# Waste-to-Carbon: Is the Torrefied Sewage Sludge with High Ash Content a Better Fuel or Fertilizer?

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## Abstract

**:**

^{−1}. Produced biochar was characterized by HHV up to 12.85 MJ·kg

^{−1}and lower H/C and O/C molar ratio. Therefore, torrefaction of SS with high ash content should not be considered as a method for improving the fuel properties. Instead, the production of fertilizer appears to be favorable. The torrefaction increased C, N, Mg, Ca, K, Na concentration in relation to raw SS. No significant (p < 0.05) influence of the increase of temperature and residence time on the increase of biogenic elements in biochar was found, however the highest biogenic element content, were found in biochar produced for 60 min, under the temperature ranging from 200 to 240 °C. Obtained biochars met the Polish regulatory criteria for mineral-organic fertilizer. Therefore SS torrefaction may be considered a feasible waste recycling technology. The calculation of torrefaction energy and the mass balance shows energy demand <2.5 GJ∙Mg

^{−1}w.m., and the expected mass yield of the product, organic fertilizer, is ~178 kg∙Mg

^{−1}w.m of SS. Further investigation should consider the scaling-up of the SS torrefaction process, with the application of other types of SSs.

## 1. Introduction

^{−1}[36,37,38,39,40], with one research indicate extremely high devolatilization in the temperature range between 260~300 °C resulting in biochars with the lower heating value (LHV) from 6.50 to 3.88 MJ∙kg

^{−1}[41].

^{−1}in a pot experiment, resulted in an increased yield of tomatoes by 64%, and a synergistic effect of increased availability of nutrients along with the improvement of soil quality. Biochar has a heterogeneous and highly porous structure. Both its internal and external surface is very extensive and has a different character both hydrophilic and hydrophobic. This makes biochar a material with high water retention capacity, which can be important for example, during the reclamation of poor soils [48].

## 2. Materials and Methods

#### 2.1. Sewage Sludge Characterization

#### 2.2. Experimental Design

_{2}was supplied at 10 dm

^{3}∙h

^{−1}to ensure inert conditions during torrefaction. CO

_{2}was introduced into the reactor by the steel 0.25-inch tube inserted through the muffle furnace chimney. The end of the tube was placed in the central point of the reactor chamber, above the crucible with the SS sample. CO

_{2}and process gases were outflowing thought the reactor chimney. The heating of the reactor began 5 min after CO

_{2}was introduced into the reactor. Heating always started at ambient temperature and took 5 to 10 min, depending on the target temperature. After torrefaction, the samples were left in the furnace to cool down for 60 to 90 min, depending on the initial process temperature. CO

_{2}flow was shut off when the temperature inside the reactor dropped below 100 °C during cooling. The cooled sample was moved from the muffle and weighed with 0.1 mg accuracy to determine the mass loss. After that, the sample was analyzed. The cooling period was incorporated because the reactor could not be opened until the temperature drops down to a certain point. Without this procedure, we would expose the biochar to atmospheric oxygen and possible self-ignition could occur.

#### 2.3. Analyses of the Biochar Properties

- Moisture content using the KBC65W (WAMED, Warsaw, Poland) laboratory dryer with Radwag PS 3500.R2 (Radwag, Radom, Poland) analytical balance following the PN-EN 14346:2011 standard [51],
- Losses on ignition (LOI) by means of model 8.1/1100, SNOL, Utena, Lithuania muffle furnace with Radwag PS 3500.R2 analytical balance following the PN-EN 15169:2011 standard [52],
- Ash content using the SNOL 8.1/1100 muffle furnace with Radwag PS 3500.R2 analytical balance following the PN-G-04516:1998 standard [53],
- Elementary C, H, N, and O composition using Perkin Elmer 2400 Series CHNS/O (Waltham, MA, USA) with Radwag, MYA 2.4 Y analyzer following PN-EN ISO 16948:2015-07 [54]
- HHV and LHV using the IKA C2000 Basic calorimeter (IKA® Poland Sp. z o.o., Warsaw, Poland) following the PN-G-04513:1981 standard [55],
- Mg, Ca, K, Na total content in solid samples were analyzed with atomic absorption spectroscopy (AAS) after dry mineralization using Varian Spektra AA 240 FS following PN-EN 14082: 2004 standard [56] (Agilent Technologies, Santa Clara, CA, USA). Dry mineralization was carried out with the procedure described below. The homogeneous sample (10 g) was incinerated on the heating plate; then the samples were mineralized in a muffle furnace for 8 h, the ash was burned for 2 h after dissolving in 2 cm
^{3}HNO_{3}. The mineralization was transferred quantitatively into 10 cm^{3}vessels using 2M HNO_{3}.

#### 2.4. Data Analysis

_{0}and M

_{t}is mass before and after process, respectively, g.

^{−1}d.m.

