Landfill gas is formed under anaerobic conditions in landfills by microbial degradation of organic material. The gas composition can vary, but at Swedish landfills the gas generally consists of 40-60% methane, 30-40% carbon dioxide and 5-20% nitrogen. Hydrogen sulphide (H2S) is a highly toxic and corrosive gas, which occur in landfill gas in varying concentrations, from 10 to 30,000 ppm (equivalent to 0.001 to 3.0%). It is desirable that the landfill gas is used for electricity and/or heat production, but to do that there is a need to clean the gas to reach <200 ppm H2S. High levels of H2S increases wear on the engine/boiler and thus the frequency of servicing. This leads to expensive maintenance costs, and ultimately shortens the economic life of the plant. To reduce corrosion, it is common to adjust the flue gas temperature, but this also leads to a lower efficiency and thus reduces the energy utilization of the gas. In some cases the gas concentration of H2S is judged to be too high to be used for energy production at all. In 2015, approximately 53 GWh of landfill gas was flared in Sweden, which in many cases is due to problems with high levels of H2S.
Cleaning of landfill gas from H2S leads to several values; the gas energy is used efficiently, maintenance and service costs of the engines/boiler are reduced, and emissions of acidifying sulphur dioxide from combustion of landfill gas decreases. There are commercial cleaning technologies for H2S but they are expensive, both in terms of capital cost and operating cost. Thus, there is a need to develop new cost efficient cleaning technologies that improve the economic outcome at landfills and that enables landfill gas with high H2S concentrations to be utilized for valuable energy transformation.
RISE (formerly JTI – Swedish Institute of Agricultural and Environmental Engineering) together with SLU develops new, potentially cost-efficient methods for upgrading biogas to fuel quality. One of the methods is based on the gas passing through a bed of moist ash (a so-called ash filter), where carbon dioxide and H2S are fixed. The hypothesis of this project was that ashes originating from the incineration of waste, recycled waste wood etc., can be used to clean the high levels of H2S in landfill gas. This type of ashes will usually be disposed of in landfills anyway and if the treatment effect is good, it would generate synergy effects in the form of the ash first being used to clean landfill gas from sulphur before it is used as a construction material at landfills.
This project performed two trials in pilot scale at a Swedish landfill with very high concentration of H2S, approximately 15,000 ppm. Different gas flow rates were studied (0.7 to 7.6 m3 / h), while the volume of ash used were similar in the two trials, 0,37 m3. The concentration of H2S in the cleaned gas was consistently very low during treatment, < 10 ppm at low gas flow rates and < 200 ppm at high gas flow rates. Two types of ash were investigated and both proved to have very good capacity to fix H2S, 44-61 g H2S/kg dry ash. In comparison with literature values, there is only one study showing an uptake capacity in the same order. Other studies report an order of magnitude lower uptake capacity.
Based on the experimental results, the technical and economic potential for an ash filter as the cleaning method was assessed. The calculations were made for various typical landfills to cover the different range of landfills. For normal sized landfills with gas flow rates of 100-1 000 m3/h and H2S concentrations between 100 and 1 000 ppm, the amount of ash needed is 10-130 tons of dry ash per year. For the special case where the H2S concentration is extremely high, the amount of ash increases and a plant with 15 000 ppm H2S and a gas flow rate of 200 m3/h requires approximately 800 tons of dry ash per year. However, overall modest amounts of ash is required and considering all Swedish landfills the requirement of ash would be only 0.2-0.3% of the annual production of ash in Sweden.
The economic calculations show that the ash filter is a competitive method for removal of H2S. For the special case of extremely high levels of H2S, it turned out that the cost of the ash filter is approximately 20% lower in comparison with the cheapest feasible conventional cleaning technology on the market. Also for the cleaning of landfill gas at more normal levels of H2S, the ash filter is competitive. At low gas flow rates (100 m3/h), the ash filter is clearly competitive compared to literature values for conventional cleaning technologies. The economy of scale seems to be higher for the conventional cleaning technologies, and consequently the difference between the cost of ash filter cleaning and other technologies is less at higher gas flow rates.
The low treatment cost of the ash filter reveals opportunities for landfills that currently do not clean the gas from H2S. During the project 15 Swedish landfills was contacted and none of these reported any form of H2S cleaning. When using cleaning, the landfill gas can be used effectively, i.e. reduced flaring, increased efficiency of electricity and heat production with reduced wear on boilers and combustion equipment as well as reduced emissions of sulphur into the atmosphere, which also reduces the potential odour problems around the landfill.
For further development, the design of an ash filter module prototype at full-scale is important. Furthermore, the treated ashes should be analysed for leaching characteristics, storability and usability as construction materials or as cover landfills along with an assessment of the overall environmental impact. Further tests at full scale should be made at other landfills with various gas flow rates and H2S concentrations to verify the performance of the conducted pilot tests.