FLUIDIZED BED INCINERATION OF SEWAGE SLUDGE IN O2

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FLUIDIZED BED INCINERATION OF SEWAGE SLUDGE IN O/N AND O/CO ATMOSPHERES 2

2

2

2

Jaroslav Moško, Michael Poho#elý, Boleslav Zach, Karel Svoboda, Tomáš Durda, Michal Jeremiáš, Michal Šyc, Šárka Václavková, Siarhei Skoblia, Zden#k Be#o, and Ji#í Brynda Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02908 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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FLUIDIZED BED INCINERATION OF SEWAGE SLUDGE IN O2/N2 AND O2/CO2 ATMOSPHERES Jaroslav Moško * a, b, Michael Pohořelý a, b, Boleslav Zach a, b, Karel Svoboda a, c, Tomáš Durda b

, Michal Jeremiáš a, Michal Šyc a, Šárka Václavková a, Siarhei Skoblia d, Zdeněk Beňo d, Jiří Brynda a, d

a

Institute of Chemical Process Fundamentals of the Czech Academy of Sciences, v. v. i.,

Rozvojová 135, 165 02 Prague 6-Suchdol, Czech Republic. b

Department of Power Engineering, University of Chemistry and Technology Prague,

Technická 5, 166 28 Prague 6, Czech Republic. c

Faculty of the Environment, University of Jan Evangelista Purkyně, Králova Výšina 7, 400 96

Ústí nad Labem, Czech Republic. d

Department of Gaseous and Solid Fuels and Air Protection, University of Chemistry and

Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic. *Corresponding author Jaroslav Moško, Institute of Chemical Process Fundamentals, The Czech Academy of Sciences, Rozvojová 135, 165 02 Prague 6-Suchdol, Czech Republic. Tel: +420 220 390 131; E-mail: [email protected] 1 ACS Paragon Plus Environment

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Keywords: sewage sludge, incineration, fluidized bed, oxygen-enriched air, oxy fuel

ABSTRACT

Sewage sludge incineration in a fluidized bed is considered to be one of the most suitable ways of sewage sludge disposal. This process reduces the volume of the waste and causes the destruction of organic contaminants such as POPs, pharmaceuticals and other compounds with endocrinedisrupting potential. Oxygen-enriched air combustion and oxy-fuel combustion can increase the combustion efficiency, reduce the amount of flue gas and make possible CO2 capture more effective. However, the influence of incineration medium composition has not yet been thoroughly investigated in case of sewage sludge incineration. In this paper, the incineration of sewage sludge in a bubbling fluidized bed reactor was studied at oxygen-enriched air conditions, oxy-fuel conditions and oxy-fuel conditions with zero and non-zero concentrations of steam, CO, NO, N2O and SO2 in the inlet combustion medium. Consequently, the effects of various operating parameters on pollutants formation were comprehensively described with emphasis on aforementioned sewage sludge incineration processes. An increase in combustion temperature resulted in an increase in NOx and SO2 emissions and in a decrease in N2O emissions. Increase in inlet oxygen concentration led to a decrease in NOx and N2O emissions. N2O and SO2 emissions were higher in CO2-rich atmosphere (oxy-fuel combustion conditions). Presence of steam in the inlet combustion medium resulted mainly in the reduction of NOx emissions. Presence of CO, NO, N2O and SO2 in the dry inlet combustion medium reduced mainly overall nitrogen-to-NOx conversion, while the effect on SO2 removal efficiency was only marginal.

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1 Introduction Sewage sludge is an inevitable waste stream from wastewater treatment and its disposal to agricultural soils is complicated due to the content of heavy metals and organic contaminants such

as

POPs,

detergent

residues,

pharmaceuticals

and

various

compounds

with

endocrine-disrupting potential.1–3 Sewage sludge with high content of heavy metals and organic pollutants may be disposed in large volumes by incineration. Additionally, some soil nutrients (e.g., phosphorus, calcium and partly potassium) are concentrated in the ash from sewage sludge mono-combustion and could be recovered from the ash4–6 and used in agriculture. Incineration contributes to sewage sludge disposal in varying degrees in individual European countries. Incineration is the major contributor to sewage sludge disposal in the Netherlands, Switzerland, Germany, Belgium, Austria and Slovenia.7 Sewage sludge incineration facilities use typically multiple hearth or fluidized bed (FB) furnaces; although, other technologies, such as rotary kilns, are also used in smaller applications.1 Fluidized bed combustors are usually suitable for the combustion of low-grade fuels (rich in moisture and ash) or fuels with variable properties, which are typical attributes of sewage sludge. Local annual production of stabilized sewage sludge at wastewater treatment plants in the Czech Republic varies from 1 000 tonnes of dry matter (county town) to 20 000 tonnes of dry matter (Prague – capital). To incinerate sewage sludge, both circulating fluidized bed (CFB) and bubbling fluidized bed (BFB) based technologies can be used.8–10 To achieve lower NOx emissions, it is necessary to control the temperature of the fluidized bed during incineration which can be done through the use of semi-dried sludge or the use of two adjustable inlet streams, dried and wet sewage sludge.1 BFB technology is favored for the mono-combustion of sewage sludge, whereas CFB technology is mainly considered for co-combustion of the sludge with coal, biomass or other fuels. Due to a 3 ACS Paragon Plus Environment

