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

Transformation of Zn and Cr during co-combustion of sewage sludge and coals: influence of coal and steam

Combustion experiments of sewage sludge (SS) blended with low-rank coal were conducted through a drop tube furnace (DTF) to explore the effects of low-rank coal type, blending ratio, and steam on the transformation of Zn and Cr. The results showed that the retention rates of Zn and Cr in ash increased from 24.35% and 71.49% for sludge combustion alone to 53.77% and 117.49%, respectively, for coal blended to SS with a mass ratio of 7:3. The greater the proportion of low-rank coal in the fuel, the greater the residual rate of heavy metals in the ash. Meanwhile, rapid diffusion of vapor occupied adsorption sites on metal mineral surfaces, reducing the retention of Zn and Cr in the co-combustion ash. The leaching toxicity analysis of ash showed that the co-combustion ash of SS with coal was free from leaching toxicity hazards in simulated scenarios. The extraction rate of Zn in co-combustion ash increased from 90.72% with hydrothermal acid leaching to 95.46% with microwave-assisted in 2 mol/L H2SO4 extract. The extraction rate of Cr in hydrothermal acid leaching was 62.80%, which was much higher than that in microwave-assisted extraction (31.76%).

Thermochemical and Toxic Element Behavior during Co-Combustion of Coal and Municipal Sludge

The thermochemical and kinetic behavior of co-combustion of coal, municipal sludge (MS) and their blends at different ratios were investigated by thermogravimetric analysis. Simulation experiments were performed in a vacuum tube furnace to determine the conversion behavior of toxic elements. The results show that the combustion processes of the blends of coal and municipal sludge are divided into three stages and the combustion curves of the blends are located between those of individual coal and municipal sludge samples. The DTG_max of the sample with 10% sludge addition reaches a maximum at the heating rate of 20 °C/min, indicating that the combustion characteristics of coal can be improved during co-combustion. Strong interactions were observed between coal and municipal sludge during the co-combustion. The volatilization rates of toxic elements decrease with an increasing proportion of sludge in the blends during co-combustion, which indicates that the co-combustion of coal and sludge can effectively reduce the volatilization rate of toxic elements. The study reflects the potential of municipal sludge as a blended fuel and the environmental effects of co-combustion of coal and municipal sludge.

Experimental Study on Co-Firing of Coal and Brewery Wastewater Sludge

The environmental pollution and high energy consumption caused by the coal-dominated energy structure in China have been the focus of attention for a long time. The co-firing of biomass with coal can save coal resources and realize effective utilization of biomass. In this paper, brewery wastewater sludge (SD) and bituminous coal (BC) were blended for an experimental study which aimed to provide basic experimental data and operational guidance as a reference for practical application in power plants. The co-firing characteristics of sludge and bituminous coal were studied. The results show that the burnout temperature and ignition temperature decrease with an increase in the sludge blending ratio. The Comprehensive Combustion Index (CCI) first rises, then decreases, reaching a maximum at about 15%. Compared with the atmosphere with 79% N2/21% O2, under the 79% CO2/21% O2 atmosphere, ignition is delayed and the burnout temperature is higher. Under an O2/CO2 atmosphere, as the O2 concentration improves, the thermo-gravimetric (TG) curve shifts to the low-temperature region, the burnout temperature drops significantly, and the comprehensive combustion characteristics are improved. With an increment of the heating rate, the curve of TG analysis shifts to the high-temperature region and the CCI increases. This study could provide helpful information on practical blending in coal-fired power plants for energy savings and emission reductions.

