Gasification of sewage sludge

Gasification of sewage sludge

25

Sebastian Werle*, Mariusz Dudziak† *Institute of Thermal Technology, Silesian University of Technology, Gliwice, Poland, † Institute of Water and Wastewater Engineering, Silesian University of Technology, Gliwice, Poland

1

Introduction

Sewage sludge is a by-product of wastewater treatment plant operations. Due to the rapid rate of urbanization and industrialization and growth of the population, the world is facing the serious challenge of disposing of this waste. For the last 50 years, the world’s population has multiplied faster than ever before, and more rapidly than it is projected to grow in the future. In 1950, the world had 2.5 billion people; and in 2005, the world had 6.5 billion people. By 2050, this number could rise to more than 9 billion (World Population Prospects, 2008). The problem of this waste material is a key issue in the majority of countries. In the European Union (EU), the problem of sewage sludge is being tackled by general directives and indicators and national legislative requirements. As a result of the implementation of the Urban Waste Water Treatment Directive (UWWTD) 91/271/CE (Council Directive, 1991), the member-countries have improved their collecting and wastewater treatment systems. The amount of sewage sludge produced increased by more than 50%, from 6.5 million tons of dry matter (DM) in 1992 to 10.9 million tons in 2015 (http://epp.eurostat.ec.europa.eu, Accessed 27 November 2017). Fig. 1 shows the amount of sewage sludge produced in EU member-countries in 2015 and the total amount produced in EU-28 (http://epp.eurostat.ec.europa.eu, Accessed 27 November 2017). According to the European Union’s and other countries’ demographic projections, the total sludge production in the union in 2020 increased to 13 million tons DM (UN DESA, 2014). In line with the 7th Environment Action Programme (http://ec.europa.eu/environ ment/action-programme/, Accessed 27 November 2017), the reduction of sewage sludge disposal in landfills by 50% from 2000 to 2050 has to be achieved. Based on these principles, the Landfill Directive (LD) 99/31/EC (Council Directive, 1999) prohibited the landfilling of both liquid and untreated waste and set restrictions. Landfill and water deposition of sewage sludge with the indicated parameters presented in Table 1 in the European Union is prohibited. Additionally, generally the member-states are applying stricter restrictions than those determined in Sewage Sludge Directive 86/278/EEC (SSD) (Council Directive, 1986), which describes the beneficial use of sludge in soil. The majority Industrial and Municipal Sludge. https://doi.org/10.1016/B978-0-12-815907-1.00025-8 © 2019 Elsevier Inc. All rights reserved.

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Industrial and Municipal Sludge

Thousand tons DM

12,000 10,000 8000 6000 4000 2000

C

ze Be ch lgi re um p G ub er lic m a Ire ny la n Sp d a C in ro a C tia yp Li r u th s u H ani N un a et ga he r rla y n Po ds l R an om d Sl ani ov a a Sw kia ed To e ta n lE U

0

Fig. 1 The sewage sludge production in EU-28 in 2015 (http://epp.eurostat.ec.europa.eu, Accessed 27 November 2017). Table 1 Criteria for the storage of sewage sludge in a nonhazardous waste landfill (Council Directive, 1999) Number

Parameter

Value limit

1 2 3

Overall organic carbon (OOC) (%DM) Loss at calcinations (LOC) (%DM) The higher heating value (MJ/kg DM)

5.0 8.0 Maximum 6.0

of EU countries have set more stringent national requirements for heavy metal concentrations in sewage sludge then required by SSD. Considering these facts, as well as the limitations of current disposal methods and the challenges of energy sustainability, it can be concluded that energy recovery is definitely going to be an important part of modern sewage sludge management. Consequently, there is an urgent need to explore low-cost, energy-efficient, and sustainable solutions for the treatment, management, and future utilization of sewage sludge. There are several thermal technologies for utilizing sewage sludge to obtain forms of energy through correct pretreatment (e.g., torrefaction, slow pyrolysis, and hydrothermal carbonization processes) (Wilk et al., 2016). In Fig. 2, the main processes (combustion and cocombustion, gasification, and pyrolysis) used for sewage sludge thermochemical conversion are presented. Based on the flowchart in Fig. 2 it seems that gasification of sewage sludge has several advantages over a traditional combustion or cocombustion process and pyrolysis (Muzyka et al., 2015). Pyrolysis requires comprehensive knowledge of the physicochemical, thermal, and kinetic decomposition characteristics of feedstock (Yaman, 2004). Gasification is the partial oxidation of biodegradable material in an air-restricted environment, which yields a mix of flammable gases such as hydrogen (H2), carbon monoxide (CO), and methane (CH4), (known as

