Characterization of Carbonate Rocks from Rio De Janeiro for Gas Emission Control

Characterization of carbonate rocks from Rio de Janeiro for gas emission control

W.C. Pereira

CETEM, R.4, Quadra.D. CidUniversitaria, Rio de Janeiro, Brazil

Abstract

The purpose of this work is a study on sulphur and potentially contaminant trace elements, present in fossil fuel, that are transferred to the environment through combustion. This study examines the performance of calcareous or dolomitic calcareous rocks as sorbents applied to SO] retention.

In the combustion process were sulphur is present there is the formation of SO%, whose emission associated to photo-oxidation phenomena in the atmosphere generates SO], that under humidity conditions changes into sulphuric acid, that on its turn can react with metallic oxide of particulate material to generate sulphate. The air pollution from the burning of fossil fuels is the major cause of acid rain. Acidic deposition, or acid rain as it is commonly known, occurs when emissions of sulphur dioxide (SO]) react in the atmosphere with water, oxygen, and oxidants to form various acidic compounds.

For the effective control of the atmospheric emissions of potentially contaminant trace elements and acid emissions, information on these elements is required, as well as what happens during combustion. There is a fundamental need for more information and a better understanding of the combustion processes relating to sulphur and trace elements in order that new environment directives can be properly formulated and implemented.

1 Introduction

This work deals with the characterization of Brazilian materials for a Flue Gas Desulphurization Technology (FGD), more specifically from Rio de Janeiro. Power stations are usually fitted with a variety of facilities for pollution control. These systems influence potentially contaminant trace element emissions. The

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use of sorbents for the removal of harmful species during combustion and incineration is becoming increasingly important. Sorbents for SC>2 capture have been the subject of various studies for many years. Sorbents for capturing alkali and various trace metals have been investigated more recently. In recent years, there have been several studies of sorbents for capturing metal vapours (Ho T.C.

[1], Sioss [2], Carvalho W.P. [3]).

Toxic (or potentially toxic) trace metallic elements such as barium, beryllium, boron, cadmium, chromium, lead, mercury, nickel, selenium, strontium,

vanadium, zinc and zirconium are usually contained in fuel or incineration wastes, in various forms (Smith I.M. [4], Clarke L.B. [5] [6], Hughes I.S.C. [7], Meij R. [8], Swaine D.J. [9] [10] ).

The use of carbonate rocks with the objective of controlling the gaseous

emissions is widely known (Thodos G. [11], Simons G.A. [12], Torres-Ordonez [13]). The advantage of using this kind of material is the low cost.

In 1992 Brazil hosted the UN Conference on Environment and Development (the

Earth Summit). Sustainable development, that is, development that meets the needs of the present without compromising the ability of future generations to meet their own needs, was the most important definition achieved in this summit. For emerging economies such as Brazil, this is no easy task, although some

efforts towards change can be observed.

The Brazilian Center for Mineral Technology (CETEM) is participating in these efforts. One of the missions of CETEM is to develop environmental technology that may assist the Brazilian mining and metallurgical sectors to find ways to

avoid and/or control the negative impacts of the production on the environment and people.

2 Environmental Cycling of Sulphur

Sulphur is the 12^ most abundant element in the earth crust. Most of it occurs in combined form as either metallic sulphide or metallic sulphate and most is quite insoluble in water. Some free (elemental) sulphur is found usually in association with gypsum or adjacent to volcanoes. The oxidation of reduced sulphur is

inevitably accompanied by formation of acid. Iron and sulphur oxidizing microorganisms catalyze this oxidation and the rates of reaction are accelerated thousands of times.

There are several ways in which sulphur forms enter the atmosphere, all of which

result directly or indirectly in acid formation. For example: 1. SO? from combustion of fossil fuel. 2. Probable SO] emission directly from high sulphur coal or refuse piles 3. SO] and reduced sulphur forms from mine or waste incineration

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4. Neutral sulphate from ocean spray 5. Acid sulphate spray or vapours from mine drainage regions 6. H?S gas generated by microbes in local environments where the organic and

sulphate content is relatively high and oxygen has been depleted.