_{2}O conversion, H-hydrogen content,%

_{daf}(on dry and on ash-free bases) was estimated as [59]:

^{−1}; M

_{f}—fuel mass (on dry basis), mass, g; M

_{ash}—the mass of ash in fuel, g.

^{−1}, 1 Mg price = 999 PLN (1 USD ~ 4 PLN), incineration efficiency = 90%) [62]. Prices were given in EURO at the exchange rate from National Polish Bank in December 2019 (EURO = 4.261 PLN). The economic analysis of the biochar was based on the unit prices of mineral fertilizers used in Poland. For this purpose, the N fertilizer prices have been converted into the cost of 1 kg of a pure component following [63].

## 3. Results

#### 3.1. The Influence of Torrefaction Temperature and Residence Time on Fuel Properties of Biochars

^{−1}) (Figure 2). However, (surprisingly) the lowest observed HHV was found in the case of the biochar produced during the 60 min at 300 °C (11.09 MJ·kg

^{−1}). As the HHV depends on C content, the similar trend of the increase (p < 0.05) of HHV (Figure 2, Table A5), and C content in biochars (Figure 3, Table A6) with the temperature and residence time can be observed.

^{−1}).

^{−1}) value was associated with the biochar torrefied at 260 °C with 60 min residence time (Figure 2). Regression analyses indicated that temperature increase affects HHV and LHV values, causing an apparent rise when short residence times were used (Figure 2). For torrefaction above 20 min at higher temperatures, devolatilization was too robust, which is confirmed by significant (p < 0.05) decrease in LOI values, hence HHV and LHV values were declining.

#### 3.2. The Influence of Torrefaction Temperature and Residence Time on Fertilizer Properties of Biochars

## 4. Discussion

#### 4.1. Proximate Analysis

#### 4.2. Fuel Properties

^{−1}to 14.4 MJ·kg

^{−1}generated at 200 °C. There was also an increase in the calorific value in relation to the dry SS. The maximum LHV value in the present study was 7% higher than the values characterizing dry SS. Such a small increase in LHV is associated with increased ash content, hence lowering volatile matter percentage in the biochars in relation to the dry SS.

_{2}, CO, and O, and VOCs containing carboxyl, carbonyl, and hydroxyl groups. Confirmation of the described trend is reported, among others, in [12,36], where the increase in temperature caused a significant reduction in the O content in the processed SS. A similar trend, but not as apparent, is visible in the work of Huang et al. [68], where despite the higher range values of the analyzed temperatures, the decreases in O content were not as high.

#### 4.3. Fertilizer Properties

^{−1}d.m., from 6.7 to 11.6 g·kg

^{−1}d.m. and from 4.1 to 5.4 g·kg

^{−1}d.m. A similar trend, although with lower growths, was presented by Hossain et al. [71], where the Mg content increased from 3.3 to 3.5 g·kg

^{−1}d.m. In the case of Ca, Hossain et al. [71] observed that Ca content increased from 30.2 to 34.7 g·kg

^{−1}d.m., while in the work of Lu et al. [70] where values for 3 different SS sample increased from 4.8 to 8.1 g·kg

^{-1}d.m., 6.7 to 11.6 g·kg

^{−1}d.m. and from 1.5 to 1.8 g·kg

^{−1}d.m. The changes of K content in biochars had similar character as results obtained in the work of Lu et al. [70], where the K content in biochars from 3 different samples of dry SS ranged from 1.2 to 2.1 g·kg

^{−1}d.m., 0.8 to 1.6 g·kg

^{−1}d.m. and 1.3 to 1.8 g·kg

^{−1}d.m. Additionally, in the case of Na, Lu et al. [70] showed a similar tendency indicating an increase in the value of generated SS biochars in relation to the dry sludge.

_{2}O), allows qualifying of the obtained biochars as solid mineral-organic fertilizers following Polish regulations [72]. Examples of effective usage of biochar from SS as fertilizers for agriculture are described in the works of Waqas et al. [73], Hossain et al. [74], Song et al. [75], and Zornoza et al. [76], where the biochars were generated from SS under the pyrolytic conditions, and their high agricultural suitability was found. However, the manuscript did not address the question concerning which of the direction of the biochars from SS utilization (fuel or fertilizer) is more preferable.

^{−1}of raw SS. Taking into account the percentage of N in the torrefied samples (240 °C, 1 h), the price of N considered as pure fertilizer was evaluated. The analysis indicates that 178.4 kg of biochar is produced from 1 Mg of raw SS, with a fertilization value of 5 EURO. It is worth mentioning that with decreasing moisture, less energy will be required for drying and torrefaction.

