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Toxic Oxide Deposits from the Combustion of Landfill Gas and Biogas. Environ.Sci.&Pollut.Res. 3 (1996) 75-77

Toxic Oxide Deposits from the Combustion of Landfill Gas and Biogas*

Dietmar Glindemann1), Peter. Morgenstern2), Rainer Wennrich2), Ulrich Stottmeister2) and Armin Bergmann1)
1) Dr. Dietmar Glindemann
Institute of Animal Hygiene and Public Veterinary Health
University of Leipzig
Semmelweisstr. 4
D-04103 Leipzig
Germany
Phone: +49-(0)341-9738-165, -158 Fax:+49-(0)341-9738198
e-mail dglinde@aol.com
2) Centre for Environmental Research Leipzig-Halle Ltd., Permoserstr. 15, D-04318 Leipzig
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Corresponding author: Dr. Dietmar Glindemann

Abstract

Oxide deposits found in combustion systems in landfill gas-fired power stations contain relatively high concentrations of elements which form volatile species such as P, As, Sb and Sn. These deposits should be handled with care because of their potential toxicity. By contrast, deposits in biogas system engines were found to contain much lower levels of such elements. The enrichment of these elements can be attributed to a hypothetical multistage process. The elements form volatile species in the landfill body. They are selectively transported as part of the landfill gas into the gas-burning devices. Inside the burners they are immobilized as nonvolatile oxides.

Key words:

landfill gas, biogas, gas combustion, gas engines, deposit, filter dust, elemental content, XRF, ICP-AES, P, As, Sb, Sn, Si, S, Fe, organometallic compounds, phosphine.

Introduction

The combustion of landfill gas and biogas in gas-fired power stations was observed to result in the accumulation of a deposit in the combustion zone in the order of a few kg per million m3 of burned gas [1]. Such deposits were found to have corrosive and abrasive effects on the equipment mainly due to by their high content of SiO2 crystals which can be attributed to the combustion of antropogenic siloxanes in landfill gas [1,2]. Therefore these deposits must be removed after accumulation periods of about a year. Furthermore, oxide deposits from domestic natural gas burners and the air in kitchens were found to contain As, Hg and Cu [3].

This raises the question of wether the toxic potential of such deposits necessiates the introduction of legal regulations to protect the health and safety of staff removing them from the system during the cleaning process. Moreover, legislation may also be required to regulate the waste management of the deposits thus removed.

Volatile compounds of As, Sb, Sn, Pb, Hg [4] and phosphorus [5,6,7] in landfill gas and biogas were recently directly detected using gas chromatography. Previous microbial investigations [5,8,9] demonstrated the ability of microorganisms to form volatile species of some of these elements. This raises the question of whether the composition of deposits from gas combustion is related to the quantity of such volatiles.
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*) Support: Centre for Environmental Research Leipzig-Halle Ltd. (joint project 08/95) and German Ministry for Education., Science, Research and Technology (02WA9375/0).

Samples: (definition, number, type, year of sampling, accumulation period)

The samples were taken from gas-fired power stations using landfill gas and biogas. The samples were divided into deposits (oxides after the burning process collected in the gas combustion zone) and filter dust (particles collected from gas filters before the combustion).

- Samples from the energy usage and filtration of landfill gas in Berlin-Wannsee, kindly provided by DEPOGAS GmbH.
No. 1.1., 1.2., 1.3.: Deposits from inside the combustion chamber and heat exchange pipes (internal and external) of a steam generator, 1984, accumulation period: nine months.
No. 1.4.: Deposit from inside internal pipes, 1983, accumulation period: one year.
No. 1.5.: Filter dust from a filter before the combustion process, 1983, accumulation period: one year.
No. 2.1.: Deposit from inside an Otto engine. No. 2.2., 2.3., 2.4.: Filter dust from 3 parallel filters for 6 main suction lines before energetic use, 1994, accumulation period: 3 years. The total mineral dust fraction of all filters was estimated at only about 10 - 100 g, whereas the quantity of each deposit was a few kilograms.

