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The latter can be solved by using an oxygen/steam mixture instead of air, but this will increase the plants costs and decrease the efficiency. Product gas from air blown gasification has an energy value up to 6 MJ/Nm3 (150 Btu/scf) after tar removal and water condensation, good enough for a gas engine but too low for a gas turbine.

Principle of the MILENA gasifier MILENA is

an indirect gasifier, meaning that the pyrolysis and combustion reactions are separated. Main advantage of this process is that the MILENA has a separate flue gas exhaust, hence the product gas is not diluted with the N2 present in air. After tar removal and water condensation, the MILENA product gas has an energy value up to 20 MJ/Nm3 (500 Btu/scf), well within specification for a gas turbine.

The 800 kW MILENA pilot at ECN

Design impression of a 30MW MILENA gasifier island

Overview Renewable energy, energy efficiency

and energy independence are hot topics nowadays. An energy source, with a large potential is biomass. Biomass is generally considered as one of the most important renewable energy sources. Biomass is the only renewable carbon source! Another important energy source is waste. Landfilling waste is limited and will be abolished in a lot of regions. Mass incineration of waste is unpopular and has a low energy efficiency. Waste is partly fossil (plastics) and partly biogenic (e.g. wood, paper, leather, cotton). Hence utilising waste is not only solving our waste related problems, it also contributes to our renewable energy mix.

Gasification of biomass & waste Gasification of biomass and waste opens the full potential of this valuable carbon source. Combustion is still used more nowadays, but gasification has more potential. Cleaning gasification product gas is more efficient than cleaning combustion flue gas. Once the product gas is cleaned, it can be used in efficient combined cycle power stations. Moreover, the product gas can also be upgraded by catalytic conversion into gaseous and liquid fuels like substitute natural gas, hydrogen or Fisher Tropsch diesel.

Principle of gasification Gasification can be

defined as the thermal conversion of carbon rich fuels into a syngas or product gas. Syngas contains hydrogen, carbon monoxide and also carbon dioxide and water. Product gas contains syngas as well as hydrocarbons like methane. Biomass and waste is normally gasified at temperatures in the range of 700-900°C (1300-1700°F). At these temperatures the solid fuel is vaporised and converted to the gaseous components of product gas. In the absence of oxygen this process is called pyrolysis. Pyrolysis is an endothermic process, it needs an energy source to maintain the temperature level. In a gasifier reactor a controlled amount of oxygen is fed, which combusts part of the fuel, therewith supplying the energy for the gasification process. An efficient process with some disadvantages:

Gasification does not have a full carbon conversion, normally this is 90-95%;

Product gas is diluted with combustion flue gas (CO2, H2O);

Product gas is diluted with nitrogen in case air is used as oxygen supply.

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MILENA process Solid fuel (biomass or waste)

enters the system in the central riser (red). A small amount of steam or air is also injected in the riser to fluidise the hot sand (catalytically active bed materials like olivine can also be used). Using the energy of the hot sand biomass reacts into three basic products:

Solids (char)

Condensables (tars)

Product gas (mainly H2, CO, CH4, C2H4, CO2, & H2O)

W W W . R E N E W A B L E T E C H N O L O G Y . N L | T . + 3 1 ( 0 ) 1 0 7 9 0 0 8 9 0 | E . J A N W I L L E M . K O N E M A N N @ S Y N O V A P O W E R . C O M 2 0 1 8

Features Benefits Full carbon conversion 100% carbon to gas ratio

Leads to a high cold gas efficiency Carbon free ash, reduction in waste

High cold gas efficiency, >80% chemical energy transfer to tar free product gas

5-10% higher than direct air blown gasifiersBetter use of a valuable carbon source

Separate flue gas exhaust classic, commercially proven flue gas treatment

High quality heat recovery Easy collection of carbon free ash

No dilution of the product gas and still using ambient air as oxygen source

N2 free gas suitable for SNG (CH4) production Smaller downstream process, less capex

High product gas energy value Up to 20 MJ/Nm3 (500 Btu/scf)

Suitable for a gas turbine combined cycle Smaller compressors, less parasitic load

Because of the volatilization; the product gas, sand and char flow upwards through the riser into the settling chamber. In this settling chamber the majority of the solids (sand, ash & char) are separated from the product gas. Raw product gas (containing gaseous tars) is flowing to downstream equipment. The solids collected in the settling chamber flow under influence of gravity to the combustion chamber (green). In this combustion chamber the char fraction burns, heating up the sand after which the hot sand flows back to the riser completing the cycle.

MILENA-OLGA: clean & efficient In MILENA

the combustion process is optimised resulting in 100% carbon conversion, boosting gasifier efficiency and minimizing ash volume. The product gas is concentrated and can therefore be cleaned efficiently. Most important cleaning step is the tar removal. Tars are known as the Achilles heel of biomass and waste gasification. Tar related problems form a major issue in efficient use of product gas. ECN and DRT also developed the OLGA technology which efficiently removes tar and is fully integrated with the MILENA process. Tars and char are removed from the product gas and recycled to the MILENA’s combustor reactor where the energy is used for the gasification process.

MILENA a patented process MILENA is an

invention of the Energy research Centre of the Netherlands (ECN part of TNO). ECN part of TNO is one of the leading institutes in the world for research & development of gasification technology. ECN & DRT cooperated in the development of MILENA and have mutual ownership to the IP.

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