Gasification of sewage sludge

577

Sewage sludge

Combustion and cocombustion

Hot gases

Steam, heat electricity

Gasification

Low-energy gas

Internal combustion engines

Mediumenergy gas

Fuel gases, methane

Pyrolysis

Char

Hydrocarbons

Liquid fuels, metanol, gasoline

Fuel oil

Sorbent material

Fig. 2 Main thermochemical processes for sewage sludge conversion (Wilk et al., 2016).

producer gas or gasification gas) and a solid fraction of carbonaceous, ash-rich char (Skorek-Osikowska et al., 2017). Producers can be used to generate mechanical or electrical power in dedicated gas engines or can be fed into the intake manifold of diesel engines in dual-fuel operations. Such gasification power systems are technologically mature, tolerant of diverse feedstocks, and practical on smaller scales than combustion-based steam power systems (Pinto et al., 2007). Taking into consideration that gasification is characterized by a low-level gasification agent environment, the volume of produced gas is less than the volume of flue gas from the combustion process. Consequently, sulfur in gasified material is transformed to hydrogen sulfide (H2S), nitrogen to ammonia (NH3), and chloride to hydrogen chloride (HCl), the formation of the dioxins sulfur dioxide (SO2) and NOx is prevented, and gas-cleaning installation is smaller and less expensive than classic combustion. This feature is very profitable, considering the possible use of such installations in wastewater treatment plants. A more efficient scenario for sewage sludge gasification is combining the fuel production process with phosphorus recovery. After sewage sludge conversion, the residue is a sanitized source of minerals and organic elements, including valuable fertilizing components, which makes them a potential substitute for natural phosphorus ore (Gorazda et al., 2017). The development of gasification technology is associated with the development of gasifiers.

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Industrial and Municipal Sludge

Gasification reactors can be divided into three main types (Gorazda et al., 2017; Werle, 2014): fixed-bed, fluidized-bed, and entrained-bed gasifiers. One of the most widely known European plants using the gasification process is a system in G€ussing, Austria, built in 2001, with power in the fuel of 8 MWth (http://biomasspower.gov.in/, Accessed 27 November 2017). Similar systems can be found in Harbore, Denmark; Spiez, Switzerland; Kokem€aki, Finland; and Skive, Denmark (Uchman and Werle, 2016). The installation by the Institute of Thermal Technology is another example of the sewage sludge gasification facility (Werle and Wilk, 2011). During sewage sludge gasification, the occurrence of inorganic and organic contamination, including the waste by-products ash and tar, is also important (Werle and Dudziak, 2014). In that context, new procedures are still generated (Werle and Dudziak, 2013; Werle et al., 2016). For example, in sewage sludge and in tars from gasification, both toxic and hazardous organic substances (primarily of polycyclic aromatic hydrocarbons in sewage sludge, and phenols and their derivatives in tars) and inorganic substances (e.g., nine heavy metals) were identified. Moreover, in ash, mainly inorganic substances (heavy metals) were detected. The inorganic and organic contamination is mainly transported within the system as sewage sludge– gasification by-products. For their determination, basic instrumental methods (gas chromatography and absorption spectrometry) (Werle and Dudziak, 2014) can be used, as well as indirect methods like photoacoustic spectrometry and ecotoxicological analysis (Werle et al., 2016). The aim of this research is a multiparametric investigation of the sewage sludge gasification in the fixed-bed gasifier. The deep characterization of sewage sludge as feedstock is presented. The influence of the gasification process parameters on the sewage sludge gasification gas quality is determined. The characterization of the gasification by-products is also included. The proposition of the reuse process of ash produced during gasification is postulated.

