Torrefaction Of Sewage Sludge Research Articles

Pine Wood and Sewage Sludge Torrefaction Process for Production Renewable Solid Biofuels and Biochar as Carbon Carrier for Fertilizers

This work presents the results of research on the thermo-chemical conversion of woody biomass–pine wood coming from lodzkie voivodship forests and sewage sludge from the Group Sewage Treatment Plant of the Łódź Urban Agglomeration. Laboratory scale analyses of the carbonization process were carried out, initially using the TGA technique (to assess activation energy (EA)), followed by a flow reactor operating at temperature levels of 280–525 °C. Both the parameters of carbonized solid biofuel and biochar as a carrier for fertilizer (proximate and ultimate analysis) and the quality of the torgas (VOC) were analyzed. Analysis of the pine wood and sewage sludge torrefaction process shows clearly that the optimum process temperature would be around 325–350 °C from a mass loss ratio and economical perspective. This paper shows clearly that woody biomass, such as pine wood and sewage sludge, is a very interesting material both for biofuel production and in further processing for biochar production, used not only as an energy carrier but also as a new type of carbon source in fertilizer mixtures.

Mercury emission profile for the torrefaction of sewage sludge at a full-scale plant and application of polymer sorbent

We evaluated mercury (Hg) behavior in a full-scale sewage sludge torrefaction plant with a capacity of 150 wet tons/day, which operates under a nitrogen atmosphere at a temperature range of 250–350 °C. Thermodynamic calculations and monitoring results show that elemental Hg (Hg0) was the dominant species in both the pyrolysis gas during the torrefaction stage and in the flue gas from downstream air pollution control devices. A wet scrubber (WS) effectively removed oxidized Hg from the flue gas and moved Hg to wastewater, and an electrostatic precipitator (ESP) removed significant particulate-bound Hg but showed a limited capacity for overall Hg removal. Hg bound to total suspended solids had a much higher concentration than that of dissolved Hg in wastewater. Total suspended solid removal from wastewater is therefore recommended to reduce Hg discharge. Existing air pollution control devices, which consist of a cyclone, WS, and ESP, are not sufficient for Hg removal due to the poor Hg0 removal performance of the WS and ESP; a further Hg0 removal unit is necessary. A commercial packed tower with sorbent polymer catalyst composite material was effective in removing Hg (83.3%) during sludge torrefaction.

Energy recovery from sewage sludge: Product characteristics, heating value prediction and reaction kinetics

Energy recovery from sewage sludge was carried out by using microwave and conventional torrefaction. The microwave torrefaction was carried out by using a laboratory-scale microwave oven that provides single-mode microwave irradiation at 2.45 GHz, and the amount of sewage sludge for each experiment was approximately 20 g. The efficiency of microwave heating can be substantially promoted at higher power level, resulting in higher heating rate and maximum temperature. According to higher energy yield and heating value of torrefied sewage sludge, the optimum power level for bioenergy produced by microwave torrefaction of sewage sludge should be 200 W. Because of lower mass yield and temperature required to obtain the same yield, microwave heating can be more effective than conventional heating for sewage sludge torrefaction. The elemental composition of torrefied sewage sludge at 400 W was similar to that of anthracite, and its low hydrogen and oxygen contents could prevent excessive formation of smoke. Two correlations were obtained to predict the HHV of SS based on proximate and elemental compositions. With the recovery of liquid and gas products as bioenergy, the energy return on investment for microwave torrefaction of sewage sludge can be up to 16.4, much higher than the minimum value required for a sustainable society. Because of lower activation energy but higher pre-exponential factor, microwave heating can be approximately five times faster than conventional heating.

Is Biochar from the Torrefaction of Sewage Sludge Hazardous Waste?

