C3d Science Nanoparticle emissions from residential wood combustation 2019, chapter introduction background conclusion (geanonimiseerd)

Renewable

and Sustainable

Contents

Renewable ELSEVIER

literature

emissions

from

Reviews

lists available

103

(2019) 515-528

ScienceDirect

at

and Sustainable Energy Reviews

journal homepage:

Nanoparticle

Energy

www.elsevier.com/locate/rser

residential wood

combustion:

A critical

®

review, characterization, and recommendations

en

Check for

Updates

ee

(geanonimiseerd) (geanonimiseerd) *

b

Center for

Life Cycle Analysis, Earth and Environmental Engineering, Columbia University, New sustainable Energy Technologies, Brookhaven National Laboratory, Upton, NY, United States

ARTICLE

INFO

York, NY, United States

ABSTRACT

Keywords:

The increased

Biomass

about potential environmental

Wood

heating fuel in residential and small commercial buildings has increased concern and safety impacts, specifically particulate matter (PM) emissions in the nanometer range. Larger particles (> 2.5 um) can be effectively removed from exhaust streams by emission control devices. However, nanoparticles (NP), due to their size, are more difficult to capture in exhaust flue gases. In addition, NPs have a higher surface to volume ratio, allowing them to absorb organic compounds, causing them 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 residential 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.

stoves

Nanoparticle Emissions

Regulations

use

of wood

as

a

1. Introduction

United

States (US)

emissions The

heating fuel in residential buildings has increased concern about potential environmental health and safety impacts. The increased use is partially due to the historically low cost of wood fuel, but also because the technology offers a renewable alternative to fossil fuel heating. In ad: ition, single room heaters using either pellets or cordwood can help offset significant amounts of home heating oil or propane without the larger investment of replacing a boiler. As the use of biomass is enc ouraged as a rewewable fuel source and considered to be an important player in the transition to renewable fuel sources, it suffers from higher particulate matter (PM) emissions in comparison to oil or natural gas fired systems. The World Health Organization estimates that more than 50% of the world population uses biomass fuels for cooking and heating [1] and these PM emissions from residential wood-smoke have corresponded to declines in local air quality in Europe [2-4]. As

increased

of

2015, residential

sponsible

for

diameter

<

*

of wood

use

more

2.5

um)

as

wood

than 45% in Europe

(RWC)

devices

were

trucks

2.

than

one-third

Protection

of

primary PMa.s

Agency (EPA)

lists the

[5].

Background

Especially of concern are emissions from wood combustion of subparticles, i.e, nanoparticles (NPs) (< 300 nm} The absorbed compounds in some cases may be potentially carcinogenic and induce a

re-

of PM».s

(particles with an aerodynamic [5]. Similarly, biomass burning in the

micron

@cotumbiacc (geanonimiseerd) EN

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

more

largest sources of PM>s as utility fuel, RWC and prescribed burning (land management tool to maintain forest health) [6]. Specifically in New York State (NYS), residential wood heating contributes institutional (ICI) 275% more PM 5 than all industrial, commercial and emissions 550% more than the electricity PMss heating combined, generation sector, and 35% more PMa.5 than the transportation sector; overall, contributing more than 90% of the PM2.5 emissions in NYS even though it only provides less than 2% of NYS overall heating needs [7]. Similar trends are reported by the Danish Ecological Council; the number of particles per unit volume of wood burning exhaust in Denmark, were found to be than 230 times greater than those from diesel

Corresponding author.

(geanonimiseerd)

for

Environmental

three

a

combustion

accounts

and the US

1

January

2019

(geanonimiseerd)

Renewable and Sustainable

more

their

the

pronounced pro-inflammatory response than larger particles of material [8-11]. Depending on the combustion source, small amounts of sulfates, nitrates, and metals may also be present along with adsorbed volatile organic carbon (OC) [12]. Perhaps the combination of metals present, organic compounds, persistent free radicals and/or low solubility or degradability are the likely source of observed health impacts of airborne PM such as oxidative stress [9,13,14]. A review by Bgman et al. discussed exposure effects such as asthma, respiratory symptoms, mortality, and declined lung function had a stronger correlation in areas where residential wood smoke combustion was a major source of PM [4]. Other reports show that NPs can become suspended in wash water, make their way into organisms and eventually cause serious health effects to humans through the food chain [15].

ditionally,

same

Many studies have looked

at

NPs,

but

to

fate of these

NPs

and

their eventual

indicates

that 99%

ambient

atmospheric

smaller

[8]. However,

pogenic

NPs

[16]. Several

date

little is known

environment is

made

of all

typically

and

contribute

NPs

al.

the chemical

on

times

carbon et

emissions (<

has

and

role in

[12] report that approxi-

are

global warming. Black carbon NPs are of particular to absorbing light, they are inefficient light scatters as they are much smaller than the 400-700 visible light spectra [19]. RWC not only contributes to photochemical smog [22,23] but has been reported to be a significant source of black and OC [24]. A recent shows that 56% of the black carbon soot in is caused from Europe study RWC and with a projected increase in its use to meet renewable energy goals, the contribution of black carbon soot is expected to grow upwards to 70% by 2030 [5]. The accumulation of NPs in the environment may not only induce climate change but also affect atmospheric visibility decreasing the sun's light intensity, and have an effect on the environmental hydroxyl radical concentration, ultimately depleting the ozone. Smita et al. [25] summarized

to

a

made

weather

extreme

in South

scenario

up of soot correlated the brown are

from clouds

conditions

production. The

authors

radicals

environment

in the

Asia

biomass

also

where

70%

combustion

of the brown

clouds

in cook

They

stoves.

pollutants

which

to

contribute

to

cloud

carry trace metals and other insoluble and immobile causing can

Residential

wood

Residential

systems may

and

residential

units

they

major

so

are

heaters. serve

to

combustion

combustion

Primary heat

an

devices

continues to increase.

systems

be

classified

heaters

include

entire home

as

and may

heaters

primary

biomass

boilers

also include

or

or

sec-

furnaces

domestic hot

are

Primary

and

do not need

an

electric

source

heaters

type of primary wood heating appliance is the wood-fired hy-

heater,

Europe

technology that

a over

the

past

has been

decade.

evolving rapidly in the U.S. Hydronic heaters heat water, or

otherworking fluid, and then pump the fluid to the home via baseboard or traditional cast iron radiators. Hydronic systems are most common in Europe and the Northeast US. Older wood-fired hydronic technology involved outdoor-installed units with large integral water volumes and updraft combustion (commonly referred to as outdoor wood boilers, OWBs). These units operate with air flow controls that set the firing rate to

re-

maintain

a

target boiler

water

temperature.