2

Gasification of sewage sludge in a fixed-bed reactor— research characteristics

A fixed-bed gasification facility located at the Laboratory of Fuels Combustion and Gasification, at the Institute of Thermal Technology of Silesian University of Technology, Gliwice, Poland, was used for the current study. A schematic of the facility is shown in Fig. 3. The main part of the system was an insulated, stainless gasifier with an internal diameter of 150 mm and a total height of 300 mm. The granular sewage sludge was fed into the reactor from the fuel container located at the top. The gasification air was fed from the bottom by the fan. The temperature inside the reactor was measured by six N-type thermoelements integrated with the Agilent temperature recording system. Thermocouples were located along the vertical axis of the reactor. Additionally, the temperature of the gasification gas at the outlet of the reactor was measured. The flow rates of gasification air and of produced gas was measured by flow meters. The gasification gas Syngas was transported by the gas pipe and then was cleaned by a cyclone, scrubber, and drop separator. Volumetric fractions of the main components

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Gas burner Fuel container

Gas to analysis Gasifier Drop separator

Gas pipe

Scrubber Thermoelement Cyclone

Flow meter

Pressure fan

Fig. 3 Schematic diagram of the experimental system (Werle and Wilk, 2011).

Table 2 Gasification process methodology (Werle, 2014, 2015)

Sewage sludge (SS)

Gasification agent

Air ratio λ

Gasification agent temperature (K)

SS1 (from the mechanicalbiological wastewater treatment plant) SS2 (from mechanicalbiological-chemical wastewater treatment plant)

Atmospheric air (molar fraction of oxygen zO2 ¼ 0.21; molar fraction of nitrogen zN2 ¼ 0.79) and oxygen-enriched air (molar fraction of oxygen zO2 ¼ 0.25; molar fraction of nitrogen zN2 ¼ 0.75)

0.12, 0.14, 0.16, 0.18, 0.23, 0.27

298 (cold), 323, 373, 423, 473 (preheated)

of produced gas were measured online using the Fisher Rosemount and ABB integrated set of analyzers. The gasification methodology is presented in Table 2.

3

Feedstock characterization

During the experiments, two types of sewage sludge feedstock were analyzed: SS1 and SS2. SS1 was taken from a mechanical-biological wastewater treatment plant, and SS2 was taken from a mechanical-biological-chemical wastewater treatment plant with phosphorus precipitation. In both analyzed cases, the biological part of the wastewater treatment plant has worked with low-load activated sludge. Such solution offers

580

Industrial and Municipal Sludge

Fig. 4 The (A) SS1 and (B) SS2 samples (Werle and Dudziak, 2015).

the chance for effective removal of phosphorus and nitrogen from wastewater. Additionally, in both cases, sewage sludge is stabilized by anaerobic digestion and dehydration. After this, sewage sludge is mechanically dried. For the SS1 sample, hot air (thotair ¼ 260°C) in a cylindrical dryer with a heated shelf was used. For the SS2 sample, hot air (thotair ¼ 150°C) in an air belt dryer was used. As a consequence of the mechanical process of drying, the form of SS1 samples is similar to granules and the form of SS2 samples is similar to small “noodle.” Fig. 4 shows the two samples.

4

Ultimate and proximate analysis and occurrences of organic and inorganic contaminants

Table 3 presents the proximate and ultimate analysis of both samples in the study. The following procedures and standards were used for the sewage sludge characterization. For the moisture PN-EN 14774-3:2010, the volatile content was determined according to the standard PN-EN 15402:2011. The ash content was obtained using PN-EN 15403:2011. The calorific value t was determined in accordance with the CEN/ TS15400:2006 standard. The infrared (IR) spectroscopy analyzer was also used to carry out the ultimate analysis of the sewage sludge. The organic contaminants in the sewage sludge analyzed by Werle et al. (2016) were characterized by the presence of polycyclic aromatic hydrocarbons, pesticides, and polychlorinated biphenyls (PCBs; Table 4). All of the analyzed sewage sludge also was characterized by the presence of inorganic contaminants (namely, various heavy metals). The technology of the wastewater treatment strongly influenced the profile of the contamination in sewage sludge. In the sewage sludge from the wastewater treatment plant operating in the mechanical-biological system (SS1), nine compounds of polycyclic aromatic hydrocarbons were identified: phenanthrene, anthracene, benzo(a)fluoranthene, pyrene, chrysene, benzo(b)fluoranthene, dibenzo(a, h) anthracene, benzo (g, h, i) perylene, and indeno (1, 2,3-cd)pyrene. In sludge from the mechanical-biological-chemical wastewater