## 5. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## Appendix A

**Table A1.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on process mass yield.

Mass Yield, % | ||||||
---|---|---|---|---|---|---|

Model | Mass Yield, % = (1.44809) + (7.03892 × 10^{-6})·T^{2} + (−0.00364092)·T + (0.000414299)·t^{2}+ (−0.0460416)·t + (0.000374584)·T·t + (−8.04432 × 10^{-7})·T^{2}·t + (−3.39151 × 10^{−6})·T·t^{2} + (7.27706 × 10^{−9})·T^{2}·t^{2}R ^{2} = 0.98 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 1.448091 | 0.285892 | 0.00 | 0.00 | 0.879256 | 2.016927 |

a2 | 0.000007 | 0.000000 | 0.00 | 0.00 | 0.000007 | 0.000007 |

a3 | −0.003641 | 0.002318 | 0.00 | 0.00 | −0.008253 | 0.000971 |

a4 | 0.000414 | 0.000201 | 0.00 | 0.00 | 0.000015 | 0.000814 |

a5 | −0.046042 | 0.016232 | 0.00 | 0.00 | −0.078339 | −0.013745 |

a6 | 0.000375 | 0.000132 | 0.00 | 0.00 | 0.000113 | 0.000636 |

a7 | −0.000001 | 0.000000 | 0.00 | 0.00 | −0.000001 | −0.000001 |

a8 | −0.000003 | 0.000000 | 0.00 | 0.00 | −0.000003 | −0.000003 |

a9 | 0.000000 | 0.000000 | 0.00 | 0.00 | 0.000000 | 0.000000 |

**Table A2.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on dry mass content in biochars.

dry mass, % | ||||||
---|---|---|---|---|---|---|

Model | d.m., % = (223.1) + (0.002)·T^{2} + (−1.053)·T + (0.083)·t^{2} + (−7.035)·t + (0.0583)·T·t + (−0.0001)·T^{2}·t + (−0.0007)·T·t ^{2} + (1.393 × 10^{−6})·T^{2}·t^{2}R ^{2} = 0.26 | |||||

Function Parameters | value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 223.1321 | 39.68104 | 0.00 | 0.00 | 144.1793 | 302.0849 |

a2 | 0.0022 | 0.00064 | 0.00 | 0.00 | 0.0009 | 0.0034 |

a3 | −1.0528 | 0.32172 | 0.00 | 0.00 | −1.6929 | −0.4127 |

a4 | 0.0828 | 0.02787 | 0.00 | 0.00 | 0.0273 | 0.1382 |

a5 | −7.0352 | 2.25299 | 0.00 | 0.00 | −11.5179 | −2.5524 |

a6 | 0.0583 | 0.01827 | 0.00 | 0.00 | 0.0219 | 0.0946 |

a7 | −0.0001 | 0.00004 | 0.00 | 0.00 | −0.0002 | −0.0000 |

a8 | −0.0007 | 0.00023 | 0.00 | 0.00 | −0.0011 | −0.0002 |

a9 | 0.0000 | 0.00000 | 0.00 | 0.00 | 0.0000 | 0.0000 |

**Table A3.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on loss on ignition content in biochars.

Loss on Ignition, % d.m. | ||||||
---|---|---|---|---|---|---|

Model | LOI, % = (−43.517) + (−0.0012353)·T^{2} + (0.702014)·t + (−0.0291921)·T^{2} + (3.83877)·T + (−0.0285369)·t·T + (4.89634 × 10^{−5})·t^{2}·T + (0.000221984)·T·t^{2} + (−3.90553 × 10^{−7})·T^{2}·t^{2}R ^{2} = 0.72 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | −43.5170 | 58.73534 | 0.00 | 0.00 | −160.382 | 73.34794 |

a2 | −0.0012 | 0.00095 | 0.00 | 0.00 | −0.003 | 0.00066 |

a3 | 0.7020 | 0.47621 | 0.00 | 0.00 | −0.245 | 1.64952 |

a4 | −0.0292 | 0.04126 | 0.00 | 0.00 | −0.111 | 0.05290 |

a5 | 3.8388 | 3.33485 | 0.00 | 0.00 | −2.797 | 10.47407 |

a6 | −0.0285 | 0.02704 | 0.00 | 0.00 | −0.082 | 0.02526 |

a7 | 0.0000 | 0.00005 | 0.00 | 0.00 | −0.000 | 0.00016 |

a8 | 0.0002 | 0.00033 | 0.00 | 0.00 | −0.000 | 0.00089 |

a9 | −0.0000 | 0.00000 | 0.00 | 0.00 | −0.000 | −0.00000 |

**Table A4.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on ash content in biochars.

Ash Content, % d.m. | ||||||
---|---|---|---|---|---|---|

Model | ash, % = (14.0912) + (−0.000486683)·T^{2} + (0.256399)·t + (−0.00923204)·t^{2} + (1.13906)·t + (−0.00882318)·T·t + (1.67301 × 10^{−5})·T^{2}·t + (4.9036 × 10^{−5})·t·T^{2} + (−3.61091 × 10^{−8})·t^{2}·T^{2}R ^{2} = 0.88 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 14.09118 | 43.69689 | 0.00 | 0.00 | −72.8519 | 101.0343 |