These

units

control

combustion

by just opening and closing a damper operated by an aquastat [7]. During periods of low-heat demand, these units go into a “slumber” or smoldering combustion mode, (described in Table 1), which can create high emissions. Also, these units have very large combustion chambers allowing them to be easily overfilled potentially causing a hundred-pound charge of wood in a boiler with a nominal output capacity in the range of 100,000 Btu/h to smolder. Hydronic heaters may be classified as single-stage or two-stage gasification boilers. Single-stage units only involve one chamber for combustion and an air damper or two. These units are typically less efficient as unburned gases leave the flue. Two-stage gasification systems involve two chambers; one chamber to burn most of the biomass

in

nuclei genin water; discontaminants [28] that are

OC

otherwise

3.

One

condensation

solved

of wood

use

dronic

and

is known

the

3.1.

glacial melting, reduced sunlight and have impacted local agricultural discussed the NPs reaction with hydroxyl and their immediate binding due to the Typically, hydroxyl radicals degrade pol-

[27] but may also interact with the natural

However,

systems’

simply rely on the operator to ignite the fuel source and control air dampers. Advanced units may include fans, lambda (oxygen), temperature sensors, and automatically controlled dampers that require an electric source to operate. Some wood heater designs also promote the use of duel fuel i.e. a manually fed cordwood and automatically fed pellets

which

eration

above.

removal

and

greenhouse gases are observed. In addition to NPs affecting the atmospheric chemistry and inducing climate changes, NPs can eventually deposit onto soils and bodies of water causing secondary contamination and other environmental effects [19,26]. Hata et al. found wood combustion NPs (in comparison to larger particles) had the largest fraction of particle-bound water soluble OC

PM

tional stoves that lack automatic controls

to

very reactive nature of both. lutants due to their strong oxidation nature, however with the NP action, the free radicals are no longer available and an increase

Ad-

heaters may be manually or automatically fed. Manual systypically fed in batches with fuel types such as cordwood (logs or split wood), cribwood (dimensional lumber}, and briquettes. Automatically fed systems transport wood chips and pellets via an auger to the combustion chamber; they may be top fed, horizontally fed or bottom fed [7]. Systems which are automatically fed or include controls, require electricity to operate the fuel screw, glow-plug ignitor and fans, whereas manual systems may not require electricity. Tradi-

since, in addition

concern

these

as

tems

between the sizes of 1 and soot, which are mostly elemental carbon (EC), non-toxic to cells but are known to absorb sunlight

are

gases.

Wood

properties

significant

flue

serts.

100 nm)

[16,19]. a

devices mentioned

have

exhaust

in

capture

heating systems (office buildings and heating) are more efficient and equipped with

Secondary heaters, also commonly referred to as ‘room heaters’, are used as a supplemental heat source, particularly for one room, and in some cases may only be used for ambience. This category typically includes wood stoves, masonry stoves, and fireplaces and fireplace in-

in the

nm

not

to

biomass

water.

atmospheric particles

in diameter

and

300

often

which

review

particles

NPs

for district

control

ondary

systematic description of anthrofate, and effects on the environment

and their occurrence, studies showed that ultrafine

15%

nm

of

up

used

emission do

difficult

more

of PM in the air. Therefore, this review paper focuses on the NPs produced from residential wood heating devices. In addition, the paper offers suggestions for future studies from an environmental perspective

environmental One

are

commercial

sources

no

It is known that black and brown altering the climate [20,21]. Nienow 1000

the

humans.

to

concentration of

are

may affect the climate and this is dependent of the aerosol [3,17,18] and their residence

mately

about

pathway

of the total number there

those

the health effects associated with wood

combustion

size,

Energy Reviews 103 (2019) 515-528

OC

carcinogenic effects on organisms [22]. Larger particles (greater than 2.5 um} typically derive from the inorganic fraction of the fuel in the form of ash, or may be agglomerations of fine and ultrafine particles, but they can be successful removed from exhaust streams by emission control devices [23] such as electrostatic precipitators (ESP), cyclones, and baghouses. Nanoparticles, due to

1some

may be sold with catalyst; however, this practice is more the US than in the EU. Recently a prototype of a residential boiler with a mini ESP was introduced in the EU but this is not a common practice yet common

[84].

516

stoves

in

(geanonimiseerd) Table

Renewable and Sustainable

Energy Reviews 103 (2019) 515-528

1

Common

Operation Conditions for Residential Wood Combustion

Operating Condition

Characteristics

Start-up

Fuel

or

ramp

up

Devices.

just ignited and the unit ramping up to its full output, fan speeds (if applicable) and/or dampers are fully open; CO to be high as well as PM with low to moderate Os values. 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 Os values. 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. 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. Fuel is almost consumed; high Os values; CO values can range from low to high depending on the unit. was

concentrations tend

Steady

state

Intermediate

operation modulation

or

or

partial load

Slumber

Burn-out

(ie, end-phase)

to

2-stage

chimney

connection

(gasification) hretubes

j— pinsulation r-water in sections

w/

turbulators

damper during gasification)

(closed

thermostatic

regulator

blower

wood

loading

ceramic

grates air

damper—

coals cast iron

b

\

\

Fig.

1.

\

+

ash

grate

pan

primary

air

flow

Schematic of hydronic wood heater. a) single-stage unit b) two-stage

gasification [29] (Courtesy of John Siegenthaler P.E.). traditional OWBs.

comparison

to

combustion

designs

advanced

control

with

system;

thermal

storage,

and/

stage combustion,

or

outdoors.

These

lume

older

of

include units units

These

new

and/or

oxygen

technologies

temperature

are

two-stage

sensors

and

an

they operate with full or partial external multiple air dampers and fans to modulate and

are

intended

contain

approximately

internally

but

are

for

use

either

one-tenth

installed

with

indoors

the water

large

or vo-

external

storage. Fig. 3 below shows an example of an advanced woodfired boiler. The concept is that these units will serve primarily to heat thermal

the

from

tegration of air supply controls, temperature, and or thermal storage, the efficiency and emissions (mass based} performance is significantly improved due to the optimization of the air supply to fuel ratio [7]. Thermal storage allows boilers to reduce cycling which in turn may reduce emissions from start-up and shut down periods. Similar to hydronic heaters, furnaces also are designed to burn cordwood, wood chips or pellets to heat an entire home and typically are controlled by a home’'s thermostat. However, the heat from a furnace as a is distributed gas (hot air} through ducts rather than as a liquid in pipes which is pumped through a home. Fig. 4 below shows the difference between a hydronic and forced air OWB. Typically wood

Fig. 2. Hydronic heater integrated with heating system. Shown here is the unit integrated with a forced hot air furnace, but other integrations may include radiant floors, radiators, or baseboards [30]. and

the

to burn the off-gases not fully combusted in the first typically involves the need for a fan or blower to push the gases into the second chamber. Burning the exhaust gases allows for a more complete combustion and ultimately a gain in efficiency. Differences between the two systems are shown in Fig. 1 below. In both scenarios below, hot water would leave the system and be piped to the home for heating and/or domestic hot water use. Fig. 2 shows how the system may be integrated with the home's heating system. The newest technologies for wood boilers are characterized by their

chamber;

low

mass

second this

(small

water

volume}

and

smaller

combustion

chambers

storage tank while running at optimal, full-output condition. Heat the storage will then be used to heat the home. With the in-

fired

furnaces

detail

common

are

located

517

boiler

indoors.