Co-combustion of industrial coal slurry and sewage sludge: Thermochemical and emission behavior of heavy metals

A combination of thermogravimetric analysis and lab-scale fixed bed combustion experiments was carried out to study the thermochemical, kinetic and heavy metals emission behavior during co-combustion of industrial coal slurry (CS) and sewage sludge (SS). The results found that the blends had integrative combustion profiles which reflected both coal slurry and sewage sludge. During co-combustion, the ignition performance of CS could be significantly improved with the addition of SS. Synergetic effects of the co-combustion were observed at lower temperature, while the high-temperature char combustion of the blends was inhibited because of high ash components of SS or formation of inactive alkali metal aluminosilicates. Kinetic analysis confirmed the improve iginition behavior of blends. Both the comprehensive combustibility index S and the activation energy suggested that the blends with 20% SS may have the best promoting effects. Compared with CS, the higher concentration of Cl in SS increased the volatilization ratios of Cu, Zn, As, and Pb. When added CS into SS, the volatilization ratios of arsenic decreased during combustion. The inhibition effects for arsenic during co-combustion might be associated with the capture of arsenic vapors by the new-formed Ca/Al from CS thermal decomposition.

Effect of blending sewage sludge with coal on combustion and ash slagging behavior

Blending sewage sludge (SS) with Zhundong coal (ZDC) for combustion in coal-fired power plants is a recent approach that can alleviate the shortage of high-quality coal resources and achieve the harmless treatment of SS, while also having a significant influence on combustion and ash slagging. Due to the high content of alkali and alkaline earth metals (AAEMs) in ZDC, its combustion ash has a strong likelihood of slagging. This study aims to investigate the effect of blending SS with ZDC on combustion and ash slagging. Thermogravimetry (TG) results indicate that blending with SS could lower the ignition and burnout temperatures of ZDC. With an increase in the ratio of sludge, the comprehensive combustion index (S) first increases and then decreases, showing that blending SS with ZDC in an appropriate proportion could improve the overall combustion. Through the analysis of the interaction, it is confirmed that SS and ZDC could complement each other during co-combustion due to their different components. X-ray fluorescence (XRF) was used to test the ash components of different blending ratios (10–30%) and combustion temperatures (800–1100 °C). Slagging indices including alkali acid ratio (B/A), silicon ratio (G), and silica–alumina ratio (SiO₂/Al₂O₃) were also calculated. The results suggest that the slagging behavior of ZDC is greatly reduced even if the blending ratio is only 10%. However, with an increase in the blending ratio, the effect on slagging gradually weakens. Considering the dual influence of SS blending on combustion and slagging, this study assumes the optimal blending ratio of 20%. Influenced by the components of the combustion ash, B/A and SiO₂/Al₂O₃ are more suitable for evaluating the slagging tendency of ash; however, there is great deviation in the results for G. This research is beneficial to coal-fired power plants for the selection of operation parameters during co-combustion with SS.

Blending sewage sludge (SS) with Zhundong coal (ZDC) for combustion in coal-fired power plants is a recent approach that can alleviate the shortage of high-quality coal resources and achieve the harmless treatment of SS, while also having a significant influence on combustion and ash slagging.

The influence of liquid plant additives on the anthropogenic gas emissions from the combustion of coal-water slurries

At present, coal is considered one of the main components for the production of cheap, high-energy and environmentally attractive slurry fuels. The latter can be produced on the basis of low-grade coal dust or coal processing wastes. Thus, coal-water slurries and coal-water slurries containing petrochemicals are produced. The involvement of coal and oil processing wastes expands the scope of raw materials, reduces the fuel costs from traditional energy sources and modifies the main economic characteristics of power plant performance. However, it also increases the impact of coal-fired thermal power stations on the environment. In the last 30-50 years, many efforts have been made to decrease the negative impact of human industrial activity on climate. Involving plant-based components in the process of energy generation to save energy and material resources looks very promising nowadays. This research studies the influence of adding typical bioliquids (bioethanol, turpentine, glycerol) on the concentration of anthropogenic emissions from coal-water slurry combustion. Relative mass concentrations of bioliquids varied in a small range below 20%. We focused on the concentration of the most hazardous sulfur and nitrogen oxides from the combustion of typical filter cakes, as well as plant-containing slurries. It was established that the concentration of sulfur oxides can be decreased (as compared to coal) by 75%, whereas that of nitrogen oxides by almost 30%. Using a generalizing criteria expression, we illustrated the main benefits of adding bioliquids to slurry fuels in comparison with coal. Adding 20% of glycerol was found to provide maximum advantages.

