Sewage sludge is a by-product generated from municipal wastewater treatment (WWT) processes. This study examines the conversion of sludge via energy recovery from gasification/combustion for thermal treatment of dewatered sludge. The present analysis is based on a chemical equilibrium model of thermal conversion of previously dewatered sludge with moisture content of 60–80%. Prior to combustion/gasification, sludge is dried to a moisture content of 25–55% by two processes: (1) heat recovered from syngas/flue gas cooling and (2) heat recovered from syngas combustion. The electricity recovered from the combined heat and power process can be reused in syngas cleaning and in the WWT plant. Gas temperature, total heat and electricity recoverable are evaluated using the model. Results show that generation of electricity from dewatered sludge with low moisture content (≤ 70%) is feasible within a self-sufficient sludge treatment process. Optimal conditions for gasification correspond to an equivalence ratio of 2.3 and dried sludge moisture content of 25%. Net electricity generated from syngas combustion can account for 0.071 kWh/m3 of wastewater treated, which is up to 25.4–28.4% of the WWT plant's total energy consumption.

INTRODUCTION

Sludge is the residue produced in municipal wastewater treatment plants (WWTPs). On average, 90 g of dry sludge per person is generated every day from the treatment of urban wastewater (Fytili & Zabaniotou 2008). Sewage sludge is a moisture-rich material (moisture content, YM >90%), composed of a volatile carbon fraction, other organic components, including N and S, and inorganic materials (Sawai et al. 2013). Before disposal and transportation, sludge volume should be reduced by thickening, conditioning and dewatering (Metcalf & Eddy 2014). When YM decreases from 98% to 60%, sludge volume reduces by ≤95%. State-of-the-art dewatering processes can reduce YM to between 60% and 80% (Werther & Ogada 1999; Neyens & Baeyens 2003; Cai et al. 2012; Li et al. 2014).

Dewatered sludge disposal methods include dumping at sea, land-filling, recycling as fertilizer and thermal treatment via combustion, also known as incineration (Werther & Ogada 1999; Stasta et al. 2006; Nipattummakul et al. 2010). The first three methods can lead to environmental problems due to the presence of toxic components and heavy metals in sludge. Sludge dumping to surface waters is prohibited in the European Union since the introduction of the Urban Wastewater Directive 91/271/EEC (EU 1991). Land-filling and recycling as fertilizer are banned or highly restricted in many countries due to concerns over soil and crop contamination (Petersen & Werther 2005). Thermal conversion processes, including combustion and gasification, are promising sludge disposal alternatives that can provide low-carbon energy to the WWTP.

In combustion, sludge is converted to a mixture of high-temperature oxidized gaseous species, known collectively as flue gas, which consists primarily of CO2, H2O, O2 and N2, as well as inorganic solids, known as ash (if solid) or slag (if molten). Through thermal conversion, waste volume is reduced by 80–90%, organic matter is fully converted to gas, and pathogens are fully eliminated, complying with disposal requirements. Furthermore, energy can be recovered as heat and/or electricity, usually via steam turbines (Marani et al. 2003). Emissions of NOx, SOx, heavy metals in fly ash and toxic gases such as PCDD/PCDF (chlorinated dioxins and dibenzofurans) are the main potential adverse environmental impacts of combustion (Fullana et al. 2004; Mininni et al. 2004; Zhu et al. 2015).

Gasification is a thermochemical conversion process in which oxygen, in air or pure O2, is reacted with sludge at high temperature to generate a synthetic gas (syngas) (Belgiorno et al. 2003) and ash/slag. H2O, present as injected water, steam or sludge moisture content, is also required for gasification. Air-blown sludge gasification usually takes place at atmospheric pressure and temperatures of 700–1,000 °C (Lumley et al. 2014), producing a syngas containing mainly CO, H2 and N2, with heating values of 4–7 MJ/Nm3 (Belgiorno et al. 2003; De Andrés et al. 2011), that can be burned in combined heat and power (CHP) processes (Dogru et al. 2002). Syngas is not an ideal fuel for CHP because of its relatively low heating value compared to natural gas (37 MJ/Nm3) (Wiliams 2005; Sun et al. 2015). However, diesel engines using duel-fuel injection or de-rated spark ignition engines can be modified to operate with low heating value gaseous fuels (Hagos et al. 2014). Similar to combustion, gasification reduces sludge volume, while it fixes the heavy metal in the ash, trapping toxic compounds (Dogru et al. 2002). Thus, gasification is considered to be a suitable technology for dewatered sludge disposal.