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Table 3 Proximate and ultimate analysis of the samples SS1 and SS2 (Werle and Dudziak, 2015) Parameter

SS1

SS2

5.30 44.20 49.00

5.30 36.50 51.50

27.72 3.81 3.59 13.53 1.81 0.003 0.033 10.75

31.79 4.36 4.88 15.27 1.67 0.013 0.022 12.96

Proximate analysis (%) (as received) Moisture Volatile matter Ash

Ultimate analysis (%DM) C H O N S F Cl LHV (MJ/kgDM)

treatment plant (SS2), there were eight compounds [acenaphthene, benzo(a) fluoranthene, pyrene, benzo(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(a) pyrene, and indeno(1,2 3-cd)pyrene] in this pollutant group [naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, benzo(a)fluoranthene, pyrene, benzo(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, dibenzo(a, h)anthracene and benzo(g, h, i)perylene, indeno(1,2,3-cd) pyrene]. However, the total concentration of polycyclic aromatic hydrocarbons was almost four times higher in SS1 than in SS2. Also, in earlier research (Werle, 2014), it was determined that polycyclic aromatic hydrocarbons are the primary form of pollutants present in sewage sludge. In addition, pesticides and PCBs were included in the identified organic pollutants in the tested sewage sludge. Content analysis shows that the total concentration of heavy metals in the sewage sludge is similar in both cases: 1841.19 mg/kg for SS1 and 1847.6 mg/kg for SS2 (Table 4).

5

Indirect methods of determining the level of sewage sludge contamination

The identification of various organic and inorganic elements in sewage sludge spurs the need to find indirect methods of determining its contamination level. Werle et al. (2016) documented that photoacoustic spectrometry (PAS) is a very effective way to determine the organic contamination level in sewage sludge. Using PAS and gas chromatography–mass spectrometry (GC-MS) analysis, a comparison of the pollution degree of SS1 and SS2 was made. Table 5 summarizes the obtained results. These results lead to the conclusion that photoacoustic spectroscopy (PAS) analysis can be used to make a preliminary assessment of the degree of contamination in

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Industrial and Municipal Sludge

Table 4 Concentration of organic and inorganic contaminants in sewage sludge (Wilk et al., 2016) Sewage sludge SS1

SS2 Concentration (μg/kg DM)

Group/compounds

Polycyclic aromatic hydrocarbons Acenaphthene Phenanthrene Anthracene Benzo(a)fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(b)fluoranthene Benzo(a)pyrene Dibenzo(a,h)anthracene Benzo(g,h,i)perylene Indeno(1,2,3-cd)pyrene Sum

n.o. 511.12 200.03 44.78 187.22 n.o. 108.14 700.51 n.o. 101.54 209.44 370.62 2433.40

80.84 n.o. n.o. 126.48 123.86 35.15 23.79 53.62 46.11 n.o. n.o. 131.48 621.33

Pesticides Heptachlor Aldrin Endrin Sum

4.14 3.13 11.58 18.85

n.o. 1.28 n.o. 1.28

PCBs 2,20,5,50-PCB 2,20,4,5,50-PCB 2,20,4,40,5,50-PCB Sum

9.75 33.33 23.78 66.86

7.90 n.d. 4.57 12.47

Heavy metal content, concentration (mg/kg DM) Zn Cu Pb Ni Cr Cd As Hg Se Sum n.o., not observed.

920.90 495.30 119.30 103.67 180.53 6.47 4.19 0.99 9.84 1841.19

991.20 183.16 59.97 18.90 584.53 3.24 3.94 0.96 1.70 1847.6

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Table 5 Comparison of the results from GC-MS and PAS analysis of the dry mass of the analyzed sewage sludge (Wilk et al., 2016) GC-MS analysis

PAS analysis

Sewage sludge

Group of pollutants

Characteristic binding

Wavelength (cm21)

Presence (+)/ absence (–)