Volatility and partitioning of Cd and Pb during sewage sludge thermal conversion

In this paper, the thermal characteristics of sewage sludge and the transformation behavior of Pb and Cd during the thermal conversion process were addressed. The incineration process and pyrolysis process of the sewage sludge were investigated by thermogravimetric analysis. The results indicated that the thermal conversion process of the sewage sludge could be divided into three stages and the presence of oxygen could accelerate the decomposition of the sewage sludge. Furthermore, the effects of thermal conditions on the concentration ratio of Cd and Pb and their species partitioning in the residual char and ash were investigated. For the pyrolysis process, the maximum concentration ratio of Cd reached 41.64% at 500 °C and the lowest one 2.92% at 700 °C. Contrary, the concentration ratio of Pb remained above 93% as the temperature increased. Thus, the suitable temperature for the sewage sludge pyrolysis was below 500 °C. For the incineration process, the incineration temperature had great influence on the concentration ratio of Cd and Pb. When the incineration temperature increased from 700 °C to 900 °C, the concentration ratio of Cd decreased drastically from 99.32% to 10.96%. The maximum concentration ratio for Pb (95.31%) was reached at 800 °C. Besides, the lowest concentration ratio of Cd and Pb were obtained at a residence time of 30 min. The partitioning analyses of the Cd and Pb contained in the ash showed that the residence time had little effect on the partitioning of Cd and Pb, and the residual fractions of Cd and Pb were both above 90%. It was concluded that Cd and Pb were properly stabilized in the ash. Thus, Cd and Pb in the ash were difficult to be released into the environment and to cause secondary pollution.

Combustion behaviors and kinetics of sewage sludge blended with pulverized coal: With and without catalysts

The combustion behaviors of sewage sludge (SS), pulverized coal (PC), and their blends were studied using a thermogravimetric analyzer. The effect of the mass ratio of SS to PC on the co-combustion characteristics was analyzed. The experiments showed that the ignition performance of the blends improved significantly as the mass percentage of SS increased, but its combustion intensity decreased. The burnout temperature (T_b) and comprehensive combustibility index (S) of the blends were almost unchanged when the mass percentage of SS was less than 10%. However, a high mass percentage of SS (>10%) resulted in a great increase in T_b and a notable decrease in S. Subsequently, the effects of different catalysts (CaO, CeO₂, MnO₂, and Fe₂O₃) on the combustion characteristics and activation energy of the SS/PC blend were investigated. The four catalysts promoted the release and combustion of volatile matters in the blended fuels and shifted their combustion profiles to a low temperature. In addition, their peak separating tendencies were obvious at 350-550 C, resulting in high peak widths. All the catalysts improved combustion activity of the blended fuel and accelerated fixed carbon combustion, which decreased the ignition temperature and burnout temperature of the fuels. CeO₂ had the best catalytic effects in terms of the comprehensive combustion performance and activation energy, followed closely by Fe₂O₃. However, the rare-earth compounds are expensive to be applied in the catalytic combustion process of SS/PC blend at present. Based on both catalytic effects and economy, Fe₂O₃ was potentially an optimal option for catalytic combustion among the tested catalysts.

Problematic issues of air protection during thermal processes related to the energetic uses of sewage sludge and other waste. Case study: Co-combustion in peaking power plant

Currently, it is necessary to deal with issues related to the emissions as there is a constantly increasing interest in combusting sludge from sewage treatment plants in the boilers for wood. An analysis of the energetic importance of the combustion of sewage sludge has already been carried out, but the effects of various treatments of the sludge are not always clear, e.g. composting and subsequent combustion to the air pollution. Investments in other thermal processes of energetic utilisation of sewage sludge and organic waste are not always successfully implemented. The objective of this paper is to point out some problematic cases for acceptance of thermal processes related to energetic use of waste in terms of the air protection. The other aim is to mention the experience with solutions of such issues in Slovakia. There are mentioned first results of the operational validation experiments during the energy generation in circulating fluidized bed boiler in peaking power plant (Power 110MW) with the addition of the so-called alternative fuel based on wood and sewage sludge to the main fuel - black coal (anthracite). And there has already been achieved the highest share of 12.4%w. (dry matter) of sewage sludge in form of compost in blend with black coal, which is technologically viable. Moreover analyzed the problems of the authorization and operation of the co-combustion of sewage sludge and of combustion of products of various kinds of pyrolysis waste - pyrolysis gas and pyrolysis oil are analyzed.

