Tar Removal through Physical Processes Physical processes
will continue to play a very important role for the successful commercial
implementation of gasification. They constitute the basic arm for removing most
of the raw gasifier contaminants, including “tar.” “Tar” is removed mainly
through wet or wet-dry scrubbing. Coalescers, demisters, and cold filtration
are also necessary supplements. These well-known commercial methods are easily
designed and applied, depending on the specific needs of any gasification
process. The main problem arising from “tar” scrubbing is that condensed “tar”
components are merely transferred into another phase (water or solids such as
scrubbing lime), which then has to be disposed of in an environmentally
acceptable manner. The problems associated with the management of these
wastewater or solid residues are summarized as follows: • “Tar” and
“tar”-contaminated solid-waste streams are considered as a special waste;
consequently, their disposal is usually cumbersome and costly. • “Tar”-bearing
wastewater is usually a bi-phasic mixture requiring various steps of treatment
before final disposal. Water insoluble “tar” skimming • Most water-soluble
“tar” components are refractory to the usual biological wastewater treatments.
The applied methods for “tar” and “tar”-containing waste streams include solid
waste stabilization and landfilling, organic phase skimming off the bi-phasic
wastewater-free surface, wastewater incineration, wet oxidation, adsorption on
activated carbon, and final biological treatment. Wastewater contaminants
include dissolved organics, inorganic acids, NH , and metals. 3 Typical values
of COD, BOD, and phenols for wastewater from fixed-bed gasifiers are given in
Hasler et al. (1997). The new BIOSYN design proposes a continuous skimming-off
of insoluble “tar” from the surface of the wastewater. tar-containing
wastewater was treated using lime and alum for the removal of in-organics,
followed by adsorption on powdered activated carbon (PAC) for the removal of
organics. At optimum concentrations, both lime and alum individually proved to
be capable of removing color, total suspended solids (TSS) and total dissolved
solids (TDS), but in both cases, pH adjustment had to be carried out after
treatment. The combination of lime and alum at the dose ratio of 0.8:0.8 g/L
was found to be optimum for the removal of inorganics. The removal efficiency
achieved at optimum concentrations were 78.6, 62.0, 62.5 and 52.8% for color,
alkalinity, TSS and TDS, respectively. Coagulation-precipitation followed by
adsorption on PAC resulted in 92.3% chemical oxygen demand (COD) removal and
100% phenol removal at equilibrium. This technology may prove to be one of the
fastest and most techno-economically feasible methods for the treatment of
tar-containing wastewater generated from BGPs. biomass gasification plant.
The specific technical objectives of this study are: 1.
Identify the optimum individual doses of lime and alum (coagulant) required to
achieve the best treatment efficiency. 2. Identify the optimum and minimum dose
of the coagulant-combination (lime and alum) required to achieve good treatment
efficiency without having to conduct final pH adjustment. 3. Determine the
efficiency of the coagulationflocculation process at optimum coagulant dosage
followed by adsorption on powdered activated carbon (PAC). The iodine value of
PAC was 1050 mg/g. The optimum dose of lime was found to be 0.8 g/L, with
maximum removal efficiency for color, TSS and TDS, and minimum amount of sludge
generation. The optimum dose of alum was observed to be 0.8 g/L with maximum
TSS, TDS and minimum amount of sludge generation. However, the optimum dose of
alum for maximum color removal was found to be 1.0 g/L. Lime was found to
remove ammonia more efficiently than alum. In the lime-alum combination,
lime:alum dose of 0.8:0.8 g/L was found to demonstrate maximum removal of TSS,
TDS, alkalinity, and minimum amount of sludge generation. However, the
lime:alum dose of 0.9:1.0 g/L provided maximum removal of color and ammonia.
The coagulation-flocculation process was found to be more efficient for the
removal of color, TDS, TSS and alkalinity than for the removal of COD and
phenol.
CHARACTERISTICS OF EFFLUENT FROM BIOMASS GASIFICATION PLANT
Parameter Unit Values pH -- 7.49 (±0.015) Color Co-Pt unit 1076.5 (±38.5) TDS
mg/L 1875 (±75) TSS mg/L 82.5 (±2.5) Alkalinity (as CaCO3) mg/L 537.5 (±12.5)
Ammonical Nitrogen mg/L 242.55 (±12.45) Nitrate nitrogen mg/L 0.62 (± 0.02)
Phosphate (as PO4 3-) mg/L 0.12 (±0.03) CODtotal mg/L 3599.5 (±57.5) CODsoluble
mg/L 3499.5 (±57.5) Phenol mg/L 465 (±10) Oil and Grease mg/L 36 (± 4.0) Note:
Values in parenthesis are standard deviation