Johansson

et

al.

[31]

describe

in

designs for up-draught combustion while a details down-draught combustion devices [7]. DownNYSERDA report draught boilers pull air in and use it to fully combust unburned gases below the active flaming area. Up-draught boilers pull air in and heat it

in

wood

(geanonimiseerd)

Renewable and Sustainable

Energy Reviews 103 (2019) 515-528

load circulator

bypas cirulator

} controller ï 1

1 1 1 Hú 1

wood-fired boiler

pressurized thermal Fig.

3.

Advanced wood-fired boiler with

thermal

the active flaming area. Modern/advanced boilers typically employ down-draught combustion while conventional boilers and wood stoves use up-draught combustion

catalyst by closing a

stove

3.3. 3.2.

Secondary Unlike

central

a

(Gourtesy of John Siegenthaler P.E.).

storage [29]

above

storage tank

a

damper

and

redirecting

the fuel gas

path based on

temperature.

places

Fire

and masonry

heaters

heaters

boiler

thermostat

third category includes units that may be used also as secondary but are not regulated by the US EPA. This category includes fireplaces and masonry heaters. Fireplaces generally have no controls A

or

to

furnace, meet

a

wood

the home's

stove

heating

is not

controlled

demand.

Wood

by

heaters

a

stoves

associated with their

only heat a small local area and are considered to free standing an enclosed, unlike an insert or open fireplace. Older stove technology {cordwood) typically uses an updraft burn which introduces primary air into

the combustion

fuel

injection causing

chamber the air

at

the bottom

and

flames

of the stove and under

due

by natural

emissions

as

design and simply exhaust flue gases up a chimney Fireplaces are not regulated in terms of their they are classified as a recreational burn, not a primary convection.

[34]. Masonry heaters similarly

source

the

of heat

fireplaces have no control mechanisms, and typically built into the home's blueprint. These units operate at a single high burn rate only, rapidly heating the brick mass and thereby avoiding some emissions typically associated with low burn rate operations. They are high mass units with heat exchanger channels for the hot flue gas to pass through the masonry bed before exhausting to the outdoors. Thus the masonry material stores heat and release the heat stored over a prolonged period of time.

naturally draft up. These stoves include a manual damper for the user to adjust to their desire heat output. Some newer stoves introduced advanced controls to adjust the dampers automatically (based on oxygen, temperature and/or thermostat setting) as well as gasification and secondary combustion techniques which push combustion gases into chambers below the fuel further burning the gases, thereby improving both efficiency an emission performance. Stoves also be categorized as may catalytic or non-catalytic. Catalytic stoves employ a ceramic honeycomb design coated with a rare-earth metal catalyst placed in the path of the flue gas to enable combustion of the volatile gases at lower temperatures [7]. Therefore, the construction and air supply vary between the two. Often in older technology especially, the user of the stove is responsible to ‘engage’ the to

are

not

4.

Fuel

a

free

standing

to

device

types

Wood heater designs range from single fuel systems (e.g., cordwood, cribwood, pellets, wood chips, and briquettes (also called biobricks)), to multi-fuel systems which have been designed to burn pellets, corn

Forced-Air Fan Inside

b. Fig.

4.

Outdoor wood boilers (OWBs)

a.

518

hydronic [32] b. forced hot

air

[33].

(geanonimiseerd) husks,

wood

chips,

Renewable and Sustainable

and olive

pits. In addition to the system design itself,

Pellets

fuel type may also impact emissions, efficiency, and performance. Fuels that contain bark for instance are known to contain many contaminants will

trees

as

push

Jensen

summary abundant

the

“unwanted”

found

mineral

matter

to

compounds

their leaves are

up

to

10

bark.

or

times

leaves

described

are

4.1.

In

{similar those

more

fuel types

common

Cordwood

or

as

without

or

species (fuel density)

in moisture and

commonly

more

bark and

air-dried

and

pieces of the

the amount

content,

referred

wood.

firewood,

Cordwood

is

are

a

oil}

to

made

content, chamber

chlorine,

ditives

used

and

corrode

to

are

in 18

or

of sawmill

kg plastic bags. High quality pellets include often have higher ash

waste; grasses, if used, sulfur levels which can

over

time, making them

cause

the

combustion

less desirable

unless

ad-

[43].

un-treated,

vary in terms further may vary

can

species covering the surface, weight,

same

of bark

systems. They

Pellets are not to be produced from recycled material such as treate painted lumber as the emissions from burning materials of this type would produce toxic emissions or large variations in burns. Require certification programs exist in most European countries (ENplus)} an Canada (CANplus} to protect pellet consumers and ensure the procurement of a high quality pellet [44]. Unlike Europe, pellets fuel in the US are only recommended but not mandated to meet a certification. The Pellet Fuel Institute (PFI) recently launched a third-party accreditation standards program to provide specifications for residential an commercial-grade pellet fuel to ensure the consumer would purchase high quality pellets [45]. The Northeast States for Coordinated Air Use Management (NESCAUM) sampled pellet fuels from 100 difference companies and, despite US standards, [7], some pellet fuels did not meet standards. In some cases, the pellets did not meet the specifie: moisture content, showed seasonal variability, exceeded standard for chloride, and many samples had higher than normal values for Arsenic (As), Cadmium (Cd), and Mecurgy (Hg).

detail.

