The Impact of Fuelwood Moisture Content on the Emission of Gaseous and Particulate Pollutants from a Wood Stove

Figures & data

Table 1. Proximate and ultimate analysis of wood fuel.

	Unit	Low moisture beech	High moisture beech
Volatile Matter	% db	75.8 ± 0.4	75.8 ± 0.4
Ash	% db	10.5 ± 0.6	10.5 ± 0.6
Fixed Carbon	% db	13.7 ± 0.4	13.7 ± 0.4
Moisture	% ar	12.4 ± 0.2	26.7 ± 2.6
C	% db	46.1 ± 0.6	46.1 ± 0.6
H	% db	5.7 ± 0.1	5.7 ± 0.1
N	% db	0.4 ± 0.01	0.4 ± 0.01
S	% db	0.00*	0.00*
O	% db	37.3 ± 0.9	37.3 ± 0.9
HHV	MJ/kg db	18.2	18.2
LHV	MJ/kg wb	14.6	11.8

*values recorded below the detection limit; db indicates dry basis; ar indicates as received basis

Figure 1. Measured mass (kg) and derived fuel conversion rate (kg/h) during the combustion of: (a) low moisture and (b) high moisture fuel. Solid vertical lines represent the point of reloading. Dashed vertical lines represent transfer from flaming to smoldering phase.

Table 2. Combustion zone temperatures.

Combustion zone and temperatures	Low moisture beech		High moisture beech
	flaming	smoldering	flaming	smoldering
combustion grate/bed † [^oC]	700	500	500	400
combustion chamber † [^oC]	900	900	810	810
upper chamber* [^oC]	850	600	500	300
flue gas temperature* [^oC]	500	450	350	250

* from experimental measurement; † derived/estimated as described; all values are ±40 ^oC

Figure 2. Gaseous emission concentrations (mg/m³) for (a) low moisture wood and (b) high moisture wood. Solid vertical lines represent the point of reloading. Dashed vertical lines represent the point of transfer from the flaming phase to the smoldering phase.

Table 3. Average emission factors evaluated over complete combustion cycles for low and high moisture wood in g/kg (dry basis).

Pollutant species	Low moisture beech EFLow (g/kg)	High moisture beech EFHigh (g/kg)	Ratio (EFHigh/EFLow)
CO	57.8 ± 13.7	102.8 ± 12.7	1.78
SO2	0.52 ± 0.20	0.94 ± 0.18	1.81
NOx	1.30 ± 0.08	1.11 ± 0.11	0.85
CH4	0.61 ± 0.28	5.39 ± 1.33	8.84
C2H6	0.12 ± 0.05	2.70 ± 1.04	22.5
C2H4	0.21 ± 0.15	1.14 ± 0.21	5.43
C3H8	0.07 ± 0.04	1.19 ± 0.46	17.0
C6H14	0.04 ± 0.02	0.61 ± 0.22	15.3
C2H2	0.23 ± 0.13	0.69 ± 0.14	3.00
C6H6	0.44 ± 0.23	3.68 ± 1.02	8.36
CH2O (formaldehyde)	0.26 ± 0.15	3.13 ± 1.07	12.0
C2H4O2 (acetic acid)	0.35 ± 0.35	5.15 ± 1.60	14.7

Table 4. Concentration of organic compounds sampled in the condensate.

Condensate composition	Low moisture beech		High moisture beech
	Concentration (mg/l)	Emission factor (mg/kg)	Concentration (mg/l)	Emission factor (mg/kg)
pH	3.6	Not applicable	3.27	Not applicable
TOC	396.1 ± 0.8	93.1	1730 ± 13	319.5
Total phenols	31.5 ± 0.5	7.4	80.0 ± 2.3	14.8
acetic acid CH3COOH	143.1 ± 25.9	33.6	109.9 ± 0.1	20.3
propanoic acid C2H5COOH	10.5 ± 5.1	2.5	28.9 ± 0.2	0.5
butyric acid C3H8COOH	1.5 ± 0.2	0.4	6.0 ± 0	0.03
isobutryic acid i-C3H8COOH	3.2 ± 0.2	0.8	8.8 ± 0.2	0.1
Total organic acid	169.7	39.9	171.2	7.4

Table 5. Soot particle size distribution (by mass fraction).

