PURWOX (USA), Inc. C 1997

Recycled Gas

The recycle gas (removed at the top of the gasification zone) and gaseous char gasification products flow upward through the reactor providing the heat for thermal destruction of the organic waste material in Zone II. The products this zone include volatiles, more steam-reformation syngas, and hydrogenation products of cracked organic chemicals such as alcohols, higher paraffins, a few organic oils and some heavier hydrocarbons that are liquid at normal temperatures.

The inorganic molecular components of Chlorine, Fluorine, Nitrogen, Sulfur and other wastes will form acid compounds such as HCL, HF, NH3, H2S, etc. in this section, as long as free oxygen is minimized inside the reactor. If not, all sorts of pollutants, such as NOx, dioxins, aldehydes, ketones, etc. will be formed in unacceptable amounts. The acid components are easily removed in downstream caustic-base scrubbers. The others are not and have presented significant problems for incineration-based waste removal processes for the last thirty years.

The pyrolysis gas is removed near the top of the pyrolysis zone (Zone II) but the location may differ according to feed type. From the pyrolysis offgas, known as pyrogas, heavier organic oils, particulates, and other non-volatile pyrocompounds are removed (cyclone separator) and returned to the gasification section of the converter. The cyclone off gas is compressed and sent to a pressurized caustic scrubbing system for removal of acid gas components. More organic liquid condensate is recovered and recycled to the converter. The spent caustic, diluted with condensed pyrogas water vapor, is flash evaporated and the water vapor recycled as superheated makeup to the restorer gas stream. The concentrated caustic sludge is pumped to the melt collection zone of the reactor for encapsulation and fixation in the primarily alkali and alkaline earth oxide molten slag material.

Above the pyrolysis zone, heat conduction from the lower bed areas drives off all of the surface water associated with the SOF, as well as substantial portion of that chemically bonded or adsorbed inside the waste components. A bleed vent of primarily steam from the top of this “drying zone” (Zone I) enhances removal of this gas without removing significant portions of the pyrogas. It also provides a bit of convective gas flow for converter temperature profile control.

Syngas Use

Clean pyrogas from the scrubber is used for three purposes:

  1. Fuel for fired heating (external to the reactor) of the restorer gas and superheating recycle steam.

  2. Product syngas for sale as a chemical feedstock or as a feed gases for conventional power generation systems, either steam driven or combustion-turbine driven.

  3. Intermittent or as a continual bleed makeup for the recirculating restorer gases. Converter control may dictate some modification to the amount of pyrogas that is recycled to the restorer gas stream.

    Conversion of municipal solid waste energy potential to product pyrogas heating value is about 75% with about half of this product needed for fired combustion gas heaters to drive the pyrolysis and gasification reactions.