Hydrothermal Carbonization as an Energy-Efficient Alternative to

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Hydrothermal Carbonization as an Energy-Efficient Alternative to Established Drying Technologies for Sewage Sludge: A Feasibility Study on a Laboratory Scale M. Escala, T. Zumbühl, Ch. Koller, R. Junge, and R. Krebs* Institute of Natural Resource Sciences, Zurich University of Applied Sciences (ZHAW), Campus Grüental, CH-8820 Wädenswil, Switzerland ABSTRACT: Hydrothermal carbonization (HTC) of stabilized and non-stabilized sewage sludge was investigated in a 25 L vessel as a pretreatment for sewage sludge before incineration, and the composition and properties of the obtained HTC coal and process water were studied. The observed values for H/C and O/C in HTC coal from stabilized and non-stabilized sewage sludge were shown to be higher than in natural coal and rather close to typical values for cellulose. The upper heating value of the stabilized sewage sludge was increased from 3.4 to 6.5%, and the upper heating value of the non-stabilized sludge was increased from 5.8 to 11.0%, after carbonization. The carbon efficiency ranged from 62 to 71% for stabilized sewage sludge and from 60 to 66% for non-stabilized sewage sludge, and the dry matter (DM) loss after carbonization was 31 and 42% for stabilized sludge and 34 and 44% for non-stabilized sludge. After carbonization, the mechanical dewaterability was increased from 30 to 70% DM content for non-stabilized sludge and from 37 to 52% for stabilized sludge. The drying process of sewage sludge including HTC needs a clearly lower energy input than established drying techniques to produce a fuel. For the drying process of 1 ton of nonstabilized sewage sludge with 9% DM, the calculated energy consumption was lowered by 99.6 kWh of thermal energy and 8.5 kWh of electric energy by introducing HTC. The results of these experiments show the feasibility of the HTC process as part of the drying process of sewage sludge and the fate of key elements in the process on a laboratory scale. However, the process has to be further optimized and developed on an industrial scale. Further important development steps include recovery steps for the carbon in the process water and adapted process water treatments.

1. INTRODUCTION Sewage sludge, the residue of wastewater treatment plants, has been traditionally used as a fertilizer in agriculture or disposed of in landfills. However, besides its valuable agronomic properties (e.g., supply of phosphorus and nitrogen), sewage sludge is often contaminated with heavy metals, microorganisms, and a range of hazardous organic substances, which can pose a threat for the soil, vegetation, animals, and humans.1 For this reason, the application of sewage sludge to agricultural land is increasingly restricted. For instance, the European Union sets maximum values of concentrations of heavy metals and bans the spreading of sewage sludge when the concentration of certain substances in the soil exceeds these values.2 More stringent regulations are left to the individual countries, and for instance, in Switzerland, the sewage sludge has to be disposed of via thermal treatment since 2006.3 At present, 53% of sewage sludge produced in the EU-15 is reused in agricultural applications, while 21% is incinerated.4 Following the regulations aimed to protect the environment and human health, the thermal treatment of sewage sludge has emerged as an appealing solution for disposal.5 The disposal alternatives range from mono-combustion, co-combustion in waste incineration plants, to use as a surrogate fuel in cement kilns. At present, the mono-combustion of sewage sludge is considered as one of the best options because it would facilitate the recovery of valuable resources, such as phosphorus (important plant nutrient with limited availability), from the ash. Besides environmental protection issues, these thermal processes enable the recovery of stored energy in stabilized and non-stabilized sewage sludge. Anaerobic fermentation is the © 2012 American Chemical Society

most applied process for stabilization of sewage sludge featuring mass reduction and methane production. On account of carbon removal in the form of methane and carbon dioxide, the end product shows a substantially better biological stability than the unfermented, non-stabilized material. Non-stabilized sewage sludge containing higher contents of carbon has normally less favorable dewatering properties.6 However, sewage sludge in general is not easily dewatered, and therefore, moisture contents are typically higher than 70% after dewatering, which means that most of the energy released during the thermal treatment is consumed to decrease the moisture.5 An efficient and costeffective energetic use of sewage sludge requires, therefore, a material with the highest possible dry matter (DM) content and calorific value. The hydrothermal carbonization (HTC) offers a pretreatment pathway for sewage sludge, which results in a product fulfilling the above requirements. In the HTC process, biomass is heated in a water medium to temperatures around 180−210 °C in a closed vessel, thus allowing for saturated pressure to build up. Under these conditions, the biogenic material reacts in an overall exothermic process, undergoing a net of reactions, which include hydrolysis, dehydration, decarboxylation, polymerization, and aromatization, although detailed characteristics of the reactions are only known for a few compounds, such as cellulose.7 The output of the HTC reaction is primarily a solid phase (HTC Received: September 17, 2012 Revised: November 13, 2012 Published: November 19, 2012 454

