Co-combustion Of Sewage Sludge Research Articles

Co-Combustion of Municipal Sewage Sludge and Hard Coal on Fluidized Bed Boiler WF-6

Abstract According to data of the Central Statistical Office, the amount of sludge produced in municipal wastewater treatment plants in 2010 amounted to 526000 Mg d.m. The forecast of municipal sewage sludge amount in 2015 according to KPGO2014 will reach 642400 Mg d.m. and is expected to increase in subsequent years. Significant amounts of sludge will create problems due to its utilization. In order to solve this problem the use of thermal methods for sludge utilization is expected. According to the National Waste Management Plan nearly 30% of sewage sludge mass should be thermally utilized by 2022. The article presents the results of co-combustion of coal and municipal sewage sludge in a bubbling fluidized bed boiler made by SEFAKO and located in the Municipal Heating Company in Morag. Four tests of hard coal and sewage sludge co-combustion have been conducted. Boiler performance, emissions and ash quality were investigated.

Reducing high-temperature corrosion on high-alloyed stainless steel superheaters by co-combustion of municipal sewage sludge in a fluidised bed boiler

One way of mitigating alkali chloride induced corrosion in biomass and waste-fired boilers is by using additives or by co-combustion and thereby decreasing the corrosiveness of the flue gas and deposits. The effect of co-firing digested sewage sludge with a mixture of biomass and waste (78% bark pellets+22% Solid Recovered Fuel, denoted “SRF”) was investigated in a 12MW circulating fluidised bed (CFB) boiler. The initial corrosion attack of the stainless steel 304L at 600°C (material temperature) was investigated during 24h exposures. The exposures were carried out in the flue gas environment from three fuel-mixes: SRF, MSS1-low (SRF with municipal sewage sludge, low dosage) and MSS2-high (SRF with municipal sewage sludge, high dosage). The results showed that the most severe corrosion attack on 304L occurred without sewage sludge in the SRF exposure. This attack was characterised by a corrosion product layer up to 100μm in thickness and signs of internal corrosion of the steel. The deposit in the SRF case was dominated by alkali chlorides. The exposures with co-combustion of sewage sludge, MSS1-low and MSS2-high, showed a significant decrease in corrosion. The steel sample of 304L performed especially well in the latter case when it was protected by a thin oxide of less than 0.3μm in thickness. This deposit was dominated by sulphate- and phosphate-containing compounds but traces of aluminium silicate compounds where also found. Furthermore, the concentration of alkali chlorides was low. Thus, the initial corrosion attack was greatly reduced by co-combustion of digested sewage sludge with SRF.

The Emission Characteristics of PAHs during Coal and Sewage Sludge Co-Combustion in a Drop Tube Furnace

The emission characteristics of Polycyclic Aromatic Hydrocarbons (PAHs) and their toxic equivalent (TEQ) concentrations during coal/municipal sewage sludge (MSS) co-combustion were investigated in a laboratory-scale drop tube furnace at temperatures in the range of 950-1250°C. The experimental results demonstrated that PAHs and their TEQ concentrations generated from 30/70 (MSS/Coal) co-combustion were lower than their linear calculated values. Meanwhile, the maximum reduction efficiency (46.09%) of the PAHs concentration was achieved at 1150°C. As for the TEQ concentration, the maximum reduction efficiency was as high as 57.77%. It was also observed that the PAHs concentration in flue gas during coal mono-combustion or 30/70 (MSS/Coal) co-combustion first increased and then decreased at 950-1250°C, and the maximum amount of PAHs occurred at about 1150°C. In the case of MSS monocombustion, the maximum PAHs concentration is 2.495 μg/L at 1050°C.

Emission, Mass Balance, and Distribution Characteristics of PCDD/Fs and Heavy Metals during Cocombustion of Sewage Sludge and Coal in Power Plants

The emission, mass balance, and distribution characteristics of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) as well as those of heavy metals (Hg, Cd, Pb, Cr, and Cu) were investigated during the cocombustion of 5%, 10%, 15%, and 20% sewage sludge (SS) in a pulverized coal power plant. The PCDD/F emissions increased from 7.00 to 32.72 pg I-TEQ/Nm(3) as the amount of SS in the mixed fuel (MF) increased. High sulfur content and relatively low chlorine levels in MF resulted in lower PCDD/F emissions. SS exhibited a remarkable difference in congener profiles compared with flue gas, bottom ash, and fly ash. The negative dioxin mass balance indicated that the cofiring of SS with coal in power plants was not a source but a sink of dioxins. The concentrations and emission factors of heavy metals in flue gas and bottom ash, as well as fly ash, all exhibited a tendency to increase with increasing input values of heavy metals in MF. The distribution characteristics of the investigated heavy metals were primarily dependent on the evaporative properties of these metals. The availability of chlorine could alter the heavy metal distribution behavior. The emitted pollutants in the power plant were below the legal limits.

