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Overview Tar is known as the Achilles heel for biomass and waste gasification. When the tar problem is solved, gasification has an important future for an independent and sustainable energy supply. The Energy research Centre in the Netherlands (ECN) tried almost every available tar removal system after which was concluded that a new approach was necessary1. Out of this research the OLGA technology was born. OLGA is a patented invention and an acronym for oil and gas scrubber. Unlike in a conventional biodiesel/RME scrubber, oils are not consumed hence lowering operational expenditures while allowing for optimization of gasification process.

Principle of OLGA The tar removal system of OLGA is based on a multiple stage scrubber in which gas is cleaned by special scrubbing oil. In the 1st loop of OLGA the raw dust containing gas is gently cooled down by scrubbing oil. Heavy tar particles condense and are collected, after which they are separated from the scrubbing oil and can be recycled to the gasifier, together with a small bleed.

In the 2nd loop lighter tars are absorbed by scrubbing oil. In the absorber column the scrubbing oil is saturated by these light tars. This saturated oil is regenerated in a stripper. Hot air or steam is used to strip the tar off the scrubbing oil. All heavy and light tars can be recycled to the gasifier where they are destructed and contribute to the energy efficiency and a tar waste stream is avoided! Process description Before entering the OLGA tar removal system the product gas is cooled by a cooler and the coarse particles are removed by one or multiple cyclones upstream OLGA. No further high-temperature dust filtration is needed. The first OLGA column accepts gas above the tar dew point (~450°C) and cools it down to a temperature close but safely above the water dew point. Heavy tars are removed by condensation. Fine particles and aerosols are removed by the

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column as well as the integrated electrostatic precipitator (ESP). The condensed tars and captured solid particles together with the scrubber oil are circulated, while an oil recovery system separates heavy tars and solids from this circulation loop. The heavy tar and dust mixture is recycled to the gasifier. After the collector and ESP the gas is free of heavy tars and solids. The light tars, under which key components phenol and naphthalene are removed in the absorber. Absorption is a limited process; the absorption oil will be saturated with tars. The saturated oil however is sent to a stripper in which tars are stripped off. The stripper uses the fluidisation air required by the gasifier, allowing light tars to be recycled and destroyed in the gasifier. Gas is now free of tar related problems and can be dewatered, further cleaned if necessary and utilized in power or synthesis applications. Because all captured solids, heavy and light tars are recycled to the gasifier, OLGA is in principle a waste free system.

Minimisation of oil consumption Within OLGA both oils are regenerated. In the 1st loop this is done by an oil recovery system, designed to maintain the properties of the oil needed for cooling the gas. By separating heavy tars and dust, both heat capacity and viscosity of the oil are controlled. Refreshing the circulating oil with fresh oil is not required. Additional benefit is that the system can handle a certain dust load, therefore not requiring advanced dust filtration upstream the OLGA. In the 2nd loop, regeneration of the oil is done by oil stripping. The light tars absorbed in the oil at low temperature are stripped of at higher temperature. Conventional biodiesel/RME would oxidise as well as vaporise at these conditions, the oil in OLGA is more stable and less volatile, resulting in only minor oil losses in stripping. In comparison, the oil evaporation loss in OLGA is <0.1% on energy basis, whereas for an RME scrubber the oil

1 L.P.L.M. Rabou, R.W.R. Zwart, B.J. Vreugdenhil and L. Bos, “Tar in Biomass Producer Gas, the Energy research Centre of The Netherlands (ECN) Experience: An

Enduring Challenge”, Energy & Fuels (American Chemical Society), 2009, Vol.23, p.6189-6198.

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consumption is ~2%. Additional benefit for the OLGA concept, in which oil is cleaned and recycled, is that no limitations are set to the initial tar load in the raw product gas.

Optimization of the gasification process Due to the consumption of oil in conventional biodiesel/RME scrubbing it is crucial to lower the initial tar load of the producer gas. This can be done by promoting tar reforming in the gasification reactor. This can be achieved by using carefully selected catalytically active bed materials such as olivine. Also steam might be applied to promote the steam reforming and gasification reactions.

To illustrate the consequences of reducing initial tar loads in a gasifier, the FICFB indirect gasifier is compared with the MILENA indirect gasifier. In the FICFB gasifier Austrian olivine as well as steam is used in order to reduce the initial tar load. Typically, this amounts to 25% steam related to the mass flow of wood on top of the steam already available from the moisture in the wood. The MILENA gasifier only needs about 5% to maintain fluidization at the bottom of the gasification zone. Tar content in gas from MILENA is not considered to be of relevance, since OLGA has proven to remove tars from high concentrations to very low concentrations. As such, MILENA generates tar loads up to 60 gr/mn³, whereas FICFB is aiming for approximately one tenth of that.

The FICFB gasification zone requires relatively high amounts of heat to supply energy for the endothermal reforming reactions as well as the heating of the relatively large amount of steam. The result is that the energy of the tar-free product gas that is available for further treatment is 78% for the MILENA-OLGA concept and 72% for the FICFB with conventional RME scrubber. Start-up and off-set conditions Besides the direct economic and indirect efficiency advantage, the OLGA system brings benefits to operations as well. As high tar loads are acceptable, initial start-up issues and off-set conditions (e.g. the bed material not being catalytically active) are not causing any issues in an OLGA based system, whereas in a conventional biodiesel/RME based system (and the need for low tar content) initial start-up and off-set conditions can lead to serious fouling issues.

Features Benefits Recycle of heavy and light tars without oil being consumed

No tar waste streams all energy recovered, resulting in lower operating costs

Oil recovery system in 1st loop, with heavy tar and solids separation

No expensive dust filter required

Oil stripping system in 2nd loop, with light tar separation

Minimization of oil losses

High tar removal efficiency and very low tar dew point in comparison to conventional biodiesel/RME scrubbing

Product gas suitable for gas engines, gas turbines and catalytic upgrading

Compatible with most industrial gasifiers

Free in gasifier selection

Upgrading of conventional biodiesel/RME scrubbers to OLGA tar removal possible

Reduction of oil consumption and allowing for optimization of the gasification process

2A. van der Drift, R.W.R. Zwart, B.J. Vreugdenhil, L.P.J. Bleijendaal, “Comparing the options to produce SNG from biomass”, Presented at the 18th European Biomass Conference and Exhibition, 3-7 May 2010, Lyon, France