Adsorption characteristics of siloxanes in landfill gas by the adsorption equilibrium test

Variations of trace metals in combustion chamber deposit in landfill gas engine a long period of time

The combustion chamber deposit (CCD) is a major problem for the gas engines that formed accumulating of the metal oxides during the oxidation of trace compounds in the landfill gas (LFG). Therefore, the LFG was purified with activated carbon (AC) before in use to reduce deposit formation in gas engines. The AC treatment demonstrated the high removal capacity by reducing to below 1 % of the mass ratios of Si and Ca in the deposit. Unfortunately, the AC treatment caused the formation of black deposit in the intercooler that was analyzed by EDS and XRD. First time in this study, the variation of the elements of the CCD was comparatively investigated over a long period of time in 2010 and in 2019 without -AC treatment of LFG. The variation of the concentrations of C, Ca, N, S, Sb, Si- and Sn in the CCD were confirmed by the analysis with ICP-OES and SEM-EDS for 9-year period. It was determined with EDS analysis that while Sb and Sn were relatively low, C and N were high based on 2010. It has been determined that there is a proportional change depending on the time of the elements forming in the deposit.

Biogas Pollution and Mineral Deposits Formed on the Elements of Landfill Gas Engines

Municipal landfills generate a significant amount of high-energy biogas, which can be used as a renewable gaseous fuel. However, it is necessary to improve the quality of this biogas due to the presence of various chemical compounds. The most common pollutants in landfill biogas include volatile compounds of silicon, sulphur, phosphorus and chlorine. The aforementioned elements, as well as other metals, were found both in the deposits and in the engine oil. The paper presents detailed characteristics of the solid residues formed in selected parts of gas engines powered by landfill biogas. Its elemental composition and morphology were investigated in order to determine the structure and influence of these deposits. In order to better understand the observed features, selected analyses were also conducted for biogas, engine oil and the condensate generated during biogas dewatering. It was found that the content of individual elements in samples collected from the same part of the gas engine but sourced from various landfills vary. The occurrence of elements in deposits, e.g., Mg, Zn, P and Cr, depends on the location of sampling sites and the type of engine. It was also observed that the deposits formed in parts that come into contact with both biogas and engine oil contain Ca or Zn, which can be related to biogas pollutants as well as different oil additives. The presence of Al, Fe, Cu, Cr, Sn or Pb in selected motor oil samples can be explained by the penetration of metallic abrasives, which confirms the abrasive properties of the formed deposits. The analysis of the characteristic deposits may contribute to the selection of an appropriate landfill biogas purification technology, thus reducing the operating costs of energy cogeneration systems. Finally, we highlight challenges for biogas purification processes and anticipate the direction of future work.

The Effects of Hydrophobicity and Textural Properties on Hexamethyldisiloxane Adsorption in Reduced Graphene Oxide Aerogels

To expand the applications of graphene-based materials to biogas purification, a series of reduced graphene oxide aerogels (rGOAs) were prepared from industrial grade graphene oxide using a simple hydrothermal method. The influences of the hydrothermal preparation temperature on the textural properties, hydrophobicity and physisorption behavior of the rGOAs were investigated using a range of physical and spectroscopic techniques. The results showed that the rGOAs had a macro-porous three-dimensional network structure. Raising the hydrothermal treatment temperature reduced the number of oxygen-containing groups, whereas the specific surface area (SBET), micropore volume (Vmicro) and water contact angle values of the rGOAs all increased. The dynamic adsorption properties of the rGOAs towards hexamethyldisiloxane (L2) increased with increasing hydrothermal treatment temperature and the breakthrough adsorption capacity showed a significant linear association with SBET, Vmicro and contact angle. There was a significant negative association between the breakthrough time and inlet concentration of L2, and the relationship could be reliably predicted with a simple empirical formula. L2 adsorption also increased with decreasing bed temperature. Saturated rGOAs were readily regenerated by a brief heat-treatment at 100 °C. This study has demonstrated the potential of novel rGOA for applications using adsorbents to remove siloxanes from biogas.

Adsorptive Biogas Purification from Siloxanes—A Critical Review

Siloxanes are among the most technologically troublesome trace compounds present in biogas. As a result of their combustion, hard-to-remove sediments are formed, blocking biogas energy processing devices and reducing the efficiency of biogas plants. The purpose of this study was to help investors and designers to choose the optimal technology for the adsorptive removal of volatile methylsiloxanes (VMSs) from biogas and to identify adsorbents worth further development. This paper critically reviews and discusses the state-of-the-art technologies for the adsorption removal of siloxanes from biogas, indicating potentially beneficial directions in their development and deficiencies in the state of knowledge. The origin of VMSs in biogas, their selected physicochemical properties, technological problems that they can cause and their typical versus limit concentrations in biogases are presented. Both the already implemented methods of adsorptive VMSs removal from landfill and sewage gases and the ones being under development are verified and systematized. The parameters and effectiveness of adsorption processes are discussed, and individual adsorbents are compared. Possible ways of regenerating spent adsorbents are evaluated and prospects for their application are assessed. Finally, zeolite-based adsorbents—which can also be used for biogas desulfurization—and adsorbents based on polymer resins, as being particularly active against VMSs and most amenable to multiple regeneration, are identified.