Evaluation of PCDD/Fs and metals emission from a circulating fluidized bed incinerator co-combusting sewage sludge with coal

The emission characteristics of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and heavy metals were evaluated during co-combustion of sewage sludge with coal from a circulating fluidized bed incinerator. The stack gas, slag and fly ash samples were sampled and analyzed. The gas-cleaning system consisted of electrostatic precipitators and a semi-dry scrubber. Results showed that the stack gas and fly ash exhibited mean dioxin levels of 9.4 pg I-TEQ/Nm3 and 11.65 pg I-TEQ/g, respectively, and showed great similarities in congener profiles. By contrast, the slag presented a mean dioxin level of 0.15 pg I-TEQ/g and a remarkable difference in congener profiles compared with those of the stack gas and fly ash. Co-combusting sewage sludge with coal was able to reduce PCDD/Fs emissions significantly in comparison with sewage sludge mono-combustion. The leaching levels of Hg, Pb, Cd, Ni, Cr, Cu, and As in the fly ash and slag were much lower than the limits of the environmental protection standard in China. These suggest that the co-combustion of sewage sludge and coal is an advisable treatment method from an environmental perspective.

Experimental Study on the Oxygen-Enriched Co-Combustion of Sewage Sludge and Coal in CFB

Oxygen-enriched combustion technology is one of the new clean coal combustion technologies that can control pollutant emission. An oxygen-enriched combustion experiment of coal and sewage sludge in a Circulating fluidized bed (CFB) incinerator was carried out. Experimental analysis on the combustion characteristics of fuels with different mass rate in the oxygen concentration of 21%~35% was done. The results showed that the combustion characteristics differ from the different mixing ratio of coal to sludge in different oxygen atmosphere. When the amount of the fuel was constant, as the increase of the oxygen concentration in the air, the furnace temperature increased gradually, the SO2 concentration and NO concentration showed increasing trend, the CO concentration and HCl concentration showed decreasing trend.

Exergy Analysis and Optimized Power Generation Applied to the Co-Combustion of Sewage Sludge and Coal

In this paper, the classical sewage sludge and coal co-combustion power generating system was optimized. The regenerative steam in the extraction turbine was partly used for the sludge drying. And then, the mathematics model was established based on the exergy theory. Several extraction points were compared to find out the most optimization parameters (drying efficiency of the dryer, pressure and tempreture). The result showed that, with different steam temperature and pressure at different extraction points, the influence to the system exergy efficiency caused by the percentage of moisture and drying efficiency was different. When the sludge moisture content went to about 35%, the generating capacity increment was moderating. After the calculation and comparison, the error between the calculation results of the mathematical optimization model and the classical thermodynamic method was very small. The calculation in mathematical optimization model based on exergy analysis was relatively simple, and analysis of the relationship between variables was easier.

Characteristics of carbonized sludge for co-combustion in pulverized coal power plants

Co-combustion of sewage sludge can destabilize its combustion profile due to high volatility, which results in unstable flame. We carried out fuel reforming for sewage sludge by way of carbonization at pyrolysis temperature of 300–500 °C. Fuel characteristics of carbonized sludge at each temperature were analyzed. As carbonization temperature increased, fuel ratio increased, volatile content reduced, and atomic ratio relation of H/C and O/C was similar to that of lignite. The analysis result of FT-IR showed the decrease of aliphatic C–H bond and O–C bond in carbonization. In the analysis result of TG–DTG, the thermogravimetry reduction temperature of carbonized sludge (CS400) was proven to be higher than that of dried sludge, but lower than that of sub-bituminous coal. Hardgrove grindability index increased in proportion to fuel ratio increase, where the carbonized sludge value of 43–110 was similar or higher than the coal value of 49–63. As for ash deposits, slagging and fouling index were higher than that of coal. When carbonized sludge (CS400) and coal were co-combusted in 1–10% according to calorific value, slagging tendency was low in all conditions, and fouling tendency was medium or high according to the compositions of coal.