SS1

Aromatic hydrocarbons Pesticides

C]C

1450–1610

+

C]C C―Cl C]C C―Cl C]C

1450–1610 600–800 1450–1610 600–800 1450–1610

+ + + + +

C]C C―Cl C]C C―Cl

1450–1610 600–800 1450–1610 600–800

+ + + +

PCBs SS2

Aromatic hydrocarbons Pesticides PCBs

the samples (e.g., before conducting GC-MS analysis). It was observed that the compared techniques have very similar results in terms of pollution detection. Thus, photoacoustic spectroscopy can be successfully used in screening analysis. Another indirect method of determining the sewage sludge contamination level, including environmental safety, is ecotoxicology analysis. The comparative results of toxicity tests using various types of assays are shown in Table 6. Based on the results of the en zymatic Microtox assay, it was determined that SS1 was characterized by low toxicity and that SS2 was toxic. Similar conclusions can be drawn from the results of the Daphtoxkit F survival test and the Phytotoxkit F growth test. Table 6 Comparison of the toxicity analysis of the sewage sludge (Werle and Dudziak, 2015) Sewage sludge SS1

SS2

Test (type/test species)

Test time

Effect (%) (evaluation of toxicitya)

Microtox (enzymatic with Vibrio fischeri) Daphtoxkit F (survival with Daphnia magna) Phytotoxkit F (growth with Lepidium sativum)

5 min

26.1 (+) 40.0 (+) 40.0 (+)

a

24 h 24 h

() nontoxic; (+) low toxic; (++) toxic; (+++) highly toxic.

57.0 (++) 60.0 (++) 52.0 (++)

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Industrial and Municipal Sludge

The source of the SS2 toxicity was probably chromium (Cr); the Cr concentration in SS2 was three times higher than in SS1.

6

Gasification process results

6.1 Influence of the air ratio on the gas composition, LHV of gas, and temperature distribution The temperature distribution in the reactor was obtained by measuring the temperatures at six characteristic points of the reactor (Fig. 5). The points, T1, T2, T3, T4, T5, and T6, were measured at 10, 60, 110, 160, 210, and 260 mm above the grate, respectively. Fig. 5 showed that the temperature at the T3 point was always the highest; thus, T3 may be located in the partial oxidation zone, which should be the hottest area in the fixed-bed gasifier. Correspondingly, the measuring points T6 and T5 may be located in the drying zone, T4 in the pyrolysis zone, T2 in the oxidation (combustion) zone, and T1 in the ash zone. A similar tendency was observed during the experiment with the SS2 sample. Fig. 6 shows the evolution of the CH4, H2, CO, and carbon dioxide (CO2), concentrations in gasification of the SS1 and SS2 samples, with varying air ratios for each sludge. Analyzing this graphs in this figure, it can be confirmed that throughout the range of analyzed air ratios, the volumetric fractions of the main combustible components of the gasification gases (CO and H2) are higher in SS1 than in SS2. At lower air ratios, SS1 350 Distance above the grate (mm)

T1 300 T2

250 200

T3

150

T4

100 T5 50 T6 0 500

600

700

800

900

1000

Temperature in the reactor (°C)

Fig. 5 Temperature distribution in the reactor for SS1 gasification (Werle, 2014).

1100

CH4; SS1

CH4; SS2

H2; SS2

H2; SS1

Gasification of sewage sludge

0.9

7 6.5

0.8 Volume fraction (%vol.)

Volume fraction (%vol.)

0.85

0.75 0.7 0.65 0.6 0.55 0.5

6 5.5 5 4.5

0.45 0.4 0.1

0.15

(A)

0.2

0.25

4 0.1

0.3

CO2; SS1

CO; SS2

CO; SS1

0.3

CO2; SS2

20 Volume fraction (%vol.)

31 Volume fraction (%vol.)

0.25

21

33

29 27 25 23 21

(C)

0.2 Air ratio l

(B)

Air ratio l

19 0.1

0.15

19 18 17 16 15 14

0.15

0.2 Air ratio l

0.25

0.3

(D)

13 0.1

0.15

0.2

0.25

0.3

Air ratio l

585

Fig. 6 The volume fraction of main components in SS1 and SS2 gasification gases as a function of air ratio; (A) methane; (B) hydrogen, (C) CO, (D) CO2 (Werle, 2014, 2015)

