The impact of extra long-term storage of logging residues on fuel quality in Estonian conditions – a case study

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Results of ANCOVA of different characteristics (Equation 1)_

Dependent variable (Y)	p-value	Standard error	R2	Figure
Ash content	0.0032	0.4825	0.1286	3
Net calorific value of dry matter	<0.0001	0.1026	0.2120	4
Net calorific value of moist fuel	<0.0001	0.6905	0.3940	5
Bulk density	<0.0001	0.0733	0.7789	6
Energy density	<0.0001	0.3962	0.7718	7
Shrinkage of diameter	0.0015	1.3550	0.0989	8

Parameter estimates for the relationship between drying shrinkage, storage time, and moisture content (Equation 2 and 3)_

Species	Parameter	Estimation	p-value
Silver birch	a0	3.851	<0.0001
	a1	0.02156	0.0432
	a2	3.572·10−283	<0.0001
	a3	−1.944·10−7	0.0175
	R2	0.240
	SE	1. 098
	p	0.0010
Norway spruce	a1	0.15284	<0.0001
	a2	0.37387	<0.0001
	R2	0.937
	SE	0.990
	p	<0.0001

Relationship between moisture content and month number (Equation 4)_

Sample	a0	a1	a2	p-value	Standard error	R2
Birch, covered	58.654	−7.687	0.423	0.0235	8.289	0.415
Birch, uncovered	78.481	−11.882	0.863	0.0967	12.862	0.284
Spruce, covered	57.448	−9.080	0.561	0.0428	8.637	0.363
Spruce, uncovered	65.693	−10.981	0.722	0.0407	9.484	0.367

Decline of energy content of logging residues during long-term storage by species and place in pile_

Storage time, year	Spruce, top	Spruce, middle	Birch, top	Birch, middle
0	100.0	100.0	100.0	100.0
1	95.9	95.8	90.5	90.4
2	92.0	91.7	81.9	81.7
3	88.2	87.9	74.2	73.9
4	84.6	84.2	67.1	66.8
5	81.2	80.6	60.8	60.4
6	77.9	77.2	55.0	54.5
7	74.7	73.9	49.8	49.3
8	71.6	70.8	45.1	44.6

Estimations and significance probabilities of parameters of ANCOVA Equation 1_

Dependent variable (Y)	Figure	Parameter	Estimation	p-value
Ash content, %	3	a0	1.561477	<0.0001
		a1	0.000941	<0.0001
		a2	0.315644	0.0230
		a5	−0.000310	0.0045
		a6	−0.000140	0.0341
		a7	−0.000122	0.0633
Net calorific value of dry matter, kWh/kg	4	a0	5.323000	<0.0001
		a1	0.000085	0.0081
		a5	−0.000039	0.0052
		a7	0.000021	0.1380
Net calorific value of moist fuel, kWh/kg	5	a0	4.162196	<0.0001
		a1	−0.001087	<0.0001
		a3	−0.643537	<0.0001
		a5	0.000423	<0.0001
Bulk density, g/cm3	6	a0	0.571200	<0.0001
		a1	−0.000160	<0.0001
		a2	0.173900	<0.0001
		a3	−0.042530	0.0013
		a5	0.000046	0.0069
Energy density, MWh/m3	7	a0	3.055000	<0.0001
		a1	−0.000846	<0.0001
		a2	0.925200	<0.0001
		a3	−0.232900	0.0012
		a5	0.000248	0.0068
Shrinkage of diameter, %	8	a0	4.284253	<0.0001
		a2	−0.791850	0.0012
		a6	0.000150	0.0973

Dates of samples_

No	Date of sampling	Days from the start of the experiment
1	16.06.2010	0
2	02.09.2010	78
3	22.11.2010	159
4	07.03.2011	264
5	02.09.2011	443
6	15.02.2012	609
7	25.04.2012	679
8	04.06.2012	719
9	24.08.2012	800
10	23.11.2012	891
11	16.05.2013	1065
12	26.11.2013	1259
13	25.06.2014	1470
14	12.12.2014	1640
15	26.08.2015	1897
16	28.08.2016	2265
17	18.03.2018	2832