dx.doi.org/10.1021/ef3015266 | Energy Fuels 2013, 27, 454−460

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Table 1. Process Parameters of Performed HTC Reactions sample

s-7-205-ca

s-7-205

ns-5-190

ns-5-205

ns-5-220

ns-7-205-a

ns-7-205-b

sewage sludge type catalyst processing time (h) time, 180 < T < 200 °C (min) time, T > 200 °C (min) total time, T > 180 °C (min) severity of the reaction, f a set temperature (°C) maximum temperature (°C) maximum pressure (bar)

stabilized citric acid (30 g) 7 68 24 92 0.177 205 202 21.8

stabilized no 7 84 18 102 0.181 205 202 23

non-stabilized no 5 141 0 141 0.165 190 192 13.2





a

Severity of the reaction calculated with t (total time above 180°) and T (maximum temperature) after the formula reported in ref 7, f = 50t0.2e(−3500/T) = [Ofeed − Ot]/[Ofeed − 6], where f is the severity of the reaction factor, t is the carbonization time (s), T is the temperature (K), Ofeed is the percentage of oxygen in biomass, Ot is the percentage of oxygen in coal, and 6 is the assumption that 6% of O in coal shows complete conversion of biomass. A total of seven carbonization reactions with continuous mechanical stirring were performed, two reactions with stabilized sewage sludge and five reactions with non-stabilized sewage sludge. The DM content of the stabilized sewage sludge samples was adjusted with water from 23.9 to 20.1% for optimum reaction conditions. The DM content for the nonstabilized sewage sludge was 9.0%. One of the experiments with stabilized sewage sludge was run by adding a catalyst (30 g of citric acid). The target temperature and the reaction duration were set in the control panel; therefore, the process started and ended automatically. As a convention and to enable a comparison of the different reaction samples, the carbonization time was defined as the time span in which the temperature in the reactor was higher than 180 °C. The process parameters of the performed HTC reactions are shown in Table 1. The produced HTC coal and the process water were separated by manual decantation, using a strainer to hold back big particles. The samples of the process water were kept in a refrigerated glass bottle, wrapped with aluminum foil to protect it from light, and analyzed within 24 h after the separation. 2.4. Analysis of Sewage Sludge, HTC Coal, and Process Water Samples. 2.4.1. Sewage Sludge and HTC Coal. One sample each of the stabilized sewage sludge, the non-stabilized sewage sludge, and the corresponding HTC coal samples were dried in an oven at 60 °C and homogenized in a swing mill (17 Hz for 3 min). In the treated samples, elemental analysis (C−H−N−O−S; precision of 0.3%) was conducted and the upper heating value was determined (precision of 120 kJ/kg). In one unconditioned (fresh) sample each of the stabilized sewage sludge, the non-stabilized sewage sludge, and the HTC coal (ns-7-205b), the following analyses were conducted: for polycyclic aromatic hydrocarbons (PAHs), the samples were dried at 40 °C and homogenized with a swing mill, following ultrasonic extraction with hexane, acetone, and toluene (10:5:1) and quantification by gas chromatography−mass spectrometry (GC−MS), with the United States Environmental Protection Agency (EPA) method 8270D; heavy metals were analyzed by microwave pressure decomposition with nitrohydrochloric acid and determination of the nitrohydrochloricacid-soluble components with inductively coupled plasma−mass spectrometry (ICP−MS) and mercury was analyzed with cold vapor atomic absorption spectrometry (CVAAS), with method EN ISO 17294-1/2; alkali metal and alkaline earth metal and iron were analyzed by microwave pressure decomposition with nitrohydrochloric acid and determination of the nitrohydrochloric-acid-soluble components with flame atomic absorption spectrometry (FAAS), with methods DIN 38406 E13, E14, E32, and EN ISO 7980; total phosphorus was analyzed by microwave pressure decomposition with nitrohydrochloric acid, with photometry EN 1189; total cyanide was analyzed by photometry for release at pH 1, with method ISO 11 262; total nitrogen was analyzed by Kjeldahl digestion and titration, with method EN 25 663; and phenols were analyzed by extraction and GC−MS, with EPA method 8270. The pH of fresh sewage sludge was measured with a multi-parameter digital meter (HQ40d, HACH LANGE).