In this study, the technical feasibility of the thermal treatment of previously dewatered sludge by combustion/gasification is evaluated using a chemical equilibrium model. This process involves: (1) a drying stage in which sludge moisture content is reduced from 60–80% to 25–55%; (2) a gasification/combustion stage in which the chemical equilibrium driven by sludge composition and oxygen concentration defines the process temperature and gas composition; and (3) an energy recovery stage consisting of gas cooling and syngas-fuelled CHP. The purpose of the study is to evaluate the process conditions in the gasification/combustion stage, including dewatered and dried sludge moisture contents and sludge–air ratios, which lead to a self-sufficient sludge thermal treatment process, while providing electricity generation on site.

METHODS

The proposed self-sufficient thermal treatment process for dewatered sludge is presented in Figure 1. Wastewater enters the WWTP, producing concentrated sludge with a moisture content of YM,cs= 94% (Neyens et al. 2004; Wang et al. 2010). The concentrated sludge undergoes dewatering after sulfuric acid hydrolysis pre-treatment to obtain a dewatered sludge moisture content (YM,dew) of 60–80%. Sludge drying is performed to bring dried sludge moisture content (YM,dry) to acceptable ranges for thermal treatment (25–55%). For combustion, extremely low YM,dry is desirable, while for gasification, which requires H2O, some amount of moisture is required. In order for the thermal treatment process to be energy self-sufficient, this sludge drying stage is performed using two sources of heat: (1) heat recovered from the desuperheating of syngas/flue gas (cooling from process temperature to 373 K) and (2) heat recovered from the combustion of syngas in a CHP system. Due to the fact that the latter source relies on the combustion of syngas, this is only considered for gasification. Besides the heat for sludge drying, the electricity consumption of the syngas cleaning system is also provided by the electricity recovered from the CHP process, to achieve a self-sufficient sludge thermal treatment system. The key step in the thermal treatment process is the combustion/gasification step. The present work treats combustion and gasification as similar processes that are differentiated by the relative amounts of sludge and air present using an equivalence ratio (ER), which is described in Equation (1). The high-temperature gas produced is cooled (desuperheated) to 373 K, providing energy for sludge drying (Q1). It is then sent for scrubbing to remove sulfurous compounds and other harmful emissions with a required electricity demand of Wscrubber. It is finally either released to the atmosphere (for combustion flue gas), or combusted in the presence of air in a CHP system (for gasification syngas). Electricity produced from syngas combustion (W) is supplied to the scrubber (W1) and WWTP (W2), while heat is recovered for sludge drying (Q2).
Figure 1

Flow diagram of proposed self-sufficient thermal treatment process for dewatered sludge.

Figure 1

Flow diagram of proposed self-sufficient thermal treatment process for dewatered sludge.

Feedstock properties and ER

The proximate and ultimate analyses and the lower (LHV) and higher heating value (HHV) of dewatered sludge considered for thermal treatment are shown in Table 1 (ECN Phyllis 2).