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relatively high content of nitrogen in sewage sludge, the formation of nitrogen oxides poses a serious problem during incineration in the fluidized bed. Oxy-fuel combustion and oxygen-enriched air combustion may be considered for incineration of sewage sludge alongside the conventional air combustion. Oxy-fuel combustion may be considered as a carbon capture technology producing a stream rich in CO2, suitable (after further cleaning) for transport and storage.11, 12 In oxy-fuel combustion, a mixture of pure oxygen (purity > 95 %) and either dry or partially wet recycled flue gas (RFG) is typically used as a combustion medium. The sufficient flue gas recirculation must be kept in order to sustain the fluidization in case of FB technologies13 and to moderate the temperature of the combustion process. The offgas from oxy-fuel combustion can be (after steam condensation and removal of remaining emission components) directly compressed and transported to a storage site.14 In the case of oxyfuel combustion of sewage sludge (with successive CO2 storage), the process can be considered even as carbon negative. Oxygen-enriched air, which is produced as a waste stream from nitrogen production in pressure-swing-adsorption units (PSA), can also be used as a combustion medium. In comparison with conventional air combustion, oxygen-enriched air combustion produces flue gas with higher concentrations of CO2 and pollutants which facilitates their separation.15 Increasing the oxygen content in the combustion medium in oxy-fuel and oxygen-enriched air combustion (in comparison with conventional air combustion) is considered to increase combustion efficiency. On the other hand, it has (together with water vapor in flue gas) various effects on the emissions of pollutants.16–19 Gaseous emissions (particularly NOx, SO2, HCl, and heavy metals) from both air combustion and oxy-fuel combustion are generally reduced by co-combustion with a fuel containing lower amount of nitrogen, sulfur, chlorine and heavy metals (e.g. coal, wood or another biomass 4 ACS Paragon Plus Environment

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fuel).12, 20 In both BFB and CFB combustion, it is reported that NOx emissions increase and N2O emissions decrease with increasing operating temperature (800–950 °C) in the fluidized bed.12, 16– 19, 21–23

Oxy-fuel conditions result in slightly higher CO emissions compared to air combustion

and the presence of steam in the inlet gas results in lower NOx emissions.12 Due to the recirculation of a part of the flue gas, the concentrations of the pollutants in the flue gas (particularly SO2) are higher than in air combustion. However, the processes for removal and reduction of pollutants are more efficient in oxy-fuel combustion, resulting in decreased emissions related to the quantity of fuel burnt and heat produced. In oxy-fuel combustion, compared to air and oxygen-enriched air combustion, optimal temperature for direct desulfurization of the flue gas by limestone addition is higher due to the increased temperature of CaCO3 decomposition in CO2-rich environment.12, 24 In fluidized bed combustion, oxygen enrichment of the gaseous inlet combustion medium leads mainly to an increase in NOx and SO2 emissions, which depends also on temperature, Ca/S ratio, entrainment of fuel particles, catalytic effects and moisture content in the flue gas.24–30 Furthermore, the SO2 emissions tend to decrease and then increase with increasing temperature. The optimal combustion temperature leading to minimal SO2 emissions is then dependent on Ca/S ratio, oxygen enrichment, ash composition and CO2 and moisture content in the inlet gas (indirect vs. direct sulfation mechanism of limestone). More detailed explanation of various effects on sulfur retention can be found in García-Labiano et al.29 and Lupiáñez et al.30. According to their results, the sulfation was more efficient under calcining conditions (indirect sulfation) than under non-calcining conditions and the major effect of CO2 partial pressure was in the change (elevation) of the CaCO3 decomposition temperature.


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The main emission problem to be solved in the combustion of sewage sludge is the efficient reduction of NOx emissions. The reduction can be done by relatively simple primary measures such as air (combustion medium) staging12, 31, re-burning by a gaseous or solid fuel with high content of volatiles12,

32, 33

and by secondary measures, such as selective non-catalytic

reduction.12, 34 Studies comparing oxy-fuel and oxygen-enriched air combustion of sewage sludge concerning NOx, N2O and SO2 emissions and the influence of inlet pollutant concentration and water vapor concentration on the emissions are very scarce and they were performed in a batch mode tubular furnace16 or by thermogravimetric analysis.35 To the best knowledge of the authors, no comprehensive experimental study on the fluidized bed incineration of sewage sludge devoted to oxy-fuel and oxygen-enriched air combustion has been published. To fill this gap, incineration experiments of sewage sludge were performed in a BFB reactor. In the experiments, the recirculation of the flue gas with and without main gaseous pollutants and water vapor was simulated. The aim was to study the effects of combustion (fluidized bed) temperature, oxygen concentration in combustion medium, combustion medium composition (O2+N2 vs. O2+CO2) and flue gas recirculation on resulting emissions and emission factors (EFs) of NOx, N2O, and SO2.

2 Materials and Methods 2.1 Fluidized bed combustor The main part of the experimental apparatus (additionally described in more details in literature36–38 and schematically described in Figure 1), is an electrically heated 130 cm long (98 cm length above the gas distributor) cylindrical reactor tube with an inner diameter of 9.36 cm. 6 ACS Paragon Plus Environment

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The theoretical thermal power is approx. 10 kWth. The reactor tube was made of a hightemperature resistant stainless steel with a maximum operating temperature of 1 000 °C. Fluidized bed particulate material is fluidized by a preheated gaseous combustion medium introduced into the bed through a perforated steel plate distributor. The apparatus is equipped with four electrical heating elements with temperature control. The first and the second elements are used to preheat the combustion medium, the third is used to heat-up and regulate the temperature of the dense FB, and the fourth element is used to heat-up and regulate the temperature of the freeboard region. Temperature is measured at different sections (including the combustion medium preheater and cyclone separator) along the height within the reactor. For this purpose, thermocouples (type K) placed in shielding tubes are used for measuring the temperature in the dense FB (5 cm above the gas distributor) and in freeboard (40 and 75 cm above the gas distributor). The feeding system used for the experiments comprises two fuel hoppers, a double-acting, pressurized gas-driven slide feeder, and a pneumatic transport of the fuel with inert gas. A detailed description of the fuel feeding device can be found elsewhere.39 Dried sewage sludge particles were introduced to the bottom of the FB by means of pneumatic transport through a water cooled feeding tube. The level of the fuel inlet nozzle is about 15 mm above the gas distributor plate. The overflow outlet hole for maintaining constant level of the dense fluidized bed is 18 cm above the distributor. Due to complexity of the feeding system (semi-continual slide feeder with controlled frequency setting of small volumetric dosing), the average fuel feeding rate is calculated as the mass difference of the fuel in the hoppers before and after the combustion experiments divided by the overall time of feeding.