coal) and a liquid phase (process water); in addition, a small amount of gas (particularly CO2) is generated. The HTC coal is a storable energy carrier with the characteristics of brown coal, and because of its rapid cycle production use, it has a virtually neutral CO2 balance.7,8 It is assumed that increased carbon content in non-stabilized sewage sludge will result in higher calorific values of the HTC coal. This work focuses on HTC as an alternative to established drying technologies for stabilized and non-stabilized sewage sludge and seeks to provide comprehensive data about the composition and properties of the obtained products (HTC coal and process water).

2. MATERIALS AND METHODS 2.1. Sewage Sludge. Two different kinds of sewage sludge were collected from two different wastewater treatment plants in the Lake Zurich region. At 2 times, 20 L of stabilized (digested) and mechanically dewatered sewage sludge (23.9% dry weight; pH between 6.9 and 7.4) was collected from the wastewater treatment plant Rietliau in Wädenswil, Switzerland. The wastewater treatment plant Rietliau in Wädenswil has a capacity of 44 000 population equivalents (PE) and is equipped with preliminary sedimentation, activated sludge tank, membrane filtration, partial nitrification, denitrification, chemical phosphate elimination, and anaerobic sludge digestion.9 At 6 times, 20 L of non-stabilized but thickened sewage sludge (9.0% dry weight; pH of 7.9) was collected from the wastewater treatment plant HombrechtikonSeewis in Hombrechtikon, Switzerland. The wastewater treatment plant Hombrechtikon-Seewis was built for a PE of 13 500, and the plant contains preliminary sedimentation, grease separator, nitrification, denitrification, filtration, and chemical phosphate elimination.10 Both types of sewage sludge were kept in refrigerated plastic buckets at 4 °C. The buckets were airtight, thus protecting the samples from oxidation. The experiments with the sewage sludge samples were accomplished within 2 weeks. 2.2. Reactor. The carbonization of the sewage sludge samples was performed with a laboratory-scale batch reactor (Grenolmatik 25, Grenol GmbH, Germany) designed for HTC. The reactor contains a double-walled pressure vessel with a detachable container of 25 L capacity. The stainless-steel reactor has a stirring unit and an built-in mantle heating system working with thermal oil. The reactor was operated with a programmable controller, which processed the data from two pressure sensors (digital and analog), a temperature sensor for the reactor container, and a temperature sensor for the heating unit. 2.3. HTC and Sample Preparation. The detachable stainless-steel container was filled with an approximate volume of 12.4−17.1 L sewage sludge, either stabilized (i.e., digested) or non-stabilized (i.e., nondigested). The space between the detachable container and the heating jacket was filled with 1.7 L of tap water to facilitate the heat transfer. 455


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2.4.2. Process Water. The process water samples of each carbonization reaction were centrifuged (4400 rpm for 10 min), and the upper layer of the supernatant was separated for further analyses. The electric conductivity (EC) and pH value were measured with a multi-parameter digital meter (HQ40d, HACH LANGE). Total nitrogen (TN, LCK 138), ammonium nitrogen (NH4 N, LCK 304), nitrite nitrogen (NO2 N, LCK 341), nitrate nitrogen (NO3 N, LCK 339), phosphate phosphorus (PO4 P, LCK 348), total phosphorus (TP, LCK 349), chemical oxygen demand (COD, LCK 414), and total phenols (LCK 345) contents of the samples were measured by photometry (DR 3800, HACH LANGE). TP and TN were determined in nonfiltered solution after the samples were kept for thermal hydrolysis in a thermostat (HT 200S) for 1 h at 100 °C. NH4 N, NO2 N, NO3 N, and PO4 P were determined after sterile filtration (0.45 μm glass fiber prefilter). The analysis of phenols was performed with nonfiltered samples protected from light and using exclusively glassware. The process water samples were diluted with ultrapure water (Water HPLC, PH Stehelin) prior to the analysis to comply with the measurement range of the used method. The readings were kept in the lower third of the measurement range for the corresponding analysis method. The dilutions were 1:10 for nitrite, 1:10 and 1:100 for phosphate, 1:100 for nitrate, 1:100 and 1:200 for total phenols and total phosphorus, 1:1000 for total nitrogen, and 1:2000 for ammonium nitrogen and chemical oxygen demand. The color of the sample background showed a value of zero for all dilutions. The process water samples of three HTC reactions (ns-5-190, ns-5205, and ns-7-205-a) were analyzed for the content of PAHs, heavy metals, mercury, alkali metal, and alkaline earth metal and iron, total phosphorus, total nitrogen, and phenols using the methods described in section 2.4.1. 2.5. Dewatering of Sewage Sludge and HTC Coal. The dewaterability of the two types of sewage sludge (stabilized and nonstabilized) and the two samples of HTC coal (s-7-205-ca and ns-7-205b) was compared using centrifugation and mechanical pressing. For the former, the samples (excluding the stabilized sewage sludge, which already had a dry weight of 20%) were centrifuged (4400 rpm for 10 min) and the supernatant was decanted. For the pressing trial, the residues from the centrifugation (or the initial stabilized sewage sludge) were wrapped in 11 μm filter paper and a maximum pressure of 40 bar was applied by a pellet press designed for X-ray fluorescence (XRF) sample preparation. The dry weight of each sample was determined after each dewatering step.

