentration was 1.9 × 108 particles/cm3 with an average Impactor (ELPI) to collect filters and analyze the PAHs. They reported GMD of 99 nm for conifer logs. In addition, the group found the PAH that the geometric mean diameter (GMD) is between 313 and 662 nm concentration was significantly higher in UFP from wood logs, with particles representing an accumulation mode [62].

The study showed conifer wood logs generating approximately 20X more PAH compared low molecular weight PAHs preferentially segregated to fine aero- to beech wood logs [67]. dynamic diameter particles while higher molecular weight PAHs pre- ferred coarse particles. 6.2. Pellet stoves Hueglin et al. [63] studied the size and particle bound PAHs. A dilution sampling system was used to collect the particles, employing a A study by Commodo et al. found the smallest particles emitted scanning mobility particle sizer (SMPS) measure the particle size dis- from a pellet stove were on the order of 7–30 nm and were also the most tribution. The stove had a 15 kW output and was fired with beech wood abundant; on the order of 1×10 4particles/cm3 [68]. The study also having a moisture content of 15–18%. The results showed the size showed the amount of particles emitted after the transient phase de- distribution was strongly dependent on the state of the combustion creased significantly, showing fewer particles (< 100 nm) in the steady process; start-up had the greatest particle concentration and largest state or stationary phase. mean diameter (approximately 260 nm), while the burn-out phase had Corsini et al. [67] also studied an 11 kW top-fed pellet stove with the smallest mean particle diameter (approximately 60 nm) and inter- two different pellets—beech and conifer.

In agreement to the results mediate phase had the lowest concentration (mean diameter from the cordwood stove, higher GMD diameter sizes were measured

521

R. Trojanowski, V. Fthenakis Renewable and Sustainable Energy Reviews 103 (2019) 515–528

with the soft-wood fuel than the hard-wood fuel with diameters of Mavrocordatos et al. [18] collected particles from a 15 kW pellet 75 nm and 38 nm. The average UFP number concentration for both the boiler flue for a detailed analysis of particles which included: particle beech and conifer pellets was 1.1 × 108 particles/cm3. These in- count, size distribution, morphology, volume, elemental composition, vestigators reported that there were lower quantities of PAHs associated and fractal analysis. Employing a SMPS, they found that the maximum with the particles generated by the pellet stove than the cordwood in number concentration measured 80 nm. From Atomic Force Micro- stove. This was most likely due to more complete combustion in the scopy (AFM) and Scanning Transmission Electron Microscopy (STEM) pellet stove that ultimately produces less inflammatory active particles the group found the individual particles of different shapes and ag- [67]. gregates with a highly compact structure. The group noticed when the particles were exposed to the electron beam in the STEM, after only a 6.3. Cordwood boilers few minutes the smaller particles (< 100 nm) would disappear and the larger would reduce in size and to an inorganic residue consisting Schmatloch and Brenn [58] found from two different cordwood mostly of K, Chlorine (Cl) or S.

Using an inductively coupled plasma boilers of 41 and 35 kW, a modern lambda (oxygen) controlled and fan atomic emission spectrometer (ICP-AES) the group was able to do an assisted boiler and a traditional boiler, respectively, that the advanced elemental analysis in addition to a total organic carbon (TOC) mea- unit produced slightly smaller particles. Using an ELPI and SMPS, the surement by using a TOC-Analyzer. The analysis showed a TOC content maximum of the number size distribution laid between 80 and 180 nm of 370 ppm, the presence of K, Ca, S, Magnesium (Mg), Cl, Phosphorus but varied significantly for different phases; highest concentrations (P), and Sodium (Na) in decreasing amounts (ppm) and traces of Iron were found during the start-up. (Fe), Aluminum (Al), Mg, and Zinc (Zn). From the analysis the group Johansson et al. [31] studied a variety of cordwood boilers which drew the conclusion that these wood combustion emission particles included old, traditional units and modern, advanced systems with were likely inorganic salts that could dissolve easily in water [18]. power outputs ranging from 6 to 34 kW. Using beech cordwood with Lamberg et al. [9] along with Kaivosoja et al. [69] studied a 25 kW moisture contents of 15, 26, or 38% moisture content, the group in- modern pellet boiler using pine pellets.