a2 | −0.00049 | 0.00071 | 0.00 | 0.00 | −0.0019 | 0.0009 |

a3 | 0.25640 | 0.35428 | 0.00 | 0.00 | −0.4485 | 0.9613 |

a4 | −0.00923 | 0.03069 | 0.00 | 0.00 | −0.0703 | 0.0518 |

a5 | 1.13906 | 2.48099 | 0.00 | 0.00 | −3.7973 | 6.0755 |

a6 | −0.00882 | 0.02012 | 0.00 | 0.00 | −0.0488 | 0.0312 |

a7 | 0.00002 | 0.00004 | 0.00 | 0.00 | −0.0001 | 0.0001 |

a8 | 0.00005 | 0.00025 | 0.00 | 0.00 | −0.0004 | 0.0005 |

a9 | −0.00000 | 0.00000 | 0.00 | 0.00 | −0.0000 | −0.0000 |

**Table A5.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on HHV of the biochars.

HHV, MJ·kg^{−1} | ||||||
---|---|---|---|---|---|---|

Model | HHV, MJ·kg^{−1} = (56.7993) + (0.000704123)·T^{2} + (−0.35836)·T + (0.0154298)·t^{2} + (−1.93107)·t + (0.0147992)·T·t + (−2.75086 × 10^{−5})·T^{2}·t + (−0.000109331)·T·t^{2} + (1.82382 × 10^{−7})·T^{2}·t^{2}R ^{2} = 0.63 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 56.79931 | 14.38158 | 0.00 | 0.00 | 28.18447 | 85.41415 |

a2 | 0.00070 | 0.00023 | 0.00 | 0.00 | 0.00024 | 0.00117 |

a3 | −0.35836 | 0.11660 | 0.00 | 0.00 | −0.59036 | −0.12636 |

a4 | 0.01543 | 0.01010 | 0.00 | 0.00 | −0.00467 | 0.03553 |

a5 | −1.93107 | 0.81655 | 0.00 | 0.00 | −3.55574 | −0.30640 |

a6 | 0.01480 | 0.00662 | 0.00 | 0.00 | 0.00163 | 0.02797 |

a7 | −0.00003 | 0.00001 | 0.00 | 0.00 | −0.00005 | −0.00000 |

a8 | −0.00011 | 0.00008 | 0.00 | 0.00 | −0.00027 | 0.00005 |

a9 | 0.00000 | 0.00000 | 0.00 | 0.00 | 0.00000 | 0.00000 |

**Table A6.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on the C content in biochars.

C, % d.m. | ||||||
---|---|---|---|---|---|---|

Model | C, % = (−6.51131) + (−0.000660336)·T^{2} + (0.304374)·T + (−0.0668472)·t^{2} + (4.04284)·t + (−0.0353957)·T·t + (7.50162 × 10^{−5})·T^{2}·t + (0.000579037)·T·t^{2} + (−1.21488 × 10^{−6})·T^{2}·t^{2}R ^{2} = 0.49 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | −6.51131 | 30.69734 | 0.00 | 0.00 | −67.5894 | 54.56676 |

a2 | −0.00066 | 0.00050 | 0.00 | 0.00 | −0.0016 | 0.00033 |

a3 | 0.30437 | 0.24889 | 0.00 | 0.00 | −0.1908 | 0.79958 |

a4 | −0.06685 | 0.02156 | 0.00 | 0.00 | −0.1098 | −0.02394 |

a5 | 4.04284 | 1.74291 | 0.00 | 0.00 | 0.5750 | 7.51068 |

a6 | −0.03540 | 0.01413 | 0.00 | 0.00 | −0.0635 | −0.00728 |

a7 | 0.00008 | 0.00003 | 0.00 | 0.00 | 0.0000 | 0.00013 |

a8 | 0.00058 | 0.00017 | 0.00 | 0.00 | 0.0002 | 0.00093 |

a9 | −0.00000 | 0.00000 | 0.00 | 0.00 | −0.0000 | −0.00000 |

**Table A7.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on the H content in biochars.

H, % d.m. | ||||||
---|---|---|---|---|---|---|

Model | H, % = (11.1242) + (8.72369 × 10^{−5})·T^{2} + (−0.0504356)·T + (0.00103414)·t^{2} + (−0.162667)·t + (0.00102235)·T·t + (−1.89313 ×·10^{−6})·T^{2}·t + (−4.26597 × 10^{−6})·T·t^{2} + (4.46763 × 10^{−9})·T^{2}·t^{2}R ^{2} = 0.79 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 11.12415 | 7.851332 | 0.00 | 0.00 | −4.49753 | 26.74584 |

