Two-in-One Fuel Synthetic Bioethanol-Lignin from Lignocellulose with Sewage Sludge and Its Air Pollutants Reduction Effects

Developing effective, economical, and environmentally sound approaches for sewage sludge management remains an important global issue. In this paper, we propose a bioethanol-lignin (nonfood biomass)-based sewage sludge upgrading process for enhancing the heating value and reducing air pollutants of hybrid sewage sludge fuel (HSF) for the effective management of sewage sludge. Sewage sludge paste with the lignin-CaO solution implies drying at 105 °C accompanied by torrefaction at 250 °C. During torrefaction, moisture and partly volatile matter begin to evaporate, and are almost vaporized out to the surface. In this study, the proposed process enhances the net caloric value (NCV) to 37%. The lignin-embedded HSF shows a two-in-one combustion peak regardless of the mixing ratio, resulting in a 70% reduction of unburned carbon (UBC) emissions, which is one of the particular matter (PM) sources of combustion flue gas. Other air pollutants, such as CO, hydrocarbon, NOx, and SOx, were also reduced by the proposed process. In particular, SOx emission remained at ~1 ppm (average value) regardless of the sulfur content of the fuel.


Introduction
The generation of sewage sludge, an unavoidable byproduct of municipal wastewater treatment, has increased rapidly over the past decades due to the rapid increase in industrialization, urbanization, and population. In China, 20 million tons of sewage sludge is generated annually. In Europe, 90 g/person/day of dry sludge is produced, while 11,432 ton/day of sewage sludge is generated in Korea (data from 2017) [1][2][3][4][5][6][7]. Currently, the commonly known sewage sludge disposal methods are agricultural recycling, landfilling, compost production, organic fertilizer production, and road surfacing [8][9][10][11][12]. However, these traditional methods may lead to serious environmental pollution and human health issues (soil, groundwater contamination, leading to plant, animal, and human health

Production of Bioethanol-Lignin Solution
We defined lignin, a solid byproduct from the bioethanol process, as bioethanol-lignin. The bioethanol-lignin (solid) was prepared by dilute acid hydrothermal treatment. The red pine and sulfuric acid solution (1 wt%) were placed into an autoclave reactor (SUS304, internal volume of 500 mL), which was then gently blended at a ratio of 1:7 (red pine: 1 wt% sulfuric acid solution) and sealed. The autoclave reactor was maintained at 180 • C for 1 h via an external heater. After the hydrothermal treatment, the solution was filtered and the solid (bioethanol-lignin) was washed three times to remove the sulfur. The bioethanol-lignin was then liquefied to impregnate the sewage sludge pores. The bioethanol-lignin and CaO solutions (1 wt%) were mixed at a ratio of 1:16 (bioethanol-lignin:1 wt% CaO solution) and kept at 180 • C for 1 h for liquefaction. Finally, a bioethanol-lignin solution was obtained.

Preparation of HSF
To prepare the HSF (Scheme 1), we first dried raw sewage sludge samples at 105 • C for 24 h in an oven to remove the moisture. The dried samples were then pulverized using a Wonder Crush/Mill D3V-10 (Taiwan), and the samples were sifted to break the particles down to 75 µm in diameter or smaller using a sieve shaker. We then mixed the dried sewage sludge samples (solid) with the bioethanol-lignin solution (liquid; lignin/sewage sludge 0.1-0.3 weight ratio on a dry basis) until the mixture was homogeneous. The mixed samples (bioethanol-lignin solution and sewage sludge mixture) were then dried at 105 • C for 10 h to vaporize the water in an oven and to allow permeation of lignin into the sewage sludge pores. Finally, torrefaction of the dried blended samples was carried out at 250 • C in a nitrogen gas atmosphere for 1.5 h. Thus, the 100 g of upgraded sewage sludge fuel (solid) was obtained. All HSF samples were prepared in 3 replicates. Aldrich Co.) were purchased as a hydrothermal catalyst for the production bioethanol-lignin and a liquefacient of bioethanol-lignin, respectively.

