Gasification Of Sewage Sludge Research Articles

Polycyclic aromatic hydrocarbon formation during the gasification of sewage sludge in sub- and supercritical water: Effect of reaction parameters and reaction pathways

The formation of polycyclic aromatic hydrocarbon is a widespread issue during the supercritical water gasification of sewage sludge, which directly reduces the gasification efficiency and restricts the technology practical application. The changes of the concentrations and forms as well as the synthesis rate of polycyclic aromatic hydrocarbons in the residues from supercritical water gasification of dewatered sewage sludge were investigated to understand influence factors and the reaction pathways. Results showed that the increase of reaction temperature during the heating period favours directly concentration of polycyclic aromatic hydrocarbon (especially higher-molecular-weight), especially when it raise above 300 °C. Lower heating and cooling rate essentially extend the total reaction time. Higher polycyclic aromatic hydrocarbon concentration and higher number of rings were generally promoted by lower heating and cooling rate, longer reaction time and higher reaction temperature. The lower-molecular-weight polycyclic aromatic hydrocarbons can be directly generated through the decomposition of aromatic-containing compounds in sewage sludge, as well as 3-ring and 4-ring polycyclic aromatic hydrocarbons can be formed by aromatization of steroids. Possible mechanisms of reaction pathways of supercritical water gasification of sewage sludge were also proposed.

Sewage sludge gasification in a bench scale rotary kiln

The goal of this work is to investigate the potentiality of energetic recovery by sewage sludge gasification. Experimental gasification tests were carried out in a bench scale rotary kiln under simulated autothermal condition. The mass rate of sludge was in the range 170–260 g/h. The gasification temperature was varied from 750 to 850 °C, equivalence ratio was increased up to 0.24. The tests run for about 3 h. Input and output streams of the process were quantified and analyzed as to assess the cold gas efficiency. Tar content in the raw gas was determined as well. The main drawbacks occurred during the tests were highlighted to have a realistic frame of the experimented technology.

Supercritical water gasification of sewage sludge in continuous reactor

In this study, a process for the continuous recovery of phosphorus and generation of gas from sewage sludge is investigated for the first time using supercritical water gasification (SCWG). A continuous reactor was employed and experiments were conducted by varying the temperature (500–600 °C) and residence time (5–60 s) while fixing the pressure at 25 MPa. The behavior of phosphorus during the SCWG process was studied. The effect of the temperature and time on the composition of the product gas was also investigated. A model of the reaction kinetics for the SCWG of sewage sludge was developed. The organic phosphorus (OP) was rapidly converted into inorganic phosphorus (IP) within a short residence time of 10 s. The gaseous products were mainly composed of H2, CO2, and CH4. The reaction followed first order kinetics, and the model was found to fit the experimental data well.

Steam gasification of sewage sludge with CaO as CO2 sorbent for hydrogen-rich syngas production

Gasification is an alternative option for sewage sludge disposal. In this paper, steam gasification of dried sewage sludge using CaO as CO2 sorbent was experimentally investigated in a fixed bed reactor to produce hydrogen-rich syngas. In the presence of CaO, tar cracking is enhanced, and the hydrogen fraction, total gas yield and cold gas efficiency increase while CO2 and CH4 in the syngas decrease. The hydrogen volume fraction could be 72.8–82.9% at 650 °C. Temperature has a strong influence on syngas quality. At higher temperature, the hydrogen yield and cold gas efficiency increase whereas the hydrogen fraction decreases. TiO2, Al2O3 and 3A molecular sieve were employed as supports of the CaO. The addition of support into CaO increases the surface area and pore volume of the sorbent particle and therefore increases the hydrogen fraction, total gas yield and cold gas efficiency. For CaO-Al2O3, CaO-TiO2, with the rise of support mass fraction from 10% to 30%, the hydrogen content in the syngas and cold gas efficiency increases, whereas for CaO-3A, the optimal support mass fraction is 20%. Metal additives Ni and Fe are favorable to tar cracking. Even though the surface area and pore volume of the sorbent particle decrease after the addition of Ni and Fe elements, the hydrogen in the syngas, total gas yield and cold gas efficiency increase with the addition of Ni and Fe.