Combustion characteristics of biodried sewage sludge

In this study, effects of biodrying on the characteristics of sewage sludge and the subsequent combustion behavior were investigated. 7-Day of biodrying removed 49.78% of water and 23.17% of VS initially contained in the sewage sludge and increased lower heating value (LHV) by 37.87%. Meanwhile, mass contents of C and N decreased from 36.25% and 6.12% to 32.06% and 4.82%, respectively. Surface of the biodried sewage sludge (BDSS) appeared granulated and multi-porous, which was thought to facilitate air transfer during combustion. According to thermogravimetric (TG) analysis coupled with mass spectrometer (MS) with a heating rate of 10 °C/min from 35 °C to 1000 °C, thermally-dried sewage sludge (TDSS) and BDSS lost 74.39% and 67.04% of the initial mass, respectively. In addition, combustibility index (S) of BDSS (8.67 × 10^-8 min^-2 K^-3) was higher than TDSS. TG-MS analyses also showed that less nitrogenous gases were generated from BDSS than TDSS. It was again showed that the average CO and NO concentrations in exit gas from isothermal combustion of BDSS were lower than those from TDSS, especially at low temperatures (≤800 °C). Based on these results, it was concluded that biodrying of sewage sludge was an energy-efficient water-removal method with less emission of air pollutants when BDSS was combusted.

Quantitative detection of powdered activated carbon in wastewater treatment plant effluent by thermogravimetric analysis (TGA)

For the elimination of potentially harmful micropollutants, powdered activated carbon (PAC) adsorption is applied in many wastewater treatment plants (WWTP). This holds the risk of PAC leakage into the WWTP effluent and desorption of contaminants into natural water bodies. In order to assess a potential PAC leakage, PAC concentrations below several mg/L have to be detected in the WWTP effluent. None of the methods that are used for water analysis today are able to differentiate between activated carbon and solid background matrix. Thus, a selective, quantitative and easily applicable method is still needed for the detection of PAC residues in wastewater. In the present study, a method was developed to quantitatively measure the PAC content in wastewater by using filtration and thermogravimetric analysis (TGA), which is a well-established technique for the distinction between different solid materials. For the sample filtration, quartz filters with a temperature stability up to 950 °C were used. This allowed for sensitive and well reproducible measurements, as the TGA was not affected by the presence of the filter. The sample's mass fractions were calculated by integrating the mass decrease rate obtained by TGA in specific, clearly identifiable peak areas. A two-step TGA heating method consisting of N2 and O2 atmospheres led to a good differentiation between PAC and biological background matrix, thanks to the reduction of peak overlapping. A linear correlation was found between a sample's PAC content and the corresponding peak areas under N2 and O2, the sample volume and the solid mass separated by filtration. Based on these findings, various wastewater samples from different WWTPs were then analyzed by TGA with regard to their PAC content. It was found that, compared to alternative techniques such as measurement of turbidity or total suspended solids, the newly developed TGA method allows for a quantitative and selective detection of PAC concentrations down to 0.1 mg/L. The method showed a linearity coefficient of 0.98 and relative standard deviations of 10%, using small water sample volumes between 0.3 and 0.6 L.

Combustion characteristics of paper and sewage sludge in a pilot-scale fluidized bed

This study characterizes the combustion of paper and sewage sludge in a pilot-scale fluidized bed. The highest temperature during combustion within the system was found at the surface of the fluidized bed. Paper sludge containing roughly 59.8% water was burned without auxiliary fuel, but auxiliary fuel was required to incinerate the sewage sludge, which contained about 79.3% water. The stability of operation was monitored based on the average pressure and the standard deviation of pressure fluctuations. The average pressure at the surface of the fluidized bed decreased as the sludge feed rate increased. However, the standard deviation of pressure fluctuations increased as the sludge feed rate increased. Finally, carbon monoxide (CO) emissions decreased as oxygen content increased in the flue gas, and nitrogen oxide (NOx) emissions were also tied with oxygen content.