2.1. Acid Precipitation

SO,

Photo-oxidation phenomena

SO] generation

Under humidity conditions change into sulphuric acid

Reaction with metallic oxide of particulate

material to generate sulphate

2.2. Acid Precipitation Consequences

Lagoons and rivers produces a reaction with Calcium and magnesium bicarbonate that act as a buffer solution forming sulphates that will limit the pH variation. On the other hand, the acidification of the environment will prevent the development of roe that need a neutral media, will affect also bacteria and algae, the soil (it can cause dissolution of elements, imposing a long range stress by calcium lixiviation of the soil and mobilizing elements such as aluminium, manganese, zinc, cadmium, lead, etc.) and the city (with the destruction of monuments and buildings)

Sulphate aerosol, which forms from sulphur dioxide emitted into the atmosphere, may also have contributed to a cooling, at least partially off-setting the greenhouse effect. It is probable that sulphur emission causes a cooling in the industrialized regions, although sulphur compounds have an atmospheric residence time limited to only a few days (contrary to greenhouse gases) (Weber

3 Sources of Trace Elements in the Atmosphere

Trace elements pass through the atmosphere as part of the biogeochemical cycling process. They are emitted by both natural sources and human activities. Most trace element emissions to the atmosphere are of airborne paniculate matter, although some trace elements (notably B, Hg, and Se) may enter the atmosphere in the vapour phase.

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Trace elements from human activities are released to the atmosphere mainly through emissions from electricity generation and industrial process. Atmospheric emissions from smelting and refining processes are major sources of trace elements from human activities. These process account for the largest

proportion of As (65%), Cd (71%), Cu (66%), Sb (40%), and Zn (55%), and also contribute significantly to emissions of Pb (19%). Incineration of waste is of particular importance to the emission budgets of Hg (32%) and Sn (16%) (Clarke L.B [5]).

Combustion of fossil fuels accounts for about 60% of the emission of Hg, Se, and Sn, and significant amounts of As, Cu, and Sn. The burning of fossil fuels to generate energy, accounted for more than 95% of the vanadium V and about 80% of the Ni (Swaine [9]).

Global emissions values for selected trace elements are listed in Table 1. That shows a estimated mobilization of trace elements in coal-fired power stations during 1990 (Bignoli) [15].

Table 1- Estimated mobilization of trace elements in coal-fired power stations during 1990.

Mean value Total Mean value Emission in coal mobilizatio in fly ash to burnt n atmosphere t/y ppm t/y % of total t/y % of total As 13.9 3,340 3,241.9 97 98.1 3 Cd 0.4 99.8 97.1 97 2.7 3 Cr 34.2 8,210 8,218 99 82 1 Hg 0.28 67.5 7.4 11 60.1 89 Mo 6.3 1,498.3 1,481 99 18.3 1 Pb 44.3 10,640 10,448.5 98 191.5 2 Sb 2.5 600 585.9 98 14.1 2 Se 2.1 505 432.8 86 72.2 14 U 1.8 440 434.9 99 5.1 1 V 57.9 13,900 13,756 99 144 1

4 Pollution Control Systems

Current practice of controlling trace metal emissions resulting from fuel combustion or waste incineration employs conventional air pollution control devices (APCDs) to collect fly ash and metal fumes. This control is post- combustion because metals are allowed to vapourize and condense before being

controlled. (Ho T.C. et al [1] ). Power stations are usually fitted with a variety of facilities for pollution control. Some form of particulate system (electrostatic

The Sustainable City 207 precipitators, fabric filter) , flue gas desulphurization - FGD and denitrification systems are installed in many power stations around the world.

In relation to Brazil, the desulphurization processes of combustion gases are still considered as the best solution in the plans to adapt existing thermoelectric plants. Traditional technology uses lime/limestone, reaching efficiency levels in the order of 90%. For an installed power of 350 MW, implementation costs of this desulphurization equipment has been estimated in approximately US$ 150 million. (ELETROBRAS [16] ).

Basically, these types of gaseous emission control technology depend on the injection of the sorbent material in an area of high temperature, being necessary the specific adoption of standard conditions, for example, granulometry of the sorbent, feeding rate respecting the proportion molar (Ca/S), temperature of the gas, adequate temperature for the sorbent and the gas flow. In laboratory conditions, the experiments are accomplished in loads and in a stationary bed.

When a sorbent particle is introduced into a furnace, slow heating takes place up to the working temperature (750 - 850°C). Them the sorbent is calcinated and sulphated in different environments of CO] and SO]. Several authors have dealt with the subject (Hartman M. [17], Alvfors P. [18], Borgwardt R. H. [19], Sun

C.C. [20] ). In this work, we have dealt only with the study of sorbent characterization, in order to determine the best areas for its supply and considering the proximity of large concentrations of industrial and energy production.

The sorption phenomenon on carbonate rocks is intimately related to the geological age, as well as to its physical texture, chemical composition and to the temperature in which happens the process. This makes necessary a complete characterization of the samples to be used.