Characteristics of the microwave pyrolysis and microwave CO2-assisted gasification of dewatered sewage sludge

ABSTRACTMicrowave drying-pyrolysis or drying-gasification characteristics were examined to convert sewage sludge into energy and resources. The gasification was carried out with carbon dioxide as a gasifying agent. The examination results were compared with those of the conventional heating-type electric furnace to compare both product characteristics. Through the pyrolysis or gasification, gas, tar, and char were generated as products. The produced gas was the largest component of each process, followed by the sludge char and the tar. During the pyrolysis process, the main components of the produced gas were hydrogen and carbon monoxide, with a small amount of hydrocarbons such as methane and ethylene. In the gasification process, however, the amount of carbon monoxide was greater than the amount of hydrogen. In microwave gasification, a large amount of heavy tar was produced. The largest amount of benzene in light tar was generated from the pyrolysis or gasification. Ammonia and hydrogen cyanide, which are precursors of NOx, were also generated. In the microwave heating method, the sludge char produced by pyrolysis and gasification had pores in the mesopore range. This could be explained that the gas obtained from the microwave pyrolysis or gasification of the wet sewage sludge can be used as an alternative fuel, but the tar and NOx precursors in the produced gas should be treated. Sludge char can be used as a biomass solid fuel or as a tar removal adsorbent if necessary.

Partial oxidative gasification of sewage sludge in supercritical water with multi-component catalyst

A new -Mn-Cu- mixture catalyst with components of 50.9wt% CuMn2O4, 46.1wt% CuO and 3.0wt% Mn2O3 was made by a co-precipitation method firstly. We adopted the multi-component catalyst together with activated carbon (AC) and Na2CO3 (N) for the first time to conduct supercritical water partial oxidative gasification (SWPO) of sewage sludge at 450°C, 25MPa, 0.3 of oxidation coefficient and 5h of residence time, so as to improve H2 production and organics removal. The results show that MnO2-AC-N, CuO-AC-N and -Mn-Cu-AC-N could obviously improve H2 yield and removal efficiency of organics in sewage sludge SWPO. At -Mn-Cu-AC-N conditions, H2 yield and XCOD (removal efficiency of chemical oxygen demand) were 202.4mL/L and 95.6wt%, much higher than 48.7mL/L and 86.2wt% obtained without catalyst, respectively. The -Mn-Cu-AC-N catalyst seemingly had the best catalytic effect on gas formation as well as on organics removal in sewage sludge SWPO among MnO2-AC-N, CuO-AC-N and -Mn-Cu-AC-N. Potential catalytic reaction pathways were proposed as well.

Experimental and economic study of small-scale CHP installation equipped with downdraft gasifier and internal combustion engine

This paper concerns the experimental and numerical analysis of a combined heat and power (CHP) installation equipped with a biomass downdraft gasifier, gas purification system and gas piston engine. The numerical studies were aimed at identifying waste heat in the CHP installation for further utilisation. In turn, a new innovative method of heat recovery inside the gasifier was proposed and subsequently applied in the experimental CHP installation. The newly developed facility is able to produce up to 75kWe of electrical power. An extensive parametric study was performed under the steady and unsteady facility operation and was supplemented by economic analysis of the process. The main objective of the study was to examine the possibility of sewage sludge gasification and the impact of produced syngas quality on gas engine performance. During the investigation a number of different types of biomass were investigated including wood pellets, sewage sludge and their blends. The results showed a stable operation of the CHP facility in terms of produced syngas load, calorific value and content – even when dried sewage sludge was used alone. Results indicate that a 40/60% blend of wood pellets with sewage sludge is recommended, which allowed the lower heating value (LHV) of 4.45MJ/Nm3. Additionally, it was confirmed that the fixed bed gasifier is able to self-adjust to temporary changes in gasifier agent load.The economic analysis was performed taking into account policies and regulations in the Polish energy market sector. The study showed that it is more profitable to use the generated electricity and heat for its self-consumption rather than selling it on the market. Even with the supporting policies the payback time is not less than about eight years. Taking into account calorific value and limited local biomass availability, it seems justified that the described CHP installation is suitable for decentralised power generation such as small farms or horticultural businesses, where the produced electricity and heat can be sufficiently utilised. This technology also contributes to reduction of CO2 emissions into the atmosphere, which is one of the most problematic issues for energy sector today.