a2 | 0.00009 | 0.000127 | 0.00 | 0.00 | −0.00017 | 0.00034 |

a3 | −0.05044 | 0.063657 | 0.00 | 0.00 | −0.17709 | 0.07622 |

a4 | 0.00103 | 0.005515 | 0.00 | 0.00 | −0.00994 | 0.01201 |

a5 | −0.16267 | 0.445779 | 0.00 | 0.00 | −1.04963 | 0.72429 |

a6 | 0.00102 | 0.003614 | 0.00 | 0.00 | −0.00617 | 0.00821 |

a7 | −0.00000 | 0.000000 | 0.00 | 0.00 | −0.00000 | −0.00000 |

a8 | −0.00000 | 0.000045 | 0.00 | 0.00 | −0.00009 | 0.00008 |

a9 | 0.00000 | 0.000000 | 0.00 | 0.00 | 0.00000 | 0.00000 |

**Table A8.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on the O content in biochars.

O, % d.m. | ||||||
---|---|---|---|---|---|---|

Model | O, % = (94.3211) + (0.00132001)·T^{2} + (−0.646472)·T + (0.110223)·t^{2} + (−7.32599)·t + (0.0612511)·T·t + (−0.000125945)·T^{2}·t + (−0.000907679)·T·t^{2} + (1.82155 × 10^{−6})·T^{2}·t^{2}R ^{2} = 0.83 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 1051.470 | 55631.35 | 0.02 | 0.99 | −113095 | 115197.6 |

a2 | 0.248 | 0.90 | 0.27 | 0.79 | −2 | 2.1 |

a3 | −47.137 | 450.67 | −0.105 | 0.92 | −972 | 877.6 |

a4 | 5.535 | 38.52 | 0.14 | 0.88 | −74 | 84.6 |

a5 | −538.670 | 3116.91 | −0.17 | 0.86 | −6934 | 5856.7 |

a6 | 10.229 | 25.24 | 0.40 | 0.69 | −42 | 62.0 |

a7 | −0.031 | 0.05 | −0.61 | 0.54 | −0 | 0.1 |

a8 | −0.122 | 0.31 | −0.39 | 0.70 | −1 | 0.5 |

a9 | 0.000 | 0.00 | 0.61 | 0.5 | −0 | 0.0 |

**Table A9.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on HHV daf of the biochars.

HHV (daf), MJ·kg^{−1} | ||||||
---|---|---|---|---|---|---|

Model | HHV (daf), MJ·kg^{−1} = (64.4828) + (0.000608875)·T^{2} + (−0.319667)·T + (0.00555782)·t^{2} + (−1.47697)·t + (0.0100922)·T·t + (−1.58506 × 10^{−5})·T^{2}·t + (−2.18888 × 10^{−5})·T·t^{2} + (−5.1846 × 10^{−9})·T^{2}·t^{2}R ^{2} = 0.71 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 64.48283 | 32.93082 | 0.00 | 0.00 | −1.03918 | 130.0048 |

a2 | 0.00061 | 0.00053 | 0.00 | 0.00 | −0.00045 | 0.0017 |

a3 | −0.31967 | 0.26700 | 0.00 | 0.00 | −0.85090 | 0.2116 |

a4 | 0.00556 | 0.02313 | 0.00 | 0.00 | −0.04047 | 0.0516 |

a5 | −1.47697 | 1.86972 | 0.00 | 0.00 | −5.19713 | 2.2432 |

a6 | 0.01009 | 0.01516 | 0.00 | 0.00 | −0.02007 | 0.0403 |

a7 | −0.00002 | 0.00003 | 0.00 | 0.00 | −0.00008 | 0.0000 |

a8 | −0.00002 | 0.00019 | 0.00 | 0.00 | −0.00040 | 0.0004 |

a9 | −0.00000 | 0.00000 | 0.00 | 0.00 | −0.00000 | −0.0000 |

**Table A10.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on LHV of the biochars.

LHV, MJ·kg^{−1} | ||||||
---|---|---|---|---|---|---|

Model | LHV, MJ·kg^{−1} = (57.3765) + (0.000737877)·T^{2} + (−0.373049)·T + (0.0172241)·t^{2} + (−2.06729)·t + (0.015999)·T·t + (−2.99897 × 10^{−5})·T^{2}·t + (−0.000125133)·T·t^{2} + (2.15415 × 10^{−7})·T^{2}·t^{2}R ^{2} = 0.61 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 57.37645 | 14.96197 | 0.00 | 0.00 | 27.60682 | 87.14608 |

a2 | 0.00074 | 0.00024 | 0.00 | 0.00 | 0.00026 | 0.00122 |

a3 | −0.37305 | 0.12131 | 0.00 | 0.00 | −0.61441 | −0.13168 |

a4 | 0.01722 | 0.01051 | 0.00 | 0.00 | −0.00369 | 0.03814 |

a5 | −2.06729 | 0.84950 | 0.00 | 0.00 | −3.75753 | −0.37704 |

a6 | 0.01600 | 0.00689 | 0.00 | 0.00 | 0.00229 | 0.02970 |

a7 | −0.00003 | 0.00001 | 0.00 | 0.00 | −0.00006 | −0.00000 |

a8 | −0.00013 | 0.00009 | 0.00 | 0.00 | −0.00029 | 0.00004 |

a9 | 0.00000 | 0.00000 | 0.00 | 0.00 | 0.00000 | 0.00000 |

**Table A11.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on the energy yield of the process.

Energy Yield, % | ||||||
---|---|---|---|---|---|---|

Model | energy yield, % = (500.253) + (0.00637136)·T^{2} + (−3.23363)·T + (0.179189)·t^{2} + (−20.5477)·t + (0.161123)·T·t + (−0.00031386)·T^{2}·t + (−0.00135047)·T·t^{2} + (2.50551 × 10^{−6})·T^{2}·t^{2}R ^{2} = 0.83 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 500.2532 | 109.4272 | 0.00 | 0.00 | 282.5274 | 717.9791 |

