Limestone scrubbing

Limestone scrubbing A process using a ground limestone and water mix to neutralize sulfur dioxide in waste gas products. 

The economics seem to be better for systems where dry powdered fresh lime plus ground recycled lime is injected along with a relatively coarse spray which impinges on and dries out from the reagent, as described by Stouffer et al. [lirEC Res., 28(1) 20 (1989)]. Withum et al. [9th Ann. Pitt. Coal Prep. Util. Env. Control Contractors Conf. (1993)] describes an advanced version of that system that has been further optimized to the point that it is competitive with wet limestone scrubbing for >90 percent flue gas desulfurization. 

An increase in magnesium concentration from 100 to 1000 m-mole/1 increases pS02 by about a factor of two. Most limestone scrubbing systems with magnesium enhancement would operate midway between these magnesium concentrations, about 200-400 m-mole/1. 

Data for magnesia-enhanced limestone scrubbing of SO2 by a Turbulent Contact Absorber were obtained from the 10-MW equivalent EPA Alkali Scrubbing Test Facility. These data and the chemical model have been used to predict the enhancement effect of magnesia-induced dissolved sulfite concentration on S02 removal. 

Borgwardt, R.H. "Limestone Scrubbing at EPA Pilot Plant" Progress Report No. 13, November 1973. 

Wet limestone scrubbing of stack gases to remove SO2 gives hydrated... 

The process for limestone scrubbing can be generally described yyJ bsorption... 

The spent sorbent from fluidized-bed combustion may be taken directly to disposal and is much easier than the disposal of salts produced by wet limestone scrubbing. Alternatively, the spent sorbent may be regenerated using synthesis gas, CO/H. ... 

There are several ways to classify processes for the removal of SO2 from stack gases wet or dry, recovery or non-recovery, and absorption, adsorption, or catalytic oxidation. Except for catalytic oxidation, each of these has been described by others in this volume. In reviewing these processes and many others. Combustion Engineering (C-E) decided that wet lime/limestone scrubbing without recovery of sulfur was worth developing because of its simplicity and low cost. 

Wet Scrubbing in Packed Towers. The packed absorption tower has been considered and used in lime and limestone scrubbing of SO2. It has demonstrated its capabilities in most of the previously listed characteristics. However, commercially available packings have difficulty maintaining a reliable operation with calcium-based wet scrubbing because of scale deposition. 

NO2—SO2 Absorption by Hydroxides and Carbonates. Several metal hyroxide slurries were screened for NO2-SO2 scrubbing potential. In addition a limestone (CaCOa) slurry was investigated. Magnesia, lime, and limestone scrubbing systems are under consideration for SO2 scrubbing, so our results on the combined scrubbing are pertinent. A summary of the data is contained in Table VI. 

The thermodynamic properties of gases, liquids, solids and aqueous solutes of interest for desulfurization processes are tabulated for temperatures from 298 K to temperatures as high as 1000 K when possible. Major emphasis has been placed on aqueous lime or limestone scrubbing, but the data can also be used for high temperature gas processes and for N0 processes. 

The experimental apparatus shown in Figure 1 was used to evaluate limestone reactivity. Conditions in the reactor approximate conditions in the reaction tank of a limestone scrubbing system. As the limestone dissolves, the pH in the reactor will begin to rise. Therefore, to insure a constant pH in the experimental reactor, a low pH test solution (see Table I) is metered to the reactor. As the low pH feed is added, an equal amount of reactor liquor is also withdrawn to maintain the reactor volume also remains constant (2.5L). In an actual scrubbing system, S02 absorbed from the flue gas will provide enough acidity such that the pH remains approximately constant. 

Borgwardt, R. H., "Limestone Scrubbing of SO at EPA/RTP Pilot Plant," Progress Report 23-27 (April 197%). 

Burbank, D.A., and Wang, S.C., "Test Results on Adipic Acid-Enhanced Lime/Limestone Scrubbing at the EPA Shawnee Test Facility", presented at the Industry Briefing on EPA Lime/ Limestone Wet Scrubbing Test Program, Raleigh, N.C., Dec. 5, 1979. 

In the case of limestone scrubbing, it logically follows that a given SO2 removal efficiency can be achieved at a lower limestone stoichiometry, thereby improving scrubber reliability. 