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Flow rates of individual inlet gases were measured and controlled by mass flow controllers. The first element of the flue gas cleaning system is a cyclone separator. Flue gas is then cooled down in a downstream water cooler with the length of 100 cm. Flue gas can be sampled at the top of the reactor tube, up-stream and down-stream of the water cooler (as illustrated in Figure 1).

2.2 Materials Dry sewage sludge obtained after mesophilic anaerobic stabilization from municipal wastewater treatment plant Brno-Modřice (the Czech Republic) was used as fuel in the experiments. Before the incineration experiments, the disintegrated dry sewage sludge was sieved. The size fraction of the sludge particles between 0.5 and 2 mm was used for the experiments. Proximate analysis, ultimate analysis, and calorific values of the sewage sludge are summarized in Table 1. The ash generated from previous incineration of the same sludge was used as a primary fluidized bed material in the experiments. The composition of the sludge ash was determined by X-Ray fluorescence (XRF) on ARL 9400 XL (Thermo ARL, Switzerland) and expressed in the forms of oxides, Table 2. The oxides mentioned in the table are to a greater extent bound (e.g. CaO with P2O5, SiO2 in glassy phases, etc.). Beside the mentioned oxides in Table 2, some relatively significant minorities like TiO2, MnO, ZnO, sulfur (mainly in the form of sulfates), chlorine, fluorine, and other elements are present in the sludge ash. Physical properties of the materials are described in Table 3. Minimum fluidizing velocities of the materials (measured and calculated for 850 °C and atmospheric pressure) were 18 cm·s-1 and 31 cm·s-1 for the ash and sewage sludge respectively. The minimum fluidizing velocities, in comparison with the complete fluidizing velocity of the sludge ash (33 cm·s-1 at 850 °C) and real gas velocity (approx. 48–76 cm·s-1 for temperatures between 800 and 900 °C) in the FB reactor, 8 ACS Paragon Plus Environment

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suggest that the sewage sludge particles burn within the fluidized bed or float on the upper part of the bed.40, 41

2.3 Flue gas sampling and analysis Concentrations of important gaseous pollutants (NOx, N2O, SO2) and oxygen in the flue gas were measured on-line. The sampling point for NOx, SO2, and O2 analysis was placed between the cyclone and flue gas water cooler, and the sampling point for N2O analysis was placed at the entry of the flue gas to the stack. A portable gas analyzer Horiba PG 350 (system of various types of analyzers) with an electrically heated portable gas sampling probe was used to analyze NOx, SO2, and O2 concentrations in the flue gas. The probe was heated to 180 °C. Fluid modulation and chemiluminescence type analyzer measured NOx concentrations, fluid modulation and infrared absorption type analyzer measured SO2 concentrations, and paramagnetic type analyzer measured O2 concentrations. N2O and CO concentrations in the flue gas were measured by IR cross interference compensating analyzer, Hartmann and Braun, type Uras-Advance Optima System.

2.4 Experimental procedure Concentrations of gaseous pollutants (emissions, mg·m-3) in the flue gas leaving the reactor chamber were normalized to dry flue gas (0 °C, 101.325 kPa) and calculated without the pneumatic transport gas used for the fuel transport into the reactor - N2 in the case of oxygenenriched air experiments; CO2 in the case of oxy-fuel experiments. Emission factors (EFs) were calculated per energy input of the raw fuel (mg·MJ-1, W ≈ 9 wt. %) and conversions of elements from fuel (or from fuel together with the elements from inlet gas simulating the flue gas recirculation) to pollutants were calculated by weight. Experimental data from online analyses 9 ACS Paragon Plus Environment

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were fitted to second-degree polynomial curve to clearly demonstrate the trends resulting from the change of main operating conditions. The trends of conversions are identical with those of EFs, therefore the conversions are included mainly in the supplementary file. Fuel feeding and combustion medium flow rates in individual experiments were chosen to attain the constant oxygen content in the raw flue gas and flue gas analysis was performed under conditions of continuously, slowly increasing bed temperature at a constant oxygen inlet concentration. Combustion medium flow rate varied according to the current oxygen concentration in combustion medium (21, 25, 28, or 30 vol. %). To simulate the effect of a partially wet flue gas recirculation on emissions, one experiment was made with an inlet water vapor concentration of 5 vol. % (without consideration of fuel transport gas) and oxygen concentration of 28 vol. % in the combustion medium. To simulate the effect of dry flue gas recirculation with gaseous pollutants on emissions, experiments were performed in a limited temperature range 800–900 °C with inlet concentrations of CO, NOx (considered as NO2 for mass concentration), N2O and SO2 on levels 150 mg·m-3, 2 000 mg·m-3, 300 mg·m-3 and 2 000 mg·m-3 respectively (without considering fuel transport gas). The following abbreviations are used in order to simplify the distinction among individual experiments and orientation in the plots: o EnAirXX – an experiment conducted in oxygen-enriched air mode, o OxyXX – an experiment conducted in simulated oxy-fuel mode, o Oxy-wetXX – an experiment conducted in simulated oxy-fuel mode with wet inlet flue gas and zero concentrations of pollutants (CO, NOx, N2O, and SO2), o Oxy-RFGXX – an experiment conducted in simulated oxy-fuel mode with dry recycled flue gas with non-zero inlet pollutants concentrations (CO, NOx, N2O, and SO2), 10 ACS Paragon Plus Environment

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where XX represents vol. % of oxygen in combustion medium. Simulated oxy-fuel mode means that a mixture of CO2 with oxygen from gas tanks was used as a gaseous combustion medium instead of a mixture of oxygen and recycled flue gas. The concentrations of water vapor and gaseous pollutants in the inlet gas were zero. Oxy-wet means condition of simulated oxy-fuel combustion with wet inlet gas (non-zero water vapor concentration in the inlet gas). Oxy-RFG means experiment with simulated oxy-fuel combustion with non-zero concentrations of gaseous pollutants (CO, NOx, N2O, and SO2) in the inlet gas, simulating recycling of dry flue gas. In all experiments, the feeding of the fuel started at the bed temperature of 450 °C with the bed heating element turned off. At first the bed temperature gradually increased self-sufficiently by the combustion process, then it was gradually increased by increasing the temperature of inlet combustion medium. Data acquisition started at the bed temperature of 750 °C. Electrical heating was used to compensate the heat loss in the freeboard section to maintain the freeboard temperature at the same level (± 10 °C) as the dense bed temperature. The average time of the temperature increase (from 750 to 940 °C) was 3 hours. The summary of operating conditions of the experiments is reported in Table 4 (EnAir experiments), Table 5 (Oxy experiments) and Table 6 (Oxy-wet experiment, Oxy-RFG experiment and relevant Oxy28 experiment).