To verify the carbonization of the sewage sludge, the change in the elemental composition of carbon, oxygen, and hydrogen was quantified. Figure 1 shows the atomic ratio of H/C versus O/C for the initial sewage sludge and the different carbonized products. For both types of sewage sludge, the carbonization caused a decrease in the H/C and O/C ratios, which can be attributed to the dehydration and decarboxylation reactions during the process.7 In the case of non-stabilized sewage sludge, the initial H/C and O/C ratios were 1.93 and 0.63, respectively. The severity of the carbonization (defined by time and temperature) was connected to the decrease of both ratios, so that between the slightest and strongest carbonized samples, a 5.5% decrease in the H/C ratio and a 7.6% decrease in the O/C ratio could be observed. However, even the lowest obtained values (H/C of 1.73 and O/C of 0.49) are far away from the natural lignite range, namely, H/C of around 0.8−1.3 and O/C of around 0.2−0.38, as reported in ref 11 and references therein. For the stabilized sewage sludge, the initial ratios of 2.30 (H/ C) and 1.00 (O/C) also decreased with the carbonization, reaching values of 1.87 and 0.80, respectively. The ratios obtained from trials s-7-205-ca and s-7-205 are nearly identical points at a good reproducibility of the carbonization and a negligible effect of the catalyst (in terms of the gross elemental composition of the coal). The values for H/C and O/C are in this case also higher than in natural coal and rather close to typical values for cellulose. These results suggest that a further carbonization of the sewage sludge is possible. Other authors have reported values for digested sewage sludge subjected to pyrolysis (30 min at 300 °C) of 1.55 for H/C (initial value of 1.82; 14.8% decrease) and 0.46 for O/C (initial value of 0.57; 19.3% decrease).11 These ratios are lower than the values obtained in the present study for digested sewage sludge but very similar to our values for non-stabilized sewage sludge. Furthermore, the relative reduction in H/C and O/C ratios in our study, namely, 18.2 and 19.7%, is also very close to values in ref 11, which suggests that the elemental composition change obtained with 30 min at 300 °C can also be attained in 100 min at temperatures between 180 and 200 °C. This comparison highlights the fact that the initial composition of the sewage sludge can vary largely and will play an important role in determining the final composition of the coal. The question remains if sewage sludge could be further carbonized by HTC by increasing the reaction time. The concentrations of other measured substances in the DM of the non-stabilized sewage sludge and the HTC coal are shown in Table 3. Because one of the currently discussed applications of HTC coal is as a soil conditioner and potential carbon sink in the soil, the substances analyzed in the DM were compared to the thresholds for organic fertilizers given in the Swiss Chemical Risk Reduction Regulation.12 The amount of detected copper and zinc was up to 2 times higher than the threshold; all of the other compared values were below the threshold. The examined phenols and nitro compounds, m- + p-cresol, were found in elevated concentrations in the non-stabilized sewage sludge, but in the HTC coal, they were decreased by half. Among the analyzed PAHs, only phenanthrene was detected above the detection limit in the stabilized sewage sludge sample (1.4 mg/kg of DM). 3.1.2. Upper Heating Value. The initial upper heating value of the stabilized sewage sludge was 10.66 MJ/kg. After two carbonization trials at temperatures between 180 and 202 °C for ca. 100 min, the obtained coal featured a heating value of 11.35 and 11.02 MJ/kg, an increase of 6.5 and 3.4%, respectively.