- A special sample from a landfill gas refining process.
No. 3.1.: Deposit. Trace gases were washed out from landfill gas. The washing solvent was regenerated by desorbing the trace gases, which were eliminated by a gas burner. The solid deposit of this burning process formed the sample.

- Samples from the energetic use of biogas from pig slurry digestion.
No. 4.1., 4.2., 4.3.: Deposit from inside the combustion chamber of a steam generator made of steel, cast iron and firebrick. Red, yellow-green and white colour, 1994, accumulation period: one year.

- A sample from the energetic use of biogas from cattle slurry digestion.
No. 5.1.: Deposit from inside the combustion chamber of a steam generator, grey, 1993, accumulation period: one year.

Analytical Methods

Most of the samples were directly analysed using the X-ray fluorescence technique (XRF) both to establish the complete composition of the solids and to avoid any loss or contamination during chemical decomposition. The analysis of samples 2.2. - 2.4. (material quantity < 1 g) was carried out with dissolved samples (leaching with aqua regie) using an inductively coupled plasma atomic emission spectrometer (ICP-AES).

Results and Discussion

The concentrations of selected elements of the samples are listed in Table 1. The measured elements are subdivided into different element groups a) - d), which are defined in the table. The deposits from landfill gas combustion contain relatively high concentrations of toxic metalloids and heavy metals, especially As and Sb. The concentrations of As and Sb found in deposits are unexpectedly high. Such concentrations do not occur in the gas; similarly the materials in contact with the landfill gas (landfill body, pipes, fittings, filters and machinery) contain much lower concentrations of these elements. The deposits from animal slurry biogas combustion contain much fewer of these elements and are less toxic and less abrasive. Some deposit samples contain high concentrations of Si, S and Fe.

Hypotheses

The levels (Table 1) of the deposits in landfill gas burners and of the filter dust preceding gas combustion enable us to suggest the varying significance of the following formation processes:

Selective volatilisation from the gas sources especially mobilises in particular the elements P, As, Sb and Sn of group c) as well as Si (but not the elements from the other groups). They can therefore pass through wet tubes or efficient dust filters as part of the landfill gas and are enriched in the deposit inside burners by mineralisation to nonvolatiles. The animal waste substrate for biogas fermentation usually contains less volatilisable heavy metals than the landfill body and the biogas contains levels of phosphine which are lower by several orders of magnitude. Therefore deposits from biogas combustion are less toxic and contain much less P.

Aerosol transport from the gas sources (landfill soil and fermentation media) ought to be negligible when the gas pipes are wet or equipped with efficient filters and thus adsorb all aerosols before the burner. Therefore all elements in the gas sources (landfill body or digester) which have no gaseous phase in the environment are differentiated in the deposit (element group a and d). The concentration and quantity of the volatiles forming elements of group c) in the filter dust was low in comparison to the deposit. Therefore it can be largely discounted that these elements entered the burner as filterable dust instead of as volatiles. It should be noted that for analytical accuracy this filtration must be performed using analytical filtration and washing procedures.

Material from the machinery used for gas transport, compression, refining and combustion is released from pipes, metallic alloys and firebrick by abrasion, corrosion or sublimation and becomes part of the deposits.

Emission through the smoke stack doubtless represents a loss process for deposit particles. Furthermore, the fractionated vaporisation of mineralized elements (for example As as AsCl3 at higher temperatures) is conceivable. Because the balance beween sources and losses of deposits is not easily observable, no correlation is drawn between the volume of waste gas burned and the quantity of deposit.

Table 2 compares the proportions of elementary concentration of volatiles forming elements P, As, Sb etc. as volatiles in landfill gas and biogas [4,7] with their concentration as solids in deposits (Table 1). The correlation supports the hypothesis that the enrichment of the elements in our samples is caused by selective volatilisation in the landfill body. Using the gas chromatographic method only a snapshot (sampling at a fixed time at one place) is possible. By contrast, our method assessed the toxicity of deposit samples representing a period of approximately one year while the deposits accumulated. It should be noted that the valididy of the correlation is limited as samples from different landfills and times were compared in Table 2 and concentrations of landfill gases change considerably with time and location.