The group found the GMD to be vestigated the effects of varying the load; running the systems at 100% 60 nm with a mean mass diameter (MMD) of 137 nm [9]. In addition, full output or firing the units with a full or half load of cordwood. The the total PAH emissions from particulate matter 1 μm and smaller group used an ELPI to determine the size distribution and Dekati Low- (PM1) emissions (PAH/PM1) were extremely low, 0.00006%, with Pressure Impactor (DLPI) to determine the number particle concentra- pyrene and fluoranthene as the most abundant. Kaivosoja et al. sup- tion, diluting the samples before measurements. In addition, the group ported this work by characterizing the PM1 by elemental analysis. The sampled for PAHs as well. Ultimately the group found older wood group found K was the main element found followed by Na, Cl, and S boilers had higher emission concentrations and un-oxidized compounds —suggesting that these elements were all in the form of alkali metal than modern wood boilers [31]. In fact, particle emissions were de- sulfates (SO4) and chlorides (Cl-) [69]. creased by 70–80 times when comparing a modern wood boiler with a Gaegauf et al. [56] also measured the particle mobility diameter ceramic-lined burn-out zone, connected to an external heat storage distribution of a 17 kW pellet boiler; diluted from a hot stack. They tank.

The PAHs were also found to be the highest in the case of the older found a peak mobility diameter occurred at approximately 85 nm, but wood boilers. Size distribution data indicated a peak mean diameter ranged from 12 nm to just below 400 nm. slightly greater than 60 nm and a decreasing trend in terms of con- centration as particle diameter grew for both the older wood boilers and 6.4.2. Chip boilers fan assisted combustion modern wood boilers. Interestingly, the dia- Using a 40 kW wood chip boiler, Torvela et al. [21] studied the meter range shifted towards smaller particle sizes in the case when chemical composition link with particle morphology and the effects of modern wood boilers were fired with a natural draught. combustion conditions on particle type. These investigators looked at three combustion phases that they classified as efficient (low level of 6.4. Automatic feed boilers CO: 0–100 mg/MJ), intermediate (elevated CO: 100–1000 mg/MJ) and smoldering (high CO: 1000–5000 mg/MJ) [21]. Using a SMPS the mode 6.4.1. Pellet boilers number size distribution was 37, 150, and 250 for the efficient, inter- In addition to the cordwood boilers studied by Johansson et al. [31], mediate, and smoldering conditions, respectively. They calculated a the group included pellet boilers ranging from 3 to 22 kW in power GMD for the efficient, intermediate, and smoldering conditions to be output.

The pellet boilers were operated at full or intermittent loads 25, 65, and 160 nm, respectively. However, even during the immediate (less than 100% output) with pellet fuel having a moisture content of and smoldering phases, a smaller number of particles with a dimension 7.6% or bark pellets having a slightly higher moisture content of 7.8% of 40 nm were observed. but significantly higher ash content of 0.5–3.7%, respectively. Two Torvela et al. also identified via TEM the three main wood com- pellet boilers run at their full output of, 11 and 22 kW power output bustion particle types discussed in Bolling et al.’s review article [10]; ignited with a pilot flame and electrical ignition, respectively, showed ash, soot, and particulate organic matter (POM). The TEM images very similar mass distributions and maximum particle sizes of 130 nm. showed the ash as crystalline structures smaller than 100 nm, the soot Experiments that involved the bark pellets showed an increase of mass particles chained as agglomerates with a primary particle size of concentration and a slight shift to larger particle sizes (approximately 30–50 nm and the POM as an amorphous type with a course surface 220 nm). No obvious trend was observed between full and intermittent [21].