Production of Bioethanol-Lignin Solution
We defined lignin, a solid byproduct from the bioethanol process, as bioethanol-lignin. The bioethanol-lignin (solid) was prepared by dilute acid hydrothermal treatment. The red pine and sulfuric acid solution (1 wt%) were placed into an autoclave reactor (SUS304, internal volume of 500 mL), which was then gently blended at a ratio of 1:7 (red pine: 1 wt% sulfuric acid solution) and sealed. The autoclave reactor was maintained at 180 °C for 1 h via an external heater. After the hydrothermal treatment, the solution was filtered and the solid (bioethanol-lignin) was washed three times to remove the sulfur. The bioethanol-lignin was then liquefied to impregnate the sewage sludge pores. The bioethanol-lignin and CaO solutions (1 wt%) were mixed at a ratio of 1:16 (bioethanollignin:1 wt% CaO solution) and kept at 180 °C for 1 h for liquefaction. Finally, a bioethanol-lignin solution was obtained.

Preparation of HSF
To prepare the HSF (Scheme 1), we first dried raw sewage sludge samples at 105 °C for 24 h in an oven to remove the moisture. The dried samples were then pulverized using a Wonder Crush/Mill D3V-10 (Taiwan), and the samples were sifted to break the particles down to 75 µm in diameter or smaller using a sieve shaker. We then mixed the dried sewage sludge samples (solid) with the bioethanol-lignin solution (liquid; lignin/sewage sludge 0.1-0.3 weight ratio on a dry basis) until the mixture was homogeneous. The mixed samples (bioethanol-lignin solution and sewage sludge mixture) were then dried at 105 °C for 10 h to vaporize the water in an oven and to allow permeation of lignin into the sewage sludge pores. Finally, torrefaction of the dried blended samples was carried out at 250 °C in a nitrogen gas atmosphere for 1.5 h. Thus, the 100 g of upgraded sewage sludge fuel (solid) was obtained. All HSF samples were prepared in 3 replicates. Scheme 1. Schematic diagram of the combined production process of hybrid sewage sludge fuel (HSF) and bioethanol.

Characterization of HSF
To analyze the fuel properties of the samples, proximate analysis, ultimate analysis, and calorific value measurement were conducted using a thermogravimeter (TGA-701, LECO Co. for moisture, volatile matter, ash and fixed carbon) on an as-received basis, an elemental analyzer (TruSpec & SC-432DR, LECO Co., St. Joseph, MI, USA for C, H, N, O and S) on a dry basis, and a calorimeter (a Parr 6320EF, Parr Instruments for higher heating value), respectively. The mesopore information, including the pore size and volume of the fuels were measured by nitrogen adsorption analysis (ASAP 2420, Micrometritics Co., Norcross, GA, USA), while the fuels were degassed to empty the pores for 7 h at 105 • C. To investigate the combustion behaviors of the fuels, we carried out thermogravimetry analysis (TGA) using Q500 (TA Instrument Ltd., New Castle, DE, USA) under a heating rate of 10 • C/min and an air flowrate of 100 mL/min. Finally, we directly compared the flue gas emissions (NO x , SO 2 , CO, and C x H y ) of dried sewage sludge and HSF samples when the mass of 1 g was burned. Each sample was put in the combustion chamber and heated up to 1000 • C at a heating rate of 16.67 • C /min under air (21% O 2 ) flowing condition (1.5 L/min), and the flue gas emission was recorded by a gas analyzer (Model 350K, Testo, Korea) for 90 min. All samples were analyzed in 3 replicates. Finally, we investigated the ash characterization (slagging, fouling, ash fusion temperature and heavy metals) using X-ray fluorescence (XRF, Primus II, RIGAKU Co., Auburn Hills, MI, USA), an ash melting test (ASTMD1857, 5E-AF4000), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES).