Improved technologies are needed for sustainable management of sewage sludge (SS). The torrefaction (also known as biomass “roasting”) is considered a pretreatment of SS before use in agriculture. However, it is not known whether the torrefaction has the potential to decrease heavy metals’ (HMs) leachability and the SS toxicity. Thus, the aim of the study was to evaluate the influences of the SS torrefaction parameters (temperature and process time) on HM contents in biochar, HM leachability, and biochar toxicity, and compare them with raw SS. The experiments were designed in 18 combinations (six temperatures, 200, 220, 240, 260, 280, and 300 °C; and three process times—20, 40, 60 min). Standard tests were used to determine HMs content, leachability, and toxicity. Results indicated that the torrefaction did not increase (p < 0.05) the HM content in comparison to the raw SS. The leachability of Zn, Ni, Cu, Cr, and Mn from SS biochars was similar to raw SS. However, the degree of leachability varied significantly (p < 0.05) from as low as 0.1% for Cu to high as 16.7% for Cd. The leachability of Cd (<16.7%) and Pb (<11.9%) from biochars was higher than from raw SS (<6.1% and <2.4%, respectively). The leachability of Cd from SS biochar, in five torrefaction combinations, was higher than the threshold value for hazardous waste. It is recommended that site-specific decisions are made for torrefaction of SS with respect to its HM content, as the resulting biochar could be considered as hazardous waste, depending on the feedstock. Moreover, the biochar produced under the whole range of temperatures during 20 min retention time significantly (p < 0.05) increased the Daphnia magna Straus mobility inhibition by up to 100% in comparison to the biochar obtained during 40 and 60 min torrefaction. Taking into account the increased leachability of specific HMs and D. magna Straus mobility inhibition, biochar should be considered a potentially hazardous material. Future research should focus on biochar dosage as a fertilizer in relation to its toxicity. Additional research is warranted to focus on the optimization of SS torrefaction process parameters affecting the toxicity.

Impact of torrefaction on biomass properties depending on temperature and operation time

Torrefaction is an effective way to upgrade biomass for producing fuels. The experimental results of torrefaction for three materials, oak wood, mixed wood mainly from deciduous trees and sewage sludge are presented. The comparison between three materials is performed to evaluate the influence of temperature and time on torrefaction operation. The influence of the operating temperature and time was analysed in order to determine optimal operation parameters for the newly developed process which has been patented. Properties, such as heating value, mass loss, chemical compositions, energy yield and enhancement factor were investigated. The results show that from an energy point of view the optimal operation time for oak and mixed wood is around 1.2 h at 260 °C. The torrefaction of sewage sludge is energetically unjustified. The highest carbon loss is shown for mixed wood, following by sewage sludge and oak wood. Torrefaction severity index was established based on the, most severe conditions. Torrefaction severity index could be applied as an indicator for prediction of torrefaction efficiency of chosen material.

Isothermal torrefaction kinetics for sewage sludge pretreatment

Torrefaction of sewage sludge, a by-product from waste water treatment plants, has been receiving increased attentions in terms of sustainable sludge treatment, energy recovery and possibility to mitigate the environmental impacts. Torrefied sewage sludge is recognized as a better solid fuel than the dry sewage sludge, and it is also beneficial for subsequent thermochemical processes. In this study, sewage sludge was subjected to different torrefaction conditions in a thermogravimetric analyzer. The thermogravimetric data were then simulated using a two-consecutive reaction model to reproduce the mass losses during torrefaction. The obtained results from the torrefaction of the non-lignocellulosic biomass are also compared with other lignocellulosic biomass including a softwood and a hardwood. The findings from this study reveal that the non-lignocellulosic biomass is less thermally resistant and degrades much faster than the lignocellulosic biomass during the first stage of torrefaction. Moreover, the solid yield from the sewage sludge torrefaction at temperatures less than 280 °C is lower than that from the woody biomass torrefaction. However, this trend is inverted when the torrefaction temperature attains 280 °C and higher. The results obtained are useful for new design or retrofit of the sewage sludge torrefaction processes.

Waste-to-Carbon: Is the Torrefied Sewage Sludge with High Ash Content a Better Fuel or Fertilizer?