some

feed

in automatic

or

Cordwood

with of

[40]. In the sections below various

in

used

consistent fuel type due to their uniform size, shape, and moisture content. They are delivered to the end-user either by delivery trucks more

in bark than in wood, with bark ash contents up to 60%wt, predominantly calcium, silica, and phosphorus [35]. Additionally, phenolic acids in bark are mostly found in outer bark cells. Low molecular weight extractives are often very abundant in bark such as alcohols, resin acids, alkaloids, and terpenes. There can also be small amounts of essential oils (volatile low molecular-weight terpenes) (e.g. incense cedar, ponderosa pine, and eucalypts, among many others} [36]. Moisture content of biomass fuels are also another key factor in the ignition time and devolatilization rate [37], as well as the overall combustion performance [38,39]. The elements that may be found biomass fuel, and the in a solid emission from their combustion, also depend on where the tree was grown. Specifically, trees living in areas with toxic air pollution or ground water contaminants may have toxic metals accumulated in the bark and

commonly

are

Energy Reviews 103 (2019) 515-528

Wood

4.3.

chips

complicate this fuel further, combustion of cordwood is chimney draught, cannot be ‘turned on or off’ like automatic fuels can with instantaneous ignition, and is subject to various lighting techniques. To minimize this variability and increase best combustion practices, stove or boiler manufacturers typically recommend their systems be

Biomass fuels with

particle sizes typically smaller than 5cm X 5 cm commonly classified as wood chips. Wood chips have 2-3 times lower bulk density than pellet fuel [39], they may contain bark and their moisture content is typically higher than that of pellets. Higher quality or lower moisture wood chips typically are chipped from seasoned cordwood by a fuel grade chipper with set screens to

fired with

size. To

subject

and

natural

to

either two main types

coniferous

include

or

evergreens

or

of cordwood:

deciduous

or

hardwoods

achieve

softwoods.

Examples

of

hardwoods

if left

or white), maple, or beech and softwoods include pine, Douglas Fir. Traditionally, round logs are cut into four to six pieces to improve the surface area and combustion performance. Manufacturers typically recommend seasoned (air-dried) cordwood for use in their devices with moisture ranges on a dry basis of 16-20%, however some units have claims to the ability to bum green wood (moisture levels above 35%). In addition, piece size, amount, and loading procedures are listed in manufacturers written instructions and device manuals. There are currently no known cordwood fuel specifior

cations

to

in either the US

is

Europe [411].

consumer's

ensure

ufacturers

fuel meet

the

recommendations

Europe, however

currently drafting

standards

for

a

of

man-

technical committee in

cordwood

(CEN/TC

the US

4.4.

Pellet fuel

commonly referred to as just pellets, are solid compressed organic matter or biomass. Biomass for pellet production includes wood shavings, wood chips,

straws,

lumber

Typically pellets use

of

a

use

a

hammer

material.

The

raw

elements

or

if the

mill

scrap,

sawdust mill is

wastes

from

type feedstock with little as a first step to

common

to

no

treat

the recommendations

for

chips

of manufacturers

technical committee

a

as

well

in Europe

(CEN/TC 335}

[41].

briquettes

similar

to

pellets,

biomass sawdust

use

the

shape

a

so

or

fiber

on), and

a

store

raw

in either

is currently

densification process making them uniform in size and easy to and transport but are pressed into a rectangular shape resembling traditional red building brick or larger disk or solid or hollow log

use

crops. bark so

of 7-25

high temperatures cause the lignin to break down and melt which will later solidify, holding the pellet together. In some pellet plants a small amount of cornstarch or flours may be added to act as additional “glue”. Finally, the processed material is pushed through a pellet die at high pressures where the pellet is cut to the desired length (typically on the

cases,

if the

cm

tends to re-absorb moisture

type feedstock (straw, paper, agricultural waste, leaves, and

to

of 0.6-3.8

size. As biomass

Briquettes

heated

raw

may be dried if material was green.

energy

meet

standards

exposed to outdoor weather During the processing state

order

material

and

fuel

Biomass

more

of

feedstock grasses,

or

made

desired

Europe, however

or

drafting

biofuels

uniform

a

consumer's

335}

Pellets

4.2.

the

or

are

uncovered, proper care is necessary to maintain lower moisture content chips after chipping. In some cases solar heat (hot air for drying) has been employed to produce high quality [46-481]. Lower quality, higher moisture content chips may be chipped by a general agricultural chipper directly after removing a tree (allowing for no seasoning of logs and in some cases leaves, grass and dirt may also be included} which tends produce a shredded material, producing longer pieces of wood, twigs, dust, and a variety of non-uniform sizes. Wood chips produced in this type of condition would have a much lower heating value than those of uniform size, lower moisture, and free of dust. Similar to cordwood there are no known specifications to ensure

oak (red

spruce

5em

Xx

cm

in

length

and

10 in 30

cm

in diameter.

If the fines

are

temperature which breaks down the biomnass lignin under high pressure no binders for the briquettes are needed [49] but in some a

fibrous

material

pressure, a binder material dung and cactus [50].

is not

is used such

heated as

Depending on the type of raw material, sieving, drying, or crushing, followed tion, cooling, and packing the briquettes are

[421). 519

or processed under high molasses, starch, or even cow

the

with

first steps of

production

preheating,

densifica-

[49]. Studies

show

higher

(geanonimiseerd) 1000

(8/6)

Renewable and Sustainable

900

850

Manual

comparison

600 400

polution Source

5.

Particle pollution of different heat Fig. data from Denmark adopted from [5].

types

sources

compared with diesel truck-

quality briquettes are produced under higher processing pressures and temperatures, longer dwell times, and increased binder contents [51,52]. Briquettes may be used in automatic or manually fed systems; the typically automatic systems only include primary heaters or commercial grade systems. Combustion

conditions

influencing nanoparticle

emissions

Wood-heating appliances have highly variable combustion condicause large variations in the physicochemical characteristics of the emitted particles [21]. The four main combustion conditions explored in studies typically include start-up (initial burn period), steady state (efficient), intermediate (a modulating stage with an output lower than the unit's maximum) and burn-out (final phase of the burn). These periods are most often identified by the carbon monoxide (CO), carbon dioxide (CO), oxygen (Oa), and mass of fuel values. The steady state phase is where the maximum efficiency is often achieved and PM tions which

emissions

are

reduced.