Table 2 below summarizes the findings of particle morphology output in terms of particle size, only a very slight increase in mass and composition from Torvela et al. concentration. Similar to the study of Mavrocordatos et al. [21] reported above, Bologa et al. [61] also characterized the particle size distribution Torvela et al. found many of their species vaporized, decomposed, and from 20 kW and 32 kW pellet boilers with flue gas temperatures during changed during TEM analysis as well to the form crystals or bubbles steady state operation in the range of 170–180 °C and 120–140 °C, re- inside particles, suggesting there may be semi-volatile hydrocarbons spectively. Similarly, to the cordwood stove tested by the same group, adsorbed on the surface of the soot. The group included an EC/OC (mg/ the steady state pellet boilers also had a mono-modal particle size MJ by thermal-optical), PAH (μg/MJ) by gas chromatography-mass distribution; however, there was a shift towards a smaller particle size, spectrometry (GCMS) and PM1(mg/MJ) by gravimetric analysis. In all peaking at about 60 nm. Furthermore, the group showed a decrease in cases, the largest counts for each were found in the smoldering condi- particle mass concentration for the pellet boilers as well; pellet boilers tion and the lowest in the efficient burn. at 10–20 mg/Nm3 vs the wood stove at 25–40 mg/Nm3. Gaegauf et al.

[56] also studied a wood chip boiler with a 70 kW

522

R. Trojanowski, V. Fthenakis Renewable and Sustainable Energy Reviews 103 (2019) 515–528

Table 2 Summarized Findings from Torvela et al. [21] Study Investigating NP PM emissions from Wood Chip Boiler. Combustion conditions Particle size and composition

Efficient Particle diameter < 100 nm, irregular shape. Most particles were ash with crystalline nuclei; some identified as zinc oxide. Intermediate Particles appeared to be more spherical in shape with coating of organic matter. Sizes ranged from 40 to 150 nm with the largest particles making agglomerates that contained ash, soot and organic matter. Smoldering Almost all large branched soot agglomerates. Some inorganic elements were detected, but were mostly a mixture of soot and POM with a minor amount of ash.

output; diluted from a hot stack. The group varied the excess air during Dispersion Spectroscopy) and OC, and the efficient burn produced the operation of the chip boiler to study its effects on particle size and particles which were merely composed of ash (mainly K, O, with larger found that lower excess air values corresponded to lower particle amounts of P, Zn, Si and S compared to the intermediate burn). counts and a shift towards higher diameters; 11.6% O2 and 2.7% O2 corresponded to diameters of 60 nm and 90 nm, respectively. 6.5. Fireplaces and masonry heaters Hueglin et al. [63] continued their size distribution and particle bound PAH study focusing on a chip boiler with an output of 60 kW, Fireplaces and masonry heaters are often used in studies due to their and enriched the study by looking at the particles morphology as well. lack of controls and so it is easy to study the effects of different types of The group coupled the SMPS with an aerodynamic particle sizer (APS) biomass without being skewed by device controls. to gain more information about the particles and measure larger dia- As early as 1982, emission measurements were to study the size and meters.

The wood chip boiler was almost always run in a steady state composition of particles were taken from two residential fireplaces and full output mode as it is computer controlled and burned chips with a one free-standing fireplace to determine the influence they potentially moisture content between 40% and 70%. Similar to the studies dis- had on Denver's air quality. Muhlbaler Dasch [71] made both in-field cussed already, particle emissions were dependent on operating con- and laboratory measurements, investigating five softwoods, nine ditions. Since the study only looked at steady state operation, the only hardwoods and two synthetic logs (similar to a biobrick construction). varying parameter was the combustion air supply to mean the excess Laboratory and residential test followed EPA method 5 protocols, clo- oxygen concentration target. The GMD size distribution had its lowest sely, so flue gas was diluted. A Sierra Series-220 cascade-impactor was value when O2 values were at 6.2% (68.6 nm) and its largest mean used to determine the size distribution of particles collected. While the diameter size at an O2 value of 11.6% (95.7 nm); the concentrations log type and size seemed to have the greatest effect on particulate also followed a similar trend [63].

