Pyrolysis and Gasification are similar processes of heating with limited oxygen. Pyrolysis for liquefaction usually uses no oxygen while gasification uses a small, controlled amount. Pyrolysis oil can be used directly as a fuel or as an intermediate for production of chemicals.
Fast pyrolysis is a thermal decomposition process operating at moderate temperatures (450-600 C) with high heat transfer rates to the biomass particles and a short residence time. Under these conditions, organic vapors, pyrolysis gases and charcoal are produced. A short residence time is required to obtain the maximum yield of the liquid. The vapors are condensed to produce pyrolysis oil (often referred to as bio-oil). Yields of liquid products as high as 79% of the initial dry weight of the biomass can be achieved. The process produces no waste and either the pyrolysis gas or charcoal is used to heat the reactor and the other can be used to supplement the other in heating, dry the feedstock, the charcoal can be sold as a byproduct or the pyrolysis gas can be used to fuel a gas engine. Pyrolysis oil is greenhouse gas neutral, does not produce SOx (sulfur dioxide) produces approximately half of the NOx (nitrogen oxide) emissions compared to fossil fuels. It is now being used for the production of chemicals and is being developed for producing liquid fuels.
EERE has a web site with a little more about pyrolysis and links to other sites.
Three companies are at the forefront of development of pyrolysis liquefaction.
DynaMotive of Vancouver, Canada claims to be the world leader in the development of technology for the production of bio-oil. A fluidized bed reactor has a bed of sand which is fluidized with a non-oxidizing gas. The shredded biomass, of sufficiently small size (1-2 mm ), is introduced simultaneously with the pre-heated gas which heats the particles to 450-500 C producing organic vapors, pyrolysis gases and charcoal. The resulting char particles are separated from the reactor effluent stream and the gases are rapidly quenched producing the bio-oil. The remaining non-condensable gases are combined with natural gas to heat the reactor. The process converts about 62% of the feedstock into bio-oil. Their objective is to produce bio-oil at cost that is competitive with diesel fuel and natural gas in North America, therefore being competitive with fossil fuels in Europe and Asia where the cost of fossil fuel is higher.
DynaMotive, as of April 27, 2005, has a 100 ton per day system in operation at West Lorne, Ontario, Canada using wood flooring residue as feedstock. The plant is supplying electricity to the grid. Commissioning will continue through the second quarter, doing minor modifications and acceptance tests. When commissioning is complete the plant will be capable of supplying 2.5 Mwe per hour to the grid. DynaMotive has an agreement in principle with Megacity Recycling of Ontario to develop a 200 ton per day plant, using forest wastes as feedstock, with an option to develop a second plant. The first plant is expected to be complete in 2006. In the event the option exercised, delivery of the second plant could occur in 2007.
BTG (the Netherlands) has developed a rotary cone reactor with a maximum oil yield of 79% at 500 C. The rotary cone is a gas-solids contactor using hot sand and biomass particles. The biomass particles must be less than 6 mm in size. The sand and feedstock are mixed together at the bottom of the reactor and moved upward by the rotating action of the cone while it is being pyrolized. The charcoal and the sand are separated from the effluent gases. The charcoal is burned to provide heat for the pyrolysis, thus having a self sustaining process. No carrier gas is required as in the fluidized bed reactors thus reducing the volume of gas that has to be handled, but this is offset by requiring the sand to be separated from the gases and then recirculated. The hot vapors are condensed to form the pyrolysis oil. The remaining non-condensable gases can be used to run a gas engine or for heating. In 2003 they were scaling up their pilot plant reactor to a 50 ton (110,000 lb) per day system.
Fortum of Finland has a pilot plant operating at 300-350 kg per hr? of pyrolysis oil. The feedstock is crushed to less than 8 mm. The reactor operates at about 500 C with a retention time of about 1 second. Non-condensable gases are used to dry the feedstock to less than 10% moisture and the charcoal is used to heat the reactor. Sixty to seventy percent of the feedstock is converted into pyrolysis oil.
Bio-oil has about 40% of the heating value of diesel, but can be used directly in diesel engines or gas turbines. Because the bio-oil has less heating value it must be fed at a higher rate than petroleum fuel and the combustion chamber may have to be larger to accommodate the higher flow rate. Bio-oil has a higher viscosity than diesel so it must be pre-heated to lower the viscosity. The injection nozzle must be redesigned and the pressure drop increased to provide a better spray pattern. Because of the low PH, the materials in the fuel lines must be of stainless steel and higher quality plastics. The solids, a combination of ash and char fines, can cause problems similar to those encountered in using heavy oil. This requires the use of additives and some minor modifications to the engine, which have been in use for decades. These modifications still result in a combination of capital cost and operating efficiency which are economically sound.