Co-combustion of coal and sewage sludge: Chemical and ecotoxicological properties of ashes

The co-combustion of sewage sludge (SS) and coal is widely used for the treatment and thermal valorization of SS produced in wastewater treatment plants. The chemical and ecotoxicological properties of the ashes produced in this thermal treatment have not been fully studied. Two combustion tests were performed in a fluidized bed combustor. Colombian coal was used as fuel in test A. A blend (1 + 1) of this coal and a stabilized SS (Biogran ®) was used in a second test B. Samples of the bottom and fly ashes trapped in two sequential cyclones were collected. The characterization of the ashes was focused on two main aspects: (1) the bulk content of a set of metals and (2) the characterization of eluates produced according to the European Standard leaching test EN 12457-2. The eluates were submitted to an ecotoxicological characterization for two bio-indicators. In what concerns the bulk content of ashes, both combustion tests have produced ashes with different compositions. The ashes formed during the co-combustion test have shown higher concentrations of metals, namely Cr, Cu, Ni, Pb, Zn and Fe for all ashes. The leaching test has shown low mobility of these elements from the by-products produced during the combustion and co-combustion tests. Cr and Cr(VI) were mainly detected in the eluates of the 1st cyclone ashes produced in both combustion tests and in the 2nd cyclone ashes produced in the co-combustion test. Considering the ecotoxicity assays, the eluates of bottom and fly ashes for both combustion and co-combustion tests have shown low ecotoxic levels. The micro-crustacean Daphnia magna was generally more sensitive than the bacterium Vibrio fischeri. CEMWE criterion has allowed to classify the bottom ashes for both combustion and co-combustion tests as non-toxic residues and the fly ashes collected in both cyclones as toxic.

Investigation of sulfur forms and transformation during the co-combustion of sewage sludge and coal using X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy was used to investigate the characteristics and evolution of sulfur (S) in mixtures of bituminous coal and sewage sludge (SS) and their chars during isothermal combustion. Five groups of mixtures with SS content of 0%, 10%, 20%, 30% and 100%, were examined at different burn-off ratios ( β) of 0, 30%, 50%, 70% and 100%. The S in the coal mainly exist as the forms of mercaptan (S1), sulfide (S2), thiophene (S3), sulfoxide (S4), sulfone (S5) and sulfate (S6). During the coal combustion process, the content of S1 and S2 decreased, while that of S3 and S5 increased in the early stage and decreased in the late stage. The S4 content increased throughout the entire process of combustion. Small amount of S6 was detected, showing a fluctuated pattern. The trend of S1, S2, S5 and S6 in SS was alike with that in coal, whereas S4 decreased at the end of combustion. The changing process of S3 in SS was opposite to that of coal, while the composition of S in the mixtures resulted from the mixing of coal and SS. The transformation of each functional group during co-combustion were correlated with their transformation characteristics during the mono-combustion of coal and SS, and no obvious interaction was observed, which demonstrated that the coal-origin and SS-origin sulfur in mixtures kept their own characteristics in the combustion. SS may accumulate on the solid surface as α increase, resulting its significant influence on the evolution of each form of S. When α was low, most of the S-contained functional groups presented the characteristics of coal. The percentage of coal-origin functional groups declined as α increased. The transforming trends of most functional groups were similar with that of SS when α reached 30%.

Isoconversional kinetic analysis of co-combustion of sewage sludge with straw and coal

In this paper, the co-combustion behaviour of sewage sludge with straw and coal were carried out in a thermogravimetric analyzer under different atmospheres and at different heating rates (10, 20 and 40 K min −1) in the temperature range from ambient temperature to 1000 °C. TG and DTG curves were analysed. The Flynn–Wall–Ozawa’s (FWO) and Kissinger–Akahira–Sunose’s (KAS) isoconversional methods were used to yield dependency of the activation energy of reduction process on the degree of conversion. The values of E α were obtained. The results indicate that: with the increase of heating rate, the maximum weight loss rate of samples increase obviously. The activation energy is practically constant in the 0.2 ⩽ α ⩽ 0.9 range, with the average values of E α = 137.27 and 132.38 kJ mol −1 calculated by FWO and KAS methods, respectively.

Co-combustion of dried sewage sludge and coal in a pulverized coal boiler

More than 1.1 million tons of municipal and industrial sewage sludge is produced annually in Poland. Most of this sewage sludge is landfilled or used for recultivation and fertilization of soil. After accession of Poland to the EU, large investments are planned for wastewater treatment, so it is expected that the amount of sewage sludge produced in Poland will grow in the near future. It is well known that the combustion of sewage sludge is becoming a more and more popular utilization method of such waste. Unfortunately, the current situation in Poland makes it impossible to incinerate the sewage sludge because of a lack of incinerators. One possible solution for Poland is the co-firing of dried sewage sludge in existing coal-fired utility boilers. This article presents results of initial Polish industrial trials of dried municipal sewage sludge and hard coal co-combustion in an OP-230 pulverized coal boiler. Such a solution was shown to be technically viable and not to require changes to the existing technological system. Cocombustion of sewage sludge with coal in power plants seems to be the best solution for sludge utilization in the near future in Poland.