Sewage sludge (SS) recycling is an important part of the proposed ‘circular economy’ concept. SS can be valorized via torrefaction (also known as ‘low-temperature pyrolysis’ or ‘roasting’). SS can, therefore, be considered a low-quality fuel or a source of nutrients essential for plant growth. Biochar produced by torrefaction of SS is a form of carbonized fuel or fertilizer. In this research, for the first time, we tested the feasibility of torrefaction of SS with high ash content for either fuel or organic fertilizer production. The research was conducted in 18 variants (six torrefaction temperatures between 200~300 °C, and three process residence times of 20, 40, 60 min) in 5 repetitions. Fuel and fertilizer properties and multiple regression analysis of produced biochar were conducted. The higher heating value (HHV) of raw SS was 21.2 MJ·kg−1. Produced biochar was characterized by HHV up to 12.85 MJ·kg−1 and lower H/C and O/C molar ratio. Therefore, torrefaction of SS with high ash content should not be considered as a method for improving the fuel properties. Instead, the production of fertilizer appears to be favorable. The torrefaction increased C, N, Mg, Ca, K, Na concentration in relation to raw SS. No significant (p < 0.05) influence of the increase of temperature and residence time on the increase of biogenic elements in biochar was found, however the highest biogenic element content, were found in biochar produced for 60 min, under the temperature ranging from 200 to 240 °C. Obtained biochars met the Polish regulatory criteria for mineral-organic fertilizer. Therefore SS torrefaction may be considered a feasible waste recycling technology. The calculation of torrefaction energy and the mass balance shows energy demand <2.5 GJ∙Mg−1 w.m., and the expected mass yield of the product, organic fertilizer, is ~178 kg∙Mg−1 w.m of SS. Further investigation should consider the scaling-up of the SS torrefaction process, with the application of other types of SSs.

The Staged Thermal Conversion of Sewage Sludge in the Presence of Oxygen

This study compares a staged thermal processing of the sewage sludge, with single step, integrated thermal processing. The aim of this study is to find the optimal conditions for drying and subsequently for carbonization/torrefaction of sewage sludge, regarding the energy consumption. This study presents the results of the drying tests performed at laboratory scale convective dryer for different parameters of drying agent (air). The tests were focused on finding and developing a method of drying that allows to minimize the energy consumption. Subsequently, both dry and vapothermal torrefaction was performed in the presence of oxygen. The kinetics of drying, using low quality heat as well as the properties of products and by-products of torrefaction in both regimes were determined. The process was characterized by mass yield and energy yield in both of the cases. There has been only scarce amount of literature studies published on the torrefaction of sewage sludge so far, without a detailed study of the composition of the torgas and tars of such origin. Performed study enables a comparison of two distinct scenarios of the processing, i.e., drying followed by dry torrefaction with a single stage of vapothermal torrefaction.

Torrefaction of Sewage Sludge: Kinetics and Fuel Properties of Biochars

We propose a ‘Waste to Carbon’ thermal transformation of sewage sludge (SS) via torrefaction to a valuable product (fuel) with a high content of carbon. One important, technological aspect to develop this concept is the determination of activation energy needed for torrefaction. Thus, this research aimed to evaluate the kinetics of SS torrefaction and determine the effects of process temperature on fuel properties of torrefied products (biochars). Torrefaction was performed using high ash content SS at six (200~300 °C) temperatures and 60 min residence (process) time. Mass loss during torrefaction ranged from 10~20%. The resulting activation energy for SS torrefaction was ~12.007 kJ·mol−1. Initial (unprocessed) SS higher heating value (HHV) was 13.5 MJ·kg−1. However, the increase of torrefaction temperature decreased HHV from 13.4 to 3.8 MJ·kg−1. Elemental analysis showed a significant decrease of the H/C ratio that occurred during torrefaction, while the O/C ratio fluctuated with much smaller differences. Although the activation energy was significantly lower compared with lignocellulosic materials, low-temperature SS torrefaction technology could be explored for further SS stabilization and utilization (e.g., dewatering and hygienization).

Torrefaction of sewage sludge by using microwave heating

In this study, sewage sludge was torrefied by using microwave heating. Both maximum temperature and average heating rate increased with increasing microwave power level. The higher heating value of the sewage sludge torrefied at 200 W was almost as twice as those torrefied at 350–400 W. Besides, mass and energy yields both decreased with increasing microwave power level. According to the high mass and energy yields and the high heating value of torrefied sewage sludge, the microwave power level of 200 W should be optimum for the production of bioenergy from sewage sludge. The mass yield of microwave heating was substantially lower than that of conventional heating, which should be attributable to the more intense reaction accomplished by microwave irradiation. The energy return on investment for microwave torrefaction of sewage sludge with the recovery of gaseous and liquid products can be up to 16.4, so the technique is energetically feasible for the production of bioenergy.