3 Results and Discussion 3.1 Effect of combustion (bed) temperature Combustion temperature is a parameter that significantly affects the formation, depletion, and sorption reactions of pollutants. Moreover, the temperature of burning fuel particles (which is higher in super-stoichiometric combustion than the temperature of the bulk of a fluidized bed) is inevitably affected by other operating parameters such as fuel particle size, gas composition, fuel 11 ACS Paragon Plus Environment

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reactivity, etc.42,

43

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Therefore, it is important to study the effect of the bed’s and burning fuel

particles’ temperature on pollutants during sewage sludge combustion. Differences between temperature of burning fuel particles and that of the bulk of the fluidized bed are less significant in denser fluidized bed with relatively larger burning particles (> 0.5 mm) at lower oxygen concentration in the flue gas.43 According to model prediction, the temperature difference between the burning fuel particles and the bulk of the FB is moderately increasing with increasing bed temperature.43 The experimental emission data exerted greater uncertainty at low temperatures (below approx. 780 °C) which is attributed to the lower combustion rate, higher concentrations of CO and organics in the flue gas; and at temperatures above 900 °C which is attributed to a very high rate of sewage sludge combustion, fuel feeder characteristics, an increased temperature of the reactor wall, and an effect of radiation on temperature measurement. The estimated uncertainty at low and high temperatures was estimated to be between 5 and 8 % of the measured values of the emissions. The relative error of emissions determination was between 3 and 5 % for temperatures between 780 and 900 °C. The effect of the measured bulk fluidized bed temperature on concentrations of pollutants in the flue gas from sewage sludge incineration is described on the basis of EnAir28 experiment in Figure 2 and expressed in mg·m-3 (the results from other experiments are provided in the supplementary file). The NOx emissions are recomputed on NO2 emissions to transform the NOx concentrations expressed in ppm-v into mass concentration in mg·m-3. When the bed temperature gradually increased from 750 °C to 930 °C, NOx and SO2 concentrations in dry flue gas also increased. On the other hand, N2O concentrations in the flue gas decreased.


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The main precursors for NO formation in the homogenous (gaseous) phase are NH3 and HCN. The main product of NH3 reactions is NO, while HCN is the precursor to both NO and N2O because they are formed from the same intermediate NCO (R1–R3). At relatively high temperatures, R1 and R2 are responsible for NO being the main formation product, whereas at lower temperatures R3 competes with R2 for the available pool of NCO. The activation energy of R3 is small when compared to the activation energy of R2. The result is that with increasing temperature R2 is strongly enhanced while R3 is slightly inhibited.44

HCN + O

→ NCO + H

R1

NCO + OH → NO + HCO

R2

NCO + NO → N2O + CO

R3

The sewage sludge exhibits some self-desulfurization ability due to high contents of calcium, magnesium, and other metals (e.g. Fe, Al) (see Table 2). However, with increasing temperature, the desulfurization ability of the metal oxides (e.g. CaO, MgO) is inhibited45 and at temperatures over approx. 850 °C, the CaSO4 formed at lower temperatures can be partly decomposed and converted into Ca-silicates and Ca-alumino-silicates46, 47, resulting in SO2 release. Additionally, similarly as reported in de Diego et al.27 and Lupiáñez et al.30, with increasing bed temperature above 750 °C, SO2 emissions decreased first and then increased in oxy-fuel experiments (Figure 3). This was probably caused by greater CO2 partial pressure (when compared to oxygenenriched air experiments) that elevates the temperature of CaCO3 decomposition and is responsible for the increase of the optimum desulfurization temperature.


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3.2 Effect of oxygen concentration in combustion medium The oxygen concentration in the combustion medium significantly affects the combustion efficiency and temperature difference between burning fuel particles and the bulk of the fluidized bed. With respect to the specific attributes of dry sewage sludge (high content of volatiles, ash and nitrogen), the effect of oxygen concentration in the combustion medium on emissions of pollutants must be verified to find the optimum set-up and conditions for oxygen-enriched air combustion and oxy-fuel combustion. The effect of oxygen concentration in combustion medium on emission factors (EFs) expressed in mg·MJ-1 is described in Figure 4a, Figure 5a, Figure 6a for oxygen-enriched air experiments and in Figure 4b, Figure 5b, Figure 6b for oxy-fuel experiments. The results expressed in mg·m-3 and conversions are available in the supplementary file. Emission factors of NOx decreased with an increase in oxygen concentration in the combustion medium. This result was surprising, but it is in accordance with the results reported for anthracite combustion.48,

49

In the case of oxy-fuel combustion, the following mechanism should be

considered: in a CO2-rich environment, the effect of Boudouard reaction (R4) is significant. Boudouard reaction is endothermic, temperature dependent, and, in the case of sewage sludge combustion, it is strongly catalyzed by the ash, particularly by higher content of CaO and Fe2O3 (see Table 2).