Low temperature circulating fluidized bed gasification and co-gasification of municipal sewage sludge. Part 2: Evaluation of ash materials as phosphorus fertilizer

The study is part 2 of 2 in an investigation of gasification and co-gasification of municipal sewage sludge in low temperature gasifiers. In this work, solid residuals from thermal gasification and co-gasification of municipal sewage sludge were investigated for their potential use as fertilizer. Ashes from five different low temperature circulating fluidized bed (LT-CFB) gasification campaigns including two mono-sludge campaigns, two sludge/straw mixed fuels campaigns and a straw reference campaign were compared. Experiments were conducted on two different LT-CFBs with thermal capacities of 100kW and 6MW, respectively. The assessment included: (i) Elemental composition and recovery of key elements and heavy metals; (ii) content of total carbon (C) and total nitrogen (N); (iii) pH; (iv) water extractability of phosphorus after incubation in soil; and (v) plant phosphorus response measured in a pot experiment with the most promising ash material. Co-gasification of straw and sludge in LT-CFB gasifiers produced ashes with a high content of recalcitrant C, phosphorus (P) and potassium (K), a low content of heavy metals (especially cadmium) and an improved plant P availability compared to the mono-sludge ashes, thereby showing the best fertilizer qualities among all assessed materials. It was also found that bottom ashes from the char reactor contained even less heavy metals than cyclone ashes. It is concluded that LT-CFB gasification and co-gasification is a highly effective way to purify and sanitize sewage sludge for subsequent use in agricultural systems.

Evaluation of Gasification Efficiency of Miscellaneous Industrial Sewage Sludges in a Down Draft Fluidized Bed Reactor

Limited information is available about energy efficiency evaluation of industrial sewage sludge gasification nowadays. The goal of this study is to determine the best proper feeding waste combination/combinations of miscellaneous industrial sewage sludges in a down draft fluidized bed pilot-scale gasification reactor to maximize energy recovery. Different industrial sewage sludge samples were taken from dewatering units of wastewater treatment plants of textile, dairy, vegetable and non-vegetable oil, glass and metal industries, located in Thrace Region of Turkey (Thrace Catchment of Area) and characterized in terms of three important indicator parameters (moisture, ash and heating value) for identifying the best feeding waste combination of the reactor which has high gasification efficiency. In addition, the same parameters were also analysed for the char and cyclone dust samples that occurred as a result of gasification process, and then, energy yields of all industrial sewage sludge combinations have been calculated. Results revealed that the majority of sludge samples, apart from metal and glass industries, were found suitable for gasification process in terms of both organic and inorganic contents either alone or in mixture form. The heating value and ash content of vegetable and non-vegetable oil industrial sewage sludge were measured approximately two times higher (average 11,622 kcal.kg−1 = 48 MJ.kg−1 for oil and average 5163 kcal.kg−1 = 21.6 MJ.kg−1 for dairy) and nearly in the same values (average 9.3% for oil and 9.5% for dairy) compared to dairy industrial sewage sludge, respectively. According to these results, four feeding sludge combinations were determined based on their ash content and heating values. It was found that Combination 1, which has high calorific value and low ash ratio, has the highest energy efficiency (97%), while the other combinations were found below this value. If the proper sludge combination can be identified for a draft fluidized bed gasification reactor, energy efficiency may be increased. Moreover, identifying the best feeding combination for sludge gasification reactor may be the best way to obtain both high energy recovery and low char/tar products and to also minimize industrial sewage sludge before being sent to landfill.