Burn-out

is

often

described

as

the

charcoal

phase. Some heaters have the ability to modulate down via control strategies such as those mentioned in the advanced boiler systems or manually controlling the draught damper in a stove for instance allowing for 30% of full output or a ‘low’ burn. The most common operating conditions and their characteristics are listed in Table 1 above. One additional common operating characteristic, but certainly a less desirable, is incomplete combustion. Manufacturers often include instructions to users which would prevent such practice. However, many units will operate in an incomplete combustion mode if poor practices are followed. Incomplete combustion is caused by small values of the three most important ‘T's’ in combustion; time, temperature, and turbulence. Too short of a residence time to allow for proper mixing, inadequate mixing of combustion and fuel in the combustion chamber due tures

to

a or

modulate down

heat limit and then electric

an

lack

of turbulence

cold

zones

in the

or

oxygen, combustion

and

low

chamber

combustion are

to

ignition other

‘slumber’

to

hand,

they

sense

feed

on

boilers

approaching

are

restart

by shutting

combustion gases are less burnt than in modern units [31]. A review of wood smoke health perspectives by Belling et al. show that

in terms smaller

particles (20-125 nm) are more common amongst logwood, chip advanced boiler units while open fireplaces and conventional (old) stoves produce nanoparticles in the range of 50-600 nm [10]. This suggests that as conventional units are displaced by modern, more efficient and “cleaner” systems, although the total particulate emissions are reduced, the number of NPs may increase. However, this is not accurately represented by current regulatory gravimetric measures as NPs contribute very little mass even when present in significant pellet

and

numbers. 6.

Nanoparticle

emissions

during residential

wood

heating

tempera-

examples

for

in-

regulations

Current emission

ulations

operating techniques are employed, or poor fuel choices are made, all causing for an incomplete combustion, higher PM emissions occur. A Swiss study pointed out emissions could be up to 100 times higher if a stove was not operated properly [5]. In addition, if an advanced system's heating load is decreased, cordwood and automatic wood-fired boilers (pellet and chips) may have trouble modulating (controlling the burn rate/output through use of fans and or dampers), causing them to cycle frequently resulting to higher PM emissions and lower efficiency. poor

if they

cannot

their fuel bed

The constituents of fine particle emissions during incomplete CO. combustion are organic matter, elemental carbon, and fine ash [55]. Typically, biomass heating systems are able to operate at a high efficiency and low emission level in their steady state, full output mode.

used,

(specifically boilers) rely

demand. Automatic

go into a burn-out period to be restarted again when there is a call for heat again. Manually fed

US and EU are based on therefore NP levels may

are

lower burn

a

complete combustion parameters [53,54]. Characteristics of incomplete combustion are also an increase in volatile hydrocarbon emissions and

However, when inefficient systems

heaters

the heat

so are automatically and 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 continuous fed boilers showed a fairly constant particle size distribution. Beyond the difference in size distribution of the emitted particles, ifferences in the particle's properties exist which will be discussed in etail below. These differences are likely to influence the biological effects induced by wood smoke particles [10]. Studies have shown combustion conditions and the resulting particles strongly depend on fuel type, combustion technology, and type of operation [3,10,20,21,56-59]. One study in particular detailed the 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 boiler and cordwood stove.

The study found significant differences in terms of emissions factor (mg/MJ), even in the cases when spruce or beech cordwood was used in the same stove run at steady state; 67 and 93 mg/MJ, respectively [60]. Additionally, the study found the period of combustion, cold start or steady state had a significant impact in terms of emission production, for some cases ten times greater emissions output (mg/MJ) were observed. Fig. 5 shows emission factors for various type of wood-heating appliances in comparison to gas and oil boilers, district heating and heat pumps. It is shown that older, conventional wood combustion technologies have the highest emission factors. The highest emissions seen in older units are due to smoldering conditions and limited heat-storing, so

the

on

forced

Particle

to meet

the

boilers

feed

automatic

can

by

in

5.

and

ifferent mechanisms

800

Energy Reviews 103 (2019) 515-528

due

their

to

above

NPs

terms

of health

are

of

an

not

low

be

mass

significant

and

from wood

overall

mass

fully

combustion devices

concentration

reflected

contribution.

interest

due

to

the environment

and

so

of

these

in

Further,

in the

particles and emission as

regmentioned

their risk of exposure in groups have begun

many

investigating (measuring, counting, and characterizing} the NPs associated with RWC. Results from a study involving 14 different appliance types,

from

cordwood

stoves

to

automatic

feed

boilers

showed

95%

(particle number) of the particle emissions were smaller than 400 nm [56]. Hata et al. found in laboratory studies that more than 30% (of cumulative

mass}

fuel

mass

had

tributed

a

greatly

of the smoke that fell

within

particles

from

the

burning

of biomass

range of < 100 nm and this conto the total levels of toxic PAHs and water soluble OC a

[22].

Bolling

et

al.

[10]

described

three

typical

classes

of

wood

(geanonimiseerd)

Renewable and Sustainable

combustion

particles based on chemical composition and morphology; spherical organic carbon particles, aggregated soot particles, and inorganic ash particles. Their review shows the smallest (aggregated soot particles, 20-50 nm} stem from combustion in conventional stoves, open fireplaces, and boilers fueled with cordwood, wood chips and pellets during high temperature and incomplete combustion phases. Saarnio compliments Belling et al’s description of particle classes by defining the main components of ultrafine particles as EC, OC, sulfates (SO4°), and trace metals [3]. The following sections provide details on nanoparticle type, size, and or concentration found in their respective

approximately

[63]. They also found that an increase in supply impacted the particle size distribution; thus more oxygen caused an increase in the concentration of particles with smaller diameter. In addition, the study showed the particle bound PAHs decreased as oxygen levels increased; organic carbon was found to be 14% for burning conditions with increased Os and 57% when there was an absence of Os [63]. Particle bound PAHs were likely resulted from adsorption or condensation processes. Kocbach et al. [64] collected particles from two wood combustion sources in Norway; one from a small farm heating from a wood stove and the other ambient sample collected from a residential neighborhood who uses wood as a major heating source, collecting all stages of the burn cycle. To confirm the particles collected were indeed all from wood sources, the group took direct samples from a common singlestage Norwegian stove with a high temperature combustion chamber {1000 °C) and another during an air-starved combustion condition with a low temperature of 500°C. The results from X-ray microanalysis {(XRMA) the study indicated Potassium (K) and sulfur (S) were the most abundant elements observed in the particles, while Silicon (Si) and Calcium (Ca) were present but slightly less. Kocbach et al. proposed the S levels in the carbon aggregates were not directly from the source but combustion

studies. 6.1.

Cordwood

Hosgood

stoves

et

al. [11]

used

coal

and

electrometer,

NPs

in the

stoves from 15 Chinese homes which aerosol monitor, portable diffuser charger Using the group measured in real-time concentrations of

wood.

or

findings suggested

surrounding

air

to two

one

studied an

ambient

total PM25

from

meters

levels

these

homes.

the main

They

also

combustion

measure

Their

source.

adequately accounted for in the total mass records due to NPs insignificant mass contribution to PMa.s measurements; this supports arguments of needed particle type an count regulations in addition to the current mass based emission regNP

were

ulations.