Fuel Characteristics of HSF
We conducted proximate, ultimate, and calorific value analyses for lignin, dried sewage sludges, and HSF samples (impregnated with lignin). The results are summarized in Table 1. The proximate analysis of the HSF samples showed that the fixed carbon content increased from around 76.81% (SL-L-10) to 185.57% (SW-L-30) compared to dried sewage sludges. This finding is mainly attributed to the fact that the bioethanol-lignin solution additives were converted into artificial fixed carbon after thermal treatment. In addition, the ash content of the upgraded sewage sludge fuels was slightly increased by CaO. Meanwhile, the moisture and volatile matter contents dramatically decreased following torrefaction treatment at 250 • C.   The ultimate analysis showed that the carbon content in HSF increased from 18.16% (SW-L-10) to 18.56% (DS-L-10), to 20.69% (SL-L-10) compared to the dried sewage sludge, while the oxygen and hydrogen contents decreased. This is attributed to the fact that the carboxyl, hydroxyl (C-O, C-H, and O-H) functional groups of volatile matter are easily broken during thermal treatment. The HSF samples showed lower nitrogen and sulfur contents than dried sewage sludge.
The van Krevelen diagram [34,35] shows the fuel rank and changes in element ratios based on ultimate analysis information (C, H, and O). The O/C and H/C molar ratios of all samples are presented in Figure 1. In general, high rank fuel indicates low O/C and H/C molar ratios due to the degree of coalification through dehydration and decarboxylation. Dried sewage sludges are ranked lower than the lignite areas. As the lignin additive ratio increases during the HSF manufacturing process, HSF steadily improved to sub-bituminous, due to the lower O/C and H/C ratios [36]. The ultimate analysis showed that the carbon content in HSF increased from 18.16% (SW-L-10) to 18.56% (DS-L-10), to 20.69% (SL-L-10) compared to the dried sewage sludge, while the oxygen and hydrogen contents decreased. This is attributed to the fact that the carboxyl, hydroxyl (C-O, C-H, and O-H) functional groups of volatile matter are easily broken during thermal treatment. The HSF samples showed lower nitrogen and sulfur contents than dried sewage sludge.
The van Krevelen diagram [34,35] shows the fuel rank and changes in element ratios based on ultimate analysis information (C, H, and O). The O/C and H/C molar ratios of all samples are presented in Figure 1. In general, high rank fuel indicates low O/C and H/C molar ratios due to the degree of coalification through dehydration and decarboxylation. Dried sewage sludges are ranked lower than the lignite areas. As the lignin additive ratio increases during the HSF manufacturing process, HSF steadily improved to sub-bituminous, due to the lower O/C and H/C ratios [36].

Combustion Behaviors of HSF at Low Temperature
To estimate the combustion behavior of dried sewage sludges, lignin, and HSF samples, the thermogravimetry analysis (TGA) analyses were conducted, and the resulting the derivative thermogravimetry (DTG) curves are presented in Figure 2 Figure 2b-e, the dried sewage sludge started to combust at ~170-180 °C and displayed two-stage combustion characteristics at ~250 °C and 450-500 °C. The first peak can be ascribed to the decomposition of volatile matter, and the second peak indicates fixed carbon combustion [37]. First, we observed the combustion behavior of the physically blended samples (torrefied sewage sludge 90% and torrefied lignin 10%; Figure 2b, blue line). They demonstrated combustion patterns in isolation. Heterogeneous combustion patterns significantly decrease the efficiency of the boiler system and increase the emission of environmental pollutants (unburned carbon). Meanwhile, the HSF samples showed a homogeneous (two-in-one) combustion pattern regardless of lignin content ranging from 10% to 30%, which is similar to that of dried sewage sludge, although the two materials were mixed (Figure 2ce). The ignition temperature was observed at ~220-230 °C by the carbonization process. Furthermore,

Combustion Behaviors of HSF at Low Temperature
To estimate the combustion behavior of dried sewage sludges, lignin, and HSF samples, the thermogravimetry analysis (TGA) analyses were conducted, and the resulting the derivative thermogravimetry (DTG) curves are presented in Figure 2. The combustion of lignin (Figure 2a Figure 2b-e, the dried sewage sludge started to combust at~170-180 • C and displayed two-stage combustion characteristics at~250 • C and 450-500 • C. The first peak can be ascribed to the decomposition of volatile matter, and the second peak indicates fixed carbon combustion [37]. First, we observed the combustion behavior of the physically blended samples (torrefied sewage sludge 90% and torrefied lignin 10%; Figure 2b, blue line). They demonstrated combustion patterns in isolation. Heterogeneous combustion patterns significantly decrease the efficiency of the boiler system and increase the emission of environmental pollutants (unburned carbon). Meanwhile, the HSF samples showed a homogeneous (two-in-one) combustion pattern regardless of lignin content ranging from 10% to 30%, which is similar to that of dried sewage sludge, although the two materials were mixed (Figure 2c-e). The ignition temperature was observed at~220-230 • C by the carbonization process. Furthermore, while the second and third peaks of the sludge are clearly visible, HSF changed its boundary peak broadly. This result indicates that volatile matter (VM) and fixed carbon (FC) are burnt at almost the same point, which facilitates FC burning. When sewage sludge and lignin in polymer form (bioethanol-lignin solid) were cofired, lignin was present on the surface of the sewage sludge, and different combustion patterns appeared simultaneously. However, when the HSF prepared using lignin in monomer or oligomer form (bioethanol-lignin solution) was burned, the lignin solution penetrated the pores of the sewage sludge, which provided a very similar combustion pattern. Nitrogen adsorption tests were conducted to support this deduction, and the results are summarized in Figure 3 and Table 2. The bioethanol-lignin solution infiltration into the intrapores was observed by the concentration difference between the inside and the outside of sewage sludge pores and capillary force. While the pore size of dried sewage sludge was mainly distributed between 3 nm and 7 nm, we observed that the pore area and volume were significantly decreased by about 30%. while the second and third peaks of the sludge are clearly visible, HSF changed its boundary peak broadly. This result indicates that volatile matter (VM) and fixed carbon (FC) are burnt at almost the same point, which facilitates FC burning. When sewage sludge and lignin in polymer form (bioethanol-lignin solid) were cofired, lignin was present on the surface of the sewage sludge, and different combustion patterns appeared simultaneously. However, when the HSF prepared using lignin in monomer or oligomer form (bioethanol-lignin solution) was burned, the lignin solution penetrated the pores of the sewage sludge, which provided a very similar combustion pattern. Nitrogen adsorption tests were conducted to support this deduction, and the results are summarized in Figure 3 and Table 2. The bioethanol-lignin solution infiltration into the intrapores was observed by the concentration difference between the inside and the outside of sewage sludge pores and capillary force. While the pore size of dried sewage sludge was mainly distributed between 3 nm and 7 nm, we observed that the pore area and volume were significantly decreased by about 30%.

