Lignocellulosic-based biofuels from synthesis gas

Solid, lignocellulosic biomass can be converted into liquid fuels by producing synthesis gas in the first step, followed by liquification, e.g. via Fischer-Tropsch-Synthesis, in a second step. The combination of gasification and the subsequent Fischer-Tropsch-Synthesis is referred to as BtL (Biomass-to-Liquids) process.

There are two options for syngas production:

• Gasification
• Biogas production via bio-chemical conversion

Since the latter option is more suitable for wet organic waste materials, only gasification will be discussed in the following.

Gasification of solid biomass

Gasification is a chemical-physical process where parts of a solid or a fluid are converted into gas by heating, in some cases under a low-oxygen atmosphere. The contained carbon is converted into a product gas that is rich in hydrogen (H2) and carbon monoxide (CO). Solid residues can be left over from the reaction. The gasification starts at temperatures above 150°C but in the hottest zones of the reactor more than 1000°C can be reached.

The process of gasification of lignite was widely used in Europe in the 19th to supply cities with gas for lighting, cooking and heating. The so called town gas was distributed via municipal pipe systems into the households.

Further, in times of oil shortage after the second world war, cars were partly equiped with improvised gasifieres to fuel them with wood gas. Therefore biomass gasification can be partly based on already existing technology.

Modes of operation

Different options exsist for gasification process management. The gasification process is an endothermic reaction which means that heat has to be supplied to the system.

In the autothermal mode of operation, the heat is provided by partial combustion of the fuel to be gasified in the gasification reactor itself.

In the allothermal mode of operation, the necessary heat is supplied from outside. Allothermal gasification processes have seperate reactors for heat production (combustion reactor) and heat consumption (gasification reactor). The thermal energy is transported from the combustion unit to the gasification reactor by circulating a hot bed material or by utilizing heat exchangers (Rauch et al. 2018). Advantages of allothermal production processes are a nitrogen-free product gas and complete carbon conversion without carbon containing waste streams. This is because, carbon containing streams from product cleaning can be converted into heat which is used to heat the gasification reactor.

The gasification process can be carried out with or without a gasification agent (e.g. air, pur oxygen or steam). If air is added a gasification agent, it is much less than needed for complete combustion. The gas obtained contains a lot of carbon monoxide, but also nitrogen from the air, as well as significant amounts of water vapor. Because of the high nitrogen content, the specific calorific value of the gas is low.

In hydrothermal gasification, water vapour is used as the gasification agent. It is also possible to use a water vapour/oxygen mix as gasification agent. The obtained gas does not contain any nitrogen and less carbon monoxide and has therefore higher heating values than gas from processes using air. However, the production of oxygen is costly and only for large facilities economically feasible.

Gasifier technologies

Before the material can be gasified in some cases it has to dried and shredded. Different gasifier technologies are available. The classification is made according to the contact as well as the flow of the gas through the reactor:

• fixed bed gasifiers
• fluidized bed gasifiers
• entrained-flow gasifier

Fixed bed gasifiers

In fixed bed gasifiers the biomass enters the reactor at the top. The fuel slowly moves downward due to gravity and decomposition processes to the bottom of the reactor where it is exposed to the gasification agent, which is directed into the reactor from the bottom in a countercurrent flow (updraft reactors, see far left figure above). If the air is fed into the reactor in an upper part, the gasification agent moves into the same direction like the fuel. These reactors are therefore called downdraft reactors (see second picture from left in the figure above). Thereby different zones are developing where the different stages of gasification process occur.

The hottest zone of the updraft reactor is at the bottom because there the gasification agent (air) is fed in. This is where the heat needed for the process is generated and the product gas is produced. Due to high temperatures ash components that do not thermally decompose liquefy and form slag which has to be removed below. The upstreaming hot gas is used for warming up and drying the fuel in the zones above. In the reduction zone above the oxidation zone the carbon dioxid (CO2) produced during oxidation is partly reduced to carbon monoxide (CO) and occuring water vapour to hydrogen. Above the reduction zone is the pyrolytic zone, where the fuel is thermo-chemically cracked with the heat from the oxidation zone. The product gas is released at the top of the reactor. On its way through the reactor the gas is cooling down. Updraft reactors have several advantages:

• high gasification efficiency
• low temperatures of the product gas (100-200°C)
• very low alkali metal contents
• low particle content in the product gas
• low requirements for fuel preparation

The disadvantages are:

• considerable amounts of undesired components, like tars in the product gas
• quite high water vapor content in the product gas
• for further use, the product gas quality requirements cannot be met so easily

In downdraft gasifiers the fuel and the gasification agent are moving in the same direction - from the top to the bottom. The fuel is dried and pyrolized in the upper part in the absence of air. Then it moves down in the very hot oxidation zone (>1000°C). Long-chain organic compounds are cracked into short-chain and low-tar gaseous compounds. This compounds and the resulting coke and ash move further in the reduction zone where they are reduced.

Advantages:

• product gas has low tar contents and can be used without further gas cleaning (e.g. for liquid fuel production)

Disadvantages:

• high product gas temperatures (600-800°C)
• high requirements for fuel size and water content (<20%)
• problems occure in big reactors (temperature in some parts too low) and in partial load operation

Commercial use of this technology has not been achieved to date. The operation is partly unsatisfactory for technical and economic reasons.

