Thermal Decomposition: Fundamentals of Gasification, Pyrolysis, and Incineration

There is sustained international interest in advanced thermal treatment technologies such as gasification and pyrolysis to treat PFAS and other emerging contaminants in biosolids. These systems, and other technologies at WWRFs such as biogas CHP systems, are underpinned by combustion science that is unfamiliar to many wastewater professionals. The information presented here provides the scientific basis to answer common questions about these technologies and other combustion applications, as well as understand common challenges and opportunities in the adoption of these technologies to the resource recovery industry. What is gasification? Pyrolysis? How are they different than incineration? Figure 1 provides a diagram of the key differences. Why do gasification and pyrolysis require little or no oxygen? High temperatures are required because chemical reactions occur exponentially faster with increasing temperature and thermal energy breaks the bonds of complex organic molecules. Organic matter is >90% carbon (C), hydrogen (H), and oxygen (O). Oxygen is the second-most reactive element (after fluorine) in terms of electronegativity, so it reacts readily with increasing temperature. Oxidizing environments are rich in electron acceptors like atmospheric O2. Examples include incinerators and campfires. Reducing environments are low in atmospheric O2 and rich in electron donors like H and C. Strongly reducing environments tend to add H to molecules through reduction (e.g. H2, H2S, CH4). Examples include anaerobic digesters, gasifiers, and pyrolyzers. Gasification occurs in a less reducing environment than pyrolysis where limited atmospheric O2 allows formation of CO. Oxygen is a constituent of organic matter and water, so oxidation and CO2 production can still occur in anerobic environments like digesters. How are biosolids similar to traditional feedstocks for gasification and pyrolysis? What are the challenges of adapting this technology to biosolids? Most commercial fuels are hydrocarbons composed of C and H with impurities like O, S, N, water, and ash that are reduced during refining. Processes analogous to gasification and pyrolysis occur naturally during the formation of anthracite coal. Buried organic matter is carbonized deep underground in an oxygen-starved environment. In addition to tectonic pressure, geothermal heat drives off H, O, S, and water, concentrating the carbon. Gasification and pyrolysis have been practiced for centuries to process coal. The coke-making process carbonizes mid-grade coal through drying followed by heating to 1000° C in an oxygen-starved oven. The gases that are released (H2 and CH4) can be upgraded and combusted as fuel. The coal charcoal product is a critical fuel for steel production. Coal gasification maximizes production of syngas. Wood gasifiers have operated since the 1700s, including wood-fired vehicles during WWII fuel embargoes that injected syngas directly into internal combustion engines. Gasification and pyrolysis technology is now expanding from processing wood and coal to biosolids. Whereas wood and coal feedstocks are dry, consistent, and low in ash, sewage sludge is wet and variable with high ash content. Wood is often less than 1% ash. Wastewater sludge is up to 40% ash, often with high alkaline metal content. Ash is a blend of solid oxidized metals and minerals that remain after combustible material is removed. It includes unoxidized carbon (charcoal), inorganic phosphorous compounds, alkali metal chlorides, alkali metals, and silica. Wastewater processes can add ash to biosolids through grit, ferric chloride, soda ash, and aluminum-based polymers. Ash often has high pH (up to 10-12) due to alkali metals. The final product from pyrolysis, gasification, and incineration have high ash content, whether ash, sand, or biochar. High ash content is a challenge for any thermal process because it melts at high temperatures forming aggregates of sticky molten metal and glass called clinker. Constituents of biosolids ash such as sodium carbonate and alkali metal chlorides act as a catalyst (flux) that promote clinker formation by reducing the melting point of Si, Al, and metal oxides. Molten fly ash and sub-micron siloxane (SiO) aerosols can adhere to downstream metal surfaces causing heat exchanger fouling. Additional challenges include variable heating value and material handling. Blending with feedstocks like wood is a potential solution. Gasifiers and pyrolyzers require some degree of feedstock drying as a pre-processing step. Operation of thermal dryers can be complex, so coupling dryers with novel technology presents further challenges. Where do gasification and pyrolysis fit within the six stages of combustion that occur in any fire? The six stages of combustion are drying, torrefaction/roasting, pyrolysis, gasification, flaming combustion of vapors, and glowing/smoldering combustion. Thermal energy is first absorbed by fuel moisture causing evaporation. Second, roasting causes the lightest compounds to volatilize and release flammable torrefaction vapors. During pyrolysis, the organic matter continues 'cracking' into pygas in oxygen-free areas. During gasification, cracking is completed with conversion to syngas in areas with limited air. When exposed to atmospheric O2, hot torrefaction vapors, pygas, and syngas spontaneously combust, causing flaming combustion (Figure 2). As the H, O, and some of the C from organic matter are converted to H2O and CO2 through combustion, solid carbon is left behind (carbonization) in the form of glowing charcoal along with non-combustible ash. All six stages are occurring simultaneously in different points of the same fire. Pyrolyzers and gasifiers are specialized reactors that optimize the third and fourth stages of combustion, respectively. How does a thermal oxidizer work? Many suppliers channel syngas and pygas to a thermal oxidizer (TO) where it is immediately combusted to avoid tar formation. The heat is often recovered to dry incoming sludge. Air is injected into the TO with turbulent mixing of atmospheric O2 to achieve complete combustion at high temperatures (982-1200 ° C). Gasifiers, pyrolyzers, and dryers can be hot enough to vaporize and/or aerosolize PFAS. Bench-scale literature suggests 1,000° C is sufficient to destroy PFAS. In addition to temperature, residence time and turbulence are critical factors for destruction of PFAS and conversion of CO to CO2. The TO is generally 200-535° C hotter than a gasifier because combustion is ~3-4 times more exothermic than gasification reactions. A TO can also refer to a device that controls air emissions and odors by burning up the flammable pollutants in a natural gas flame. Why are the air emissions from gasification and pyrolysis easier to permit than incinerators? Syngas burns 10-100 times cleaner than dried biosolids (depending on the air pollutant), so it is easier to get an air permit for these clean burning fuels. The stoichiometric line is the balance point with exactly enough air molecules to combust every molecule of fuel into CO2 and H2O. Above and below the line, the ratio of air and fuel is not balanced. Examples of fuels that achieve complete combustion include: - Upgraded biogas (CH4 + 2O2 -> CO2 + 2H2O) - Syngas and pygas (2H2 + O2 -> 2H2O; 2CO + O2 -> 2CO2) Gasification reactions occur at 20-40% of the 100% stoichiometric balance point. With additional air, the reactions transition from syngas production to combustion. Gasification is analogous to choking a carburetor with an air-restricted fuel-rich mixture. Wood and biosolids have too much C and O when combusted as solid fuel compared with 21% O2 in the atmosphere. Instead of generating CO2 and H2O, the imbalanced ratio of oxygen to carbon causes the C, H, and O to rearrange and form numerous CnHnOn toxic constituents such as volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), acidic gases, tars, carbon particulates, and unburned syngas. Products of incomplete combustion are highly flammable, including carbon particulates (soot), which are the visible fraction of smoke. In structural fires, a fire can consume the available oxygen in a building with inadequate ventilation, allowing smoke and gases to build up with a temperature well-above the ignition point. Opening a door allows air to rush in, causing instant combustion and explosion. Firefighters are trained to manage this dangerous phenomenon, which is called backdraft. Incinerators generate products of incomplete combustion which creates permitting and regulatory challenges. In conclusion, the core science presented here enhances the ability to evaluate thermal treatment technologies, reduce confusion during project development, improve design, permits, communicate with the public, and train confident operators.