Analysis of the combustion and pyrolysis of dried sewage sludge by TGA and MS

In this study, the combustion and pyrolysis processes of three sewage sludge were investigated. The sewage sludge came from three wastewater treatment plants. Proximate and ultimate analyses were performed. The thermal behaviour of studied sewage sludge was investigated by thermogravimetric analysis with mass spectrometry (TGA-MS). The samples were heated from ambient temperature to 800 °C at a constant rate 10 °C/min in air (combustion process) and argon flows (pyrolysis process). The thermal profiles presented in form of TG/DTG curves were comparable for studied sludges. All TG/DTG curves were divided into three stages. The main decomposition of sewage sludge during the combustion process took place in the range 180-580 °C with c.a. 70% mass loss. The pyrolysis process occurred in lower temperature but with less mass loss. The evolved gaseous products (H2, CH4, CO2, H2O) from the decomposition of sewage sludge were identified on-line.

Study of thermal pre-treatment on anaerobic digestion of slaughterhouse waste by TGA-MS and FTIR spectroscopy

Thermogravimetric analysis coupled to mass spectrometry (TGA-MS) and Fourier-transform infrared spectroscopy (FTIR) were used to describe the effect of pasteurization as a hygienic pre-treatment of animal by-products over biogas production. Piggery and poultry meat wastes were used as substrates for assessing the anaerobic digestion under batch conditions at mesophilic range. Poultry waste was characterized by high protein and carbohydrate content, while piggery waste presented a major fraction of fat and lower carbohydrate content. Results from anaerobic digestion tests showed a lower methane yield for the pre-treated poultry sample. TGA-MS and FTIR spectroscopy allowed the qualitative identification of recalcitrant nitrogen-containing compounds in the pre-treated poultry sample, produced by Maillard reactions. In the case of piggery waste, the recalcitrant compounds were not detected and its biodegradability test reported higher methane yield and production rates. TGA-MS and FTIR spectroscopy were demonstrated to be useful tools for explaining results obtained by anaerobic biodegradability test and in describing the presence of inhibitory problems.

Combustion characteristics of Malaysian oil palm biomass, sub-bituminous coal and their respective blends via thermogravimetric analysis (TGA)

The combustion characteristics of Malaysia oil palm biomass (palm kernel shell (PKS), palm mesocarp fibre (PMF) and empty fruit bunches (EFB)), sub-bituminous coal (Mukah Balingian) and coal/biomass blends via thermogravimetric analysis (TGA) were investigated. Six weight ratios of coal/biomass blends were prepared and oxidised under dynamic conditions from temperature 25 to 1100°C at four heating rates. The thermogravimetric analysis demonstrated that the EFB and PKS evolved additional peak besides drying, devolatilisation and char oxidation steps during combustion. Ignition and burn out temperatures of blends were improved in comparison to coal. No interactions were observed between the coal and biomass during combustion. The apparent activation energy during this process was evaluated using iso-conversional model free kinetics which resulted in highest activation energy during combustion of PKS followed by PMF, EFB and MB coal. Blending oil palm biomass with coal reduces the apparent activation energy value.

Volatilization behavior of Cd and Zn based on continuous emission measurement of flue gas from laboratory-scale coal combustion