Air Pollutants Emissions of HSF
As seen in Figures 4-6, the most flue gas (CO, C x H y , NO x , and SO 2 ) of the sewage sludge samples (black line) was measured in the temperature range in which the volatile components were burned. Unlike other sewage sludges, the SO 2 emission temperature of SW-D was measured at higher than 800 • C.

Air Pollutants Emissions of HSF
As seen in Figures 4-6, the most flue gas (CO, CxHy, NOx, and SO2) of the sewage sludge samples (black line) was measured in the temperature range in which the volatile components were burned. Unlike other sewage sludges, the SO2 emission temperature of SW-D was measured at higher than 800 °C.

Air Pollutants Emissions of HSF
As seen in Figures 4-6, the most flue gas (CO, CxHy, NOx, and SO2) of the sewage sludge samples (black line) was measured in the temperature range in which the volatile components were burned. Unlike other sewage sludges, the SO2 emission temperature of SW-D was measured at higher than 800 °C.   In comparison, the release of flue gas from the HSF samples was lower. The reduction of the CO gas emissions were calculated to be 27.69% (SW-L-10), 32.18% (DS-L-10), and 23.04% (SL-L-10), as the O/C ratios decreased (see Figures 4, 5 and 6a). In the case of CxHy, the emission decreased as the H/C ratio decreased (the reduction ratio of CxHy gas: SW-L-10 39.50%, DS-L-10 58.54%, and SL-L-10 74.65%, see Figures 4, 5 and 6b). The experimental results show that oxygen and hydrogen contents In comparison, the release of flue gas from the HSF samples was lower. The reduction of the CO gas emissions were calculated to be 27.69% (SW-L-10), 32.18% (DS-L-10), and 23.04% (SL-L-10), as the O/C ratios decreased (see Figures 4, 5 and 6a). In the case of C x H y , the emission decreased as the H/C ratio decreased (the reduction ratio of C x H y gas: SW-L-10 39.50%, DS-L-10 58.54%, and SL-L-10 74.65%, see Figures 4, 5 and 6b). The experimental results show that oxygen and hydrogen contents in the fuel affect CO and C x H y emissions. In addition, the decrease in CO emissions of HSF compared to those of raw sewage sludge can be attributed to a higher rate of complete fuel oxidation. These characteristics can be confirmed indirectly from the DTG graph ( Figure 2). The combustion boundary of VM and FC changed broadly, and the complete combustion of FC proceeded [38]. CO and C x H y emissions and unburned carbon (UBC) are closely related. In this study, we investigated the effects of lignin impregnation on UBC emission. Figure 7 shows UBC emission after burning at 1000 • C for dried sewage sludges and HSF samples. UBC emission was calculated as [30]: where Msample, Mchar, and Mash denote the initial sample weight, the intermediate residue weight after combustion for 10 min at 1000 • C, and the ash weight after perfect combustion for 7 h at 1000 • C, respectively [39,40]. UBC in HSF decreased as the impregnated-lignin ratio increased. When we examined the factors influencing UBC emission, we found that the UBC of HSF is closely related with the biomass ratio. These results are in good agreement with those of a previous study. According to Lee [41] and Park [38], biomass-impregnated fuels improve combustion responsiveness and reduce UBC emissions. Therefore, HSF with lignin would be beneficial for application in power plants from an environmental perspective in terms of particulate matter emission reduction.
dried sewage sludges and HSF samples. UBC emission was calculated as [30]: where Msample, Mchar, and Mash denote the initial sample weight, the intermediate residue weight after combustion for 10 min at 1000 °C, and the ash weight after perfect combustion for 7 h at 1000 °C, respectively [39,40]. UBC in HSF decreased as the impregnated-lignin ratio increased. When we examined the factors influencing UBC emission, we found that the UBC of HSF is closely related with the biomass ratio. These results are in good agreement with those of a previous study. According to Lee [41] and Park [38], biomass-impregnated fuels improve combustion responsiveness and reduce UBC emissions. Therefore, HSF with lignin would be beneficial for application in power plants from an environmental perspective in terms of particulate matter emission reduction.