C + CO2 → 2CO

R4

The higher the temperature, the higher the rate of the Boudouard reaction results in higher CO yield, which is together with char a significant agent in the reduction reactions of NO. 14 ACS Paragon Plus Environment

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Therefore, a possible explanation for the reduction of NOx with increased oxygen concentration in the combustion medium can be as follows: with a higher O2 concentration in inlet gas, the temperature of individual burning particles increases and promotes the Boudouard reaction under conditions of high CO2 partial pressure. Consequently, CO concentration locally increases in the environment of the stationary fluidized bed and promotes the reduction of NOx. In oxy-fuel experiments, NOx emissions (depicted in the supplementary file) decreased with increasing oxygen inlet concentration, however, they remained unaffected in the case of oxygenenriched air experiments. That suggests enhanced impact of Boudouard reaction in CO2-rich environment. With increasing oxygen concentration in combustion medium, EFs of N2O decreased in both the oxygen-enriched air experiments (Figure 5a) and the oxy-fuel experiments (Figure 5b). Fuel-N to N2O conversion is substantially affected by fluidized bed (bulk) temperature and by burning particle temperature increasing with increase in inlet oxygen concentration. The higher the temperature, generally, the lower fuel-N to N2O conversion (Chapter 3.1). Most of the sulfur in the sewage sludge (Table 1) is contained in the species that are easily combustible at temperatures typical for fluidized bed combustion (800–900 °C). Consequently, it may be presumed that fuel-S to SO2 conversion will be mostly independent of the oxygen concentration in the combustion medium and that the temperature will be the main parameter influencing the sulfur conversion to SO2 during sewage sludge combustion with no limestone addition. Such an assumption is supported by the trends displayed in Figure 6a and Figure 6b. The data curves almost overlap in oxygen-enriched air experiments. There was only slight increase in SO2 EFs when inlet oxygen concentration increased in oxy-fuel experiments and so the effect of inlet oxygen concentration on EF can be considered relatively small during oxy-fuel 15 ACS Paragon Plus Environment

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combustion of sewage sludge. On the other hand, when expressed in mg·m-3, the effect of oxygen concentration in combustion medium is noticeable and SO2 emissions increase with an increase in inlet oxygen concentration, in accordance with literature.25, 26 This is due to an increase in the ratio of sulfur introduced into the reactor to flow rate of the flue gas.

3.3 Effect of balance gas (N2 vs. CO2) CO2 and N2 as balance gases may influence the combustion process differently, particularly due to different specific heats, radiative heat transfers in their environments, different adsorption behavior on solid surfaces and gas diffusivities (including different effective diffusivities of N2 and CO2 in pores of solids). In addition, CO2 can directly participate in the combustion/gasification process via the Boudouard reaction. To understand the combustion process, it is necessary to study the influence of the balance gases on emissions. The EFs of NOx were lower during oxy-fuel combustion at lower bed temperatures (up to ≈ 860 °C), see Figure 7. There are two probable reasons: (1) the specific heat of CO2 is greater than the specific heat of N2 and (2) radiative heat transfer in CO2-rich environment will exceed that of N2-rich environment (especially at temperatures over 800 °C). That means that the flue gas rich in CO2 will exhibit more significant cooling effects than the same amount of flue gas rich in N2. Additionally, the effect of Boudouard reaction is more significant in CO2-rich atmosphere and also the catalytic properties of solid surfaces are changed due to presence and adsorption of CO2. Free CaO can react at such temperatures with CO2 forming less active CaCO3. The Boudouard reaction is endothermic and intensifies the cooling effect of CO2-rich atmosphere. Lower temperature of burning fuel particles results in lower NOx EFs, as described in Chapter 3.1. On the other hand, EFs of NOx were higher in oxy-fuel combustion at higher bed temperatures (above approx. 860 °C) in comparison with oxygen-enriched air combustion with 16 ACS Paragon Plus Environment

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the same inlet oxygen concentration. With an increase in bed temperature, more fuel-N is released to the flue gas because at temperatures over 850 °C, char-N is released. NO is formed from char-N mainly via HCN precursor, similarly to reactions in homogenous phase (R1–R3).44 The reaction product then depends on the pathway R2 or R3. Due to lower temperatures of burning particles in CO2-rich atmosphere, CO and N2O concentrations were locally higher in the environment of the fluidized bed during oxy-fuel combustion than during oxygen-enriched air combustion with the same inlet oxygen concentration. Greater CO and N2O partial pressure may cause the inhibition of the reaction R3 in CO2-rich atmosphere at higher bed temperatures more significantly than in N2-rich atmosphere. The effect of greater CO and N2O partial pressure seems to dominate over the effect of temperature and, as a result, R2 nitrogen pathway prevails. In addition, less NO is consumed by R3. It was observed that, with the same inlet oxygen concentration, emission factors of N2O were significantly higher during the oxy-fuel combustion than during oxygen-enriched air combustion (see Figure 8). The temperature of burning particles was lower in CO2-rich atmosphere than in N2-rich atmosphere due to greater specific heat of CO2, more intensive radiative heat transfer in CO2 atmosphere, and the endothermic nature of the Boudouard reaction. Consequently, lower temperatures resulted in higher N2O EFs in CO2-rich environment as it was described in Chapter 3.1. As mentioned in Chapter 3.1, sewage sludge has some sorption capacity for flue gas desulfurization. The desulfurization by alkali compounds (mainly compounds of Ca, partly Mg and K related compounds) in the flue gas is influenced by the reaction atmosphere. CO2-rich atmosphere (oxy-fuel combustion) negatively influences efficiency of desulfurization. The SO2 emission factors from oxy-fuel combustion were higher than the EFs from oxygen-enriched air 17 ACS Paragon Plus Environment

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combustion with the same inlet oxygen concentration (see Figure 9). With an increase in bed temperature above 750 °C, the emission factors of SO2 increased monotonically in oxygenenriched air experiments, however, emission factors of SO2 from oxy-fuel combustion decreased and then increased with increase in temperature, with the minimum emission factor at the bed temperature around 810 °C. Similar convex shape of dependence of SO2 EFs on temperature can be expected in oxygen-enriched air combustion, however the optimum temperature will be lower (probably less than 750 °C). As mentioned earlier in the introduction, the sulfation conversions are higher under calcining conditions (sorbent in the form of CaO) than under non-calcining conditions and the major effect of CO2 partial pressure is in the shift of the CaCO3 decomposition temperature. The optimum temperature for SO2 sorption depends not only on temperature and CO2 conncentration in the flue gas (influencing CaCO3 thermal decomposition), but also on sewage sludge ash composition (e.g. presence of Al2O3, SiO2, Fe2O3, phosphorus content, etc. that contribute to the formation of calcium phosphates, Ca-alumino-silicates, glassy (silicate) minerals, ferrites, etc.). The content of “free” CaO in sewage sludge ash (available for efficient flue gas desulfurization) is usually relatively small.50 Therefore, the ash desulfurization behavior and optimum temperature in reactions of sewage sludge ash with SO2 are different in comparison with flue gas desulfurization by natural limestone.