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Industrial and Municipal Sludge

the CO fraction was found to start out low, and then it rises until the optimum air ratio of 0.18 is reached, and later drops again for higher air ratios. The maximum CO average composition values of 31.3% for SS1 (and 26.9% for SS2) were obtained for gasification at λ ¼ 0.18. CO2 shows an inverse relation with CO, as the reactions that produce those gases are competing for the same reactant (namely, C). The concentration of CO2 is generally expected to be minimum of the optimal air ratio range between 0.18 and 0.24. The rapid growth of CO observed with an air ratio equal to 0.18 is caused by the dominant role of the primary water gas reaction. The reactions that can occur in the gasifier as a result of the gasification agent flow can be categorized as the reaction of gasification agents and C in the fuel and the reaction of gasification agents and CO in the gas. The reaction of gasification air and C is an endothermic reaction that generates mainly CO, whereas the reaction of gasification air and CO is an exothermic reaction that generates mainly CO2 and H2. When gasification air is fed with the fuel into the reactor, the endothermic reaction of air and carbon occurs first (e.g., the primary water gas reaction CO + H2O ! CO + H2), and the CO in a gaseous state produced from the fuel reacts with the residuals, causing the next reactions (e.g., water gas shift CO +H2O $ CO2 + H2). Thus, the composition of H2, CO, and CO2 in the gasification gas changes according to the amount of the air supplied to the reactor. Fig. 7 illustrates the dependence of the collated based on the volume fraction of the gas component’s lower heating value (LHV) for obtaining the gas versus an air ratio. The following formula was used (Kim et al., 2011): LHV ¼ 0:126  CO + 0:108  H2 + 0:358  CH4 SS1

SS2

6

LHV (MJ/m3 n)

5 4 3 2 1 0 0.12

0.14

0.16

0.18

0.23

0.27

Air ratio l

Fig. 7 The LHV of the SS1 and SS2 gasification gases as a function of the air ratio (Werle, 2014, 2015).

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587

SS1; lambda = 0.18

SS1; lambda = 0.27

5.5

LHV (MJ/m3 n)

5

4.5

4

3.5

3 0

50

100 150 200 Gasification agent temperature (°C)

250

300

Fig. 8 The LHV of the SS1 gasification gas as a function of gasification agent temperature (Werle, 2014, 2015).

Analyzing Fig. 7, it can be concluded that taking into consideration the LHV of the gasification gas, the optimum value of the air ratio is equal to 0.18, from which the LHV takes its maximum value. This is true regardless of the sewage sludge type. Above that optimal value, the thermochemical process can shift from gasification to combustion.

6.2 Influence of the gasification temperature on the LHV of gas and temperature distribution Fig. 8 shows the LHV of the SS1 gasification gas as a function of gasification agent temperature. Analyzing the presented data reveals that the increase in the gasification temperature causes a corresponding increase in the calorific value of the obtained gas. This is mainly because the increase in gasification gas temperature is profitable for heat-cracking reactions, which act to promote high-calorific value hydrocarbon production in the gas. Such components exert a strong influence on the LHV. Further, these reactions are exothermic so the medium reactor temperature is also increasing (Fig. 9).

6.3 Influence of the oxygen content in gasification agents on gas composition, LHV of gas, and temperature distribution In Fig. 10, the influence of oxygen concentration of the gasification agent on the composition of the SS1 gasification gas is shown.

588

Industrial and Municipal Sludge

SS1; cold air

SS1; preheated air up to 250°C

350 Distance above the grate (mm)

T1 300 T2

250 200

T3

150 T4 100 T5 50 T6 0 600

700

800

900

1100

1000

1200

Temperature in gasifier (°C)

Fig. 9 The influence of the gasification agent temperature on the temperature distribution in the reactor for SS1 gasification (Werle, 2014, 2015). SS1; CH4

SS1; CO

SS1; CO2

SS1; H2

Volume fraction (%vol.)

30 25 20 15 10 5 0 0.21

0.25

Oxygen fraction in gasification agent (%vol)

Fig. 10 The influence of oxygen concentration of the gasification agent on the SS1 gasification gas composition (Werle, 2014, 2015).

Analyzing the data presented in Fig. 10 reveals that the use of a gasifying agent with increased O2 content (zO2 ¼ 0.25) causes an increase in the proportion of flammable components in gasification gas. This occurs because under such conditions, the effect of dilution with nitrogen is balanced, thereby increasing the share of flammable gases in the produced gas. Analyzing the graph shown in Fig. 11 shows that an increase in the proportion of oxygen in the gasifying agent results in an increase in the calorific value of the gas compared to when atmospheric air is used as the gasifying agent.

