The present disclosure is directed to a process and system for the generation and treatment of syngas. In particular, the present disclosure is directed to a process and system for the treatment of a crude syngas stream produced by the plasma gasification of waste, including municipal solid waste (MSW).
The effective management and utilization of waste is a global issue. Current waste management techniques, as suggested by regulatory agencies, such as the U.S. Environmental Protection Agency (EPA), include source reduction first, recycling and composting second, and, finally, disposal in landfills or waste combustors. Other techniques of managing waste include converting the waste to energy involving processes such as incineration and pyrolysis. There are many types of waste including municipal solid waste, commercial and industrial waste, refuse derived fuel (RDF), construction and demolition waste, electronic waste, medical waste, nuclear waste, and hazardous waste. Municipal solid waste (MSW), also called urban solid waste, trash, rubbish, or garbage, mainly comprises household/domestic waste. MSW is generally in solid/semi-solid form and includes paper and card, plastic, textiles, glass, metals, biodegradable waste (e.g., kitchen waste, yard sweepings/trimmings, wood waste), inert waste (e.g., dirt, rocks) and may include small quantities of miscellaneous materials such as batteries, light bulbs, medicines, chemicals, fertilizers, among other materials. Typically MSW is found to be predominantly paper/card and kitchen waste, although exact compositions can vary from one region to another (e.g., depending upon the levels of recycling carried out in that region). RDF refers to MSW and/or commercial and/or industrial waste which has been pre-sorted, typically to remove recyclable material.
Another form of waste management includes gasification. Gasification is a process for the conversion of a carbonaceous feedstock such as coal, petroleum, biofuel, biomass, municipal solid waste (MSW), and other wastes into a combustible gas such as synthesis gas. Synthesis gas, commonly referred to as syngas is a mixture of varying amounts of carbon monoxide and hydrogen (CO+H2) and has a variety of applications. The syngas can be used to generate power by combusting directly in a gas turbine, boiler or reciprocating engine, by feeding into a fuel cell, and/or waste heat can be used in the generation of steam which can provide additional power through a steam turbine. Syngas can also be used for the production of hydrogen or liquid fuels or chemicals, gaseous fuels, synthetic natural gas, and/or carbon monoxide, some of which may be used as raw materials in the manufacture of other chemicals such as plastics. Gasification is thus a process for producing value added products and/or energy from organic materials.
A particular form of gasification includes plasma gasification. Plasma gasification is a waste treatment technology that uses electrical energy and the high temperatures created by a plasma arc to break down waste into a gaseous product which contains syngas and molten, glass-like by-product (slag) in a vessel called the plasma gasification reactor. Plasma is a high temperature luminous gas that is partially ionized and is made up of gas ions, atoms and electrons. Slag is produced from the vitrification of inorganic mineral matter such as glass and metals which are often contained in waste.
Depending on the composition of the waste used as feedstock and the gasification process employed, the gaseous product containing syngas may comprise CO, H2, H2O, HCN, CO2, N2, O2, CH4, H2S, COS, NH3, HCl, Ar, Hg, CxHy, and other heavier hydrocarbons (tars), particulates comprising char, ash, and/or unconverted fuel. Heavy hydrocarbons, heavier hydrocarbon compounds or tars refers to hydrocarbons that may be saturated, unsaturated, or partially saturated as well as hydrocarbons that may include other atoms such as, but not limited to, oxygen, nitrogen, or sulfur. One of the challenges of carrying out a waste plasma gasification process is that the widely varying nature of the waste can lead to a widely varying gas and and hydrocarbon compositions as well as particulate amounts and sizes coming out of the gasifier, which shall herein be defined as the “crude syngas stream”. The clean-up of this widely varying crude syngas stream is particularly challenging if the final product of the overall process is power, especially power that is produced with a gas turbine, reciprocating engine, or an internal combustion engine. In all of these power production processes, particulates are desirably removed down to at 50 ppm or less regardless of the widely varying amount of particulates in the crude syngas stream. Despite the potential for a wide variation of hydrocarbon compositions and/or tars, the cooling system of a 24/7 power generation facility cannot afford to be shut down due to fouling issues caused by the formation of tars at an undesired location in the process. Thus, the production of power from waste requires a different and unique syngas clean-up system relative to those of solid and liquid fossil fuels such as coal, pet coke, asphaltenes, and many biomass gasification clean-up systems.
US Patent Publication No. 2009/0133407 A1, which is hereby incorporated by reference in its entirety, discloses a system for producing and processing syngas from waste using a plasma gasifier. US Patent Publication No. 2009/0133407 A1 generally refers to a gas clean-up train prior to conversion of the syngas to energy but do not disclose co-scrubbing of HCl and NH3. US Patent Publication No. 2009/0133407 A1 discloses a particulate removal system but do not address the high particulate loads that are typical of syngas formed from waste. In addition, the system disclosed in US Patent Publication No. 2009/0133407 A1 discloses a bioreactor for H2S removal and involves a heat recovery steam generator coupled to the syngas cooler immediately downstream of the gasifier to provide heat energy input back to the plasma cupola or to provide heat to the integrated electric generation cycle. The heat recovery system of US Patent Publication No. 2009/0133407 A1 is less desirable and less robust for waste gasification as it may be less tolerant to the wide range of metal, particulate and condensable by-products present in the stream exiting the gasifier which may lead to tar formation.
US Patent Publication No. 2008/210089 A1, which is hereby incorporated by reference in its entirety, discloses a gas conditioning system for processing an input gas from a gasification system. US Patent Publication No. 2008/210089 A1 fails to disclose co-scrubbing of HCl and NH3 and does not disclose COS hydrolysis for sulfur removal. The limitation of this patent's process is that it will not allow for complete sulfur removal, particularly that in the form of COS. The separate removal of HCl and NH3 in the US Patent Publication No. 2008/210089 A1 requires more unit operations to remove HCl and NH3 to desired levels, which requires more equipment, more energy and less efficient processing.
Gas treatment processes and systems for treating high particulate-containing and widely varying crude syngas streams are needed to efficiently treat syngas streams formed from plasma gasification of waste streams, including, but not limited to municipal solid waste. The cleaned syngas composition will vary depending upon whether one intends to render it suitable for power generation, fuel or chemical manufacture, hydrogen production, or other applications that utilize CO and/or H2.