Table 1

Dewatered sludge proximate and ultimate analyses and heating values

PropertyUnitValueData range (ECN Phyllis 2)
Proximate analysis 
 Moisture content wt% a.r. 60.00, 70.00, 80.00  
 Ash content wt% d.b. 35.00 26.15–54.50 
 Volatile matter wt% d.b. 53.50 39.70–74.20 
 Fixed carbon wt% d.b. 11.50 2.10–11.56 
Ultimate analysis 
 C wt% d.b. 34.00 23.10–40.75 
 H wt% d.b. 4.90 2.60–5.82 
 O wt% d.b. 20.01 10.01–26.26 
 N wt% d.b. 4.70 1.40–6.22 
 S wt% d.b. 1.30 0.50–2.90 
 LHV MJ/kg d.b. 14.03 6.50–16.53 
 HHV MJ/kg d.b. 15.10 7.20–18.20 
PropertyUnitValueData range (ECN Phyllis 2)
Proximate analysis 
 Moisture content wt% a.r. 60.00, 70.00, 80.00  
 Ash content wt% d.b. 35.00 26.15–54.50 
 Volatile matter wt% d.b. 53.50 39.70–74.20 
 Fixed carbon wt% d.b. 11.50 2.10–11.56 
Ultimate analysis 
 C wt% d.b. 34.00 23.10–40.75 
 H wt% d.b. 4.90 2.60–5.82 
 O wt% d.b. 20.01 10.01–26.26 
 N wt% d.b. 4.70 1.40–6.22 
 S wt% d.b. 1.30 0.50–2.90 
 LHV MJ/kg d.b. 14.03 6.50–16.53 
 HHV MJ/kg d.b. 15.10 7.20–18.20 

a.r.: As received. d.b.: Dry basis.

The amount of air reacted with sludge is quantified by the ER, which is defined as the ratio of the stoichiometric air–fuel mass ratio to the actual air–fuel mass ratio used in the process. Stoichiometric refers to the air–fuel mass ratio such that the minimum amount of air is present to cause full oxidation (combustion) of the fuel. ER can be expressed as: 
formula
1
where AFstoi is the stoichiometric air–fuel mass ratio and AF is the actual air–fuel mass ratio. In the present work, we define combustion as occurring at ER ≤1, while gasification takes place when ER > 1.

Mass conservation and chemical equilibrium

In this model, is the elemental composition of the organic fraction of the sludge. The global sludge combustion/gasification reaction can be expressed as follows: 
formula
2
where x, y, z, m, and n are the mole fractions of the elements defined by the ultimate analysis of the sludge feedstock, and a to j are the stoichiometric coefficients in the global reaction. All the mass balance, chemical equilibrium and energy balance equations are referenced from Higman & Burgt (2011).

Gas product composition is determined assuming that chemical equilibrium is attained. Implicit in this assumption is that sufficient time is allowed to elapse for an equilibrium state to be reached. This is a valid limiting assumption in the case of a well-designed and well-sized thermal treatment system, and is a commonly used first step in process design.

The equilibrium constant can be estimated using the Gibbs free energy of the reaction , the reaction temperature (T) and the universal gas constant (R): 
formula
3
For a particular reaction, is determined by the standard-state Gibbs free energy of formation of reactants and products: 
formula
4
where i corresponds to species, represents standard-state Gibbs free energy of formation of the pure species at T, and and represent stoichiometric coefficients of products and reactants, respectively. Empirical expressions for the estimation of are taken from the literature and are presented as polynomials of the form shown in Equation (5) (Syed Janajreh et al. 2011). The coefficient values A to G and the enthalpy of formation at 298 K and 1 atm are tabulated in the literature (Probstein & Hicks 2006). 
formula
5

Energy balance during thermal conversion

The process is considered to be adiabatic and at constant pressure (1 atm) under steady-state operation. The energy balance for thermal conversion is shown in Equation (6). 
formula
6
where , are the mass fractions of reactants and products, , are the specific enthalpies of formation of reactants and products, and , are the sensible enthalpies at temperature T. The sensible enthalpies can be estimated as follows: 
formula
7
where is the specific heat capacity of component i at T and it can be determined by empirical expressions as a function of temperature (Shi et al. 1996). Specific heat capacity (Seggiani 1998), latent heat of melting (Vargas et al. 2001) and temperature- and composition-dependent viscosity (Monaghan & Ghoniem 2012a, 2012b) of ash are all considered in this model.
The enthalpy of formation of sludge can be calculated following the Hess Law (Negi & Anand 1985) for the following overall reaction: 
formula
8
 
formula
9
where the Δhf terms represent the enthalpies of formation of product species, is the known LHV of sludge, and the coefficients x, y, m, n are defined from mass conservation.