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6.6

LHV (MJ/m3 n)

6.5 6.4 6.3 6.2 6.1 6 0.21 0.25 Oxygen fraction in gasification agent (%vol)

Fig. 11 The influence of the oxygen concentration of the gasification agent on the LHV of the SS1 gasification gas (Werle, 2014, 2015).

7

Solid and liquid waste by-product analysis

Werle and Dudziak (2014) concluded that both solid (ash and charcoal) and liquid (tar) gasification products do not consist of the organic contaminants that have been identified primarily in sewage sludge (as shown earlier in this chapter in Table 4). The tests of the liquid products (tar) formed during sewage sludge gasification determined TOC, which directly measures the amount of the various organic substances in the sample. The indicator was very high for the tar produced during gasification of both SS1 and SS2, being 20,950 and 22,390 mg TOC/L, respectively. A chromatographic analysis of the tar samples showed their contamination, mainly by phenols and their derivatives (Table 7). The inorganic contaminants (heavy metals) identified in sewage sludge were also found in solid by-products from gasification (ash, charcoal). In addition, it has been documented that the concentrations of some heavy metals have increased significantly due to cumulative effects. This applied to seven of the nine heavy metals investigated (zinc, lead, nickel, copper, chromium, cadmium, and arsenic; the others were selenium and mercury), regardless of the type of sewage sludge. Due to reductive atmosphere during the gasification the migration of heavy metals from fuel to solid phase is observed. Similar conclusions were drawn by Li et al. (2012), who found an increase in the concentration of cadmium (from 0.93 to 1.67 mg/kg dry basis), chromium (from 80.82 to 247.95 mg/kg dry basis), copper (from 580.36 to 922.14 mg/kg dry basis), lead (from 78.27 to 125.09 mg/kg dry basis), and zinc (from 402.09 to 637.50 mg/kg dry basis) in sewage sludge and ash samples after gasification. However, some differences in the accumulation of the solid waste by-products of particular heavy metals were detected. For instance, zinc concentration in an ash sample increased twofold for SS2 gasification, and even threefold for SS1. The liquid by-products (tar) produced during sewage sludge gasification demonstrated high conductivity, which proves their high contamination with various inorganic substances.

590

Industrial and Municipal Sludge

Table 7 Concentrations of phenols and their derivatives in the tar produced during the gasification of sewage sludge (Werle and Dudziak, 2014) SS1

SS2

Compound

Concentration (μg/L)

2-Chlorophenol 2-Nitrophenol 2,4-Dichlorophenol 4-Chloro-3-methylphenol 2,4,6-Trochlorophenol Pentachlorophenol Sum

211.84 89.52 361.56 9.27 62.32 57.97 792.48

57.20 n.o. n.o. 1.98 46.33 53.90 100.23

n.o., not observed.

The conductivity of the tar was 9800 μS/cm for SS1, and 8170 μS/cm for SS2. The hazardous inorganic contaminants occurring in the tar also included ammonia, whose concentrations were 1090 (tar from SS1) and 950 mg NH4 + =L (tar from SS2). Ecotoxicological analysis of solid and liquid by-products of gasification has shown that they can pose a danger to the natural environment (Table 8). By-products were obtained during the gasification process under constant conditions (temperature of the gasifying agent, 298 K, and excess air ratio λ ¼ 0.18). The decrease in bioluminescence intensity (shown by the Microtox test) caused by the gasification by-products Table 8 Comparison of the gasification by-products (gasification parameters: T ¼ 298 K, λ ¼ 0.18) (Werle and Dudziak, 2015) Gasified sewage sludge SS1

SS2

SS1

SS2

SS1

Gasification by-products Ash Test (type/test species) Microtox (enzymatic with Vibrio fischeri) Daphtoxkit F (survival with Daphnia magna) Phytotoxkit F (growth with Lepidium sativum) a

Test time

Tar

Charcoal

Effect (%) (evaluation of toxicitya)

5 min

2.1 (–)

64.7 (++)

56.8 (++)

66.3 (++)

9.4 (–)

24 h

40.0 (+)

70.0 (++)

80.0 (+++)

100.0 (+++)

20.0 (–)

24 h

20.0 (–)

50.0 (+)

44.0 (+)

40.0 (+)

20.0 (–)

(–) nontoxic; (+) low toxic; (++) toxic; (+++) highly toxic.

