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A better utilization of biogas with specifications for siloxanes

2014-07-10

A research team has contributed to the scientific understanding of the combustion of siloxanes, compounds that can be found in biogas.

Dr. Howard Levinsky, a principal specialist at DNV GL (photo: EDGaR/Jan Buwalda) Dr. Howard Levinsky, a principal specialist at DNV GL (photo: EDGaR/Jan Buwalda)

“Having sound specifications for siloxanes is an enabler for biogas,” said Howard Levinsky, a principal specialist at DNV GL. Dr. Levinsky and his team have researched the technical basis for specifications of silicon-containing compounds in biogas. The idea of this research was “to discover the limits on siloxane concentration, so that biogas can be used the same way as natural gas in domestic appliances,” he said. “Biogas can become a full-grown, fully accepted sustainable replacement for natural gas.” 

Dr. Levinsky’s team carried out two years of research at DNV GL’s laboratories in Groningen, supplemented with transmission electron microscope data from the University of Groningen and cooperation with the Joint Institute for High Temperatures in Moscow. Gas infrastructure company Gasunie and gas distribution companies Enexis, Liander and Stedin participated to this project of the Energy Delta Gas Research (EDGaR). 

The problem of siloxanes

Households have been equipped with appliances — central heating boilers, flow-through water heaters, etc. — designed to perform with a specific range of natural gas composition. Siloxanes are not present in natural gas but in some biogases. The introduction of biogas requires determining an acceptable level of siloxanes in the fuel, and the subsequent monitoring of siloxanes concentrations in the grid.

The problem is that siloxanes, when burned in combustion appliances, form silicon dioxide. Silica, the main component of sand, “can deposit as a white powder or glassy layer that can interfere with the performance of combustion equipment,” Dr. Levinsky said.

Dr. Levinsky’s team studied the silica growth and deposition processes in natural gas fired domestic equipment. They developed theoretical models to estimate the growth of silica fractal aggregates in flames and to analyze particle deposition on surfaces in heating equipment.

Through laboratory experiments, they determined the effects of silicon deposition on the performance of a number of domestic appliances. Finally, they defined an unambiguous unit for the specification of the concentration of siloxanes in gas.  In short, they contributed to the scientific understanding and practical consequences of the differences in composition between natural gas and biogas. 

Specifications for siloxanes

How appliances deteriorate over time because of siloxanes deposition? The team used models developed in the project. “The model insights were used to guide the design and interpretation of the experiments on appliances,” said Sander Gersen, a consultant at DNV GL and one of the contributors to the research.

“These were performed at concentrations seen in some practical situations, and allow extrapolation of the results to even lower concentrations,” he said. “Otherwise we must do experiments for fifteen years.”

Dr. Levinsky explained further the method: “Based on the best insight from the experimental work, we estimate the time it would take for the performance of appliances to deteriorate at different concentrations of siloxanes. We compared those times with the average lifetime of the appliance.” 

“A piece of the puzzle”

“This research is one of the pieces of the puzzle,” said Albert van der Molen, an Asset Manager at Stedin and participant in the research. “It is part of a broader picture on the effects of biogas that is made from certain types of biomass.”

Biomass feed into digesters to produce biogas determine the concentrations of siloxanes. Biogas produced from sewage treatment has, as a general rule, a higher concentration of siloxanes than biogas made out of landfill and dairy waste.

The gas industry has to specify the allowed concentration of siloxanes in the network. Mr van der Molen said that there is no compromise about security. Gas distribution companies are looking for ways to make sure that involved parties in the production, transport and distribution of gas are satisfied with security.

“There are no immediate safety issues,” he said on the current concentration of siloxanes in biogas.  “The whole sustainable energy distribution system in the Netherlands can only be a success if the public image of biogas remains positive.”

By Jean-François Auger