Low temperature circulating fluidized bed gasification and co-gasification of municipal sewage sludge. Part 1: Process performance and gas product characterization

Results from five experimental campaigns with Low Temperature Circulating Fluidized Bed (LT-CFB) gasification of straw and/or municipal sewage sludge (MSS) from three different Danish municipal waste water treatment plants in pilot and demonstration scale are analyzed and compared. The gasification process is characterized with respect to process stability, process performance and gas product characteristics.All experimental campaigns were conducted at maximum temperatures below 750°C, with air equivalence ratios around 0.12 and with pure silica sand as start-up bed material.A total of 8600kg of MSS dry matter was gasified during 133h of operation. The average thermal loads during the five experiments were 62–100% of nominal capacity. The short term stability of all campaigns was excellent, but gasification of dry MSS lead to substantial accumulation of coarse and rigid, but un-sintered, ash particles in the system. Co-gasification of MSS with sufficient amounts of cereal straw was found to be an effective way to mitigate these issues as well as eliminate thermal MSS drying requirements. Characterization of gas products and process performance showed that even though gas composition varied substantially, hot gas efficiencies of around 90% could be achieved for all MSS fuel types.

O-201 Ni触媒の下水汚泥を用いた低温接触ガス化による軽質ガス製造および窒素化合物の乾式ガス処理法(セッション3:技術・導入-ガス化等-,口頭発表)

O-201 Ni触媒の下水汚泥を用いた低温接触ガス化による軽質ガス製造および窒素化合物の乾式ガス処理法(セッション3:技術・導入-ガス化等-,口頭発表)

Investigation of sewage sludge treatment using air plasma assisted gasification

This study presents an experimental investigation of downdraft gasification process coupled with a secondary thermal plasma reactor in order to perform experimental investigations of sewage sludge gasification, and compare process parameters running the system with and without the secondary thermal plasma reactor.The experimental investigation were performed with non-pelletized mixture of dried sewage sludge and wood pellets. To estimate the process performance, the composition of the producer gas, tars, particle matter, producer gas and char yield were measured at the exit of the gasification and plasma reactor. The research revealed the distribution of selected metals and chlorine in the process products and examined a possible formation of hexachlorobenzene.It determined that the plasma assisted processing of gaseous products changes the composition of the tars and the producer gas, mostly by destruction of hydrocarbon species, such as methane, acetylene, ethane or propane. Plasma processing of the producer gas reduces their calorific value but increases the gas yield and the total produced energy amount. The presented technology demonstrated capability both for applying to reduce the accumulation of the sewage sludge and production of substitute gas for drying of sewage sludge and electrical power.

Sewage sludge gasification as an alternative energy storage model

Renewable energy production, storage and utilization has been studied in connection with fluctuating renewable power production and demand. The coupling has been considered of surplus renewable power with sewage sludge gasification and renewable syngas storage. High quality syngas produced with power support can be introduced into a gas distribution network or stored and utilized on site. Both systems enable gas consumption at power and heat demand periods. The efficiency of power conversion to syngas produced from high moisture sewage sludge has been studied using a developed equilibrium thermodynamic model for the gasification process providing the calculated prediction of the syngas amount and composition. Furthermore, the total energy efficiency has been studied of the syngas production and utilization system. The results show that the optimum scenario to get syngas suitable to introduce into a natural gas distribution network would be to gasify sewage sludge with 35–40wt.% moisture, while the optimal parameter for syngas to be valorized onsite is, in terms of efficiency, with SS dried to 20wt.% of moisture content. In the first scenario, a little more power is produced from stored gas in a natural gas distribution network than consumed, and almost twice as much is produced with on-site production, storage and utilization. The study has showed a technically feasible management option for energy production, storage and consumption with available technologies and infrastructure.