a2 | 0.0064 | 0.0018 | 0.00 | 0.00 | 0.0028 | 0.0099 |

a3 | −3.2336 | 0.8872 | 0.00 | 0.00 | −4.9989 | −1.4684 |

a4 | 0.1792 | 0.0769 | 0.00 | 0.00 | 0.0262 | 0.3321 |

a5 | −20.5477 | 6.2130 | 0.00 | 0.00 | −32.9097 | −8.1858 |

a6 | 0.1611 | 0.0504 | 0.00 | 0.00 | 0.0609 | 0.2614 |

a7 | −0.0003 | 0.0001 | 0.00 | 0.00 | −0.0005 | −0.0001 |

a8 | −0.0014 | 0.0006 | 0.00 | 0.00 | −0.0026 | −0.0001 |

a9 | 0.0000 | 0.0000 | 0.00 | 0.00 | 0.0000 | 0.0000 |

**Table A12.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on the H/C ratio in biochars.

H/C | ||||||
---|---|---|---|---|---|---|

Model | H/C = (6.15786) + (6.41009 × 10^{−5})·T^{2} + (−0.0338503)·T + (0.00373692)·t^{2} + (−0.263384)·t + (0.0021366)·T·t + (−4.4153 × 10^{−6})·T^{2}·t + (−3.03887 × 10^{−5})·T·t^{2} + (6.18517 × 10^{−8})·T^{2}·t^{2}R ^{2} = 0.76 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 6.1579 | 4.1683 | 0 | 0 | −2.358 | 14.4515 |

a2 | 0.0001 | 0.0001 | 0 | 0 | −0.0001 | 0.0002 |

a3 | −0.0339 | 0.0338 | 0 | 0 | −0.1011 | 0.0334 |

a4 | 0.0037 | 0.0029 | 0 | 0 | −0.0021 | 0.0096 |

a5 | −0.2634 | 0.2367 | 0 | 0 | −0.7343 | 0.2075 |

a6 | 0.0021 | 0.0019 | 0 | 0 | −0.0017 | 0.0060 |

a7 | 0.0000 | 0.0000 | 0 | 0 | 0.0000 | 0.0000 |

a8 | 0.0000 | 0.0000 | 0 | 0 | −0.0001 | 0.0000 |

a9 | 0.0000 | 0.0000 | 0 | 0 | 0.0000 | 0.0000 |

**Table A13.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on the O/C ratio in biochars.

O/C | ||||||
---|---|---|---|---|---|---|

Model | O/C = (2.85356) + (4.1663 × 10^{−5})·T^{2} + (−0.0202011)·T + (0.0038009)·t^{2} + (−0.246543)·t + (0.00208228)·T·t + (−4.3088 × 10^{−6})·T^{2}·t + (−3.16778 × 10^{−7})·T·t^{2} + (6.4223 × 10^{−8})·T^{2}·t^{2}R ^{2} = 0.80 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 2.8536 | 1.8909 | 0 | 0 | −0.9087 | 6.6159 |

a2 | 0.0000 | 0.0000 | 0 | 0 | 0.0000 | 0.0001 |

a3 | −0.0202 | 0.0153 | 0 | 0 | −0.0507 | 0.0103 |

a4 | 0.0038 | 0.0013 | 0 | 0 | 0.0012 | 0.0064 |

a5 | −0.2465 | 0.1074 | 0 | 0 | −0.4602 | −0.0329 |

a6 | 0.0021 | 0.0009 | 0 | 0 | 0.0004 | 0.0038 |

a7 | 0.0000 | 0.0000 | 0 | 0 | 0.0000 | 0.0000 |

a8 | 0.0000 | 0.0000 | 0 | 0 | −0.0001 | 0.0000 |

a9 | 0.0000 | 0.0000 | 0 | 0 | 0.0000 | 0.0000 |

**Table A14.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on the N content in biochars.

N, % d.m. | ||||||
---|---|---|---|---|---|---|

Model | N, % = (13.737) + (0.000154911)·T^{2} + (−0.0716041)·T + (−0.00987428)·t^{2} + (0.322928)·t + (−0.00346963)·T·t + (7.1607×10^{−6})·T^{2}·t + (9.22803 × 10^{−5})·T·t^{2} + (−1.91518 × 10^{−7})·T^{2}·t^{2}R ^{2} = 0.83 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 13.73703 | 7.673398 | 0.00 | 0.00 | −1.53063 | 29.00468 |