Application of torrefaction for recycling bio-waste formed during anaerobic digestion

The problem of waste recycling becomes more serious every year. As the population grows, the amount of organic bio-waste is increased proportionally. Even the subsequent energy utilization of such bio-waste leaves a huge amount of unprocessed organic mass, as in the case of anaerobic digestion. In this paper, the results of experimental studies of the torrefaction of sewage sludge and animal waste (mixture of horse manure and crop waste) after anaerobic digestion are presented. The aim of this work was to investigate the possibility of applying torrefaction to improve the thermotechnical properties of bio-waste products. The research results showed that the torrefaction allows to improve heating value and hygroscopicity, but with increasing of process temperature these values grow insignificantly. In addition, the conclusion about the ash content value of the bio-waste in which it is effectually to apply torrefaction treatment was made.

Torrefaction as a Valorization Method Used Prior to the Gasification of Sewage Sludge

The gasification and torrefaction of sewage sludge have the potential to make the thermal utilization of sewage sludge fully sustainable, thus limiting the use of expensive fossil fuels in the process. This includes sustainability in terms of electricity consumption. Although a great deal of work has been performed so far regarding the gasification of sewage sludge and some investigations have been performed in the area of its torrefaction, there is still a gap in terms of the influence of the torrefaction of the sewage sludge on its subsequent gasification. This study presents the results from the torrefaction tests, performed on a pilot scale reactor, as well as two consecutive steam gasification tests, performed in an allothermal fixed bed gasifier, in order to determine if torrefaction can be deemed as a primary method of the reduction of tar content for the producer gas, from the aforementioned gasification process. A comparative analysis is performed based on the results obtained during both tests, with special emphasis on the concentrations of condensable compounds (tars). The obtained results show that the torrefaction of sewage sludge, performed prior to gasification, can indeed have a positive influence on the gas quality. This is beneficial especially in terms of the content of heavy tars with melting points above 40 °C.

Novel Concept of an Installation for Sustainable Thermal Utilization of Sewage Sludge

This study proposes an innovative installation concept for the sustainable utilization of sewage sludge. The aim of the study is to prove that existing devices and technologies allow construction of such an installation by integration of a dryer, torrefaction reactor and gasifier with engine, thus maximizing recovery of the waste heat by the installation. This study also presents the results of drying tests, performed at a commercial scale paddle dryer as well as detailed analysis of the torrefaction process of dried sewage sludge. Both tests aim to identify potential problems that could occur during the operation. The scarce literature studies published so far on the torrefaction of sewage sludge presents results from batch reactors, thus giving very limited data of the composition of the torgas. This study aims to cover that gap by presenting results from the torrefaction of sewage sludge in a continuously working, laboratory scale, isothermal rotary reactor. The study confirmed the feasibility of a self-sustaining installation of thermal utilization of sewage sludge using low quality heat. Performed study pointed out the most favorable way to use limited amounts of high temperature heat. Plasma gasification of the torrefied sewage sludge has been identified that requires further studies.

Microwave torrefaction of sewage sludge and leucaena

Abstract This study attempted to investigate and compare sewage sludge and leucaena torrefaction using a single-mode microwave oven. To discuss reactivity, product characteristics, and energy recovery, microwave torrefaction of the two feedstocks was carried out at various microwave power levels. Besides, thermal properties of raw feedstocks and torrefied products were observed by the deconvolution of derivative thermogravimetric profiles. The volatile matter contents of the two biomass feedstocks were substantially reduced at relatively low microwave power levels. Because of high heating value and fuel ratio as well as low atomic H/C and O/C ratios, the biochar could replace coal. Compared with sewage sludge, microwave torrefaction of leucaena produced thermally stable biochar at lower microwave power levels, which means that the microwave heating performance of leucaena is better. Compared with conventional torrefaction, mass and energy yields of microwave torrefaction were lower, which could be attributable to the more severe reaction accomplished by microwave heating. The atomic H/C and O/C ratios of leucaena biochar were close to those of anthracite, so combustion or gasification of the biochar could form less smoke. After microwave torrefaction at relatively high microwave power levels, a high HHV (dry ash-free basis) of sewage sludge biochar was found.

Is the biochar produced from sewage sludge a good quality solid fuel?

Abstract The influence of sewage sludge torrefaction temperature on fuel properties was investigated. Non-lignocellulosic waste thermal treatment experiment was conducted within 1 h residence time, under the following temperatures: 200, 220, 240, 260, 280 and 300°C. Sawdust was used as lignocellulosic reference material. The following parameters of biochar have been measured: moisture, higher heating value, ash content, volatile compounds and sulfur content. Sawdust biochar has been confirmed to be a good quality solid fuel. High ash and sulfur content may be an obstacle for biochar energy reuse. The best temperature profile for sawdust torrefaction and fuel production for 1 h residence time was 220°C. At this temperature the product contained 84% of initial energy while decreased the mass by 25%. The best temperature profile for sewage sludge was 240°C. The energy residue was 91% and the mass residue was 85%. Higher temperatures in both cases caused excessive mass and energy losses.