Steam gasification of wood pellets, sewage sludge and manure: Gasification performance and concentration of impurities

In the dual fluidised bed steam gasification process a product gas with a relatively high calorific value is produced. For clean biomass such as wood pellets or wood chips this process has been previously demonstrated in pilot-scale. Within this work, the applicability of waste biomass such as sewage sludge or manure as a fuel for this gasification process was investigated experimentally. A special focus was given to the concentration of impurities in the product gas of the gasifier (such as tar, NH3, H2S and Cl). The gasification experiments have shown, that the steam gasification of both biogenic waste materials is possible. For all fuels a high gas yield, close to the gas yield of wood pellets, was achieved and the product gas composition did not vary much between the fuels. Due to the different structures of the fuels and their different nitrogen, sulphur and chlorine contents, the concentrations of the impurities such as tar, NH3 H2S and Cl can vary significantly: tar concentrations of up to 100 g ⋅ m−3, NH3 concentrations of up to 0.06 m3 m−3 and H2S concentrations of up to 7000 ⋅ 10−6⋅m3⋅m−3 were measured.

Investigation of sewage sludge gasification with use of flue gas as a gasifying agent

The paper presents results of investigation of low-temperature sewage sludge gasification with use of flue gas as a gasifying agent. Tests were conducted in a laboratory stand, equipped with a gasification reactor designed and constructed specifically for this purpose. During presented tests, gas mixture with a composition of typical flue gases was used as a gasifying agent. The measuring system ensures online measurements of syngas composition: CO, CO2 , H2 , CH4 . As a result of gasification process a syngas with combustible components has been obtained. The aim of the research was to determine the usability of sewage sludge for indirect cofiring in power boilers with the use of flue gas from the boiler as a gasifying agent and recirculating the syngas to the boiler’s combustion chamber. Results of presented investigation will be used as a knowledge base for industrial-scale sewage sludge gasification process. Furthermore, toxicity of solid products of the process has been determined by the use of Microtox bioassay. Before tests, solid post-gasification residues have been ground to two particle size fractions and extracted into Milli-Q water. The response of test organisms (bioluminescent Aliivibrio fischeri bacteria) in reference to a control sample (bacteria exposed to 2% NaCl solution) was measured after 5 and 15 minutes of exposure. The obtained toxicity results proved that thermal treatment of sewage sludge by their gasification reduces their toxicity relative to water organisms.

An Energy Analysis on Gasification of Sewage Sludge by a Direct Injection in Supercritical Water

Supercritical Water Gasification is an efficient technology in converting wet biomass into H2 and CH4 in comparison to other conventional thermochemical processes. Coke deposition, however, remains as a major challenge in this technology. Coke formation is the result of polymerization reactions that take place at sub-critical conditions. Directly injecting the relatively unheated wet biomass feed into supercritical water increases the heating rate and reduces the residence time of the feed in the sub-critical condition. This leads to a minimized coke formation in the process. However, a non-isothermal mixing takes place during this direct injection that is less energy-efficient. In addition, the biomass feedstream experiences less pre-heating that means less heat recovery from the product gas. These two aspects might reduce the overall process performance. Parametric studies of key operating parameters, such as operating temperature, dry matter content, bypass water ratio and heat exchanger effectiveness, are carried out to investigate the influence of direct injection to the thermal efficiency of the system. Subsequently, optimization using pinch analysis is conducted to the system with direct injection. Finally, an operating window for optimum performance of the optimized direct injection gasification system is proposed.

Sewage Sludge Gasification as an Alternative to Incineration

Sewage Sludge Gasification as an Alternative to Incineration

Co-production of clean syngas and ash adsorbent during sewage sludge gasification: Synergistic effect of Fenton peroxidation and CaO conditioning