Cold gas efficiency and energy recovery

The performance of gasification is often expressed in terms of its cold gas efficiency (CGE): 
formula
10
where corresponds to syngas LHV and can be estimated by the LHV of CO, H2, and CH4 in the gas (kJ/m3) (Basu 2006), represents sludge LHV (kJ/kg) and H2SO4 LHV (0 kJ/kg), Fgas is syngas volumetric flow rate (m3/s), and are sludge and H2SO4 mass flow rates (kg/s).

RESULTS AND DISCUSSION

Flue gas/syngas temperature

Combustion/gasification temperatures below 1,023 K lead to very slow reaction rates, requiring extremely long residence times and impractically large reactors (Manyà et al. 2005; De Andrés et al. 2011). In this study, only operational conditions that lead to gas equilibrium temperatures Tgas > 1,023 K are considered. Figure 2 presents Tgas as a function of ER and YM,dry during the thermal treatment process. When ER= 1 (complete combustion) and YM,dry= 25%, Tgas,max= 1,891K, while Tgas,min= 1,023 K is attained at ER= 1.8 for YM,dry= 45%, and at ER= 2.3 for YM,dry= 25%. Oxidation reactions in combustion are exothermic (heat of reaction, ΔhR < 0), providing thermal energy required for the endothermic reactions (ΔhR > 0) in the gasification process. Table 2 shows the heat of reaction values of key reactions considered. Thus, when ER ≈ 1 (complete combustion), oxidation reactions predominate and temperature is higher than that observed when ER >1 (gasification). High-moisture sludge contains more liquid water than dry sludge, which must be evaporated prior to thermal treatment. This requires thermal energy that ultimately lowers the achieved Tgas. In lean combustion (ER < 1), even though reaction equilibrium is mainly limited by the sludge feed, excess air leads to lower temperatures due to the presence of large quantities of nitrogen that is mostly chemically inert and serves only to absorb heat.
Table 2

Chemical reactions and their heats of reaction of importance to combustion and gasification

Combustion
Gasification
ReactionΔhRoReactionΔhRo
 −393.77 kJ/mol Water–gas reaction  +131.38 kJ/mol 
H2 + ½O2 = H2−241.80 kJ/mol Boudouard reaction  +172.58 kJ/mol 
  Shift reaction  +41.98 kJ/mol 
Combustion
Gasification
ReactionΔhRoReactionΔhRo
 −393.77 kJ/mol Water–gas reaction  +131.38 kJ/mol 
H2 + ½O2 = H2−241.80 kJ/mol Boudouard reaction  +172.58 kJ/mol 
  Shift reaction  +41.98 kJ/mol 
Figure 2

Combustion/gasification gas temperature (Tgas) as a function of ER and moisture content (YM,dry).

Figure 2

Combustion/gasification gas temperature (Tgas) as a function of ER and moisture content (YM,dry).

Heat and electricity recovery within the self-sufficient sludge thermal treatment process

Thermal energy recovery is accomplished in two stages (Figure 1): (1) the gas desuperheating prior to gas cleaning (Q1); and (2) heat recovery from the syngas-fuelled CHP process (Q2). A heat transfer efficiency of 80% is considered for gas desuperheating. CHP thermal efficiency of 35% based on syngas LHV is assumed for the estimation of Q2 (Lantz 2012). Figure 3 presents the available thermal energy in the syngas (Q1+Q2, kJ/kg dry sludge) as a function of ER and YM,dry. As previously shown in Figure 2, higher Tgas is achieved for ER ≤ 1. Consequently, more thermal energy is recovered from the desuperheating of the flue gas (Q1 = 7,902–9,596 kJ/kg dry sludge), than from the syngas (Q1 = 2,379–8,272 kJ/kg dry sludge). Syngas, however, benefits from the capture of Q2 (418–3,268 kJ/kg dry sludge), which increases its feasibility.
Figure 3

Heat available for sludge drying (Q1+Q2) as a function of ER and moisture content (YM,dry).

Figure 3

Heat available for sludge drying (Q1+Q2) as a function of ER and moisture content (YM,dry).

