Gasification: Refining Safety

Waste to energy developers and operators could do well to take note of the hard learned lessons from the petro-chemical industry Around the world the need to reduce the carbon footprint of energy generation, as well as to treat waste in the most environmentally sound way possible has given rise to a growing number of waste gasification facilities Deb Pal looks at the health and safety challenges present in such facilities. Gasification is the process by which solid organic matter is converted into a highly flammable synthesis gas in a partially oxidising environment. The energy industry has many years experience of combustion as a power generation method, but has limited experience of the hazards created during the production of synthesis gas (syngas) by gasification. These hazards, however, are well understood in the chemical process industries, whereby safety techniques have evolved to ensure the safe design, construction and commissioning of plants. Capitalising on this experience is essential to ensure that the emerging waste to energy industry is not derailed by the type of major incidents that have occurred on a number of occasions in the chemical industry. Using process industry safety techniques a number of key issues relating to gasification plant design have been identified. Incorporation of the necessary safety features at an early stage in the design will ensure that the plant is safe, meets the required legislative standards, satisfies the ALARP (As Low As Reasonably Practicable) criteria, and is able to handle a variable feedstock. The most critical process safety issues applicable to waste to energy plants are explosion hazards, fire hazards and toxic gas. Explosion hazards Equipment overpressure can occur due to a range of events which must be identified to enable adequate pressure relief to be specified according to pressure relief standard API 521. The likelihood of external or internal explosion depends upon whether the gasifier operates at above atmospheric pressure or below atmospheric pressure. At underpressure there is a risk of air ingress whereby an explosion may occur inside a particular plant section. As there are likely to be sources of ignition within equipment, for example the gasifier, a key explosion prevention measure is avoidance of an explosive atmosphere. In order for an explosion to occur the syngas concentration needs to occur in the flammable range, and oxygen is required. A gasifier plant routinely passes through the flammable range, hence the need for purging to remove oxygen. Therefore a potential explosion risk may occur during plant start-up, at shutdown or in the case of uncontrolled air intake, for instance due to leakages. As such, gas tightness is an important prevention measure to avoid air intake, however, equally the design of protection measures to ensure the containment and venting of an explosion is essential. In plant sections where a positive pressure exists (e.g. after a blower), there is a risk of gas escape to the atmosphere, which may lead to an external explosion. Gas tightness is again critical to avoid gas escape, as well as hazardous area zoning to avoid external ignition sources. Fire hazards Potential fire hazards arise due to the formation of flammable mixtures both inside and outside of the equipment. The nature of the gasification process means that ignition sources, and controlled, localised "combustion zones" are typically present within the equipment (e.g. gasifier). At underpressure there is a risk of air ingress and a fire may occur inside a particular section. Under the right circumstances wooden feedstocks offer the potential for self ignition In plant sections where pressure build up exists, there is a risk of gas escape to atmosphere, which may lead to an external fire. Syngas from gasification can auto-ignite at temperatures above about 600°C - 650°C. Toxic gas The gasification process produces a highly flammable cocktail of gaseous components, including hydrogen, and the very toxic gas, carbon monoxide. In plant sections where pressure build up exists, there is a risk of gas escape to atmosphere, which may lead to a toxic atmosphere. Gas tightness is therefore important to ensure containment and avoid the release of toxic gas. The areas outside the equipment must be adequately ventilated to prevent build up of an explosive atmosphere, but also to ensure that there is no toxic atmosphere build up to cause carbon monoxide poisoning of employees. Carbon monoxide detection equipment should be provided to detect possible leaks. Flare system function and features In addition, key technical safety challenges include emergency venting/flaring and start-up and shutdown. The combination of boiler gas feed and upstream syngas generation leads to control issues in balancing syngas usage versus the generation rate and also syngas quality. In the case of an unplanned boiler shutdown (e.g. boiler "lock-out"), an alternative venting route is required, due to the time lag between shutting off gasifier feeds and the cessation of syngas generation. To allow safe venting of the syngas generated in these circumstances a flare system, or equivalent, is required. As well as handling/venting gas when the boilers are shut down, the flare system forms the final section of the pressure relief system in the event of a plant emergency shutdown (ESD). The flare system should incorporate a water seal vessel, automatic ignition system and flame monitoring with alarm. Additionally, during start-up and shutdown the gasifier may pass through the flammable range unless it is purged with an inert gas. It is essential that the equipment is designed and maintained in a gas tight state to avoid the formation of an explosive mixture inside equipment. Hazards of the feed material The feed materials to a waste to energy plant typically comprise of biomass, Municipal Solid Waste (MSW), Refuse Derived Fuel (RDF) or Solid Recovered Fuel (SRF). These feed materials have their own hazards including fire, dust explosion and toxic gas formation. For example, where feed materials such as biomass wood are stored in large piles, there is potential for self-heating. Wood fuel is a source of nutrients for microbial activity, which, in the presence of moisture, over extended time periods, can lead to the generation of heat, and self-ignition. Other feed safety considerations include hazards associated with dust, such as explosion hazards requiring protection by, for example, hot particle detection and explosion venting. Cross industry co-operation The gasification of waste introduces a whole new series of safety issue more akin to chemical process plants than conventional combustion technologies. These hazards are well understood in the chemical process industries, where safety techniques, including HAZOP (Hazard and Operability), LOPA (Layers of Protection Analysis) and SIL (Safety Integrity Level), have evolved to ensure safe design of plants. Cross industry co-operation will result in quicker, safer implementation of new technology, greatly reducing the risk of a catastrophic incident. Adapting the established practice of the power sector and supply chain to apply controls that at first may be regarded as restrictive or over-engineered may be a significant challenge. The lack of "standard/generic" design solutions available for waste gasification plants, unlike conventional energy plants, where many plants have been built, makes this particularly challenging. Although guidance is now becoming available, due to the multitude of gasification technologies and corresponding reactor configurations available, these recommended practices are often not directly applicable. The process industry sector has a wealth of experience of the tools and techniques required to ensure, as far as is reasonably practicable , that plants meet the high standards of safety expected by the public. Applying these techniques to gasification will result in a safe and green future. Deb Pal is process director at petrochemical, pharmaceutical, biotechnology and environmental engineering consultants, Stopford Energy and Environment, and is a Fellow of the Institute of Chemical Engineers.Web: www.stopfordenergyandenv.co.uk More Waste Management World Articles Waste Management World Issue Archives