Prediction of slagging potential of sewage sludge ash using Fu and AFI indices Cover

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Prediction of slagging potential of sewage sludge ash using Fu and AFI indices

Environmental Protection and Natural Resources

Volume 36 (2025): Issue 1 (March 2025)

By: Barbara Jagustyn, Sławomir Stelmach and Izabela Mazur

Open Access

|Nov 2025

Figures & Tables

Statistical characterisation of the basic properties of the studied sewage sludge samples

Parameter	Average value	Standard deviation	Median	Minimum value	Maximum value
Total moisture content (as received) [%]	45.1	31.9	41.2	3.7	86.6
LHV¹ (as received) [kJ/kg]	6,849	5,475	7,710	0.0²	15,215
Ash content (dry) [%]	35.0	9.8	32.8	20.7	57.7
Carbon content (dry) [%]	33.2	5.7	34.5	20.4	39.7
Hydrogen content (dry) [%]	4.5	0.7	4.5	3.0	5.5
Nitrogen content (dry) [%]	4.9	1.0	5.0	3.5	6.5
Sulphur content (dry) [%]	1.4	0.3	1.3	0.8	1.9

Characteristic ash fusion temperatures of the tested sewage sludge samples

Sample No.	Parameter
	oxidizing atmosphere				semi-reducing atmosphere
	sintering temperature (t_S) [°C]	softening temperature (t_A) [°C]	melting temperature (t_B) [°C]	flow temperature (t_C) [°C]	sintering temperature (t_S) [°C]	softening temperature (t_A) [°C]	melting temperature (t_B) [°C]	flow temperature (t_C) [°C]
1	960	1,080	1,300	1,320	900	1,170	1,280	1,300
2	910	1,110	1,220	1,250	850	1,090	1,200	1,300
3	950	1,090	1,200	1,210	890	1,070	1,160	1,180
4	930	1,050	1,200	1,300	900	1,130	1,220	1,310
5	910	1,130	1,260	1,270	870	1,030	1,140	1,180
6	920	1,040	1,180	1,230	900	1,080	1,160	1,220
7	920	1,180	1,220	1,240	870	1,180	1,220	1,240
8	940	1,160	1,290	1,410	890	1,230	1,360	1,450
9	700	1,120	1,240	1,300	670	1,030	1,080	1,220
10	920	1,080	1,230	1,260	820	1,030	1,080	1,110
11	900	1,090	1,260	1,310	850	1,050	1,120	1,190
12	890	1,030	1,240	1,270	810	1,030	1,150	1,200
13	910	1,300	1,390	1,450	740	1,080	1,290	1,380
14	930	1,220	1,310	1,390	880	1,170	1,310	1,400
15	880	1,160	1,240	1,250	730	1,030	1,150	1,180
16	930	1,150	1,250	1,270	860	1,050	1,180	1,230
17	910	1,160	1,320	1,370	820	1,050	1,280	1,390

Evaluation of slagging and fouling tendencies of tested sewage sludge samples

Sample No.	Fu	risk	AFI (oxidizing)	risk	AFI (semi-reducing)	risk
1	2.17	high	1,124	very high	1,192	high
2	2.24	high	1,132	very high	1,112	very high
3	2.60	high	1,112	very high	1,088	very high
4	4.58	high	1,080	very high	1,148	very high
5	2.76	high	1,156	high	1,052	very high
6	0.62	medium	1,068	very high	1,096	very high
7	0.43	low	1,188	high	1,188	high
8	0.31	low	1,186	high	1,256	medium
9	49.08	very high	1,144	very high	1,040	very high
10	3.13	high	1,110	very high	1,040	very high
11	5.95	high	1,124	very high	1,064	very high
12	4.81	high	1,072	very high	1,054	very high
13	1.61	high	1,318	medium	1,122	very high
14	1.74	high	1,238	medium	1,198	high
15	1.80	high	1,176	high	1,054	very high
16	1.88	high	1,170	high	1,076	very high
17	1.59	high	1,192	high	1,096	very high

Characteristics of ash composition in the tested sewage sludge samples

Sample No.	Parameter
Sample No.	SiO₂ [%]	Al₂O₃ [%]	Fe₂O₃ [%]	CaO [%]	MgO [%]	P₂O₅ [%]	SO₃ [%]	Mn₃O₄ [%]	TiO₂ [%]	BaO [%]	SrO [%]	Na₂O [%]	K₂O [%]
1	25.90	11.49	8.64	18.99	1.57	16.19	4.20	0.12	1.02	2.09	0.13	0.98	1.64
2	25.29	11.87	9.55	17.19	2.05	20.64	1.94	0.12	0.84	2.59	0.14	1.04	1.66
3	31.73	12.35	7.93	17.36	5.24	19.68	1.64	0.16	0.90	0.16	0.08	0.94	2.50
4	29.24	8.37	5.16	11.64	5.07	21.81	1.33	0.11	0.92	0.11	0.06	1.07	5.20
5	29.98	7.00	16.87	16.40	2.54	19.78	2.45	0.16	0.90	0.09	0.07	0.85	1.86
6	30.80	10.02	14.89	8.34	4.34	5.38	24.15	0.55	0.18	0.71	0.04	0.81	0.09
7	32.29	17.13	7.07	15.70	2.72	1.54	17.59	2.86	0.23	0.67	0.09	0.69	0.12
8	48.93	5.87	5.12	9.34	3.96	4.07	17.49	0.96	0.26	0.47	0.07	0.80	0.09
9	6.23	4.29	26.83	17.60	1.97	30.70	2.88	0.14	0.26	0.09	0.06	3.58	5.89
10	26.12	8.55	19.58	13.36	2.60	20.49	3.06	0.18	0.95	0.14	0.05	1.05	1.85
11	25.52	6.00	26.98	11.82	2.77	21.70	1.65	0.11	1.28	0.06	0.05	1.36	2.90
12	21.50	8.18	13.76	20.55	4.26	26.72	2.14	0.19	0.86	0.21	0.06	1.08	2.41
13	36.13	3.28	16.98	8.67	6.10	22.60	1.47	0.17	0.37	0.05	0.05	0.65	1.25
14	44.09	5.58	11.35	11.26	3.06	19.14	1.17	0.16	0.55	0.07	0.05	0.97	2.08
15	39.54	8.50	10.77	12.04	2.63	13.31	3.55	0.35	0.70	0.06	0.03	0.94	2.14
16	37.03	6.53	11.70	15.17	2.93	19.12	2.10	0.15	1.00	0.11	0.05	0.81	1.78
17	46.08	5.48	12.88	12.86	2.89	17.65	1.11	0.14	0.59	0.07	0.06	1.05	1.60

DOI: https://doi.org/10.2478/oszn-2025-0014 | Journal eISSN: 2353-8589 | Journal ISSN: 1230-7831

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Language: English

Page range: 21 - 28

Published on: Nov 21, 2025

Published by: National Research Institute, Institute of Environmental Protection

In partnership with: Paradigm Publishing Services

Publication frequency: 4 issues per year

Keywords:

sewage sludge ash,

ash fusibility,

phosphorus pentoxide,

coal-fired boilers

Related subjects:

© 2025 Barbara Jagustyn, Sławomir Stelmach, Izabela Mazur, published by National Research Institute, Institute of Environmental Protection
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

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