PURWOX (USA), Inc. C 1997Technical Concept Analysis
The net product of the PURWOX process is a mixture of primarily carbon monoxide and hydrogen with varying but smaller amounts of methane, water and carbon dioxide. The gas has a net heating value of around 300 BTU per standard cubic foot as produced. This type of gas is commercially known as synthesis gas or syngas for short.
The process has some chemistry that has been proven viable in similar type organics-gasification processes for producing syngas over the last 70 years. Syngas is a basic building block for several types of commercial processes. Ammonia-urea, methanol, synthetic natural gas, hydrogen, several oxo-alcohols, and some synthetic motor gasoline processes all start by generating some form of syngas. Most use steam reforming and/or partial oxidation of natural gas (primarily methane). A significant portion however, use other organic material, mostly coal, though several projects using biomass and municipal solid waste have also been funded and even commercialized to some extent.
Several plants have been started and successfully run for several years. Some are still operating, especially in areas of the world where indigenous oil and gas are rare but coal is plentiful. “Water gas” production from the reaction of steam with coal or coke was the principal means of providing municipal heating and lighting gas for the first thirty years of this century. South Africa is an example where the SASOL coal gasification complex has successfully supplied gasoline, chemicals and synthetic natural gas to that nation for over 40 years. Germany supplied much of its war needs in the 1940's from coal pyrolysis and gasification. Dakota Gasification Co. has run a large 300 MW high sulfur lignite gasification plant for several years in Beulah, North Dakota. On a less spectacular scope, the Baltimore Municipal Waste District ran a prototype trash gasification unit successfully, though it was judged uneconomical in terms of energy production savings. This was back in the late 1970's and early 1980's before eastern U.S. landfill problems became a major environmental concern. Federal, state and municipal governments and private investors have funded several similar type of waste to product schemes since the early 1970's - again most being viewed as alternative fuel plants.
Nearly all of these plants used some type of free-oxygen based combustion internal to the gasification process. While this can generate a great amount of heat inexpensively and reduce some of the reactor space-time parameters, these design advantages are offset by the need for more complicated environmental and safety control equipment. Most of these syngas processes also require expensive oxygen-generation and pressurized air-blowing equipment. The presence of free oxygen and its direct waste-combustion products also present potential corrosive situations that will complicate the process design.
PURWOX uses very high temperature steam and a high temperature recycle stream to provide enough heat to drive the complete gasification of all carbonaceous material leaving the reactor. In addition, the bottoms materials of the reactor are in a molten metal phase that can be recovered as a quality recycle material and molten slag that is suitable for sale as clean building material or construction fill.
The process uses clean syngas (externally to the reactor) to provide all necessary process heat and has approximately 50% of the net gasifier to make gas available for sale or conversion into steam, electrical power, etc. As an example, for a typical mid-size city mix of municipal solid waste, a 10-ton/hr trash gasification facility could support a 6-7 Megawatt (MW) power generation system.
The syngas product burns very cleanly in commercially available power or steam generation equipment.
Potentially toxic agents found in trash (chlorine, sulfur, fluorine, nitrogen, pesticides, batteries, heavy metals, etc.) are reduced by the steam and hydrogen to a form that is readily removed and recovered from the process gas product or in the liquid slag bottoms. The high temperature gasification process breaks down the molecular structure of the most chemically complicated feed material into very basic molecular forms CO, H2, H20, CH4 and CO2. The lack of free oxygen in the process stream eliminates the formation of dioxins, ketones, aldehydes, phenols and other toxins characteristic of direct waste combustion or oxygen-blown gasification processes.
Finally, the PURWOX process is amenable to mixed-type organics processing. Adding various amounts of coal, used tires, spent lube oil, coal tailings, petroleum-based insecticides or refinery waste, medical waste, biomass, to a feed base of municipal solid waste (or one of the other solids mentioned) is an exciting possibility. Very few adjustments to the “garbage-gasification” process equipment would be required to handle any type of above-mentioned wastes.