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depended noticeably on the type of sample (ash, tar, or charcoal). The observed correlation is also greatly affected by the type of sludge that was gasified. The samples of ash and tar produced during the gasification of SS1 turned out to be nontoxic. Comparing them to the toxicity of the sludge, their toxic effect decreased. The reverse phenomenon was observed for the SS2 sample, as well as for ash that formed during the gasification of this sludge. The tar samples were toxic regardless of the type of gasified sewage sludge. The results of the Microtox test were the opposite of the results of the Daphtoxkit F survival test and the Phytotoxkit F growth test. The tars proved to be highly toxic to the crustacean Daphnia magna (Daphtoxkit F test). The dicot plant Lepidium sativum was the least sensitive to the effect of tars (Phytotoxkit F test).

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Management and purification of the gasification by-products

Dudziak and Werle (2016) have demonstrated that solid gasification by-products can be used as an adsorption material for the elimination of toxic organic substances from the water streams (e.g., phenol). Table 9 presents the comparison of the maximum adsorption capacity of the phenol monolayer on various adsorbents. Based on the presented data, it can be concluded that the efficiency of phenol adsorption on the ash was greater than for the other unconventional adsorbents (i.e., bagasse fly ash, neutralized red mud, and olive pomace). The adsorption of phenol was found for commercially available activated carbon and activated carbon derived from waste materials such as beet pulp or rice husk. However, only ashes that are nontoxic (Table 8) can be used as the adsorption material. Taking into account the profile of contaminants identified in the tars from sewage sludge gasification (Table 7), they should be subjected to deepen purification processes. This also indicates the ecotoxicological analysis of the tar samples that confirms their toxicity (Table 8). Advanced oxidation methods that are effective in coking Table 9 Comparison of the maximum monolayer adsorption capacity of phenol onto various adsorbents (Dudziak and Werle, 2016) Adsorbents

q (mg/g)

Activated carbon fiber Beet pulp carbon Commercial activated carbon Rice husk carbon Chemically modified green macro algae Baggase fly ash Neutralized red mud Olive pomace Sewage sludge ash

110.20 90.61 49.72 22.00 20.00 12.00–13.00 5.13 4.00–5.00 42.22

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Industrial and Municipal Sludge

wastewater treatment have been proposed (Krzywicka and Kwarciak-Kozłowska, 2014). There is some analogy between the profile of contaminants in tar from sewage sludge gasification and coke wastewater. However, the issue raised is currently in the field of new research.

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Conclusions The operating conditions (amount of the gasification agent) of the sewage sludge gasification process greatly influence the gasification gas composition distribution. Higher values of the main components (especially C and H) in the sewage sludge plant affect increases in the LHV of gasification gases. Taking into consideration the LHV of the gasification gas, the optimum value of the air ratio is equal to 0.18, in which the LHV takes its maximum value. The yield of the main gasification gas components, CO, H2, and CH4, was enhanced by increasing the gasification agent temperature and the oxygen concentration of the gasification agent. Both standard and indirect evaluation methods can be used to assess the degree of contamination of sewage sludge and the by-products of gasification (i.e., ash, charcoal, tar). In this respect, ecotoxicological analyses should be included that allow an assessment of environmental safety. Originally, sewage sludge contains a variety of organic pollutants (PAHs, pesticides, PCBs) and inorganic ones (heavy metals). Solid waste by-products from sewage sludge (ashes) can be used as an adsorbent for the elimination of toxic organic substances from water streams such as phenol. The sludge from sludge gasification should be subjected to deep purification processes.

Acknowledgments The paper has been partially prepared within the frame of the National Science Centre project based on Decision No. DEC-2011/03/D/ST8/04035 and within the framework of the Ministry of Science and Higher Education Iuventus Plus Programme Project No. 0593/IP2/2011/71 and within the frame of the project “Study on the solar pyrolysis process of the waste biomass,” financed by the National Science Centre, Poland (registration number 2016/23/B/ST8/02101).

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