Pyrolysis

Fixed bed reactors were traditionally used to produce charcoal due to slow and poor heat transfer resulting in very low liquid charcoal yields. The technology is simple and reliable and utilises relatively uniform particle size. This type of kiln/furnace is divided into downdraft and updraft. In a downdraft fixed bed reactor, the solid material moves slowly down a vertical shaft, with the solid and product gas move downward in a co-current mode which produces a relatively clean gas with low tar and high carbon conversion rate. In an updraft fixed bed reactor, the feedstock also moves down a vertical shaft but in a counter-current mode. This results in a dirty product gas with high levels of tars which can be alleviated using tar crackers. However, due to the feedstock not moving during the process, it is difficult to obtain heating of large quantities of feedstock material on an industrial scale. This could be solved if the reactor used methods that enable better heat transfer such as heat pipes.

In bubbling fluidised beds, particles are entered into a bed of hot sand fluidised by a recirculated product gas. The high heat transfer rates from the fluidised sand cause rapid heating of biomass particles and some ablation with the sand particles occurs. This type of reactor is characterised by good temperature control, limited turn down capacity and efficient heat transfer to biomass particles due to high solid density.

In circulating fluidised beds, biomass particles are introduced into a circulating bed of hot sand. The recirculated product gas, sand and biomass move together in the reactor. The high heat transfer rates from the sand ensure rapid heating of the biomass and ablation is more prevalent than bubbling fluidised beds. This reactor is characterised by good temperature control and high heating rates. The residence time for the tar produced is around the same for the vapour and gas and can be separated by cyclone.

For a vacuum furnace reactor, the biomass is thermally decomposed under reduced pressure. This produces vapours which are quickly removed from the vacuum and recovered as bio-oil via condensation. This reactor has the ability to produce larger particles than most fast pyrolysis reactors and there is less tar in the bio-oil product due to lower gas velocities. The typical liquid yields for dry biomass feed produced from vacuum furnace reactors range from 35% to 50%.

The ablative reactor provides high relative motion between the reactor wall and the particles in the presence of high pressure exerted by the particles of the hot reactor wall. The system is intensive and the process is mechanically driven so the reactor is complex. This type of reactor has advantages over fluidised bed:

• No pre-treatment/sorting of the biomass is required because it is in direct contact with the hot surface so is not influenced by particle size
• They have good heat transfer with high heating rates and relatively small contact surface because their compact design
• They have high energy and cost efficiency as no heating and cooling of fluidizing gases is required
• They allow installation of condensation units with a small volume, requiring less space at lower costs

The rotating cone reactor is a novel type for flash pyrolysis. It is characterised by rapid heating, a short residence time for solids and negligible char formation. Biomass materials (wood, rice husks, olive stones) are pulverised and fed into the rotating cone pyrolyzer. There is also a riser for sand recycling and a bubbling char combustor for char burn off in which carrier gas is required. Relatively fine particles are needed to produce good liquid yields of 60% to 70% of dry feed. However, this type of reactor is still at pilot scale as there is no commercial applications as of yet.

In an auger reactor, hot sand and biomass particles enter at one end of a screw. The screw mixes these inputs and conveys them along, providing good control of the biomass residence time. The process does not dilute the pyrolysis products with the carrier or fluidising gas, but the sand must be reheated in a separate vessel. Mechanical reliability is a concern and so there is currently no large-scale implementation of this reactor type. Advantages of this reactor:

• Compact size
• No carrier gas required
• Lower operating temperature (400°C)[7]