Overall

the

group

between

cooking

and

heating

not

concluded activities

there and

was

a

elevated

clear

rather

NP

levels

Bologa et al. [61] tested an 8 kW stove with dry beech log wood an: the particle size distribution as well as the particle mass concentration. Their study showed the short start-up time of five to seven minutes produced more particles than the steady state and burnburnout phases. Stack temperatures for the start-up, steady state and out and 160-180°C, phases were —100°C, 300-330 respectively. °C The group showed the start-up and steady state phases to have a monomodal particle size distribution of 180 nm while the burn-out phase had the highest concentration of nano-sized particles with a bi-modal discharacterized

of 180-200

Stack emissions

and

nm

30-40

a

welded construction stove

and

to

have

total PAH

a

Corsini

out.

polycyclic aromatic hydrocarbons (PAHs) associated with both Douglas Fir and White Oak at high (> 24%) and low (13%) moisture contents in a freestanding steel woodstove. Their experiments were run in compliance with US test standards, measuring only a steady state condition, and used an Electrical Low Pressure Impactor (ELPI) to collect filters and analyze the PAHs. They reported that the geometric mean diameter (GMD) is between 313 and 662nm particles representing an accumulation mode [62]. The study showed low molecular weight PAHs preferentially segregated to fine aeroynamic diameter particles while higher molecular weight PAHs preferred coarse particles. Hueglin et al. [63] studied the size and particle bound PAHs. A ilution sampling system was used to collect the particles, employing a scanning mobility particle sizer (SMPS) measure the particle size diset

al. studied

tribution. The

having

a

stove

moisture

istribution

had

the

a

15 kW

content

of

output and 15-18%.

The

was

nominal

showed

the

mean

et

al.

count

[67]

of 9745 ng/mg. an advanced

studied

6.2.

output of 8 kW. The

stove

has

cordwood

stove

with

a

triple air supply with both and air system primary secondary supply. They found the ultra-fine particle (UFP) number concentration was 7.0 X 10° particles/ em?° with an average GMD of 50nm for beech logs while the UFP number concentration was 1.9 X 10° particles/cm° with an average GMD of 99 nm for conifer logs. In addition, the group found the PAH concentration was significantly higher in UFP from wood logs, with conifer wood logs generating approximately 20X more PAH compared to beech wood logs [67].

state-of-the-art

Pellet stoves A

study by Commodo et al. found the smallest particles emitted pellet stove were on the order of 7-30 nm and were also the most abundant; on the order of 1x10“particles/cm? [68]. The study also showed the amount of particles emitted after the transient phase decreased significantly, showing fewer particles (< 100 nm) in the steady state or stationary phase. Corsini et al. [67] also studied an 11 kW top-fed pellet stove with two different pellets—beech and conifer. In agreement to the results from the cordwood stove, higher GMD diameter sizes were measured from

fired with beech wood

results

gases in ambient air. While the

A later study by Kochback et al. [65] collected NPs from a common single-stage Norwegian wood stove in a laboratory at high-temperature burn conditions, following Norwegian standards for testing wood stoves using a dilution tunnel. The group examined the NPs collected with High Resolution Transmission Electron Microscopy (HRTEM) to further determine the microstructure of the NPs. Similar to an earlier study [64], only carbon aggregates were found with the primary particles having a mean geometric diameter was 31 + 7 nm from TEM analysis. From HRTEM analysis the group found the wood smoke NPs had one nuclei and a turbostractic structure; a concentric arrangement of layer places with a two-dimensional graphitic crystal structure, lacking the ordered stacking of graphite [66]. In addition, the group found the NPs

analyzed for their size distribution and total number concentration in the Gaegauf et al. study [56]. The start-up period of the stoves showed the largest particle size (i.e, average 200 nm), followed by the steady state period with an average of approximately 175 nm and the smallest particles (— 30 nm} were found in the burn-out period. Similar to the study above, the lowest number concentration of particles less than {0.8 um) was seen in the steady state period and the highest in the burnHays

reaction with

for

forest fires.

nm.

diluted from

were

a

primary combustion particles for both locations remained very similar, roughly 39 nm + 11 and showed consistent number of particles (226-239), the morphology differed from Transmission Electron Microscopy (TEM) analysis. The high temperature combustion produced carbon aggregations while the low temperature combustion did not have aggregates but carbon dominated spherical particles with diameters between 50 and 600 nm [64]. The NPs produced during the low-temperature combustion were similar to particle from smoldering

[LL].

tribution

107 nm)

in air

caused from

diameters

association

local

Energy Reviews 103 (2019) 515-528

size

strongly dependent on the state of the combustion process; start-up had the greatest particle concentration and largest mean diameter (approximately 260 nm), while the burn-out phase had the smallest mean particle diameter (approximately 60 nm) and intermediate phase had the lowest concentration (mean diameter was

521

a

(geanonimiseerd) with

the

75 nm beech

and

soft-wood

Renewable and Sustainable

diameters

of

38 nm. The average UFP number concentration for both and conifer pellets was 1.1 X 10° particles/cm°. These

fuel

than

the

hard-wood

fuel

with

the

Mavrocordatos

[18] collected particles from a 15 kW pellet analysis of particles which included: particle count, size distribution, morphology, volume, elemental composition, and fractal analysis. Employing a SMPS, they found that the maximum boiler

in-

vestigators reported that there were lower quantities of PAHs associated with the particles generated by the pellet stove than the cordwood stove. This was most likely due to more complete combustion in the pellet stove that ultimately produces less inflammatory active particles

Cordwood

boilers

Schmatloch

and

Brenn

[58]

found

from

two

different

produced slightly

cordwood

particles. Using an ELPI and SMPS, the size distribution laid between 80 and 180 nm

significantly for different phases; highest concentrations during the start-up. Johansson et al. [31] studied a variety of cordwood boilers which included old, traditional units and modern, advanced systems with power outputs ranging from 6 to 34 kW. Using beech cordwood with of 15,

contents

vestigated the

effects of

26,

or

moisture

the group inthe systems at 100% half load of cordwood. The

38%

content,

varying the load; running

full output or firing the units with a full or group used an ELPI to determine the size distribution Pressure

Impactor (DLPI)

to

determine

tion, diluting the samples before

sampled

for

PAHs

well.

as

and Dekati

Low-

the number

particle concentraIn addition, the group

measurements.

Ultimately

the

group

found

older

wood

boilers had

higher emission concentrations and un-oxidized compounds than modern wood boilers [31]. In fact, particle emissions were decreased by 70-80 times when comparing a modern wood boiler with a ceramic-lined tank. The wood

Automatic

6.4.1.