Fluidized Bed Gasifiers

Fluidized bed gasifiers are most frequently used for syngas production. They have a bed material which is chemically inert and does not participate in combustion. Mostly it is sand. The fluidized bed is formed when the gasification agent flows rapidly through the gasifier it swirls up the bed material and flows around the added fuel. The fuel particles are much smaller (up to 40 mm) than in fixed bed reactors and are completely mixed with the bed material. Fluidized-bed gasifiers do not form distinct temperature and reaction zones. The reactions taking place during thermochemical conversion run in parallel in the entire reactor. The tempertures are around 700 to 900°C. The process can be controlled more easily. Due to the intensive heat transfer from the bed material to the fuel particles, the temperature and residence time of the fuel in the reactor is reduced. This enables a high material conversion even with smaller reactor dimensions.

Tar contents in product gas from fluidized bed gasifiers are higher than in that from downdraft gasifiers, but significantly lower than for updraft gasifiers. But the particle content in the product gas is significantly higher than for fixed bed reactors, since fine-grained fuel, fine-grained ash or abraded bed material is entrained with the product gas during fluidized bed gasification.

Two different engineering approaches can be realized:

Stationary fluidized bed reactors

• Gasification agents are air, oxygen, steam or mixtures thereof
• Technology have been tested on a large scale (coal and peat, waste wood, bagasse, olive kernels)
• Not suitable for biomass with low ash softening temperatures (e.g. straw)

Circulating fluidized bed reactors:

• Gasification agents are air, oxygen, steam or mixtures
• High gas velocities cause the bed material to be discharged from the reactor
• Product gas has to be cleaned with cyclones and bed material is fed back into the reactor
• Technology commercially proven (co-firing, paper and pulp industry, power generation from biomass - Integrated Gasification Combined Cycle technology)

Entrained flow gasifiers

In entrained flow gasifiers, the gasification of the fuel takes place “on the fly”. Here, the finely ground or pasty biogenic fuel is blown through the reactor together with the gasification agent in co-current. Almost complete gasification takes place. See illustration on the far right in the picture above. The technology is only of minor importance nowadays, although entrained flow gasifiers with oxygen as gasification agent have excellent conditions for producing product gases suitable for Fischer-Tropsch processes.

Product gas requirements

There are some requirements for the product gas to be suitable for further conversion into liquid fuel. 

Important properties are:

• H2/CO ratio of 2
• Very low nitrogen content (inert gas reduces conversion efficiency)
• Low methane and hydrocarbons content (behave like inert gases)
• Low levels of catalyst poisons (e.g. sulphur, nitrogen or chlorine)

This results in certain requirements for the gasification reactor:

• Suitable gasification agents are oxygen, steam, CO2 and mixtures of them
• High gasification temperatures are needed to avoid methane, hydrogcarbons and sulphur occurance

Cleaning and upgrading

After leaving the gasifier the product gas has to be cleaned (purified) and upgraded (conditioned).

Purification eliminates substances negatively affecting downstream processes. Especially catalysts are very sensitiv to small amounts of sulfur, halogens, nitrogen components, metals, dust and organic compounds. Particles can be removed from the gas by using cyclones, barrier filters, electrostatic precipitators or scrubbers. Tars or hydrocarbons (methane, ethylene, benzene and toluene) can be removed for example at temperatures above 1000°C or with the help of catalysts. In entrained flow gasifierers all organic compounds are thermally destructed during the process. Inorganic components lice sulfur can be removed by wet scrubbing (absorption by physical, chemical or solid solvents - e.g. rectisol, amins or zinc oxide).

Conditioning removes undesired gas components like CO2 and adjust gas components (e.g. H2) to the wanted ratios. After finishing cleaning and upgrading the syngas is ready for liquefaction.

Water gas shift reaction can be used for product gas conditioning. A gas consisting mainly of carbon monoxide (CO) and hydrogen (H2) is called “water gas”. "Shift” means to change the ratio of carbon monoxide to hydrogen. The ratio can be increased by adding CO2 or reduced by adding steam.

The water gas shift reaction (WGSR) can be used to adjust the CO/H2 ratio in synthesis gas, i.e. to reduce the carbon monoxide content (see upgrading) or to produce hydrogen. During WGSR carbon monoxide reacts with water vapour to carbon dioxide and hydrogen. The following equation shows the water gas shift reaction, also known as conversion equilibrium:

The reaction is moderately exothermic and reversible. As the temperature increases, the chemical equilibrium shifts from reaction products to reaction reducts and the speed of the reaction increases. Further, the reaction shifts towards carbon monoxide as temperature increases. The equilibrium for H2 production is favored by high moisture content and low temperatures. Different catalysts are used for low temperature shift (200-250°C) and high temperature shift (350-500°C). Chromium or copper promoted iron-based catalysts are used for high temperature shift and low temperature shift deploys a copper-zinc-aluminum catalyst.

WGSR is also used in steam reforming processes to elevate the hydrogen content.