Overall, the NPs concentrations were emissions in terms of total particulate emissions (g/kg), there were no reduced when the boiler was operated at an optimized O2 value. The obvious differences in terms of size fractionation for the three types of group found particles with a diameter greater than 1000 nm to be wood; the mass median diameter was found to be 170 nm [71]. From negligible when using the APS. Particles emitted from the chip boiler SEM imaging, Muhlbaler Dasch also found the particles to be agglom- contained low amounts of semi-volatile compounds as well. SEM erates and spherical in shape. images revealed different types of particles; agglomerates of spherical A study in Italy focused on investigating the particle size distribu- compact particles, perfectly spherical particles representing coal fly tion of NP emissions from an 11 kW commercially available fireplace ash, or irregular shaped crystalline platelets. burning beech wood (the fuel for European standards) [2]. The group Another study by Leskinen et al. [70] focused their investigation on analyzed 15 tests over three different burn periods: start-up, steady a wood chip-fired appliance. The group created a novel modular com- state, and burn-out.

Similar to US test methods the group measured bustion unit which included a reactor and moving grate (not a marketed from a dilution tunnel and further diluted the flue gas to achieve less boiler with given output) which allowed them to produced three dif- particle coincidence. To determine NP concentrations an ELPI and a ferent combustion conditions: efficient (representing optimal condi- Fast Mobility Particle Sizer (FMPS) were used. It was concluded NPs tions), intermediate (representing a partial load or poor air-stage set- accounted for 67% of the total particles measured with the mode par- tings) and smoldering (burn-out or smothering situation observed in ticle diameter of the average size distribution to be around 10–20 nm, batch combustion) with similar CO levels as seen in the work done by in all three phases, but the highest concentrations were found in the Torvela et al. [21]. The group provided detailed information on the start-up phase and lowest in the steady state period [2]. The group system and fuel type which was both bark and stem wood chips from found particles in the range of 100–326 nm as well but suggested they spruce and typical Finnish broadleaved trees with a moisture content of were likely primary soot particles that underwent condensation in the 24.3%. The group was able to measure the number and mass con- diluted and cooled fuel gas [2].

A dip in O2 values corresponded to a centrations as well as the size and mass distribution in addition to decrease in number of NPs as well. Interesting in terms of environ- characterizing the particle morphology, EC/OC, elemental and ion mental fate of wood combustion NPs, the group showed that due to analyses, PM1 and PAH compounds, similar to Torvela et al. [21] re- dilution and cooling of flue gas, stable distinct size distributions during ferenced above. Data from each of the burn periods indicated the each separate phase were achieved with only slight changes during the smallest mass emission concentration (mg/MJ, (amount of particles per transition from one phase to another. kilogram of dry wood combustion)) and smallest particles were pro- In a later study by Ozgen et al. [72], using the same 11 kW stove as duced during the efficient period (average diameter of 35–75 nm) and mentioned above [2], nanoparticles with a GMD of 28 nm accounted for the largest particle diameter size and mass emission concentration were 32% of the emissions whereas larger particles with a GMD of 127 nm observed during the smoldering condition (average diameter accounted for 68% of the emissions. Similar sampling techniques were ~200 nm). From the groups chemical analysis, they found Zn was the employed using a dilution tunnel.

The investigators found the ignition most abundant metal with the highest relative fraction occurring during phase was responsible for 46% of the NP burden and cold starts, which the efficient conditions. The fraction of PAHs were found to be lowest are common practice in the field, resulted in a magnitude higher in during the efficient burn and highest during the smoldering. TEM emissions. They reported that larger soot particles emitted during the analysis showed the smoldering burn produced particles were made up flaming combustion phases were coated by condensed semi-volatile of mostly large agglomerates of soot (EC and OC), intermediate burns organic species that were diluted and cooled in the flue gas; this finding produced a variety of particles from soot agglomerates and ash or suggests that minimizing incomplete combustion is enough for particle composite particles of ash (mainly K, O, Zn determined from Electron number reduction. Similar to other cited studies, the Ozgen et al. [72]