Investigation of volatile methyl siloxanes in biogas and the ambient environment in a landfill

Landfill biogas is a potential alternative for fossil fuel, but the containing impurities, volatile methyl siloxanes (simplified as siloxanes), often cause serious problems in gas turbines when applied to generate electricity. In this research, a collecting and analyzing method based on solvent adsorption and purge and trap-gas chromatography-mass spectrometry was established to determine the siloxanes in biogas from a landfill in Jinan, China, and adjacent ambient samples, such as soil, air, and leachate of the landfill. The results showed that, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) accounted for 63% of total siloxanes; and without considering D4 and D5, the order of detected siloxanes in concentration was found relating to Gibbs free energies of molecules, namely that higher abundant siloxane (except for D4 and D5) usually had lower Gibbs free energy. Additionally, the mass ratio between D4 and octamethyltrisiloxane (L3) in the biogas varied with different garbage age in landfills, possibly revealing the breaking-down of larger siloxane molecules with time. The samples, which were collected from environmental samples adjacent to the landfill, such as soil, water, and air, presented much higher siloxane level than urban or rural area away from landfills. The current H₂S scrubber of the landfill biogas could decrease the total siloxanes from 10.7 to 5.75 mg/m³ due to Fe₂O₃ and a refrigerant drier in a purification system and cyclic siloxanes were more easily removed than linear ones.

Gaseous adsorption of hexamethyldisiloxane on carbons: Isotherms, isosteric heats and kinetics

Volatile methylsiloxanes (VMS) are a special kind of impurity that exist in biogas and seriously hamper its utilization; therefore, their removal has attracted great attention in recent years. Adsorption is the only technology that is currently capable of industrial-scale removal of VMS. In this research, three carbons with various porous structures, including ordered mesoporous carbon (OMC), activated carbon fiber (ACF) and granular activated carbon (GAC), were selected as potential adsorbents to investigate their adsorption properties toward hexamethyldisiloxane (L2), which is a typical linear VMS pollutant. The adsorption isotherms and kinetics of L2 on the three carbons were studied, and the isosteric heats of adsorption were calculated in accordance with the isotherms under different temperatures by using the Van't Hoff equation. Additionally, the influences of the topological structures of the carbons on the adsorption kinetics were compared. Generally, adsorption isotherms of the three carbons can be well-fitted by the Dubinin-Astakhov equation, and the variation of the isosteric heats and adsorption kinetics are presumed to be closely related to the pore sizes of the carbons. These new findings reveal the adsorption mechanisms of L2 on carbons and make it possible that the proper adsorption system is set up to fulfill higher removal efficiency.

Biogas Purification: A Comparison of Adsorption Performance in D4 Siloxane Removal Between Commercial Activated Carbons and Waste Wood-Derived Char Using Isotherm Equations

Biogas production from organic waste could be an option to reduce landfill and pollutant emissions into air, water, and soil. These fuels contain several trace compounds that are crucial for highly efficient energy generators or gas injection into the grid. The ability of adsorbents to physically remove such adsorbates was investigated using adsorption isotherms at a constant temperature. We experimentally modelled isotherms for siloxane removal. Siloxanes were considered due to their high impact on energy generators performance even at low concentrations. Octamethylcyclotetrasiloxane was selected as a model compound and was tested using commercially available carbon and char derived from waste materials. The results show that recyclable material can be used in an energy production site and that char must be activated to improve its removal performance. The adsorption capacity is a function of specific surface area and porous volume rather than the elemental composition. The most common adsorption isotherms were employed to find the most appropriate isotherm to estimate the adsorption capacity and to compare the sorbents. The Dubinin-Radushkevich isotherm coupled with the Langmuir isotherm was found to be the best for estimating the adsorption capacity.