Size distribution

feed

data

indicated

a

mean

diameter

by Johansson et al. [31],

group included pellet boilers ranging from 3 to 22kW in power output. The pellet boilers were operated at full or intermittent loads a

moisture

content

of

pellets having a slightly higher moisture content of 7.8% but significantly higher ash content of 0.5-3.7%, respectively. Two pellet boilers run at their full output of, 11 and 22kW power output ignited with a pilot flame and electrical ignition, respectively, showed very similar mass distributions and maximum particle sizes of 130 nm. Experiments that involved the bark pellets showed an increase of mass concentration and a slight shift to larger particle sizes (approximately or

bark

[61]

also characterized

and 32 kW

pellet boilers with

et

al.

the

particle

size distribution

steady state pellet boilers also had a mono-modal particle size distribution; however, there was a shift towards a smaller particle size, peaking at about 60 nm. Furthermore, the group showed a decrease in particle mass concentration for the pellet boilers as well; pellet boilers

mg/Nm®

vs

the wood

stove

at

25-40

Micro-

noticed

(STEM) and

ag-

(ICP-AES) the group

was

when

able

to

do

an

and

chlorides

al.

[56]

also measured

the

distribution

of

a

17 kW

pellet boiler;

diluted

peak mobility

a

from

12

nm

to

(CI)

diameter

just

below

[69].

occurred 400

particle mobility

at

from

a

hot

approximately

diameter

stack.

They

85 nm,

but

nm.

nm

were

et

observed. al. also identified

via

TEM

the three

main

wood

com-

particle types discussed in Bolling et al.’s review article [10]; ash, soot, and particulate organic matter (POM). The TEM images showed the ash as crystalline structures smaller than 100 nm, the soot particles chained as agglomerates with a primary particle size of 30-50 nm and the POM as an amorphous type with a course surface [21]. Table 2 below summarizes the findings of particle morphology and composition from Torvela et al. Similar to the study of Mavrocordatos et al. [21] reported above, Torvela et al. found many of their species vaporized, decomposed, and changed during TEM analysis as well to the form crystals or bubbles inside particles, suggesting there may be semi-volatile hydrocarbons adsorbed on the surface of the soot. The group included an EC/OC (mg/ MJ by thermal-optical), PAH (Hg/MJ) by gas chromatography-mass spectrometry (GCMS) and PM,‚(mg/MJ) by gravimetric analysis. In all cases, the largest counts for each were found in the smoldering condi-

the

10-20

Force

bustion

flue gas temperatures during state in the of 170-180 °C and 120-140 °C, resteady operation range spectively. Similarly, to the cordwood stove tested by the same group,

at

spectrometer

The

et

Torvela

concentration.

Bologa

structure.

a

Gaegauf

of 40

220 nm). No obvious trend was observed between full and intermittent output in terms of particle size, only a very slight increase in mass

from 20 kW

Atomic

Chip boilers Using a 40 kW wood chip boiler, Torvela et al. [21] studied the chemical composition link with particle morphology and the effects of combustion conditions on particle type. These investigators looked at three combustion phases that they classified as efficient (low level of 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 number size distribution was 37, 150, and 250 for the efficient, intermediate, and smoldering conditions, respectively. They calculated a GMD for the efficient, intermediate, and smoldering conditions to be 25, 65, and 160 nm, respectively. However, even during the immediate and smoldering phases, a smaller number of particles with a dimension

the

7.6%

From

nm.

6.4.2.

Pellet boilers

{less than 100% output} with pellet fuel having

emission

ranged

boilers

In addition to the cordwood boilers studied

atomic

found

peak slightly greater than 60 nm and a decreasing trend in terms of concentration as particle diameter grew for both the older wood boilers and fan assisted combustion modern wood boilers. Interestingly, the diameter range shifted towards smaller particle sizes in the case when modern wood boilers were fired with a natural draught. 6.4.

boilers.

zone, connected to an external heat storage also found to be the highest in the case of the older

were

gregates

with

sulfates (SO4)

burn-out

PAHs

80

analysis in addition to a total organic carbon (TOC) measurement by using a TOC-Analyzer. The analysis showed a TOC content of 370 ppm, the presence of K, Ca, S, Magnesium (Mg), Cl, Phosphorus {P), and Sodium (Na) in decreasing amounts (ppm) and traces of Iron (Fe), Aluminum (Al), Mg, and Zinc (Zn). From the analysis the group drew the conclusion that these wood combustion emission particles were likely inorganic salts that could dissolve easily in water [18]. Lamberg et al. [9] along with Kaivosoja et al. [69] studied a 25 kw modern pellet boiler using pine pellets. The group found the GMD to be 60 nm with a mean mass diameter (MMD) of 137 nm [9]. In addition, the total PAH emissions from particulate matter 1 pm and smaller {PM} emissions (PAH/PM;) were extremely low, 0.00006%, with pyrene and fluoranthene as the most abundant. Kaivosoja et al. supported this work by characterizing the PM, by elemental analysis. The group found K was the main element found followed by Na, Cl, and S —suggesting that these elements were all in the form of alkali metal

found

moisture

measured

concentration

elemental

varied

but

were

al.

the highly compact group particles were exposed to the electron beam in the STEM, after only a few minutes the smaller particles (< 100 nm) would disappear and the larger would reduce in size and to an inorganic residue consisting mostly of K, Chlorine (CI) or S. Using an inductively coupled plasma

smaller

of the number

maximum

et

detailed

a

scopy (AFM) and Scanning Transmission Electron Microscopy the group found the individual particles of different shapes

boilers of 41 and 35 kW, a modern lambda (oxygen) controlled and fan assisted boiler and a traditional boiler, respectively, that the advanced unit

flue for

in number

671. 6.3.

Energy Reviews 103 (2019) 515-528

tion

mg/Nm®.

and the lowest

Gaegauf 522

et

al.

in the efficient burn.

[56] also studied

a

wood

chip

boiler with

a

70 kw

(geanonimiseerd)

Renewable and Sustainable

Energy Reviews 103 (2019) 515-528

Table 2 Summarized

Findings from Torvela

et

conditions

Particle

size and

Efficient

Particle

diameter

Intermediate

Particles

Combustion

al. [21]

Study Investigating

NP

PM

emissions

from Wood

Chip Boiler.

composition

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

Smoldering

of ash.

output; diluted from

a hot stack. The group varied the excess air during operation of the chip boiler to study its effects on particle size and found that lower excess air values corresponded to lower particle counts and a shift towards higher diameters; 11.6% O2 and 2.7% Oa corresponded to diameters of 60 nm and 90 nm, respectively. Hueglin et al. [63] continued their size distribution and particle bound PAH study focusing on a chip boiler with an output of 60 kw, and enriched the study by looking at the particles morphology as well. The group coupled the SMPS with an aerodynamic particle sizer (APS) information about the particles and measure larger diato gain more meters. The wood chip boiler was almost always run in a steady state full output mode as it is computer controlled and burned chips with a

Dispersion Spectroscopy} and OC, and the efficient burn produced particles which were merely composed of ash (mainly K, O, with larger amounts of P, Zn, Si and S compared to the intermediate burn).

the

moisture

between

content

40%

and

70%.

