Determination of organic silicon compounds in biogas from wastewater treatments plants, landfills, and co-digestion plants

Abstract

The study determined the organic silicon compounds in biogases from landfills, wastewater treatment plants (WWTPs), and biogas plants processing different organic material. The aim was to provide information for gas utilisation applications, as siloxanes are reported to shorten the life time of engines when biogas is used for energy production. In total, 48 samples were measured. The total concentration of organic silicon compounds in landfill and WWTP gases varied from 77 to 2460 μg/m³ while the concentrations in biogases from biogas plants varied from 24 to 820 μg/m³. The total concentration of organic silicon compounds was lowest (24 μg/m³) in the biogas plant processing grass and maize, and highest (2460 μg/m³) in one of the studied WWTP. The most common compounds in WWTPs and in biogas plants processing also sewage sludge were D4 and D5 while in landfills the most common compounds were D4 and L2 followed by trimethyl silanol. The effect of condensation of biogas on concentrations of organic silicon compounds was studied in one of the landfills and a negligible effect on concentrations was detected.

Introduction

Biogas is produced in landfills, wastewater treatment plants (WWTPs), and biogas plants by microorganisms during the degradation process of organic material. In WWTPs and biogas plants, the gas produced is generally used in energy production. Moreover, in several countries, methane rich gas from landfills must be collected and burnt or used for energy production in order to prevent the methane from being released into the atmosphere. For this reason, and also due to increasing interest in renewable fuels, biogas has become a notable alternative to conventional fuels in the production of electricity and heat. Indeed, biomethane, upgraded from biogas, has also become an interesting alternative for vehicle fuel.

Biogas contains methane and carbon dioxide, from 40 to 60% and 40 to 55% respectively. Landfill gas may also contain nitrogen and oxygen [1]. Along with the main compounds, biogas may also contain trace compounds such as hydrogen sulphide, halogenated compounds, and organic silicon compounds. Biogas can be used for electricity and heat production, but hydrogen sulphide and halogenated compounds can cause problems such as engine corrosion [2]. Depending on the utility application, it may be required to upgrade biogas so that no compounds harmful to engines end up to the combustion process. Furthermore, the engine manufacturers have set the minimum limits for methane content to ensure engine performance.

The organic silicon compounds present in biogas are oxidized during the biogas combustion into microcrystalline silicon dioxide, a residue with chemical and physical properties similar to glass. Silicon dioxide collects in deposits on valves, cylinder walls, and liners, causing the abrasion and blockage of pistons, cylinder heads, and valves. In gas turbines, siloxane deposits usually form on the nozzles and blades, causing erosion of the turbine blades and subsequently decreasing the operating efficiency [3]. Moreover, the glassy residues can de-activate the surface of the emission control system catalyst [1], [4]. The use of spark ignition engines has increased the number of engine failures caused by siloxanes, as previous dual fuel engines were less vulnerable to silica deposits [5]. According to Tower (2003) [3], there is a correlation between increasing CO emissions and the build-up of silicate-based deposits from siloxane combustion in generator engines. Some organic silicon compounds can end up in the engine oil after the combustion process. In this case, the engine oil needs to be changed more frequently [6], leading some gas engine manufacturers to introduce a limit value for silicon of 1 mg/l in the oil of the gas engine [7].

The subgroups of the silicones containing Si–O bonds with organic radicals bonded to the Si are called siloxanes. The organic radicals can include methyl, ethyl, and other organic functional groups [6]. The structure of a siloxane can be linear or cyclic. Siloxanes have an abbreviation L if the structure of the compound is linear and D if the structure is cyclic. Most siloxanes have a high vapour pressure and low water solubility (Table 1), hence they also have high Henry’s law constant even though their molecular weights are high. This indicates that they move easily from water to gas [8]. In WWTP digesters and landfills, siloxanes are volatilised into biogas, which is affected e.g., by temperature [9]. Along with the most common silicon compounds in biogases (Table 1), biogases may also contain other organic silicon compounds than siloxanes that cause similar detrimental effects in the combustion process. For example methoxytrimethylsilane, tetramethylsilane, trimethylfluorosilane, and trimethylpropoxysilane have been detected from WWTP digester biogases [3].

The use of siloxanes is increasing, for example, in household/industrial cleaning products because volatile methylsiloxanes (VMSs) solvents are aroma-free, widely available, and they are not included in volatile organic compound (VOC) regulations and therefore not considered a health risks to humans. They are also used in personal care products. VMSs can also originate from the hydrolysis of polydimethylsiloxane (PDMS). PDMS is an organosilicon compound. It is the most widely used silicon compound in household and industrial applications. It is used for example as softener and wetting agent in textile manufacturing, as a component for many surface treatment formulations, and in many domestic products such as shampoos, deodorants, gels, and so on. It is estimated that one million tons of siloxanes are produced annually worldwide [4], [9], [10].

Most siloxanes are very volatile and can decompose in the atmosphere into silanols, which are eventually oxidised into carbon dioxide. These compounds are insoluble in water and have a high adsorption coefficient. Some of the siloxanes end up in the wastewater and are adsorbed onto the extracellular polymeric substances (EPS) of the sludge flocks. Siloxanes are volatilised from the sludge during the anaerobic digestion and end up in biogas [5], [7]. Silicones are also sometimes added in the digesters as anti-foaming agents, where they can biodegrade into siloxanes [5]. Organic silicon compounds end up in landfills from sources such as shampoo bottles and other containers in which some of the product remains, through landfilling of wastewater treatment sludge, and from packaging, construction and so on [6], [11]. Degradation of high molecular silico-organic compounds in landfills may also form volatile silicon compounds [1]. Because of wide use of siloxanes, they are commonly found in air, water, sediment, sludge, and biota, and the variation in concentrations can be high [12].

The objective of this study was to determine the amounts and the types of organic silicon compounds in three different types of biogas production sites. Organic silicon compounds were measured with same analysing methods from landfills, WWTPs and biogas plants, to provide conclusive information for gas utilising applications.