Adipic acid-enhanced limestone scrubbing has lower projected capital and operating costs than unenhanced limestone or MgP-enhanced limestone scrubbing. This is due primarily to the reduced limestone consumption at the lower operating pH, the reduced grinding cost, and the reduced quantity of waste sludge generated. 

Figure 4. Flow diagram for adipic acid-enhanced limestone scrubbing in the spray tower system...

Bleed stream oxidation of unenhanced lime or limestone slurry is usually not feasible because the pH rise caused by the residual alkali in the oxidation tank makes it difficult to redissolve the solid calcium sulfite. With adipic acid-enhanced limestone scrubbing, however, this constraint is removed because of the low operating pH and low residual alkali in the bleed slurry. Thus, the oxidation tank can be maintained at a low pH for good sulfite oxidation, while achieving high SO2 removal efficiency with a sufficiently high concentration of adipic acid in the scrubber liquor. 

The economics of limestone scrubbing, with or without additive, have been projected for forced oxidation systems designed to... 

As shown In Table 10, both the total capital investment and the first-year revenue requirement are the lowest for adipic acid-enhanced limestone scrubbing at low pH (Case 4). The total capital investment is reduced by 4.8 percent, and the first year revenue requirement reduced by 5.8 percent for the limestone/ adipic acid/low pH case (Case 4), compared with the conventional... 

Conditions for Economic Analysis of Limestone Scrubbing with Forced Oxidation and with or without Additive... 

These cost figures are cited as representative of typical scenarios only, and some variation from them would be normally expected. Moreover, the differences in total capital investments and operating costs between these cases are small. The principal conclusion from these evaluations is that adipic acid addition to a limestone scrubbing system decreases cost consistently when compared on the same basis. 

Lime/Limestone Scrubbing Model Users Manual," EPA-600/8-... 

Since the conventional MgO process has not gained widespread acceptance, efforts are underway to make the process more energy efficient, less dependent on fuel oil and, most importantly, more competitive economically with a limestone scrubbing system. This paper is the result of TVA s initial investigation into an improved MgO process. The purpose is to describe the new MgO process, to detail some of the technical uncertainties requiring further development work, and to present preliminary conceptual design economics for this improved MgO FGD system. 

Since the MgO spray dryer FGD process collects particulate matter as an inherent part of the FGD system, it can be evaluated either as a combined particulate-S02 removal system or, with the inclusion of an ESP credit, as an FGD-only process. In this study the spray dryer MgO process evaluation is based on the combined particulate-502 removal system. The primary reasons are that the combined system is easier to explain and that similar evaluations for the limestone scrubbing/ESP and conventional MgO/ESP systems are available from previous studies (2, jj). The conventional MgO/ESP process cost information from the previous study was updated using area scale factors, relative product and gas rates, etc., to put the results on a consistant basis with the current evaluation of the spray dryer MgO process. 

The capital investment for the spray dryer MgO FGD process is 139.5M ( 279/kW) in mid-1982 dollars while that for a comparable limestone scrubbing/ESP process is 122.0M ( 244/kW) in mid-1982 dollars. The capital investment for the conventional MgO FGD process (including particulate control) is 149.7M ( 299/kW) in mid-1982 dollars. These costs are... 

The capital investment for the spray dryer MgO process is approximately 14K higher than that for the comparable limestone scrubbing process. This is not an unexpected result since the MgO process is a regenerable system while the limestone scrubbing process is a throwaway system. 

There are no comparable areas in the limestone scrubbing process. The 22 million investment for these areas in the MgO process offsets the lower investment for the other processing areas. 

The first-year annual revenue requirements for the spray dryer MgO FGD process are 28.8M (10.47 mills/kWh) in mid-1984 dollars. The levelized annual revenue requirements for the spray dryer MgO process are 36.1M (13-13 mills/kWh). The first-year annual revenue requirements for the comparable limestone scrubbing process are 32.4M (11.78 mills/kWh) in mid-1984 dollars. The levelized annual revenue requirements for the limestone scrubbing process are 45.2M (16.43 mills/kWh). The first-year annual revenue requirements for the conventional MgO FGD process (including particulate control) are 40.4M (14.69 mills/kWh) in mid-1984 dollars. Levelized annual revenue requirements are 56.7M (20.61 mills/kWh). These costs are summarized in Table III. The complete details are presented in Tables A-IV, A-V, and A-VI in the Appendix. 

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