5 Case Study

Rio de Janeiro has some areas of calcareous rock occurrence that, according to the Brazilian Association of Lime Producers, are: Barra Mansa, Barra do Pirai, Cabo Frio, Cambuci, Campos, Cantagalo, Cordeiro, Itaborai, Itaguai, Itaocara, Itaperuna, Mage, Marica, Marques de Valenga, Miguel Pereira, Mongao, Paraiba do Sul, Pirai, Rezende, Rio de Janeiro, Sao Fidelis, Sao Goncalo, Sao Pedro da Aldeia, Sao Sebastiao do Alto, Saquarema, Tres Rios, Vassouras

Of the above mentioned areas, a large part is of shell deposits. Of these areas, the most favourable, are the ones that have tertiary origin. According to some authors, if a calcareous rock is more recent, in geological terms, it increases its possibilities to adequate for the use in gaseous emission control technology, due to its larger porosity.

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The tectonic activity that influenced the evolution of Serra do Mar has originated small basins (Sao Paulo, Taubate, Resende) and the deposits in Baixada Fluminense (Falcao H. [21])

The Itaborai Formation, is to the north of Guanabara Bay and has its genesis associated to tertiary tectonic movements. Lithologically is constituted of a carbonated sequence with the predominance of calcareous beds intercalated to clastic calcareous banks and layers (Asmus H.. et al [22].

For the FGD utilization we can work with calcitic or dolomitic calcareous. Magnesium carbonate is usually found in association with calcium in a form of double carbonate, CaCC^.xMgCO], which is known as dolomite when x = 1 or dolomitic limestones when x < 1. On heating, carbonates dissociate to a solid

residue of oxide and CC^. Differential thermal analysis (DTA) is the usual technique for studying the behaviour of carbonates on heating. Another technique used in the characterization of sorbent for FGD is the porosimetry. These techniques are the starting point in the sorbent characterization.

For the analysis it was used a MICROMERITICS ASAP 2000 equipment, for BET-Nitrogen specific surface area and micropore properties, and a poresizer 9310 unit, in order to determine the total volume by the Mercury Intrusion

technique. It was also used Perkin-Elmer, 7 Series Thermal Analysis System.

5.1. Sorbent Particle Size

As the sulphation reaction occurs, the principal resistance on the reaction rate changes from pore diffusion and surface reaction to diffusion (Adanez [23], Gallagher P.K.,[24], Ulerich N.H. [25], Zarkanitis S.[26],) Therefore, sorbent particle size is a very important variable in the study of the sorbent sulphation reactivity.

The Figure 1 shows a relation between the porosity of three occurrences from Rio de Janeiro State. Whereas it is not yet a conclusive result, Itaborai calcareous, at first, presents better results. The results in the study of the obtained porosity seem coherent with the aspect of the samples. The most crystalline have

been the less porous ones and of smaller specific surface. In the most crystalline samples, the biggest porosity percentage corresponds to the biggest sizes of pores, and possibly almost everything is intergranular porosity (Alonso J. [27]). In the other samples the most important fraction in porosity also corresponds to the intergranular pores (those of larger size) that contribute very little to increase

the specific surface (Gregg S.J. [28], Ulerich N.H. [24], Zarkanitis S [25].

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impos

Fig. 1 - Relation between the porosity of calcareous rocks from Rio de Janeiro.

5.2 TGA Analysis

The thermal decomposition processes have been studied in order to gain an understanding of the structural changes that occur during firing and thereby helping with the definition of work temperatures. As it is shown in Fig. 2, the Itaborai sample presents a change at 750- 800°C indicating the lime formation. This result indicates a mixture behaviour, probably due to the presence of clay, which in this case may not be a disadvantage on its use, because clays can represent an important role in the sorption of potentially contaminants elements (Carvalho W.P. [3]). The other samples also presented similar results in relation to temperature.

Fig. 2 - TGA Results for Itaborai Formation

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6 Conclusion

As exposed above, the used techniques are just the first steps in the characterization

of the sorbent. However incipient, this work is necessary for the definition of the areas in which the adequate materials can be found, aiding in the control of effluents. This means the beginning of a strategy for industrial planning for the cities.

Therefore, in agreement with the accomplished studies, we can conclude that the city of Rio de Janeiro could have sorbent materials in its surrounding areas. These calcareous occurrences have different geological ages, and they could serve not only for the control of gaseous effluents, but also for the liquid ones, being a

subject for specific studies. The Itaborai Formation deserves more in-depth studies, according to the presented results The advantages of using this kind of material would be, then, the easy access and low cost.

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