Nonisothermal torrefaction kinetics of sewage sludge using the simplified distributed activation energy model

Sewage sludge can be converted into bio-material or bio-fuel by means of thermal–chemical technique. Torrefaction characteristics of municipal sewage sludge were investigated based on a thermogravimetric analysis technique in nitrogen atmosphere. The whole torrefaction process could be divided into two regions, which referred to coexistence and devolatilization. Moreover, the torrefaction kinetics of sewage sludge was evaluated based on the distributed activation energy models with differential and integral methods. There was a linear dependency between activation energy and pre-exponential factor at different conversion levels. The Gauss-Peak function was employed to depict the distribution curves of the activation energies for both methods. The distributed activation energy model developed by the integral method predicted accurately the conversion degree of the sewage sludge torrefaction, whereas the model developed by the differential method only had a good prediction for high conversion degrees.

A study on torrefaction of sewage sludge to enhance solid fuel qualities

Torrefaction is a treatment which serves to improve the properties of biomass in relation to thermochemical processing techniques for energy generation. In this study, the torrefaction of sewage sludge, which is a non-lignocellulosic waste was investigated in a horizontal tubular reactor under nitrogen flow at temperature ranging from 150 to 400°C, for torrefaction residence time varying from 0 to 50min. The torrefaction kinetics of sewage sludge was studied to obtain the kinetic parameters. The torrefied sewage sludge products were characterized in terms of their elemental composition, energy yield, ash content and volatile fraction. The energy and mass yields decreased with an increase in the torrefaction temperature. From an elemental analysis, the weight percentage of carbon in the sewage sludge increased with an increase in the torrefaction temperature. On the other hand, the weight percentages of hydrogen and oxygen tended to decrease. The gaseous products from torrefaction of sewage sludge were also analyzed. From this work, it was found that the compounds with oxygen were emitted at a temperature lower than that for hydrocarbon gases and the temperatures of 300–350°C were the optimum torrefaction temperatures for sewage sludge.

Sewage Sludge Torrefaction in an Auger Reactor

A lab-scale auger reactor was used for the study of dry sewage sludge torrefaction. The influence of the torrefaction temperature (between 250 and 300 °C) and the solid residence time (between 13 and 35 min) on the product distribution and properties was investigated. The results have shown that both parameters affect dry sewage sludge torrefaction products to a similar extent within the ranges of study. The yield of torrefied sewage sludge decreases when increasing the torrefaction temperature and the solid residence time, while the yields of liquid and noncondensable gases show the opposite trend. Carbon dioxide and hydrogen sulfide are the major noncondensable products. The yield of water is higher than the initial moisture content of sewage sludge. Organic compounds are also released during torrefaction, especially under severe conditions. Torrefaction liquid separates into an organic phase and an aqueous phase. The former is rich in oxygen-containing aliphatic compounds and steroids and their derivat...

Sewage sludge torrefaction in a fluidized bed reactor

Torrefaction of sewage sludge in a lab-scale fluidized bed reactor was studied in order to know if this pre-treatment could enhance the properties of this waste in subsequent thermochemical processing, such as pyrolysis. The influence was studied of two important torrefaction operational parameters, temperature (220–320°C) and solid residence time (3.6–10.2min), on the product distribution and properties. Taking into account the operation conditions evaluated in this work, torrefaction temperature affects solid product properties at long solid residence times (longer than 6.1min) and that the effect of solid residence time is only significant at the highest temperature (320°C). Severe torrefaction conditions result in the release of bonded water which could enhance some properties of the liquid obtained in the ensuing pyrolysis process. However, the torrefaction pre-treatment also implies that part of the extractives is lost from the raw material. Compared to dry raw sewage sludge, the energy density of sewage sludge after torrefaction increases under certain conditions. The removal of H2O and CO2 during the torrefaction step reduces the O/C ratio in the torrefied solid up to 0.12 (66.70% reduction compared to 0.37 in raw sewage sludge) which could be a benefit for subsequent thermochemical treatments. For example, one of the main drawbacks of pyrolysis liquids is their high oxygen content.