Although the nitrogen content of the HSF was lower than that of dried sewage sludges, the nitrogen content had a minor impact on the reduction of NOx emission (Figures 4, 5 and 6c). When the fuel burned, most of the nitrogen oxides were derived from N2 in the air. Finally, because the added CaO was a highly reactive desulfurization agent, SO2 emission was reduced drastically regardless of the sulfur content (the reduction ratio of SO2 gas: SW-L-10 74.16%, DS-L-10 81.82%, and SL-L-10 77.53%; see Figures 4,5 and 6d).
The desulfurization mechanism can be expressed as [42]: CaO (s) + SO 2 (g) → CaSO 3 (s). ( In this study, we confirm that combustion of HSF could reduce the emissions of substances that cause pollution in the atmospheric environment compared to dried sewage sludges.  Although the nitrogen content of the HSF was lower than that of dried sewage sludges, the nitrogen content had a minor impact on the reduction of NO x emission (Figures 4, 5 and 6c). When the fuel burned, most of the nitrogen oxides were derived from N 2 in the air. Finally, because the added CaO was a highly reactive desulfurization agent, SO 2 emission was reduced drastically regardless of the sulfur content (the reduction ratio of SO 2 gas: SW-L-10 74.16%, DS-L-10 81.82%, and SL-L-10 77.53%; see Figures 4,5 and 6d).
The desulfurization mechanism can be expressed as [42]: In this study, we confirm that combustion of HSF could reduce the emissions of substances that cause pollution in the atmospheric environment compared to dried sewage sludges.

Ash and Heavy Metal Characterizations
The fouling or slagging characteristic is one of the most important issues in combustion systems [43]. To evaluate the ash characteristics of HSF, XRF and ash fusion temperatures were measured and shown in Table 3. According to the XRF results, most of the other components were reduced by the addition of CaO compared to sewage sludge. The added CaO contributed to an increase in initial deformation temperature (IDT) to~200 • C. The ash composition, the calculated fouling/slagging indices, and the measured ash fusion temperature representing the melting point of ash from HSF were improved compared to those of dried sewage sludge. Having reported that, the addition of lignin and CaO in the proposed synthesis process positively influences the ash characteristics, and accordingly, HSF is expected to ameliorate behaviors of dried sewage sludge, such as sintering, agglomeration, fouling, and slagging. Heavy metals in sewage sludge lead to serious environmental pollution and human health problems. We investigated heavy metals characteristic of dried sewage sludges and HSFs and summarized in Table 4. The industrial city of Suwon, had a higher heavy metal content compared to other cities. According to the heavy metal analysis, the measured heavy metal components from HSFs were almost unvaried compared to those from dried sewage sludges. The proposed upgrading sewage sludge fuel synthesis process does not influence heavy metal characteristics. Thus, heavy metals' release from HSFs is expected to occur similarly from dried sewage sludges in the combustion system.

Conclusions
In the current study, we developed a bioethanol-lignin with a CaO impregnated sewage sludge upgrading process, and experimentally validated its enhanced performance as fuel. Starting by producing a bioethanol-lignin solution, we prepared modified HSF samples by: (1) mixing sewage sludge with the bioethanol-lignin solution, drying the mixture at 105 • C for 10 h, and applying a thermal treatment at 250 • C for 1.5 h.