a2 | 0.00015 | 0.000124 | 0.00 | 0.00 | −0.00009 | 0.00040 |

a3 | −0.07160 | 0.062214 | 0.00 | 0.00 | −0.19539 | 0.05218 |

a4 | −0.00987 | 0.005390 | 0.00 | 0.00 | −0.02060 | 0.00085 |

a5 | 0.32293 | 0.435677 | 0.00 | 0.00 | −0.54393 | 1.18979 |

a6 | −0.00347 | 0.003532 | 0.00 | 0.00 | −0.01050 | 0.00356 |

a7 | 0.00001 | 0.000000 | 0.00 | 0.00 | 0.00001 | 0.00001 |

a8 | 0.00009 | 0.000044 | 0.00 | 0.00 | 0.00001 | 0.00018 |

a9 | −0.00000 | 0.000000 | 0.00 | 0.00 | −0.00000 | −0.00000 |

**Table A15.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on the Mg content in biochars.

Mg, mg·kg^{−1} | ||||||
---|---|---|---|---|---|---|

Model | Mg, mg·kg^{−1} = (1051.47) + (0.247677)·T^{2} + (−47.1368)·T + (5.53494)·t^{2} + (−538.67)·t + (10.2288)·T·t + (−0.0310345)·T^{2}·t + (−0.122493)·T·t^{2} + (0.000382962)·T^{2}·t^{2}R ^{2} = 0.50 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 94.32113 | 57.65115 | 0.00 | 0.00 | −20.3866 | 209.0288 |

a2 | 0.00132 | 0.00093 | 0.00 | 0.00 | −0.0005 | 0.0032 |

a3 | −0.64647 | 0.46742 | 0.00 | 0.00 | −1.5765 | 0.2835 |

a4 | 0.11022 | 0.04050 | 0.00 | 0.00 | 0.0296 | 0.1908 |

a5 | −7.32599 | 3.27329 | 0.00 | 0.00 | −13.8388 | −0.8132 |

a6 | 0.06125 | 0.02654 | 0.00 | 0.00 | 0.0084 | 0.1141 |

a7 | −0.00013 | 0.00005 | 0.00 | 0.00 | −0.0002 | −0.0000 |

a8 | −0.00091 | 0.00033 | 0.00 | 0.00 | −0.0016 | −0.0003 |

a9 | 0.00000 | 0.00000 | 0.00 | 0.00 | 0.0000 | 0.0000 |

**Table A16.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on the Ca content in biochars.

Ca, mg·kg^{−1} | ||||||
---|---|---|---|---|---|---|

Model | Ca, mg·kg^{−1} = (316658) + (5.82666)·T^{2} + (−2648.24)·T + (341.228)·t^{2} + (−23391.2)·t + (213.523)·T·t + (−0.468153)·T^{2}·t + (−3.09572)·T·t^{2} + (0.00673435)·T^{2}·t^{2}R ^{2} = 0.32 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | 316658.0 | 485972.1 | 0.65 | 0.52 | −680474 | 1313790 |

a2 | 5.8 | 7.9 | 0.74 | 0.47 | −10 | 22 |

a3 | −2648.2 | 3936.8 | −0.67 | 0.51 | −10726 | 5429 |

a4 | 341.2 | 336.5 | 1.01 | 0.32 | −349 | 1032 |

a5 | −23391.2 | 27228.4 | −0.86 | 0.40 | −79259 | 32477 |

a6 | 213.5 | 220.5 | 0.97 | 0.34 | −239 | 666 |

a7 | −0.5 | 0.4 | −1.06 | 0.30 | −1 | 0 |

a8 | −3.1 | 2.7 | −1.14 | 0.27 | −9 | 2 |

a9 | 0.0 | 0.0 | 1.24 | 0.23 | 0 | 0 |

**Table A17.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on the K content in biochars.

K, mg·kg^{−1} | ||||||
---|---|---|---|---|---|---|

Model | K, mg·kg^{−1} = (−7811.08) + (0.0132002)·T^{2} + (35.0213)·T + (−5.9302)·t^{2} + (432.779)·t + (−0.0448789)·T·t + (−0.00580631)·T^{2}·t + (0.002207)·T·t^{2} + (7.25633 × 10^{−5})·T^{2}·t^{2}R ^{2} = 0.48 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | −7811.08 | 33686.23 | −0.23 | 0.82 | −76929.5 | 61307.36 |

a2 | 0.01 | 0.55 | 0.02 | 0.98 | −1.1 | 1.13 |

a3 | 35.02 | 272.89 | 0.13 | 0.90 | −524.9 | 594.95 |

a4 | −5.93 | 23.33 | −0.25 | 0.80 | −53.8 | 41.93 |

a5 | 432.78 | 1887.38 | 0.23 | 0.82 | −3439.8 | 4305.37 |

a6 | −0.04 | 15.28 | 0.00 | 1.00 | −31.4 | 31.32 |

a7 | −0.01 | 0.03 | −0.19 | 0.85 | −0.1 | 0.06 |

a8 | 0.00 | 0.19 | 0.01 | 0.99 | −0.4 | 0.39 |

a9 | 0.00 | 0.00 | 0.19 | 0.85 | 0.0 | 0.00 |

**Table A18.**The statistical parameters of polynomial regression analysis of the influence of SS torrefaction temperature and residence time on the Na content in biochars.