The thermal energy recovered can be compared with the energy required for the drying stage that reduces YM,dew to YM,dry. The required drying energy (Qdrying) is directly dependent on both YM,dew and YM,dry. For example, when YM,dew is 60%, Qdrying = 1,233–3,273 kJ/kg dry sludge, and when YM,dew is 80%, Qdrying = 7,664–9,757 kJ/kg dry sludge, with ranges depending on YM,dry. Available thermal energy (Q1+Q2) and Qdrying are compared via the drying heat ratio , which is defined as follows: 
formula
11
When , Q1+Q2 is enough to provide for Qdrying, and the sludge drying operation is self-sufficient. Drying heat ratio is shown in Figure 4 as a function of ER and YM,dry with values of YM,dew of 60% (Figure 4(a)) and 70% (Figure 4(b)).
Figure 4

Drying heat ratio as a function of ER and dried moisture content (YM,dry) for sludge with dewatered moisture contents (YM,dew) of (a) 60% and (b) 70%.

Figure 4

Drying heat ratio as a function of ER and dried moisture content (YM,dry) for sludge with dewatered moisture contents (YM,dew) of (a) 60% and (b) 70%.

Electricity is recovered from the combustion of syngas in a CHP system. Electrical efficiency of a syngas-fuelled CHP system is assumed at 30% (Lantz 2012). Figure 5 presents the electricity production, also the available electricity for gas scrubbing (kWh/m3 of wastewater treated), attained from a CHP system that uses syngas generated from sludge gasification in a facility of the scale of the Ringsend WWTP in Dublin, Ireland. This plant has a wastewater treatment capacity of 150 Mm3/year and produces on average 17,260 tonnes/year of dry sludge, using sequencing batch reactors as the wastewater treatment process (EPA 2013). As expected from the LHVsyngas, the maximum electricity generation of 0.090 kWh/m3 wastewater treated is attained when ER= 2.3 and YM,dry= 25%.
Figure 5

Electricity available for scrubber (W) as a function of ER and moisture content (YM,dry).

Figure 5

Electricity available for scrubber (W) as a function of ER and moisture content (YM,dry).

The electrical energy recovered can be compared with the electricity required for the syngas cleaning stage. As a conservative estimate of scrubber performance, the required electrical energy for syngas cleaning at a coal-fed gasification plant of 93 kJ/kg (Monaghan et al. 2011) was increased by a factor of two to 186 kJ/kg. The maximum required electricity for scrubbing is therefore 0.067 kWh/m3 of wastewater treated at ER= 0.5, YM,dry= 55%, and is 0.018 kWh/m3 at ER= 2.3, YM,dry= 25%. The available electrical energy (W, shown in Figure 5) and Wscrubber are compared via the scrubber electricity ratio (φ), which is defined in Equation (12) and shown in Figure 6: 
formula
12
When , W is enough to provide for Wscrubber, and the gas cleaning stage is self-sufficient.
Figure 6

Scrubber electricity ratio as a function of ER and dried moisture content (YM,dry) for sludge with dewatered moisture contents (YM,dew) of 60% and 70%.

Figure 6

Scrubber electricity ratio as a function of ER and dried moisture content (YM,dry) for sludge with dewatered moisture contents (YM,dew) of 60% and 70%.

In this study, only operating conditions in which and are considered suitable for the process. Figures 4 and 6 present and as functions of ER and YM,dry for two sludge feedstocks (YM,dew= 60%, 70%) for which both thermal and electrical conversion are feasible. When YM,dew is low (60%), increases from 1.73 (ER= 2.3, YM,dry= 25%) to 4.89 (ER= 1.4, YM,dry= 55%). However, with the increase of YM,dew, the drying energy required for the process increases and reduces significantly. For YM,dew= 70%, varies from 1.04 (ER= 2.3, YM,dry= 25%) to 1.80 (ER= 1.3, YM,dry= 52.5%). Scrubber electricity ratio increases from 1.01 (ER = 1.3, YM,dry= 52.5%) to 4.85 (ER= 2.3, YM,dry= 25%) regardless of YM,dew= 60% or 70%. For YM,dew= 80%, the thermal and electrical conversion processes are found to be unfeasible with and .