3. RESULTS AND DISCUSSION 3.1. HTC Coal. 3.1.1. Elemental Composition. The main elements of the sewage sludge and HTC coal are listed in Table 2. Table 2. Elemental Composition of the Sewage Sludge and HTC Coal sample

C (%)

H (%)

N (%)

O (%)

S (%)

stabilized sewage sludge HTC coal s-7-205-ca HTC coal s-7-205 non-stabilized sewage sludge HTC coal ns-5-190 HTC coal ns-5-205 HTC coal ns-5-220 HTC coal ns-7-205-a HTC coal ns-7-205-b

24.89 26.56 25.86 38.11 40.95 40.22 41.15 41.02 41.12

4.78 4.14 4.06 6.12 6.03 5.77 5.8 5.93 5.72

3.91 4 3.83 3.31 3.6 2.76 2.69 2.66 2.3

33.08 28.24 27.59 31.99 28.82 28.04 26.98 26.88 26.73

1.09 1.15 1.16 0.36 0.37 0.36 0.38 0.36 0.43

The main difference in composition between both sewage sludges lies in the percentage of carbon, lower for the stabilized material, which can be explained by the anaerobic digestion to which it was subjected. The coal obtained with a catalyst has a slightly higher C content than the sample without a catalyst (see Table 2), although the total reaction time (T >180 °C) was lower (see Table 1). 456


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Figure 1. Van Krevelen diagram of the carbonized sewage sludge samples.

catalyst should be tested to clearly quantify the effect. For the non-stabilized sewage sludge, the difference between the initial heating value (17.2 MJ/kg) and the final heating values (18.20− 19.09 MJ/kg), was higher; the increase in this case was from 5.8 to 11.0%. As a comparison,13 reported heating values were close to 9 MJ/kg for HTC coal from stabilized sewage sludge and 12 MJ/kg for HTC coal from non-stabilized sewage sludge, which both lie below the heating values in the present study. However, the initial values for their trials are not known. For all carbonized samples, the overall increase of the heating value is closely related to the rise in the carbon concentration during the process (linear correlation with a coefficient R2 of 0.996). 3.2. Process Water. The concentrations of all components in the process water resulting from HTC were very high, except for nitrite (Table 4). The high values of conductivity reveal the presence of salts in all samples of process water, with the two samples from stabilized sewage sludge featuring an average conductivity of 14.1 mS/cm (n = 2), at 17.6% higher as the average of 11.6 mS/cm (n = 5) for non-stabilized sewage sludge. The pH of the process water samples from stabilized sewage sludge was 6.9 (n = 2), remarkably more basic than that for the non-stabilized sewage sludge, with a pH of 5.0 (n = 5). Initial pH values were around 7 and 7.9 in stabilized and non-stabilized sludge, respectively. Most substances were present in higher concentrations in the process water from stabilized sewage sludge, especially phenols, with an average concentration at 44% higher for stabilized than for non-stabilized material. This can be generally attributed to the higher concentration of the stabilized starting material (20.1 versus 9.0% DM). However, the phosphorus content was remarkably higher for the process water samples obtained from non-stabilized material. Because the nitrogen contents of both types of samples were very similar, the resulting N/P ratio was much higher for the process water samples from stabilized sewage sludge (average of 165.3; range from 152.2 to 181.5) than for process water from the non-stabilized material (average of 31.0; range from 19.9 to 54.4). These differences could be, however, attributed to the different composition of the original sewage sludge. The dissolved organic carbon, between 31.4 and 53.0 g/L, was very elevated in all surveyed samples. 3.3. Mass Balance and Carbon Efficiency. The solid mass inside the reactor is bound to decrease during the carbonization. This is primarily due to the chemical dehydration of the biomass

Table 3. DM Analysis (All Values in mg/kg of DM) sample calcium magnesium potassium sodium total nitrogen total phosphorus, P total phosphorus, P2O5 antimony arsenic cadmium chrome cobalt copper lead lithium manganese mercury molybdenum nickel silver tin thallium total cyanide tungsten vanadium zinc m- + p-cresol phenanthrene phenols a

non-stabilized sewage sludge 35100 3500 2410 890 46600 25200 57700 2.4 1.5 0.66 26 14 229 26 3 447 0.5 3.8 17 9 21 0.06 3.7 13 17 547 834

Hydrothermal Carbonization as an Energy-Efficient Alternative to

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