Na, mg·kg^{−1} | ||||||
---|---|---|---|---|---|---|

Model | Na, mg·kg^{−1} = (−41466.8) + (−0.577248)·T^{2} + (318.433)·T + (−49.4931)·t^{2} + (3620.2)·t + (−27.0631)·T·t + (0.0542841)·T^{2}·t + (0.390176)·T·t^{2} + (−0.000809744)·T^{2}·t^{2}R ^{2} = 0.29 | |||||

Function Parameters | Value | Standard Error | t Value | p Value | Lower Confidence Limit | Upper Confidence Limit |

a1 | −41466.8 | 141846.8 | −0.29 | 0.77 | −332512 | 249,578.7 |

a2 | −0.6 | 2.3 | −0.25 | 0.80 | −5 | 4.1 |

a3 | 318.4 | 1149.1 | 0.28 | 0.78 | −2039 | 2676.2 |

a4 | −49.5 | 98.2 | −0.50 | 0.62 | −251 | 152.0 |

a5 | 3620.2 | 7947.5 | 0.46 | 0.65 | −12,687 | 19,927.1 |

a6 | −27.1 | 64.4 | −0.42 | 0.68 | −159 | 105.0 |

a7 | 0.1 | 0.1 | 0.42 | 0.68 | 0 | 0.3 |

a8 | 0.4 | 0.8 | 0.49 | 0.63 | −1 | 2.0 |

a9 | 0.0 | 0.0 | −0.51 | 0.61 | 0 | 0.0 |

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**Figure 1.**Mass yield, dry mass, loss on ignition, the ash content in torrefied SS in relation to process temperature, and residence time.

**Figure 2.**HHV, LHV, HHV daf., energy yield, and H/C, O/C molar ratios of torrefied SS in relation to process temperature, and residence time.

**Figure 3.**C, H, N, O percentage content in torrefied SS in relation to process temperature and residence time.

**Figure 4.**Van Krevelen diagram of dry sludge and torrefied sewage sludge generated at three retention times (20, 40, 60 min), in six temperature variants (200, 220, 240, 260, 280, 300 °C), in relation to coal (anthracite) and biomass (cellulose and lignin) reference.

**Figure 5.**Mg, Ca, K, Na content in torrefied SS in relation to process residence time and temperature.

Property | Value |
---|---|

dry mass,% | 20.3 |

loss on ignition,% d.m. | 57.2 |

ash,% d.m. | 38.5 |

LHV, MJ·kg^{−1} | 0.4 |

HHV, MJ·kg^{−1} | 12.2 |

HHV daf. MJ·kg^{−1} | 20.6 |

C,% d.m. | 27.9 |

H,% d.m. | 3.7 |

N,% d.m. | 4.3 |

S,% d.m. | 1.6 |

O,% d.m. | 23.9 |

H/C ratio | 1.6 |

O/C ratio | 0.6 |

Mg, mg·kg^{−}^{1}, d.m. | 2,643 |

Ca, mg·kg^{−}^{1}, d.m. | 14,640 |

K, mg·kg^{−}^{1}, d.m. | 1535 |

Na, mg·kg^{−}^{1}, d.m. | 3511 |

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**MDPI and ACS Style**

Pulka, J.; Manczarski, P.; Stępień, P.; Styczyńska, M.; Koziel, J.A.; Białowiec, A.
Waste-to-Carbon: Is the Torrefied Sewage Sludge with High Ash Content a Better Fuel or Fertilizer? *Materials* **2020**, *13*, 954.
https://doi.org/10.3390/ma13040954

**AMA Style**

Pulka J, Manczarski P, Stępień P, Styczyńska M, Koziel JA, Białowiec A.
Waste-to-Carbon: Is the Torrefied Sewage Sludge with High Ash Content a Better Fuel or Fertilizer? *Materials*. 2020; 13(4):954.
https://doi.org/10.3390/ma13040954

**Chicago/Turabian Style**

Pulka, Jakub, Piotr Manczarski, Paweł Stępień, Marzena Styczyńska, Jacek A. Koziel, and Andrzej Białowiec.
2020. "Waste-to-Carbon: Is the Torrefied Sewage Sludge with High Ash Content a Better Fuel or Fertilizer?" *Materials* 13, no. 4: 954.
https://doi.org/10.3390/ma13040954