Conclusions

Oxide deposits from the combustion of landfill gas contain high concentrations of toxic elements such as As and Sb. These deposits have to be removed periodically from the combustion systems by the service staff. Therefore, procedures to eliminate occupational health risks emanating from the removal of the deposits and from their waste management must be drawn up.

The deposits may be hypothetically derived from a long-term transfer and accumulation process of elements forming volatile compounds in the landfill body and nonvolatile oxides in the burner. The collection and burning of landfill gas prevents the landfill body from permanently emitting traces of toxic volatiles of P, As, Sb and Sn. Consequently the toxic potential is focused on the deposits. Moreover it cannot be ruled out from the outset that in addition to the group of elements classified in c), other elements found in the deposits may also exist as volatile compounds under environmental conditions. For example, iron can form volatile carbonyles, chelates and ferrocenes, which can be analysed by means of gas chromatography [10]. Future investigations will establish the applicability of this approach to other landfills depending on their state and composition.  

1. Schneider,J.: Untersuchung der Ursachen von Ablagerungen in einem mit Deponiegas befeuerten Kessel. In: Franzius,V., Stegmann,R. (ed.) Deponiegasnutzung, pp.55-66. BMFT, Bonn, 1995.

2. Arendt,G., Kohl,E.G.: Spurenstoffe in Deponiegas. Neue Erkenntnisse über siliziumorganische Verbindungen. In: Rettenberger,G. (ed.) Deponiegas 1995: Nutzung und Erfassung. Trierer Berichte zur Abfallwirtschaft 9, pp.9-20. Economica, Bonn, 1996

3. Kucha, H., Slupczynski, K., Prochaska, W.: Health risks and natural gas. Nature 363 (1993) 680

4. Feldmann, J., Hirner, A.V. (1995) Occurrence of volatile metal and metalloid species in landfill and sewage gases. Intern.J.Environ.Anal.Chem. 60 (1995) 339-359

5. Devai, I., Felföldy, L., Wittner, I. & Plosz, S.: Detection of phosphine: new aspects of the phosphorus cycle in the hydrosphere. Nature, 333 (1988) 343-345

6. Glindemann, D., Bergmann, A.: Spontaneous emission of phosphane from animal slurry treatment processing. Zbl.Hyg.Umweltmedizin 198 (1995) 49-56

7. Glindemann, D., Stottmeister, U., Bergmann, A.: Free phosphine from the anaerobic biosphere. Environ.Sci.&Pollut.Res. 3 (1996) 17-19

8. Brinckman, F.E., Olson, G.J., Iverson, W.P.: The production and fate of volatile molecular species in the environment: Metals and Metalloids. In: Atmospheric Chemistry, ed. Goldberg, E.D., pp.231-249. Dahlem Konferenzen 1982. Berlin, Heidelberg, New York: Springer

9. Gassmann, G., Glindemann, D.: Phosphane in the biosphere. Angew.Chem.Intern.Edit. 32 (1993) 761-763

10. Sun, X.Y., Aue, W.A.: Selective detection of volatile iron compounds by flame photometry. J.Chromatogr. 467 (1989) 75-84

Captions for Tables

Table 1 (file "elemente.xls)
Elemental content (mg/kg) of deposits from the combustion of landfill gas and biogas

Table 2 (file "elemente.xls)
Elements as volatiles in waste gases and as nonvolatile oxides in deposits from inside gas burners.a)

Footnote to Table 2

a) The element concentration (mg/kg) found in deposits is used to estimate the average elemental concentration (µg/m3) required as volatile in a waste gas volume of 1 million m3 to accumulate 10 kg of the deposit during combustion. This is compared with the elemental concentration (µg/m3) of volatiles actually measured in waste gas. Maximum values from the investigations of steam generators are used. The volatile phosphorus is measured as phosphine [7]. The other volatiles are measured as alkyl compounds [4].

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