Electricity reused in WWTP

As mentioned above, the maximum electricity generation from the CHP process is 0.090 kWh/m3 wastewater treated. Net electricity generated from syngas combustion (gross CHP generation minus scrubber electricity consumption) can total 0.071 kWh/m3 of wastewater treated, as illustrated in Figure 7. Conventional WWTPs consume 0.25–0.28 kWh/m3 wastewater treated (Metcalf & Eddy 2014), so the proposed system could provide 25.4–28.4% of WWTP electricity, assuming that a dewatered sludge with low YM,dew is produced in the WWTP (≤70%). As mentioned previously, the generation of electricity through the proposed process becomes unfeasible when a dewatered sludge of YM,dew= 80% is used, since additional thermal or electrical energy would be required.
Figure 7

Electricity reused in a WWTP of the scale of the Ringsend plant in Dublin, Ireland.

Figure 7

Electricity reused in a WWTP of the scale of the Ringsend plant in Dublin, Ireland.

Comparison of model result with existing plants

In order to evaluate the model, the key results are compared with the data obtained from the demonstration plant in Balingen, Germany, and the pilot plant in Mannheim, Germany. The plant in Balingen, built in 2002, is the first demonstration plant of sludge gasification. Six surrounding wastewater plants deliver their sludge to Balingen for drying and gasification. The pilot plant in Mannheim was subsequently built in 2010 (Judex et al. 2012). As shown in Table 3, the model results (gasification temperature, syngas LHV and CGE) are in ranges similar to those obtained in the two demonstration plants.

Table 3

Comparison of key results with existing sludge gasification plants

 BalingenMannheimModel results
Population equivalent 2,50,000 6,00,000 1,700,000 
Throughput (t/a dry solids) 1,955 5,000 17,260 
Gasification temperature (K) 1,123 1,123–1,173 1,145 K (ER= 2.1, YM,dew= 70%, YM,dry= 25%) 
Syngas LHV (MJ/Nm33.2 4.7 2.9 (ER= 2.3, YM,dew= 70%, YM,dry= 25%) 
CGE 66% 70% 66.6% 
 BalingenMannheimModel results
Population equivalent 2,50,000 6,00,000 1,700,000 
Throughput (t/a dry solids) 1,955 5,000 17,260 
Gasification temperature (K) 1,123 1,123–1,173 1,145 K (ER= 2.1, YM,dew= 70%, YM,dry= 25%) 
Syngas LHV (MJ/Nm33.2 4.7 2.9 (ER= 2.3, YM,dew= 70%, YM,dry= 25%) 
CGE 66% 70% 66.6% 

CONCLUSIONS

The present study evaluates the effect of three process variables: (1) moisture content of dewatered sludge (YM,dew), (2) moisture content of dried sludge (YM,dry), and (3) ER, on the performance of a proposed self-sufficient thermal treatment process for wastewater sludge. The impact of these variables on key process performance indicators, including combustion/gasification temperature (Tgas), the quantities of heat (Q1+Q2), net CHP electricity generation (W) recoverable for use, and the fractions of sludge drying energy and scrubber electricity consumption that can be provided by the process, are studied. The use of dewatered sludge with low YM,dew (≤ 70%) allows for the self-sufficient thermal treatment of sludge to recover both heat and electricity. In this context, processes using sludge materials with high YM,dew (≥80%) would rely on external heat/electricity sources. ERs around 2 and low YM,dry (∼25%) are preferable when energy recovery as electricity is desired. Under these gasification conditions, the produced syngas can contain up to 2.9 MJ/Nm3, corresponding to a CGE of 66.6%. Likewise, the net electricity derived from CHP can account for up to 0.071 kWh/m3 wastewater treated, providing potentially up to 25.4–28.4% of the energy consumption in a conventional WWTP.

ACKNOWLEDGEMENTS

Dr Dussan acknowledges financial support from the Environmental Protection Agency of Ireland under grant number 2014-RE-DS-3. Mr. Yang acknowledges funding from the College of Engineering and Informatics at NUI Galway.

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