Pyrolysis is the thermal decomposition of organic fuels (such as biomass resources) into volatile compounds (gases such as carbon monoxide, carbon dioxide, and water vapor; and liquids such as bio-oil), and solids (char) in the absence of oxygen and usually water. Pyrolysis types (slow, flash, and fast) are differentiated by the temperature, pressure, and residence (processing) time of the fuel source, which determines the types of reactions that dominate the process and the mix of products produced. Currently, most of the interest in pyrolysis focuses on fast pyrolysis, which involves rapid heating rates (>400°F per second) and temperatures usually in excess of 1,000oF. Due to the short residence time, the main products are ethylene-rich gases that can be used to produce alcohols and bio-oil, rather than char and tar.
The types of products produced during pyrolysis depend critically on how fast the biomass material is heated (the heat rate), as well as the temperature and length of time (residence time) at which the reaction is maintained. At slow heat rates and low reaction temperatures, solid material (char) is the dominant product. As the heat rate and temperature increase, the yields of volatile products (gases and liquids such as tars) increase. The conversion of tar to gas occurs at high heat rates and temperatures, and is maximized at temperatures between 1000°F and 1300°F. Heat rates are lower for large particle sizes (more char) and higher for small particles (more liquids). High pressure increases residence times (more gas and less tar) and low pressure decreases residence times (more char). The atmosphere in which the reactions occur (i.e., vacuum, inert, or reactive) as well as the presence of inorganic salts and acid or alkali reagents can affect temperate, and thus influence the mix of products.
The composition of the biomass feedstock (i.e., percent cellulose, hemicellulose, and lignin) is a key determinant of the mix of products produced during biomass pyrolysis. Cellulose is converted to char and gases (carbon monoxide, carbon dioxide, and water vapor) at low temperatures (<600oF), and to tars and oils at high temperatures (>600oF). The pyrolysis of cellulose is complete at temperatures above 1150°F. Hemicellulose is the most reactive component of biomass. It decomposes at temperatures between 400 and 525oF and produces more gases and less tar than cellulose. Lignin is the least reactive component of biomass, with decomposition occurring between 560°F and 960°F, and it yields more char and tar than cellulose. The natural level of ash (inorganic chemicals) in biomass resources acts as catalysts and influences the overall mix of products.
The biomass pyrolysis product of most interest is bio-oil, which can be used as heating oil and to produce liquid transportation fuels and organic chemicals. Bio-oil is formed when volatile compounds (gases and tars) are cooled and condensed. It contains less nitrogen than petroleum, and almost no metal or sulfur. Bio-oil can not be readily mixed with petroleum derived fuels. It is acidic and corrodes most metals other than stainless steel. Bio-oil has low viscosity and can be pumped and transported through pipelines. NOx emissions are low when combusted. Bio-oil cannot be dissolved in water. It is relatively unstable compared to fossil fuels. Table 1 summarizes select properties of bio-oil derived from the pyrolysis of wood.
A number of different kinds of pyrolysis reactors are available. Pyrolysis is a precursor to gasification, and the same reactors can be used for pyrolysis. Bubbling fluidized bed reactors are simpler to design and construct than other reactor designs, providing good bio-oil yields but requiring small biomass particles (as the rate at which the biomass particles are heated is the limiting factor). Circulating fluidized bed reactors are similar to bubbling fluidized bed reactors, but have shorter residence times. The heat supply typically comes from a secondary char combustor. Ablative pyrolysis reactors transfer heat from the hot reactor wall to biomass particles in contact with the wall, permitting pyrolysis of larger biomass particles. The process is dependent on surface area and the use of mechanical drivers, making it more complex and more expensive to scale up to larger facilities. In rotating cone reactors, room temperature biomass particles and hot sand are introduced near the bottom of the cone, mixed, and transported upwards by the rotation of the cone. Thus, rapid heating and short residence times can be achieved.