Particulate size range	From low moisture beech (wt.%)	From high moisture beech (wt.%)
PM10.0 >	0.76 ± 0.51	0.41 ± 0.22
PM2.5 – PM10.0	1.61 ± 0.53	0.37 ± 0.10
PM1.0 – PM2.5	4.60 ± 0.29	1.26 ± 0.93
<PM1.0	93.0 ± 0.8	98.0 ± 1.1

Table 6. Elemental Carbon, Organic Carbon and Total Carbon composition of the soot samples presented on a weight percent basis (wt. %).

Sample	EF(PM1) g/kg	EC (wt.% of PM1)	OC (wt.% of PM1)	EC/TC	Color	MCE
Low Moisture 1	3.0	75.4	24.6	0.75	Black	0.94
Low Moisture 2	3.7	84.0	16.0	0.84	Black	0.96
Low Moisture 3	4.6	92.9	7.1	0.93	Black	0.94
High Moisture 1	6.5	26.9	73.1	0,27	Brown	0.92
High Moisture 2	9.0	27.2	72.9	0.27	Brown	0.84
High Moisture 3	17.6	21.6	78.4	0.22	Brown	0.87

Table 7. Published data from (Jones et al. 2020) on optical properties and MCE.

Fuel	Absorption Angstrom Exponent (AAE)	EC/TC	Color of sample	MCE
Furfural	1.18	0.98	Black	1.0
Pine wood, flaming	0.99	0.5	Black	0.93
Pine wood, transient	1.91	0.3	Brown	0.65
Pine wood, smoldering	3.33	0.1	Yellow	0.63

Figure 3. SEM images of soot collected by impaction from low moisture wood (a, b) showing long chains of spherical particulate material (high EC/TC) and high moisture wood (c, d) showing the formation of amorphous tar-like material (high OC).

Figure 4. Chromatogram of the Py-GC-MS of species desorbed at 300oC from the ‘raw tar’ collected from the combustion of High Moisture wood showing the sugar hydrides, oxidized species and polyphenol species: 1, methoxy-toluenes; 2, methoxy-phenols; 3, guaiacyl acetone;4, 1-(3,4,5-trimethoxyphenyl)ethenone; 5, levoglucosan, d-glucopyranose, anhydro-d-mannosan; 6, Syringaldehyde; 7, acetosyringone, acetophenone.

Figure 5. Normalized Raman spectra of soot from the combustion of low and high moisture woods.

Table 8. Relationship of OC fraction to ID/IG value.

Sample	D Band Intensity	G Band Intensity	ID/IG	OC%
Low Moisture 1	269	315	0.85	24.6
Low Moisture 2	382	404	0.95	16.0
Low Moisture 3	317	353	0.90	7.1
High Moisture 1	1671	2356	0.71	73.1
High Moisture 2	1211	1738	0.70	72.8
High Moisture 3	1912	2924	0.65	78.4

Table 9. Soot characteristics from low and high moisture wood.

Low moisture beech combustion cycle
Time (min)	38	40	42	44	46	48	50	52	54	56
Darkness (0–100)	26.5	44.2	32.7	27.6	26.2	21.7	26.8	35.3	55.9	20.6
Burning rate, kg/h	1.5	5.1	3.9	3.6	3.6	3.3	2.7	2.1	2.4	1.5
Combustion chamber Temperature (°C)	240	799	799	752	722	687	708	605	679	506
High moisture beech combustion cycle
Time (min)	62	64	68	72	76	80	88	96	104
Darkness (0–100)	15.5	21.5	18.5	13.9	13.7	18.6	14	19.1	4.4
Burning Rate, kg/h	1.5	3.3	3	2.1	1.8	1.2	1.2	0.9	0.6
Combustion chamber Temperature (°C)	300	514	562	470	408	351	320	403	272

Figure 6. UV Vis results for: (a) low moisture wood and (b) high moisture wood.

Jones, J. M., E. J. S. Mitchell, A. Williams, E. Kumi-Barimah, G. Jose, K. D. Bartle, N. Hondow, and A. R. Lea-Langton. 2020. Examination of combustion-generated smoke particles from biomass at source: relation to atmospheric light absorption. Combust. Sci. Technol. 192:130–43.

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