Gasification technologies

Gasification technologies

SFC focuses on technologies relevant to industrial interests, namely indirect fluidized bed gasification, entrained flow gasification, direct fluidized bed gasification. These technologies are shortly described in the following.

Direct fluidized bed gasification

In fluidized bed gasifiers the reaction space contains a sand-like bed material that is fluidized (the sand is lifted by the gas stream and gets a liquid appearance) or entrained by the oxidant gas (air or oxygen), steam or mixtures thereof being added in the bottom. Autothermal or direct gasifiers use an oxidant, and allothermal or indirect gasifiers, see below, use steam without an oxidant being fed to the gasifier section.

The feedstock for fluidized bed gasifiers can be very broad, but too much of small fuel particles are undesirable. In the case of a stationary (or bubbling) fluidized bed the bed material is kept suspended by the gas in a defined bed volume through which gas in the form of interstitial gas and bubbles pass. Above the bed there is a freeboard section used for disengagement of particles mainly ejected by bubbles erupting on the bed surface.

In a circulating fluidized bed, the gas velocity is higher than for a stationary (bubbling) bed and the bed material is carried up in the gasifier shaft by the gas. Some of this material moves radially to the wall and transported back to the bottom by gravity as part a wall layer sliding down. The remainder of the solid-gas suspension is carried out by the gas to an external primary particulate separator, typically a cyclone, from which it is returned to the bottom of the gasifier by means of a recycle line with a moving bed of solids. The effect is huge net circulation rate between the bottom and the top of the reactor.

The vigorous movement of the bed material in combination with the large circulation of bed materials gives a high internal heat transfer rate that assists in maintaining an even temperature in the entire bed. This avoids hot spots and thereby avoiding or restricting agglomeration. The temperature is in the range 750-950°C, and is limited by ash melting properties.

Even if the feedstock is fed in to a hot environment, the temperature is not enough to completely decompose tars. The raw product gas typically contains tar in a magnitude of 5-20 g/Nm3. properties and handling.

Direct fluidized beds can be built at large scale. The largest biomass gasifier in operation at atmospheric pressure, at Vaasa, Finland, has a capacity of 140 MW thermal. They can also be pressurized up to 1-3 MPa to increase the single vessel capacity further.

Indirect fluidized bed gasification

Allothermal, or indirect fluidized beds (i.e. that use a gasifier-combustor combination, e.g. Milena, FICFB and Batelle designs) can have different combinations of fluidization types in the two beds, such as two stationary fluidized bed reactors, one stationary fluidized bed gasifier and circulating bed combustor, one circulating fluidized bed gasifier and one stationary bed combustor or two circulating fluidized beds. Indirect double fluidized beds function in the same way as a fluidized bed. The main difference is that there is no oxidant, instead typically steam is added to the gasifier: The energy required is provided by hot sand bed material being transferred at high rate from the second, combustor bed.

After releasing heat in the gasifier, the sand is returned to the combustor from the bottom of the gasifier and where entrained carbonaceous char material from the gasifier and other fuels are combusted with air to generate the temperature required to re-heat the massive flow of sand. Since the combustor is subjected to the same limitations on operating temperature as other fluidized beds to avoid agglomeration, the gasifier is operated at slightly lower temperature, compared to when air or oxygen is used directly, to have a temperature difference that matches the sand flow. Therefore the tar content is of the same magnitude or slightly higher than for a direct fluidized bed. The heated sand is the returned to the gasifier via a bed overflow in the case of a stationary fluidized bed combustor, or via the primary separator for circulating fluidized beds.

The need for a close-coupled circulation loop in practice restricts the capacity of such reactors from layout limitations to somewhere above 50 MW thermal. Due to the small pressure differential between the two reactors, pressurizing such units become very challenging. The main advantage of these systems is that a medium calorific value (MCV) gas can be produced without the recourse to the use of oxygen.

Other forms of indirect fluidized bed gasifiers use indirect heat transfer into the bed via heat exchanger tube bundles immersed in the bed using flue gases from combustion of part of the product gas outside of the gasifier section, or so-called heat pipes immersed in the bed that work as an intermediate heat transfer unit.

The advantages of indirect gasifiers relate to that for synthesis gas, or that a medium calorific heating value gas, can be produced without the use of pure oxygen and that for some designs total conversion of the fuel can be achieved.

Entrained flow gasification

Most entrained flow gasifiers are vessels where the fuel is injected and partially oxidized in one or more burners and where the residence time is sufficient to complete the reactions, including both the gasification of the solids the decomposition of tars and other hydrocarbons, within a few seconds. For this reason, the fuel must either be a pumpable liquid that can be dispersed to droplets in the burner nozzle or small particles (< 1 mm) that can be consistently fed by means of dense phase transport to the fuel register of the burner.

Entrained flow gasifier also typically operates in an ash melting mode, i.e. the ash is removed as slag in the bottom of the reactor. This requires operation at very high temperature, e.g. 1 500 °C, to reduce the slag viscosity and make it free-flowing to avoid build-up of slag in the reactor. To reach such high temperatures with low energy content feedstocks and still retain a significant heating value the use of oxygen mixed with steam is required. Air is not possible to use.

Entrained flow gasifiers are also typically pressurized up to 1-3 MPa for solids or even higher for some high-energy liquids. The high gas velocity and short reaction time at pressure makes it possible to scale this type of technology to large capacities, up to 400 MW or more. However, the complexity of fuel preparation in the case of solids, and the requirement of using oxygen typically do not make small installations feasible.

One advantage of this gasifier type is the ash melting which yields a dense slag by-product with less leaching rate. Since many lighter inorganic species and heavy metals are evaporated at the high temperatures in the gasifier, such dense slag may even be recycled as construction material.

There are also other forms of entrained flow gasifiers where fuel is injected into a gasifier or pyrolizer and the resulting gas and char solids are heated indirectly by radiation and convection in several heated tubes. The tubes are then heated on the other side by firing product gas and/or char to heat the tubes.

In addition, a type of “entrained flow” post-treatment is common downstream of fixed or fluidized bed gasifiers as a mean to reduce the tar content from the gasifier itself. Such devices can be air- or oxygen-blown and sometimes are assisted by a plasma generator.