3.4 Effect of water vapor presence in combustion medium Higher concentration of water vapor in the flue gas affects the concentration of radicals (·OH, ·H, ·O) in the flue gas and exerts effects on the sorption and catalytic heterogeneous reactions. In our experiments with the inlet combustion medium containing 5 vol. % of water vapor, we focused mainly on the effect of the moisture in inlet gas on NOx emissions and nitrogen conversion to NOx, because there was relatively small effect of inlet water vapor concentration on 18 ACS Paragon Plus Environment

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SO2, CO and N2O emissions in the temperature range 800–900 °C, particularly at temperatures above 850 °C. Reduction of NOx in the flue gas is the most important effect of non-zero water vapor concentration in the inlet gaseous combustion medium. It is obvious from Figure 10, the effect of water vapor presence in inlet combustion medium on NOx emissions increases with increasing operating temperature (between 800 and 900 °C). This is qualitatively in agreement with conclusions from literature.24 According to conclusions from oxygen-enriched air combustion of organic fuels with high nitrogen content19, the effect of water vapor on NOx emissions is more significant at oxygen concentrations in the inlet combustion medium below approx. 30 vol. %. We believe that the effect of water vapor on NOx emissions is also dependent on output oxygen concentration in the flue gas, which in our case was relatively low.

3.5 Effect of pollutants presence in combustion medium Under the conditions summarized in Table 6, recirculation of the flue gas with gaseous pollutants was simulated by addition of CO, N2O, NO and SO2 to the inlet gaseous combustion medium. The inlet nitrogen overall conversion (conversion of the sum of fuel-N and inlet NOx) to the resulting NOx is a good indicator of the effect of elevated NOx concentration in inlet gas on resulting NOx emissions. As it is shown in Figure 11, the nitrogen conversion of inlet nitrogen sources (fuel-N + input gaseous NO) decreases with increasing overall nitrogen input. On the other hand, the NOx concentration in the flue gas (in mg·m-3) increases under the condition of NOx addition into the inlet combustion gas. The combined influence of CO and SO2 has additional effect on the reduction of NOx emission, so the effect of inlet NOx reduction on the elevation of resulting NOx emissions is lower than the effect for the case of only NO addition into the inlet gas (as can be found in literature for coal combustion24). On the other hand, it seems that attaining the conversions of input fuel + NOx nitrogen into output NOx below approx. 5 % at 19 ACS Paragon Plus Environment

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temperatures above 850 °C is difficult in oxy-fuel combustion without gas staging or re-burning. The overall nitrogen conversion into NOx in oxy-fuel combustion of nitrogen rich solid fuels are still incomparable with the low conversions of nitrogen into NOx (below 1 %) attainable e.g. in chemical looping combustion of sewage sludge for CO2 separation.51 In the case of SO2 emissions and emission factors for FB oxy-fuel combustion of sewage sludge without addition of limestone for desulfurization, the SO2 emission values (expressed in mg·m-3) in the flue gas increase only slightly less than the addition of SO2 caused by inlet SO2 concentration (Figure12). Therefore, the influence of higher inlet SO2 concentrations on desulfurization efficiency is lower at such conditions (combustion of sewage sludge without limestone addition) than for FB combustion with limestone addition for desulfurization, where the ratio of “free” calcium derived from limestone and sulfur in the flue gas can be typically high (Ca/S > 2).24

4 Conclusions Dry (W ≈ 9 wt. %), anaerobically stabilized sewage sludge was combusted in a bubbling fluidized bed reactor and the effects of operating parameters on emissions of NOx, N2O, and SO2 were studied. The sewage sludge was incinerated in oxygen-enriched air (O2/N2), and oxy-fuel (O2/CO2) combustion atmospheres at temperatures from 750 to 930 °C. For the combustion experiments, three different concentrations of oxygen in the inlet combustion media were used (25, 28, and 30 vol. %). The following bullet points summarize our most important observations: •

With an increase in combustion temperature, NOx and SO2 emissions increased while N2O emissions decreased.


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•

The conversion of fuel-N to NOx-N did not exceed 10 wt. % and 16 wt. % in oxygenenriched air combustion and oxy-fuel combustion respectively at the highest combustion temperatures (≈ 930 °C).

•

At the lowest combustion temperatures (750–800 °C), the conversion of fuel-N to N2O-N was very low (3–3.5 wt. %) in both oxygen-enriched air and oxy-fuel combustion atmospheres.

•

With an increase in oxygen concentration in inlet combustion medium, NOx and N2O emission factors decreased (at relatively low oxygen concentrations in output flue gas) and increased O2 concentration in the inlet gaseous combustion medium had no significant influence on SO2 emission factors.

•

During oxy-fuel combustion experiments, NOx emission factors were lower (when compared with oxygen-enriched air experiments with the same oxygen inlet concentration), particularly at combustion temperatures below 850 °C, and N2O emission factors were higher within whole interval of temperatures.

•

SO2 emission factors were higher in oxy-fuel combustion due to the negative effect of CO2 on the ability of sewage sludge to capture SO2 from the flue gas. The desulfurization ability of sewage sludge ash is only weak (incomparable with natural limestone) due to the low content of active “free” CaO in sewage sludge ash.