Similar

to

the

studies

6.5.

early as 1982, emission measurements were to study the size and composition of particles were taken from two residential fireplaces and one free-standing fireplace to determine the influence they potentially had on Denver's air quality. Muhlbaler Dasch [71] made both in-field and laboratory measurements, investigating five softwoods, nine hardwoods and two synthetic logs (similar to a biobrick construction). Laboratory and residential test followed EPA method 5 protocols, closely, so flue gas was diluted. A Sierra Series-220 cascade-impactor was used to determine the size distribution of particles collected. While the log type and size seemed to have the greatest effect on particulate emissions in terms of total particulate emissions (g/kg), there were no As

dis-

already, particle emissions were dependent on operating conditions. Since the study only looked at steady state operation, the only varying parameter was the combustion air supply to mean the excess The

diameter

of 11.6%

size

at

also followed reduced

a

when

O2 value

an

similar trend the

boiler

found

particles negligible when using group

contained

images

low

revealed

at

size distribution had

(68.6 nm)

and

its

its lowest

largest

operated a

the APS.

at

an

were

optimized O» value.

diameter

greater

than

Particles

emitted

from the

semi-volatile

mean

the concentrations

(95.7 nm};

[63]. Overall, the NPs concentrations

was

with

GMD 6.2%

1000nm

compounds

to

The

obvious

be

wood;

chip boiler

SEM

well.

erates

amounts

of

different

types of particles; agglomerates of spherical

as

particles, perfectly spherical particles representing coal fly irregular shaped crystalline platelets. Another study by Leskinen et al. [70] focused their investigation on a wood chip-fired appliance. The group created a novel modular combustion unit which included a reactor and moving grate (not a marketed boiler with given output} which allowed them to produced three different combustion conditions: efficient (representing optimal conditions), intermediate (representing a partial load or poor air-stage settings) and smoldering (burn-out or smothering situation observed in batch combustion) with similar CO levels as seen in the work done by Torvela et al. [21]. The group provided detailed information on the system and fuel type which was both bark and stem wood chips from spruce and typical Finnish broadleaved trees with a moisture content of or

24.3%.

The

centrations

state, from

The

burn

fraction and

of PAHs

were

found

to

dilution

mental

PAH

efficient

was

found

to

for the three types of 170 nm [71]. From

be

tunnel

Similar and

to US further

test

methods

the group measured flue gas to achieve less concentrations an ELPI and a

diluted the

of the average

size distribution

to

be around

NPs par-

10-20 nm,

phases, but the highest concentrations were found in the start-up phase and lowest in the steady state period [2]. The group found particles in the range of 100-326 nm as well but suggested they were likely primary soot particles that underwent condensation in the diluted and cooled fuel gas [2]. A dip in Os values corresponded to a decrease in number of NPs as well. Interesting in terms of environ-

particle morphology, EC/OC, elemental and ion compounds, similar to Torvela et al. [21] referenced above. Data from each of the burn periods indicated the smallest mass emission concentration (mg/MJ, (amount of particles per kilogram of dry wood combustion}} and smallest particles were produced during the efficient period (average diameter of 35-75 nm} and the largest particle diameter size and mass emission concentration were observed the diameter during smoldering condition (average 200 nm). From the groups chemical analysis, they found Zn was the most abundant metal with the highest relative fraction occurring during the

diameter

in all three

the

the efficient conditions.

median

burn-out.

and a

ticle diameter

group was able to measure the number and mass conwell as the size and mass distribution in addition to and

mass

particle coincidence. To determine NP Fast Mobility Particle Sizer (FMPS) were used. It was concluded accounted for 67% of the total particles measured with the mode

as

characterizing analyses, PM;

differences in terms of size fractionation the

imaging, Muhlbaler Dasch also found the particles to be agglomand spherical in shape. A study in Italy focused on investigating the particle size distribution of NP emissions from an 11 kW commercially available fireplace burning beech wood (the fuel for European standards} [2]. The group analyzed 15 tests over three different burn periods: start-up, steady

SEM

compact

ash,

and masonry heaters

Fireplaces and masonry heaters are often used in studies due to their lack of controls and so it is easy to study the effects of different types of biomass without being skewed by device controls.

cussed

oxygen concentration target. value when O» values were

Fireplaces

fate

of wood

combustion NPs, the group showed that due to of flue gas, stable distinct size distributions during cooling separate phase were achieved with only slight changes during the

dilution and each

transition

one phase to another. study by Ozgen et al. [72], using the same 11 kW stove as mentioned above [2], nanoparticles with a GMD of 28 nm accounted for 32% of the emissions whereas larger particles with a GMD of 127 nm accounted for 68% of the emissions. Similar sampling techniques were employed using a dilution tunnel. The investigators found the ignition phase was responsible for 46% of the NP burden and cold starts, which are common practice in the field, resulted in a magnitude higher in emissions. They reported that larger soot particles emitted during the flaming combustion phases were coated by condensed semi-volatile organic species that were diluted and cooled in the flue gas; this finding suggests that minimizing incomplete combustion is enough for particle number reduction. Similar to other cited studies, the Ozgen et al. [72]

In

be lowest

the

during highest during smoldering. TEM analysis showed the smoldering burn produced particles were made up of mostly large agglomerates of soot (EC and OC), intermediate burns produced a variety of particles from soot agglomerates and ash or composite particles of ash (mainly K, O, Zn determined from Electron 523

a

from

later

(geanonimiseerd)

Renewable and Sustainable

Energy Reviews 103 (2019) 515-528

Table 3

Summary of average particle

size

Equipment

from

cited

Summary:

Cordwood

Stoves

sources.

particle sizes are given unless otherwise specified

average

Start up:

Steady

180-260

state:

[56,61-65,671

nm

30; 107; 175-180

Burn

out:

30-60

Burn

out

or

References

1m

nm;

180-200

up

showing the highest concentrations of NPs and steady

start

nm

state

having the lowest concentrations of

NPs In

studies, aggregations of NPs with diameters of 313-662 ms PAHs favored small NP diameters & High MW PAHs favored coarse particles Increase in oxygen in combustion chamber caused for smaller particles some