Reducing of CO2 and H2s for Biogas Upgrading Using Accelerated Mineralization

To provide renewable electric and thermal energy regardless of weather condition biogas is a promising addition to solar power systems and wind farms. For upgrading biogas to natural gas level for storing it in the natural gas grid, carbon dioxide and hydrogen sulfide fractions have to be reduced. For carbon dioxide sequestration a lot of research has been conducted to capture and store CO₂ from mainly flue gas using classic carbon capture and storage (CCS) processes. What all these technologies have in common is that they are energy-intensive and require long-term monitoring, verification and permanency. Also they are selective and only reduce carbon dioxide content. For the reduction of hydrogen sulfide from biogas, additional upgrading processes are necessary.

But there is another possibility of capture carbon dioxide and hydrogen sulfide, so far receiving little attention: Accelerated mineralization processes use alkaline earth metals to capture and store carbon dioxide in the form of ecofriendly carbonates. The advantage of a solid reaction product makes unnecessary the liquefaction as well as the search for a secure storage location. Calcium carbonate is stable long term and can be stored in an environmentally compatible manner.

This research project has as its goal to use alkaline earth metals from combustion ashes like calcium and magnesium to upgrade biogas to natural gas level. The ashes with a very good surface-to-volume ratio are thermally activated and don’t need to be produced by mining like classic alkaline minerals. At the same time also sulfur from hydrogen sulfide can be captured and stored using heavy metals like lead from the combustion ash.

For investigating on influencing factors like pH number, reaction temperature and ash content for better separation efficiency a test field concept was developed and constructed. The test field consists of three tanks, two batching tanks and a settling tank. In the first batching tank (the extraction tank), the ash is mixed with water, and metals like calcium, magnesium and lead are extracted from the solid phase. The ionized water from the extraction tank is filtered and pumped into the reaction tank. There, the water is purged by a mixture of nitrogen, carbon dioxide and hydrogen sulfide. Calcium ions react with the carbonate ions to form calcium carbonate. In another reaction, the lead from the ash and the sulfur ions from hydrogen sulfide form lead sulfide. Since the solubility of both calcium carbonate and lead sulfide is low under standard conditions, both reaction products precipitate in the reaction tank. The suspension of process water and precipitation product is pumped into a settling tank where the precipitation solids sediment. First analyses of the product show that both carbon dioxide and hydrogen sulfide were reduced by about 25 mol.-%. From the settling tank, the process water flows back to the extracting tank and the water cycle starts anew.