•

Elevated concentration of water vapor in the inlet combustion medium had positive effect mainly on the reduction of NOx, particularly in terms of decreasing emission factors of NOx and conversion of fuel-N to NOx.

•

Elevated concentrations of NOx, N2O, CO and SO2 in the inlet combustion medium caused mainly a reduction of overall nitrogen (fuel-N + inlet NOx related N) conversion 21 ACS Paragon Plus Environment

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into NOx. Emissions of SO2 (in mg·m-3) were influenced by elevated SO2 concentrations in inlet combustion medium close to additional concentration. It means that the sorption of SO2 by the sewage sludge ash was very low. This implies necessity of limestone utilization for efficient flue gas desulfurization in oxy-fuel combustion.

Acknowledgement This work was supported by Waste to Energy Competence Centre funded by the Technology Agency of the Czech Republic (project TE02000236), and financial support from specific university research (MSMT No 20-SVV/2016 and MSMT No 20-SVV/2017).

Supporting Information Supporting information file provides figures describing the influence of combustion temperature on emissions expressed in mg·m-3 and on fuel nitrogen and sulfur conversion to the flue gas.

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25. Xin, S. W.; Lu, X. F.; Liu, H. Z. Experimental research of the oxygen-enriched combustion of sewage sludge and coal in CFB. Proceedings of the 20th International Conference on Fluidized Bed Combustion (I), Xi’an, China, 2009, 612–617. 26. Duan, L.; Zhao, C.; Zhou, W.; Qu, C.; Chen, X. O2/CO2 coal combustion characteristics in a 50kWth circulating fluidized bed. Int. J. Greenhouse Gas Control 2011, 5, 770-776. 27. de Diego, L.F.; de las Obras-Loscertales, M.; Rufas, A.; García-Labiano, F.; Gayán, P.; Abad, A.; Adánez, J. Pollutant emissions in a bubbling fluidized bed combustor working in oxy-fuel operating conditions: Effect of flue gas recirculation. Appl. Energy 2013, 102, 860-867. 28. Han, X; Niu, M.; Jiang, X.; Liu, J. Combustion characteristics of sewage sludge in a fluidized bed. Ind. Eng. Chem. Res. 2012, 51, 10565-10570. 29. García-Labiano, F.; Rufas, A.; de Diego, L. F.; de las Obras-Loscertales, M.; Gayán, P.; Abad, A.; Adánez, J. Calcium-based sorbents behaviour during sulphation at oxy-fuel fluidised bed combustion conditions. Fuel 2011, 90, 3100-3108. 30. Lupiáñez, C.; Guedea, I.; Bolea, I.; Diez, L. I.; Romeo, L. M. Experimental study of SO2 and NOx emissions in fluidized bed oxy-fuel combustion. Fuel Process. Technol. 2013, 106, 587-594. 31. Houshfar, E.; Lovas, T.; Skreiberg, O. Experimental investigation on NOx reduction by primary measures in biomass combustion: straw, peat, sewage sludge, forest residues and wood pellets. Energies 2012, 5, 270–290.


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41. Hartman, M.; Pohořelý, M.; Trnka, O. Fluidization of dried wastewater sludge. Powder Technol. 2007, 178, 166-172. 42. Maffei, T.; Khatami, R.; Pierucci, S.; Faravelli, T.; Ranzi, E.; Levendis, L. A. Experimental and modelling study of single coal particle combustion in O2/N2 and Oxyfuel (O2/CO2) atmospheres. Combust. Flame 2013, 160, 2559-2572. 43. Svoboda, K.; Hartman, M.; Pohořelý, M.; Trnka O. Modelling of effects of operating conditions and coal reactivity on temperature of burning particles in fluidized bed combustion. Acta Geodyn. Geomater. 2004, 1, 261-274. 44. Pels, J. R.; Wójtowicz, M. A.; Kapteijn, F.; Moulijn, J. A. Trade-off between NOx and N2O in fluidized-bed combustion of coals. Energy Fuels 1995, 9, 743-752. 45. Folgueras, M.B.; Diaz, R.M.; Xiberta, J. Sulphur retention during co-combustion of coal and sewage sludge. Fuel 2004, 83, 1315-1322. 46. Aadraoui, M.; Elbaghdadi, M.; Rais, J.; Barakat, A.; Ennaji, W.; Karroum, L.A.; Oumenskou, H. Effect of incineration of sewage sludge on the evolution of physicochemical characterization and mineralogical properties. J. Mater. Environ. Sci. 2017, 8, 2800-2806. 47. Wang, L.; Skjevrak, G.; Hustad, J.E.; Grønli, M.G. Sintering characteristics of sewage sludge ashes at elevated temperatures. Fuel Process. Technol. 2012, 96, 88-97. 48. Díez, L. I.; Lupiáñez, C.; Guedea, I.; Bolea, I.; Romeo, L. M. Anthracite oxycombustion characteristics in a 90 kWth fluidized bed reactor. Fuel Process. Technol. 2015, 139, 196-203. 28 ACS Paragon Plus Environment

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Table 1 Proximate and ultimate analyses and calorific values of the sewage sludge (d - dry basis, daf - dry and ash free basis, * - calculated by difference) Proximate analysis Moisture, W

wt. %

9.45

Ash, Ad

wt. %

44.6

Volatiles, Vdaf

wt. %

87.3

Fixed Carbon, FCdaf

wt. %

12.7

Calorific value Higher heating value, HHVd MJ.kg-1

12.8

Lower heating value, LHVd MJ.kg-1

11.8

Ultimate analysis Cd (Cdaf)

wt. %

28.9 (52.2)

Hd (Hdaf)

wt. %

4.39 (7.94)

Nd (Ndaf)

wt. %

4.10 (7.40)

Od (Odaf) *

wt. %

16.2 (29.3)

Stotald (Stotaldaf)

wt. %

1.75 (3.16)

Scombustd (Scombustdaf)

wt. %

1.30 (2.35)

Cld (Cldaf)

mg.kg-1

433 (782)