Sewage biogas efficient purification by means of lignocellulosic waste-based activated carbons

The present paper evaluates the efficiency of sustainable activated carbons obtained from the valorization of lignocellulosic waste in removing siloxanes and volatile organic compounds for the purification of anaerobic digester biogas. Pyrolized and non-pyrolized lignocellulosic residues generated in food and wood industries were used as precursor materials to obtain experimental adsorbents by a chemical activation process using several activating agents. The highest porosity was obtained by non-pyrolized residue activated by K₂CO₃ at 900 °C. The performance of the experimental materials was compared with that of commercial activated carbons in gas adsorption tests of siloxanes (octamethylcyclotetrasiloxane and hexamethyldisiloxane) and volatile organic compounds (toluene and limonene). The waste-based activated carbons developed in this work proved to be more efficient for the removal of both siloxanes and VOCs than the commercial samples in most of the conditions tested. Adsorption capacities correlated with porosity, while the more relevant pore size depends on the adsorbate.

A review on removal of siloxanes from biogas: with a special focus on volatile methylsiloxanes

The occurrence of siloxanes is a major barrier to use of biogas as renewable energy source, and removal of siloxanes from biogas before combustion is needed. The siloxane can be transformed into silicon dioxide (SiO₂) through the combustion process in engine, which will be deposited on the spark plug, cylinder, and impeller to form the silica layer, causing the wear and damage of the engine parts, and shorten the life of the engine and affect the utilization efficiency of the biogas. This paper reviewed some methods and technologies for siloxanes removal from biogas. There are three commercial available technologies to remove siloxanes: adsorption, absorption, and cryocondensation. Other newer technologies with better prospects for development also have made a research progress, including membrane, catalysts, biotrickling filters. This work introduces the source and characterization of siloxanes in biogas, reviews the scientific progress of siloxanes removal, and discusses the development direction and further research of removal siloxanes.

Design and analysis of siloxanes removal by adsorption from landfill gas for waste-to-energy processes

Separation of volatile methyl siloxanes from landfill gas using fixed adsorption beds was modeled with the objective of identifying appropriate technology and the economics associated with this purification step. A general adsorption model assuming plug flow and radial symmetry was developed and used to conduct a parametric sweep of 162 unique cases. The varied parameters were adsorbent type (activated carbon and silica gel), bed height (3.05-9.15 m/10-30 ft), inlet siloxane concentration (5-15 mg/m³), moisture content (0-100% relative humidity at STP or RH), and siloxane tolerance limit (0.094-9.4 mg/m³) that correlated to three distinct energy conversion technologies (electricity production using engines or fuels cells or catalytic conversion to liquid hydrocarbon fuels). Due to the detrimental effect of RH on siloxane absorption, the maximum allowable moisture content of LFG before purification is 50% RH and moisture removal processes are also required. The design calculations using a selected case study show that the adsorption bed height required needed for 6 months minimum breakthrough time for catalytic fuel production is twice that for engine applications. Fuel cell applications require 3 times the bed height compared to engine applications. However, the purification costs amounted to 94%, 16% and 52% of recovered product value for engine, liquefaction, and fuel cell applications, respectively indicating the need for a high value product to justify purification costs. The approaches and conclusions can be extended to specific process conditions for landfill gas purification and to other processes that use biogas produced from waste as a feedstock.

Insights into siloxane removal from biogas in biotrickling filters via process mapping-based analysis

Data process mapping using response surface methodology (RSM)-based computational techniques is performed in this study for the diagnosis of a laboratory-scale biotrickling filter applied for siloxane (i.e. octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5)) removal from biogas. A mathematical model describing the process performance (i.e. Si removal efficiency, %) was obtained as a function of key operating parameters (e.g biogas flowrate, D4 and D5 concentration). The contour plots and the response surfaces generated for the obtained objective function indicate a minimization trend in siloxane removal performance, however a maximum performance of approximately 60% Si removal efficiency was recorded. Analysis of the process mapping results provides indicators of improvement to biological system performance.

Biogas upgrading: optimal activated carbon properties for siloxane removal

A total of 12 commercial activated carbons (ACs) have been tested for the removal of octamethylcyclotetrasiloxane (D4) in dynamic adsorption experiments using different carrier gases and D4 concentrations. Characterization of the ACs included several physical and chemical techniques. The D4 adsorption capacities were strongly related with the textural development of the ACs. Results showed that the optimum adsorbent for D4 is a wood-based chemically activated carbon, which rendered an adsorption capacity of 1732 ± 93 mg g(-1) using 1000 ppm (v/v) of D4 with dry N2 as the carrier gas. When the concentration of D4 was lowered to typical values found in biogas, the adsorption capacity was halved. The presence of major biogas compounds (i.e., CH4 and CO2) and humidity further reduced the D4 adsorption capacity. The polymerization of D4 over the surface of all ACs was found to be relevant after prolonged contact times. The extent of this phenomenon, which may negatively affect the thermal regeneration of the AC, correlated reasonably well with the presence of phenolic and carboxylic groups on the carbon surfaces.