Co-combustion of different sewage sludge and coal: A non-isothermal thermogravimetric kinetic analysis

The kinetics of the combustion of coal, two different sewage sludge and their blends (containing different dried weight percentages of sewage sludge) was studied by simultaneous thermogravimetric analysis. Once the weight percentage of sludge in the blend was ⩽10%, the effects on the combustion of coal were hardly noticeable in terms of weight loss. The Arrhenius activation energy corresponding to the co-combustion of the blends was evaluated by non-isothermal kinetic analysis. This showed that, though differences between coal and sewage sludge, the combustion of their blends kept kinetically alike to that of the coal. This work illustrates how thermogravimetric analysis may be used as an easy rapid tool to asses, not only mass loss, but also kinetics of the co-combustion of sewage sludge and coal blends.

Leaching of ashes from co-combustion of sewage sludge and wood—Part I: Recovery of phosphorus

Combustion of sewage sludge with subsequent recovery of phosphorus is a relatively new sludge treatment option. In the work described in this paper, recovery of phosphorus by acid leaching of fly ashes from co-combustion of sewage sludge with wood in a circulating fluidized bed combustor was investigated. The results showed that it is possible to find a pH range (0.5–1) in which one can extract a significant fraction of the phosphorus from these ashes without precipitation of secondary phosphates. The type of flocculation agent used in the waste water treatment plant where the sludge is formed has a significant effect on the phosphorus recovery. Ashes from combustion of sewage sludge that was formed using aluminium sulphate as flocculating agent released nearly all phosphorus at a pH value of 1. When iron sulphate was used as flocculating agent, this affected the chemistry of the resulting ashes, making phosphorus recovery more difficult. The yield of phosphorus from those ashes was 50–80%. In Part II of the project [Pettersson et al. Leaching of ashes from co-combustion of sewage sludge and wood. Part II: The mobility of metals during phosphorus extraction. Biomass and Bioenergy 2007 in press, doi:10.1016/j.biombioe.2007.09.006 [1].] focus is on the trace element release to the leachate. This determines if the leachate can be used directly as a fertilizer or if further removal of trace elements is necessary.

Leaching of ashes from co-combustion of sewage sludge and wood—Part II: The mobility of metals during phosphorus extraction

Sewage sludge and its ashes after combustion are contaminated with metals in various concentrations. In the work described in this paper, the mobility of metals during recovery of phosphorus by acid leaching of fly ashes from co-combustion of sewage sludge with wood was investigated. The metal concentrations in two sewage sludges, fly ashes and leachates from acid phosphorus extraction, were compared with phosphorus rock and different fertilisers used in agriculture. The secondary cyclone ashes were found to be less contaminated with trace elements than the bag-filter ashes. The largest problem is cadmium, which in all ashes studied has a too high level to meet the legislative limits for the maximum dosage in Sweden and the proposed limits in the European Union. The legislation includes limits for cadmium (Cd), mercury (Hg), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) calculated for an amendment of X kg of fertiliser per year and hectare of land. The solubility of the trace metals in the cyclone ash, with Cd as an exception, is in general much lower than the solubility of phosphorus. A decrease in pH results in an increased release of Cd but has just a slight influence on the other metals analysed in this study. The Cd yield increases by 30% when pH is lowered from 2.5 to 1.0, whereas the Hg release is not affected at all. The trace element concentrations in the leachates are far below the European and Swedish limitations of metal concentrations in fertilisers. The leachates thus fit as fertilisers and also as raw material to the industry, if Cd is removed. In Part I of the project [Pettersson A, Åmand L-E, Steenari B-M. Leaching of ashes from co-combustion of sewage sludge and wood. Part I: recovery of phosphorus. Biomass and Bioenergy, 2007, accepted for publication [1]] focus was on the phosphorus recovery only.

Effects of sewage sludge blending on the coal combustion: A thermogravimetric assessment

Combustion of urban sewage sludge together with coal in existing infrastructures may be a sustainable management option energetically interesting for these materials, usually considered wastes. Thermogravimetric analysis was used to study the combustion of a semianthracite coal and the modifications undergone when adding a small percentage (2%, 5%, 10%) of sewage sludge. Both Differential Scanning Calorimetric analysis and Differential Thermogravimetry burning profiles showed differences between coal and sewage sludge combustion. However, the effects of adding a percentage of sewage sludge equal or smaller than 10% was hardly noticeable in terms of heat release and weight loss during the coal combustion. This was further proved by non-isothermal kinetic analysis, which was used to evaluate the Arrhenius activation energy corresponding to the co-combustion of the blends. This work shows that thermogravimetric analysis may be used as an easy rapid tool to asses the co-combustion of sewage sludge together with coal.