Fd (Fdaf)

mg.kg-1

255 (460)


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Table 2 Composition of the ash from the sewage sludge incineration, determined by XRF analysis Species

Unit

Content

Al2O3

wt. %

17.5

CaO

wt. %

14.5

Fe2O3

wt. %

14.0

K2O

wt. %

1.63

MgO

wt. %

2.71

P2O5

wt. %

18.7

SiO2

wt. %

27.2

Sum

wt. %

96.2

Table 3 Physical properties of the materials used in the experiments (a 25 °C; 101.325 kPa, b 850 °C; 101.325 kPa, c theoretical value from calculation) Material

Ash

Sewage sludge

0.71–1.6

0.5–2.0

Mean particles size (mm)

1.16

1.25

Loose poured bulk density (kg·m-3)

581

768

Apparent density (kg·m-3)

1027

1542

True solid density (kg·m-3)

2354

1740

Particle porosity (%)

56

11

Bed voidage (%)

43

50

Minimum fluidizing velocitya (cm·s-1)

30

45

Minimum fluidizing velocityb (cm·s-1)

18

31c

Complete fluidizing velocitya (cm·s-1)

45

74

Complete fluidizing velocityb (cm·s-1)

33

68c

Size range of particles (mm)


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Table 4 Operating conditions of experiments in oxygen-enriched air mode (a calculated without pneumatic transport gas) Experiment

EnAir25

EnAir28

EnAir30

Fluidized bed material

Ash from sewage sludge combustion

Fuel feeding rate (g.h-1)

616

617

617

Bed temperature (°C)

750–890

750–930

750–940

Pneumatic transport gas flow, N2 (m3.h-1)

1.0

1.0

1.0

Average O2 in raw flue gasa (vol. %)

6.8

6.8

6.3

Average O2 in dry flue gasa (vol. %)

7.8

8.0

7.5

Combustion medium (O2/N2, vol. %)

25/75

28/72

30/70

Combustion medium flow rate (m3.h-1)

2.3

2.0

1.8

Raw flue gas flowa (m3.h-1)

2.6

2.3

2.1

Dry flue gas flowa (m3.h-1)

2.3

2.0

1.8


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Energy & Fuels

Table 5 Operating conditions of experiments in oxy-fuel mode (a calculated without pneumatic transport gas) Experiment

Oxy25

Oxy28

Oxy30



Fuel feeding rate (g.h-1)

697

699

697


750–940

750–930

750–930

Pneumatic transport gas flow, CO2 (m3.h-1)

1.0

1.0

1.0


4.3

3.6

2.6


5.1

4.3

3.1

Combustion medium (O2/CO2, vol. %)

25/75

28/72

30/70

Combustion medium flow rate (m3.h-1)

2.3

2.0

1.8

Raw flue gas flowa (m3.h-1)

2.7

2.3

2.2

Dry flue gas flowa (m3.h-1)

2.3

1.9

1.8


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 34 of 42

Table 6 Operating conditions of experiments in Oxy-wet mode and Oxy-RFG mode with 28 vol. % oxygen in inlet combustion medium and comparison with operating conditions of corresponding oxy-fuel conditions (a calculated without pneumatic transport gas) Experiment

Oxy-wet28


Oxy-RFG28

Oxy28


Fuel feeding rate (g·h-1)

699

699

699


800–900

800–900

750–930

Pneumatic transport gas flow, CO2 (m3·h-1)

1.0

1.0

1.0


3.6

3.6

3.6


4.5

4.3

4.3

Combustion medium (inlet O2/CO2/H2O, vol. %)

28/67/5

28/72/0

28/72/0

Mass conc. of pollutants (CO, N2O, NO2, SO2) in inlet gas (mg/m3)

0,0,0,0

150, 300, 2000, 2000

0,0,0,0,

Combustion medium flow rate (m3·h-1)

2.0

2.0

2.0

Raw flue gas flowa (m3·h-1)

2.3

2.3

2.3

Dry flue gas flowa (m3·h-1)

1.8

1.9

1.9


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Energy & Fuels

Figure 1 Experimental apparatus


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 42

Figure 2 Effect of combustion (bed) temperature on NO2, N2O and SO2 concentrations in the flue gas leaving the combustor chamber

Figure 3 SO2 concentrations in the flue gas leaving the combustor chamber from oxy-fuel combustion of the sewage sludge


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Energy & Fuels

Figure 4a NO2 emission factors from and oxygen-enriched air combustion of the sewage sludge

Figure 4b NO2 emission factors from oxy-fuel combustion of the sewage sludge


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 42

Figure 5a N2O emission factors from and oxygen-enriched air combustion of the sewage sludge

Figure 5b N2O emission factors from oxy-fuel combustion of the sewage sludge


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Energy & Fuels

Figure 6a SO2 emission factors from and oxygen-enriched air combustion of the sewage sludge

Figure 6b SO2 emission factors from oxy-fuel combustion of the sewage sludge


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 42

Figure 7 NO2 emission factors from EnAir28 and Oxy28 experiments

Figure 8 N2O emission factors from EnAir28 and Oxy28 experiments


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Energy & Fuels

Figure 9 SO2 emission factors from EnAir28 and Oxy28 experiments

Figure 10 Effect of inlet water vapor content in combustion medium (5 vol. %) on emission factors (EF) of NO2 expressed in mg of NO2/MJ of inlet fuel


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 42

Figure 11 Effect of content of CO, N2O, NO and SO2 in inlet combustion medium on overall conversion of inlet nitrogen (fuel-N + NO2-N) to NO2 in simulated oxy-fuel combustion of sewage sludge

Figure 12 Effect of content of SO2 in inlet combustion medium (2000 mg·m-3) and non-zero concentrations of CO, N2O and NOx in inlet combustion gas (Table 6) on concentration of SO2 in the flue gas leaving the combustion chamber in simulated oxy-fuel combustion of sewage sludge 42 ACS Paragon Plus Environment

FLUIDIZED BED INCINERATION OF SEWAGE SLUDGE IN O2

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