Pyrolysis meaning is given as the chemical decomposition of organic (carbon-based) materials through heat application. Pyrolysis is also the first step in combustion and gasification, which occurs in the absence or near absence of oxygen, and thus differs from combustion (burning), which occurs only when adequate oxygen is present. The rate of pyrolysis increases with the temperature. In industrial applications, the temperatures that are used are often 430 °C (about 800 °F) or higher, whereas, in the smaller-scale operations, the temperature may be very lower.
Let us look at the pyrolysis process in detail.
Pyrolysis transforms the organic materials into their gaseous components, which are a solid residue of ash and carbon and a liquid known as pyrolytic oil (otherwise as bio-oil). Pyrolysis contains two primary methods for removing contaminants from a substance: destruction and removal. Whereas, in destruction, the organic contaminants are broken down into compounds having lower molecular weight, and in the case of the removal process, they are not destroyed; however, they are separated from the contaminated material.
Pyrolysis can be a useful process for treating organic materials that either “crack” or decompose under the presence of heat; some pyrolysis examples include dioxins, polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs). Although pyrolysis is not more useful either for removing or destroying inorganic materials like metals, it may be used in techniques that render those materials inert.
Known Products of Pyrolysis
Two of the well-known products, which are created by pyrolysis, are a charcoal form known as biochar, created by heating coke and wood (used as a heat shield and an industrial fuel), created by heating coal. Also, pyrolysis produces condensable liquids (or even tar) and noncondensable gases.
When the organic matter gets heated at increasing temperatures in open containers, generally, the below-given processes occur, either in successive or overlapping stages:
About below 100 °C, volatiles, including some amount of water, evaporate. Heat-sensitive substances, such as proteins and vitamin C, may partially either change or decompose already at this particular stage.
About 100 °C or slightly higher, any remaining water, which is merely absorbed in the material, gets driven off. This process consumes an excessive amount of energy. Hence the temperature can stop rising until the entire water has evaporated. Water trapped in the hydrates crystal structure can come off at somewhat higher temperatures.
Some solid substances, such as waxes, sugars, and fats, may melt and separate.
Between a temperature of 100 and 500 °C, several common organic molecules break down. Most of the sugars start decomposing at a temperature of 160–180 °C. Cellulose, which is a major component of paper, cotton fabrics, and wood, decomposes at around 350 °C. Lignin, which is another major wood component, starts decomposing at a temperature of about 350 °C but continues releasing the volatile products around 500 °C.
Methane Pyrolysis for Hydrogen
Methane pyrolysis is given as a non-polluting industrial process for the production of hydrogen from methane by removing the solid hydrogen carbon from natural gas. This is a one-step process, and it produces non-polluting hydrogen in high volume at a low cost. Only the water is released when hydrogen is used as the fuel for the transportation of fuel-cell electric heavy trucks, gas turbine electric power generation, and hydrogen for industrial processes, including ammonia fertilizer and cement production.
Methane pyrolysis is the other process, which operates up to 1065 °C for the production of hydrogen from natural gas, which allows the removal of carbon easily (solid non-polluting carbon is given as a byproduct of the pyrolysis process). The carbon, which is of industrial quality, can then either be sold or landfilled, and it is not released into the atmosphere, with no emission of greenhouse gas (GHG).
Pyrolysis contains numerous application counts of interest to green technology. It may be useful in the materials extraction from goods such as vehicle tires, creating biofuel from crops and waste products, and removing organic contaminants from soils and oily sludges. Also, pyrolysis can help in the breakdown of vehicle tires into some useful components, hence reducing the environmental burden of discarding the tires. Tires are a significant landfill component in many areas, and when burned, they release heavy metals and PAHs into the air.
When tires are pyrolyzed, however, they break down into oil (usable for fuel) and gas and carbon black (that may be used as filler in rubber products, including new tires, and as an activated charcoal in fuel cells and filters). Pyrolysis, in addition, may remove the organic contaminants, such as synthetic hormones, from sewage sludge (it means the semisolid materials, which remain after wastewater is treated and the content of water reduced) and make the heavy metals remaining in the sludge inert that allows the sludge to be safely used as fertilizer.
Moreover, the pyrolyzing biomass (biological materials such as sugarcane and wood) holds great promise for producing energy sources, which could either supplement or replace petroleum-based energy. Pyrolysis causes the hemicellulose, cellulose, and part of the lignin in the biomass (pyrolysis of biomass) to disintegrate to the smaller molecules in gaseous form. Those gases, when cooled, condense to the liquid state and become bio-oil, while the original mass remainder (primarily, the remaining lignin) is left as noncondensable gases and solid biochar.