The accumulation of toxic metals generated by coal-fired power stations presents a serious threat to the environment. The volatilization behavior of two representative metals (Cd and Zn), and the influence of temperature were investigated during coal combustion. An inductively coupled plasma atomic emission spectrometric (ICP-AES) method was developed to continuously measure the heavy metal concentrations quantitatively in flue gas under combustion conditions in order to track the metal release process. This continuous heavy metal analysis system was implemented by coupling it to two types of high temperature reactors: a bubbling fluidized bed reactor and a fixed bed reactor with diameter of 0.1 m and 0.08 m respectively. For the two metals considered in this study (Cd and Zn), the experimental setup was successfully used to continuously monitor the metal vaporization process during coal combustion independent of reactor design, and at different temperatures. Cd is more easily vaporized than Zn during coal combustion. Temperature significantly influences the metal vaporization process. In general, the higher the temperature, the higher the metal vaporization, although the vaporization is not proportional to temperature. In addition to the experimental study, a thermodynamic calculation was carried out to simulate the heavy metal speciation during coal combustion process. The theoretical volatilization tendency is consistent with the experiment. The thermodynamic calculation identified the formation of binary oxides retarding heavy metal vaporization.

Technological options for the management of biosolids

BACKGROUND, AIM, AND SCOPE

Large quantities of biosolids (sewage sludge), which are produced from municipal wastewater treatment, are ever-increasing because of the commissioning of new treatment plants and continuous upgrades of the existing facilities. A large proportion of biosolids are currently landfilled. With increasing pressure from regulators and the general public, landfilling of biosolids is being phased out in many countries because of potential secondary pollution caused by leachate and the emission of methane, a potent greenhouse gas. Biosolids contain nutrients and energy that can be used beneficially. Significant efforts have been made recently to develop new technologies to manage biosolids and make useful products from them. In this paper, we provide a review of the technologies in biosolids management.

MATERIALS AND METHODS

A survey of literature was conducted.

RESULTS

At present, the most common beneficial use of biosolids is agricultural land application because of inherent fertilizer values found in biosolids. Expansion of land application, however, may be limited in the future because of more stringent regulatory requirements and public concern about food chain contamination in some countries. Perceived as a green energy source, the combustion of biosolids has received renewed interest. Anaerobic digestion is generally a more effective method than incineration for energy recovery, and digested biosolids are suitable for further beneficial use through land application. Although conventional incineration systems for biosolid management generally consume more energy than they produce because of the high moisture content in the biosolids, it is expected that more combustion systems, either monocombustion or cocombustion, will be built to cope with the increasing quantity of biosolids.

DISCUSSION

Under the increasingly popular low-carbon economy policy, biosolids may be recognized as a renewable fuel and be eligible for 'carbon credits'. Because ash can be used to manufacture construction materials, combustion can provide a complete management for biosolids. A number of advanced thermal conversion technologies (e.g., supercritical water oxidation process and pyrolysis) are under development for biosolids management with a goal to generate useful products, such as higher quality fuels and recovery of phosphorus. With an ever-increasing demand for renewable energy, growing bioenergy crops and forests using biosolids as a fertilizer and soil amendment can not only contribute to the low-carbon economy but also maximize the nutrient and carbon value of the biosolids.

CONCLUSIONS

Land application of biosolids achieves a complete reuse of its nutrients and organic carbon at a relatively low cost. Therefore, land application should become a preferred management option where there is available land, the quality of biosolids meet regulatory requirements, and it is socially acceptable. Intensive energy cropping and forest production using biosolids can help us meet the ever-increasing demand for renewable energy, which can eliminate the contamination potential for food sources, a common social concern about land application of biosolids. In recent years, increasing numbers of national and local governments have adopted more stringent regulations toward biosolid management. Under such a political climate, biosolids producers will have to develop multireuse strategies for biosolids to avoid being caught because a single route management practice might be under pressure at a short notice. Conventional incineration systems for biosolids management generally consume more energy than they produce and, although by-products may be used in manufacturing, this process cannot be regarded as a beneficial use of biosolids. However, biosolids are likely to become a source of renewable energy and produce 'carbon credits' under the increasingly popular, low-carbon economy policy.

RECOMMENDATIONS AND PERSPECTIVES

To manage biosolids in a sustainable manner, there is a need for further research in the following areas: achieving a higher degree of public understanding and acceptance for the beneficial use of biosolids, developing cost-efficient and effective thermal conversions for direct energy recovery from biosolids, advancing technology for phosphorus recovery, and selecting or breeding crops for efficient biofuel production.