Biomass to Liquids (BtL)

Introduction

The term BtL is applied to synthetic fuels made from biomass through a thermo chemical route. The objective is to produce fuel components that are similar to those of current fossil-derived petrol (gasoline) and diesel fuels and hence can be used in existing fuel distribution systems and with standard engines. They are also known as synfuels.

Although the processes for production of BtL are well known and have been applied using fossil-feedstocks, such as methane (GtL) or coal, commercial biofuels based on these technolgies are not currently available in the market place.

However, BtL RD&D in Europe is gathering momentum, and the world's first commercial BtL Plant is under construction in Frieberg Saxony, utilising the Choren Carbo-V ® Process.

The Choren Carbo-V ® Process

The Carbo-V® Process is a three-stage gasification process resulting in the production of syngas:

low temperature gasification,
high temperature gasification and
endothermic entrained bed gasification.

The Fischer-Tropsch (FT) process is then used to convert the synthesis gas into an automotive fuel SunDiesel®. The waxes formed during the FT synthesis are further processed using hydrocracking.

Choren BTL plant - reactor being delievered

The Choren website lists a number of advantages for SunDiesel®:

High cetane number and therefore much better ignition performance than conventional diesel fuel,
No aromatics or sulfur and significantly reduces pollutants from exhaust emissions,
Can be used without any adjustment to existing infrastructure or engine systems,
Largely CO₂-neutral.

SunDiesel® demonstration fuel station at Choren BTL plant

Presentation on Choren BtL plant from EBTP SPM2

CEA Bure Saudron Pilot Plant

In December 2009, CEA (Atomic and Alternative Energy Commission) France announced the construction of a pilot BTL plant in Bure Saudron. The plant will use 75000 tonnes per foresty and agricultural residues to produce ~23000 tonnes/year of biofuel (diesel, kerosene and naptha). The Bure Saudron project will add hydrogen during the synthesis stage to optimise the ratio with carbon monoxide [Source CEA].

NSE Biofuels Demonstration Plant

NSE Biofuels Oy - a joint venture between Neste Oil and Stora Enso - is building a BtL demonstration plant at Stora Enso’s Varkaus Mill in Finland, due to open in 2009. As well as providing test data and operating experience, the plant will also reduce greenhouse gas emissions as wood-based gas from the plant will replace oil in the pulp mill’s lime kiln, making the Varkaus integrate virtually fossil fuel free. Following trials, the joint venture will assess the viability of building a commercial production plant at one of Stora Enso’s mills.

Presentation on the Neste Oil Stora Enso joint venture from EBTP SMP2

Stora Enso Neste Oil BTL demonstration plant

The Dutch Biorefinery Initiative (DBI)

In 2009, a demonstration facility will be initiated in the Port of Rotterdam by WUR and ECN with support from the Netherlands government. This will include a 10 MWth entrained-flow gasification based syngas production platform for heat and power, base chemicals and BtL. Information on this project is included in the IEA Bioenergy Task 42 publication on Biorefineries

BtL Facts and Figures

BtL fuels may be produced from almost any type of low-moisture biomass, residues or organic wastes such as short rotation trees, perennial grasses, straw, forest thinnings, bark from paper-pulp production, bagasse, waste paper or reclaimed wood or fibre based-composites.

It is estimated that over 4m³ of BtL-fuels can be produced per hectare of land per annum. Hence, in future if 4-6 million hectares of land were used to grow energy crops, one could replace 20-25 % of the liquid transport fuel currently used.

The advantage of the BtL route to liquid transport fuels lies in the ability to use almost any type of biomass, with little pre-treatment other than moisture control. This is because the feedstock is gasified in the first stage of the process. The gas produced is then treated further to clean it, remove tars, particulates and gaseous contaminants, and to adjust the ratio of the required gases (hydrogen and carbon monoxide) to that required. The result is a balanced syngas that can be used in the second, catalytic, stage. Syngas may also be obtained by pyrolysis to form charcoal. The hot charcoal is then reacted with steam to produce watergas.

The two main catalytic processes for BtL production are Fisher-Tropsch and the Mobil Process

Fisher-Tropsch

The Fischer-Tropsch process is a catalyzed chemical reaction in which carbon monoxide and hydrogen are converted into liquid hydrocarbons of various forms. Generally the catalysts used, for the following reaction, are based on iron and cobalt.

(2n+1)H₂ + n(CO) -> C_nH_2n+2 + nH₂O

The FT process is an established technology and is already applied on a large scale from coal or natural gas. Developed in the 1920s in Germany, it was used by both Germany and Japan during World War II and later by South Africa and to a lesser extent in the United States.

One problem is the high capital cost of the multistage process. This may be greater when biomass is used as feedstock, since the scale of operation may be limited by the distance over which biomass can be transported to the factory at an economic price. Hence, the economy of scale is decreased compared to a large coal or gas-based operation. Running and maintenance costs are also comparatively high.

Mobil Process

This is a two stage catalytic process. In the first stage LINK methanol is produced. The methanol is then used as feedstock to generate hydrocarbons of varying chain length, using a zeolite catalyst. In the conversion, a number of reactions take place in the gas phase. The conversion is initiated by the removal of water to produce dimethyl ether:

2CH₃OH(g) -> CH₃OCH₃(g) + H₂O(g)

This is followed by various other reactions in which further molecules of water are removed resulting in gradual increase in chain length.

These reactions include the following.

2CH₃OCH₃(g) + 2CH₃OH(g) -> C₆H₁₂(g) + 4H₂O(g)

3CH₃OCH₃(g) -> C₆H₁₂(g) + 3H₂O(g)

As a result of other dehydration reactions occuring in parallel a mixture of hydrocarons is produced of which about 80% is suitable for petrol production. The mixture contains (w/w) around 50% highly branched alkanes, 12% highly branched alkenes, 7% cycloalkanes and 30% aromatics. This process has been commercialised by Methanex in New Zealand using methanol produced from natural gas.