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Diagram illustrates the test structure of the 100-Nm3/h plant at the Vereinigte Ville landfill site.
© Fraunhofer UMSICHT

Silicon dioxide deposits on the cylinder head of a gas motor after 3,500+ operating hours resulting from the combustion of organic silicon trace compounds in the engine compartment.
© Fraunhofer UMSICHT

The research team tested both catalytic separation on activated aluminium oxide (cat-ox process) and adsorptive purification using activated carbon with subsequent regeneration (activated carbon process).
© Fraunhofer UMSICHT

The two materials, aluminium oxide and activated carbon, were each examined by the research team in an isolated fixed-bed reactor with landfill gas.
© Fraunhofer UMSICHT
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Test plant at the landfill site

The researchers developed a test plant in which two methodological approaches can be investigated: The catalytic separation of trace compounds of organic silicon from activated aluminium oxide (cat-ox process) and adsorptive purification through activated carbon with subsequent regeneration (activated carbon process).

The research team worked with a small 0.6-Nm³/h test bench and a large 100-Nm³/h test plant. The two of these were connected to one another and were supplied with crude gas via a partial flow from the main landfill gas line. The test plant was situated at the Vereinigte Ville landfill site. The small test bench was used by the researches to perform several test cycles consecutively using small quantities of adsorbent.

Activated carbon and activated aluminium oxide were investigated in an isolated fixed-bed reactor in the 100-Nm³/h circuit. This involved close scrutiny of the adsorptive properties and the transformation of trace elements of organic silicon under practical conditions.

Catalytic separation

The cat-ox process as a concept involves the separation of organic silicon compounds from activated aluminium oxide at temperatures of 250 to 350 °C. The organic silicon compounds on the surface of this material react to form silicon dioxide. This solid reaction product is deposited, however, in the pores of the material. The aluminium oxide is consequently deactivated. The team at Fraunhofer UMSICHT also tested other possible materials for catalytic separation under laboratory conditions in the preliminary stages. Aluminium oxide, however, proved to be the most suitable. Calcium oxide (CaO) and magnesium oxide (MgO) are ruled out as the carbon dioxide contained in the landfill gas reacts to form a carbonate layer on the materials, which reduces the reactivity. Titanium dioxide and vanadium pentoxide showed good reactivity over extended periods, though are ten to twenty times as expensive as aluminium oxide.

In the large test plant, the landfill gas is initially heated to max. 350 °C via a thermal oil heater. It is then fed into a reactor filled with activated aluminium oxide. Through reactive adsorption, the compounds of organic silicon on the aluminium oxide are converted to silicon dioxide. The purified landfill gas is cooled to approx. 50 °C in the heat exchanger and fed back into the main gas line of the landfill site.

The tests show that the process is essentially suitable for separating organic silicon compounds. Results to date lead researchers to conclude that the operating costs in this regard are roughly comparable to those of the adsorptive activated carbon procedure with on-site regeneration. Further technical considerations, however, need to be addressed before use in practice. These include for instance questions around how effective heat transfer to the reactor can be achieved and how dust deposition can be prevented in plant components behind the reactor.

Adsorptive purification with subsequent regeneration

The approach of the activated carbon procedure in the large test plant involves initially concentrating the landfill gas and heating it to max. 70 °C. The gas is then fed through an adsorber filled with activated carbon. Trace elements of organic silicon contained in the gas are adsorbed on the activated carbon. The purified gas is cooled before being fed back into the main line of the landfill site. The desorption of the activated carbon occurs through a combination of thermal regeneration and vacuum desorption. In the process, a partial flow of the gas is heated to max. 160 °C and fed into the adsorber. A vacuum pump at the outlet of the adsorber ensures a vacuum of 700 mbar. The gas filled with desorbent is cooled to approx. 80 °C in a heat exchanger and fed through a control filter, which is an adsorber filled with activated carbon. The gas is then fed back for activated carbon adsorption.

The field tests have shown that the on-site regeneration of activated carbon is partially achievable in the method and constant residual loading is attained. As expected, however, the activated carbon exhibited very limited retention of polar organic compounds such as trimethylsilanol, meaning that additional purification must be considered for these for technical application. The proportion of trimethylsilanol in the total load of organic silicon compounds can be up to 50 per cent in the landfill gas. The research team recommended as a solution a process of pre-purification by means of seepage water washing. A system such as this has already been implemented by industry partner Siloxa in Ihlenberg in Mecklenburg-Western Pomerania, Germany and separates virtually all contained trimethylsilanol and other polar organic silicon compounds.

Recommendations for a gas purification plant

The research group developed a theoretical procedural concept for a gas purification plant with a throughput of 1,000 Nm³/h: A process of seepage water washing is proposed as a pre-purification step. In the process, the landfill gas is cooled to 4 °C in order to enhance the absorptive capacity.

If the landfill gas has a temperature of 41 °C, a moisture saturation of 100 %, a pressure of 1,013 mbar and if an air temperature of 35 °C prevails, the energy requirement at a throughput of 1,000 m³/h would be approx. 18 kWhel. Two alternately operated adsorbers are located downstream for adsorption and desorption. The desorbent is ultimately disposed of via a high-temperature gas burner.

By integrating an upstream gas purification process of this nature, modern gas motors can achieve a rate of efficiency of approx. 43 % in the conversion to electricity of landfill gas. For this procedural concept, the research team also produced an operating costs analysis for an average silicon load present in the landfill gas. On the basis of this, the specific operating costs were estimated to be 1.06 euro cents per kWhel. This is considered to be a significant saving over a conventional adsorption plant involving the exchange of activated carbon, the specific operating costs of which were calculated to be 1.65 euro cents per kWhel according to the research team’s data. Further tests are needed under boundary application-like conditions to develop the market viability for the combination of activated carbon procedure and seepage water washing.

Projektinfo 11/2014:
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Addresses

Project management, planning, development and commissioning of mobile test plant
Siloxa Engineering AG

Scientific support for gas analytics and plant operation
Fraunhofer UMSICHT

Links

Funding programme Biomass energy use
Website of the Funding programme of the German Federal Ministry for  Economic Affairs and Energy (BMWi)

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