In order to achieve the complete utilization of both sludge organic and inorganic components, this study proposed a novel method for synchronous production of clean syngas and ash adsorbent during steam gasification process based on composite conditioning. Sewage sludge was pretreated by Fenton’s reagent (Fe2+/H2O2) and CaO, then gasified in a lab-scale fluidized bed at 873–1273K. The ash adsorption performance was also evaluated in a self-designed reactor. According to the results, both residual iron and calcium salts were able to promote H2-rich fuel gas generation. The effect of iron compounds became more strongly with temperature increased, while the influence of calcium substances tend to be more obviously with temperature decreased. Thus after combined conditioning, very good sludge gasification performance and the highest cold gas efficiency were achieved at a wide temperature range. Moreover, separate gasification of Fenton-oxidized or CaO-treated sludge released H2S emission respectively from sulfonic acid/sulfone/heterocyclic-S and inorganic sulfide in char. However, H2S releasing amount was less than 1.8% of total sulfur with the co-usage of two conditioners due to oxidation and fixation reactions. Meanwhile, although CaCxNy that formed in CaO-added sludge char reacted with steam to produce NH3, the residual iron and calcium compounds could synergistically inhibit the final emissions of NH3 and HCN. In this case, no nitrogenous gas was generated at 1273K. Besides this outstanding role, the remaining conditioners were still able to boost the adsorption performance of sludge gasification ash and significantly prolong its life.

Evaluation of the possibility of the sewage sludge gasification gas use as a fuel

Abstract Biomass is one of the major sources of energy that is estimated to contribute between 10% and 14% of the world’s energy supply. Over the past several years, many societies have established policy targets to increase their production of renewable energy from biomass. The thermo-chemical utilization of biomass includes 4 technologies: the most popular combustion and co-firing, and unconventional: pyrolysis and gasification. Gasification is considered to be the perspective technology because has many advantages in comparison to traditional process of combustion: (1) limited emission of the SO2, NOx, oxides of the heavy metals and no risk of the dioxins and furans emission due to reducing atmosphere in the gasification reactor, (2) volume of the gasification gas is smaller in comparison to flue gases from combustion due to the reducing atmosphere, (3) gasification process produce gas which is potential gaseous fuel in power engineering (engines, gas turbines and boilers) and chemistry. Unfortunately, composition of the gasification gas is always described as a variable. Moreover, it depends on the conditions of the process and quality of the base fuel. For this reason, the use of gasification gas can’t be very easy. For this reason, the knowledge of the basic properties of the gas is very important. Laminar burning velocity is assumed as an important quantity for in the process of the design equipment for the gas utilization. The numerical and experimental results of the laminar burning velocity of sewage sludge gasification gases were presented. Experimental Bunsen burner method was used. Cosilab 3© software for numerical analysis was used. GRI-Mech 3.0 mechanism of gas oxidation was implemented. As a result of the work, the set of the parameters where the sewage sludge gasification gas combustion process is stable with effective heat release, were presented.

Characterisation of tar from sewage sludge gasification. Influence of gasifying conditions: Temperature, throughput, steam and use of primary catalysts

This work shows the influence of parameters such as temperature, throughput (TR), the gasifying agent and the use of catalysts (olivine, alumina and dolomite) on the products resulting from the gasification of sewage sludge on a fluidised bed. An increase in temperature of between 750 and 850°C rises the production of combustible gas and reduces the production of gravimetric tar (by 65%, at TR=110kg/hm2, and 49% at TR=322kg/hm2), while the increase in TR at a given temperature produces the contrary effects. An analysis of the composition of the tar detected by gas chromatography (GC), shows that higher temperatures increase the concentration of polycyclic aromatic hydrocarbons (PAH). In spite of all the catalysts being active in tar removal, dolomite showed the most marked results in the air tests, with a reduction in the production of gravimetric tar of 40–50% and a conversion of the heavy PAHs (4–7 aromatic rings) of up to 90%. The combined use of a catalyst and air+steam increases the H2 and CO2 content in the synthesis gas and reduces the CO, CH4 and CnHm production, in addition to improving by 20% the catalysts performance in gravimetric tar removal compared to the tests with catalyst and air. Under the evaluated gasification conditions, the dew point of the gas was never below 108°C, which means that technical problems could be expected due to tar condensation if the synthesis gas were to be used in combustion engines without applying additional tar removal measures.