Ozonation of coal gasification plant wastewater - Environmental

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Ronald D. Neufeld’ Department of Civil Engineering, University of Pittsburgh, Pittsburgh, Pa. 15261

Anthony A. Spinola United States Steel Research Center, Monroeville, Pa.

A laboratory investigation is conducted on the ozonation of a liquid waste sample from an experimental coal gasification plant. A 30-in.-high, 2.75-in. i.d. packed glass column is used to countercurrently scrub the wastewater with a gas stream of 74 mg/L ozone. Specific component analyses are presented of the wastewater and its ozonated end products. These were done via both wet analyses and gas chromatography. Data examination indicates that higher ozone levels yield substantial reduction of cyanogen compounds and polyduric phenols, with a simultaneous increase in the concentration of low boiling compounds measured as acetone, benzene, and methanol. Synthane, a second generation coal gasification process, has been under development by the Department of Energy (DOE) Pittsburgh Energy Research Center, formerly the Bureau of Mines, since 1961. Figure 1is a block diagram of the Synthane process. Wastes are generated as follows: Coal enters a fluidized bed maintained in the lower section of the gasifier. Oxygen for partial combustion and steam are fed into the bottom of the fluidizing reactor. Raw gas produced in the reaction area passes up through the column and, prior to passing through an internal cyclone, devolatilizes the pretreated coal. The raw gas, a t about 1400 O F , exits the gasifier and is quenched and scrubbed with water in a venturi scrubber. Raw gas and condensates, as well as particulate matter not removed in the cyclone, enter a scrubber surge tank for gas-liquid effluent separation. This liquid is gasifier wastewater. Synthane gasifier waters are amber in color, having a strong ammonia and cresol-like odor. Chemical compositions vary somewhat, with operating conditions of the gasifier and with the type of coals being gasified (1-3). The objectives of the research were to characterize such process liquid wastes from a coal gasification plant and to characterize the influence of severe ozonation on these wastes as part of an overall waste management plan.

Experimental Procedure Synthane gasifier water was supplied by the Pittsburgh Energy Research Center’s Synthane bench scale unit a t Bruceton, Pa. This facility is located in the vicinity of the University of Pittsburgh; thus, a fresh sample was assured and degradation due to long holding times was minimized. The sample of process water used in the tests was a typical sample of wastewaters from the Synthane PDU gasification of Illinois # 6 coals (2, 3). Specifically, it was obtained from a Synthane bench scale operation run number 40 CHPFI-198, which was conducted using Illinois # 6 coal at a coal feed rate of 18.2 lb/h, a steam feed rate of 35 l b h , an oxygen rate of 80 standard ft3/h, an average temperature of 890 “C, and a pressure of 40 atm. A countercurrent packed absorption tower was constructed and used to provide intimate contact for mass transfer of ozone with Synthane plant water, as shown on Figure 2. Ozone was generated using a Welsbach laboratory ozonator. 470

Environmental Science 8 Technology

Commercial grade oxygen was dried to a dew point of -100 OF in a Lectrodryer prior to being fed into the ozonator. The ozonator was operated at a pressure of 8 psig, 100 AC V, 126 W, and a calibrated gas flow rate of 1standard L/min. At these conditions, the amount of ozone in the gas stream, as determined by the Iodometric method ( 4 ) , was 74 mg/L. The absorption tower was constructed of Pyrex glass and had an inside diameter of 2.75 in., a height of 30 in., and was packed with l/d-in. Intalox saddles. The top of the tower was equipped with a liquid distribution plate to prevent channeling. Gasifier wastewater at a temperature of 77 O F was fed to the plate from a metered, calibrated, FMI reciprocating piston pump. The number of runs was limited by the quantity of sample made available by DOE. Therefore, for this experimental design only the liquid feed rate into the tower was changed for each run.

Analytical Approach Tests and analyses began the day the samples were received. All samples were refrigerated at 4 OC and preserved for each particular analysis, according to prescribed procedures. The method of analysis was as outlined in “Standard Methods for the Examination of Water and Wastewater”, 14th ed. ( 4 ) . Seed for use in the 5-day BOD was obtained daily from a local coke plant biological waste treatment facility. Metal analysis was conducted using an atomic absorption unit. Specific organic component analyses were done by extracting liter portions of the samples with methylene chloride under basic and then acidic conditions, as described by Schmidt et al. (1).These extracts were examined by flame ionization gas chromatography, and similar portions of the water were also examined directly without extractions. The chromatograms of the direct analysis were very similar to the chromatograms of the extracts. The quantitative results reported are based on application factors using phenol calibration data, and identifications are based on relative retention time data using pure components. Results Tables I and I1 compare the results of this analysis of Synthane wastewater with available analysis for the Illinois # 6 coal, as reported by Forney et al. (2).Table I lists wet chemical analyses for components and atomic absorption analyses for metals, while Table I1 lists results of gas chromatograph component analysis. As indicated above, Synthane wastes may be characterized by having a dark amber to tea-like color, being high in total organic carbon, alkalinity, and phenolics. Heavy metals do not appear to pose severe environmental threats from Synthane wastes, as is shown in Table I. Examination of Table I1 indicates that the organic matrix of gasifier wastes are composed of simple phenol and polyduric phenolics, many of which are not revealed via the “Standard Methods” ( 4 ) tests for phenol. It should be noted that these results, as well as those of Forney et al. (2), are from gasification of a discrete number of samples of Illinois coal. More recent analyses reported by Johnson et al. ( 3 )and Neufeld et al. ( 5 )indicate that the composition of 0013-936X/78/0912-0470$01 .OO/O

0 1978 American Chemical Society

Spray tower


Shift converter

Hot carbonate scrubber

Cool peparotion

Table II. Gas Chromatography Analysis of Synthane Process Wastewater from Illinois #6 Coal This work as recalved, mglL





Gasifier Steam Oxygen


Figure 1. Block diagram of Synthane process

Figure 2. Schematic of ozonation apparatus

Table 1. Analvsis of Synthane Process Wastewater Illinois #6 Co’al PH Suspended solids Phenol Cyanide Thiocyanate Ammonia Chloride Carbonatea Bicarbonate COD BODS TOC Iron Manganese Calcium Nickel Copper Barium Cobalt Magnesium Zinc Potassium Chromium Aluminum Strontium Tin a

As CaC03.

mg/L mglL mg/L mglL mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L PS/L Pg/L P W PSIL PS/L Pg/L

This work as received

AHer Forney (2)

9.4 268 2320 2.28 418 4250 333 6576 0.00 17 162 420

8.6 600 2600 0.6 152.0 8100 500 6000 11 000 15 000

5800 3.52 0.19 0.33