The behaviour of phosphorus in the flue gas during the combustion of high-phosphate fuels

Cost effectiveness forces operators of coal fired power plants to switch to more economic fuels. This generated a trend in Germany to co-combust secondary waste fuels as an alternative to incineration. The co-combustion of sewage sludge became quite common. Additionally, the combustion of animal residues has been practiced. These fuels, however, are reported to cause operational problems such as increased slagging and damage on the adjacent SCR catalysts. As main reason, the high phosphorus content of these fuels has been identified. These observations were in good agreement with problems reported during the combustion of some PRB coals, which are similar in terms of phosphorus content and compounds. In order to obtain information on the release and the condensation of phosphorus in the boiler, bench scale combustion tests and full-scale measurements were carried out with the focus on the behaviour of phosphorus in the flue gas duct after the boiler. The results gave information on the gas-phase concentration as well as on the concentration of particle bound phosphorus.

05/00786 Metal emissions from co-combustion of sewage sludge and coal/wood in fluidized bed

05/00786 Metal emissions from co-combustion of sewage sludge and coal/wood in fluidized bed

Heavy Metals Behavior during Monocombustion and Co-Combustion of Sewage Sludge

This paper presents the study of the combustion of granular dry sewage sludge performed on a pilot fluidized bed system. The results of monocombustion of sludge and co-combustion with coal were compared with those of coal combustion for ash partitioning, the formation of gaseous pollutants, and heavy metals behavior. It was found that the mineral matter of sludge was essentially retained as bottom ash. The production of fine ash was small during the monocombustion but was high during co-combustion due to the tendency of coal to produce fine ash, which also contained unburned char. The degree of heavy metal volatilization was found to be slightly higher during co-combustion than in monocombustion; however, most of the metals were retained in the ash and their emissions were found to be below the regulated levels. Hg was completely volatilized. However, during combustion trials involving coal, Hg was captured in the cyclone ash at temperatures below 300°C. During sludge monocombustion the retention of Hg in cyclone ash containing low loss on ignition (LOI) was not enough to decrease emissions below the EU regulated levels; hence, it is necessary to install dedicated flue gas treatment for Hg removal. The leachability and ecotoxicity of sludge and ash were compared with the new regulatory limits for landfill disposal in the European Union (EU). It was found that the release of organic matter and heavy metals, found in the sludge, was low from granular bed ash. Hence, except for sulphate release, bed ash was converted into inert and non-ecotoxic materials. Ash from tests with limestone and cyclone ash seemed to be more problematic because of pH effects and contamination with steel corrosion products. The recovery and reutilization of sludge bed ash could, therefore, be possible, as long as the release of sulphate does not interfere with the process.

Emission Characteristics of Co-combustion of Sewage Sludge with Olive Cake and Lignite Coal in a Circulating Fluidized Bed

In this study, a circulating fluidized bed (CFB) of 125 mm diameter and 1800 mm height was used to find the combustion characteristics of sewage sludge (SS) produced in Turkey. Sludge + olive cake, and sludge + lignite coal mixtures were burned separately. Various sludge-to-lignite coal and sludge-to-olive cake ratios (5/95, 10/90, 15/85, 20/80) were tried. On-line concentrations of major components (O2, SO2, CO2, CO, NO x , C m H n ) were measured in the flue gas, as well as temperature and pressure distributions along the bed. Combustion efficiencies of sludge + olive cake and sludge + lignite coal mixtures were calculated, and the optimum conditions for operating parameters were discussed. The results have shown that the combustion mainly takes place in the upper regions of the main column where the temperature reaches 900°C. SS + Coal burn in the CFB with an efficiency of 95.14% to 96.18%, which is considered to be quite good. When burning sludge mixed with olive cake, appreciable amounts of CO and unburned hydrocarbons are formed and the combustion efficiency drops to 92.93%. CO and C m H n emissions are lower when lignite coal is mixed with various amounts of SS than the emissions when the coal is burned alone. As the %SS is increased in the fuel mixture, the SO2 emission decreases. NO x emissions are slightly higher. When burning sludge mixed with olive cake, SO2 and NO x emissions are slightly higher. CO and C m H n emissions decrease sharply when SS is mixed with 5%wt. olive cake. With increasing sludge ratio these emissions increase due to the unburned hydrocarbons. As a result of this study, it is believed that SS can be burned effectively in a CFBC together with other fuels, especially with olive cake (OC). OC will be a good additive fuel for the combustion of lower quality fuels.