Purox process

Industrial gasification achievements by themselves have succeeded in upgrading munidpai wastes. Union Carbide s Purox process represents a typical technology in the field. Many developments have occurred with wood Table 1.6 provides an idea of the dry gas compositions obtained, which vary with the type of gasifier and the feedstock. 

The pyrolysis and melting process developed by Nippon Steel Co., Ltd. was adopted at Ibaraki City to treat 450 tons/day (150 tons/ day X 3) of MR, and is under construction. Funabashi City has decided to use the dual fluidized bed reactor system (Pyrox process) to treat 450 tons/day (150 tons/day X 3) of MR, and Chichibu City has decided to use the Purox process developed by Showa Denko Co., Ltd. and Union Carbide Co., Ltd.. The design and construction of these plants were started this April. 

A block flow diagram for production of fuel grade methanol from biomass is depicted in Figure I. The gasification step is based upon the Purox process and is followed by shift conversion and gas purification steps. The clean gas, which is shifted to a H2/CO ratio of approximately 2/1, is converted to methanol in the ICI low-pressure methanol synthesis process. The process yields approximately 98% pure methanol with the remaining 2% consisting of water and some higher carbon number alcohols. 

The medium-Btu gas from the Purox process need not be desulfurized prior to entering the shift reactor since a sulfided catalyst is used. The shifted gas goes to the purification system, where a hot-carbonate scrubbing system is... 

Thermal efficiencies for converting corn stover (8), furfural residue (8), and wood residue to methanol were estimated from the published data to be 48.0, 48.0, and 45.3% respectively. The efficiency data used for converting wood to methanol via the Purox process is also in good agreement with recently published data (12). The Purox process may not have been the best choice for the gasification (1 ). but process and economic data are available for all three feedstocks considered in this paper. 

Purox A process for partially combusting organic wastes in a shaft furnace with oxygen, thereby producing a fuel gas and a molten slag. The gas, not diluted with nitrogen, is suitable for use as a chemical feedstock. Developed by Union Carbide Corporation in 1974 and piloted in Charleston, WV. 

Fisher, T,F., Kasbohm, M,L, and Rivero, J.R, "The PUROX System," in Proceedings 1976 National Waste Processing Conference, Boston, May 23-26, 1976, American Society of Mechanical Engineers, 1976,... 

To meet the abovementioned local conditions, Showa Denko has established a process whereby the pyrolysis gas is directly combusted and the generated heat is utilized for evaporation and oxidation of the wastewater from the refuse pit (hereinafter called the Dry Process). The schematic flow of the Dry Process PUROX System is as shown in Figure 2. 

Figure 2. Schematic flowsheet of the dry process PUROX system...

Although the pilot plant had a refuse treatment capacity of 20 Mg/d, it was usually operated at reduced load due to limitations coming from the refuse preparation site. Also schedule constraints prevented replacement of the desuperheater and scrubber employed to clean fuel gas in the pilot plant, by a waste heat boiler and electrostatic precipitator, such as are employed in the Dry Process PUROX System design. Consequently, dust was not recycled. 

The combustor off-gas corresponds to the throughput to the gas cooler (e.g., waste heat boiler) of a commercial plant. The gas analysis is given in Table VII, The values for the conventional stoker type incinerator are those in the case of large cities in Japan. They are average values with the exception of HC1, which is slightly higher. The amounts of particulate, SOx and HC1 contained in the off-gas of the Dry Process PUROX System are much less than those of the stoker incinerator. The reason is considered to be that the Cl", SO , etc. combine with alkali metals and shift into the slag in the converter. 

Furthermore, it was confirmed that addition of limestone to the refuse as the neutralization agent reduces the volume of HC1 in the off-gas to about 60%. Further study is required, however, of the relationship between the HC1 removal rate and the amount of limestone added and of the method of such addition as well. With the exception of its particulates, off-gas from the Dry Process PUROX System can satisfy Japanese national emission and effluent standards without any special treatment. 

On the basis of the price levels as of spring 1979 and at the translation rate of 200 yen to one dollar, the construction cost of the Dry Process PUROX System in Japan is estimated to be about 13,000,000 for a 200 Mg/d facility ( 65,000 for Mg of refuse), exclusive of the land, utility supply facilities down to the battery limits, and fixtures and supplies. This is about 10% higher than the construction cost in Japan of the stoker incinerator, which is estimated to be about 58,500 per Mg of refuse. 

Table X shows the running costs of the Dry Process PUROX System as well as the stoker incinerator. In Japan solid refuse treatment plants are operated by local municipalities and taxes and depreciation are not considered components of the running costs of solid refuse treatment plants. They, therefore, comprise electricity, water, chemicals, auxiliary fuel, labor, and maintenance costs. When a comparison is made on the basis of these costs, the Dry Process PUROX System is somewhat higher than the stoker incinerator.

TABLE X. RUNNING COSTS OF DRY PROCESS PUROX SYSTEM (Design capacity 150 Mg/d)... 

Unit Price Dry Process PUROX System Stoker Incinerator ... 

Union Carbide developed a pyrolysis reactor schematically shown in Fig. 4 The reactor uses pure oxygen rather than air and hence the Purox name. Before being fed into the reactor the waste is shredded and magnetic separator removes metals. The waste is fed at the top of the reactor. As it descends it is first dried and then pyrolysed by raising hot gases. Gas produced by the process is cleaned in two steps ... 

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