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Waste to energy technology: Burning up your garbage

Waste to energy technology: Burning up your garbage

Raw material for biogas energy is available cheaply and abundantly. As Thomas Elsenbruch explains, the technology and new products are evolving and can be easily harnessed.

More countries than ever before are now looking for alternatives to fossil fuel as a means of generating power. The goals of these initiatives are usually to protect the environment and to gain independence from foreign suppliers. Above all, in agriculture and waste management, the use of organic waste products in biogas plants is opening up undreamed-of potential for the generation of electricity and heat. Germany, for instance, is taking a leading role in these international efforts with its renewable energy law. Many countries are following this example, and biogas plants are achieving a never seen before boom.

Characteristics of Biogas

Biogas is a natural, purely ecological product that is extracted from biomass by anaerobic digestion. When organic waste material undergoes anaerobic fermentation—a controlled decomposition process—a mix of gases is given off, consisting of 50-70 per cent methane (CH4) and 30-50 per cent carbon dioxide (CO2), depending on the makeup of the source material. A large variety of organic substances can be transformed into biogas in this way, although the yield may vary significantly based on the source material. Input materials range from food waste, effluents from beverage industry (draff, distiller's wash, pomace) via the separated organic household waste to animal manure.

Different anaerobic digestion processes

As a metabolic product of the participating methane bacteria, the prerequisites for its production are absence of oxygen,
a pH value from 6.5-7.5 and a constant process temperature of 15-25° C (psychrophile), 30-40° C (mesophile) or 50-55° C (thermophile). The fermentation period is approximately 10 days for thermophile, 25-30 days for mesophile and 90-120 days for psychrophile bacteria. The fermentation systems of today operate largely within the mesophile temperature range. The process of biogas generation is divided into three steps (Figure 1: Anaerobic digestion process for biogas in CHP).

• Preparation of the bio-input
• Fermentation
• Post-treatment of the residual material

At the beginning, the organic material is collected in a primary pit, sterilised to remove harmful germs in case of food waste and moved to the digester. The biogas produced in the digester is collected in a gas storage tank to ensure a continuous supply of gas, independent of fluctuations in the gas production. Finally, the biogas is fed into a gas engine. For safety reasons, the installation of a gas flare is recommended so that excess gas can be burned off in the event of excessive gas production. The end product from the fermentation of the biomass can be utilised as fertilizer. The gas mixture produced in the digester consists of 50-70 per cent methane (CH4) and 30-50 per cent carbon dioxide (CO2). This composition makes biogas well suited for combustion in gas engines.

Biogas is an especially high-value fuel that is ideally suited for running gas engines. The energy produced can be used either for the company's own electricity needs or can be fed into the public power grid. The heat generated is typically used as process heat supply for the plant. And finally, the material left over from the fermentation process can be used as a high-quality fertilizer in agriculture.

Critical substances in biogases

Gases, respectively their constituents, have different properties which can be assessed through their characteristic values, such as methane number, heat value and laminar flame speed, to mention only a few (Table 1: Characteristics of different gases). To be able to achieve an ideal degree of energy conversion, these values have to be considered during design of the engine.

Formation of hydrogen sulphide and its impact on gas engines: The anaerobic fermentation of biomass generally produces not only methane and carbon dioxide but also a certain amount of hydrogen sulphide (H2S) from the protein component of the biomass. This gas is toxic, attacks ferrous materials and has an unpleasant smell of rotten eggs. One assumes a H2S content from 0.01-0.5 per cent by volume (corresponds to 150-7,500 mg/Nm³ gas).

The relatively high content of hydrogen sulphide (H2S) in biogas and sewage gas causes the development of SO2 during the combustion. When biogas is used in gas engines, an excessive H2S content has harmful effects:

• The lubricating oil acidifies and has to be changed very frequently.
• If the oil is not changed in good time, moving parts of the engine become excessively worn.
• When the H2S content is very high (above 2,000 ppm), some engine components suffer direct damage.

Where condensates arise in the gas flow (due to insufficient drying of the biogas in the gas control system or starting procedures in the exhaust gas heat exchanger), the result is an acid attack on metal parts. By further oxidation especially in the catalytic converter SO2 reacts to SO3. Together with water it forms sulphuric acid (H2SO2) which is among the most aggressive acids. These acids can cause massive corrosion problems in exhaust gas heat exchangers. Therefore it is important to avoid condensation. (Figure 2: Acid dew point in dependence on SO3-concentration)

Different solutions to remove hydrogen sulphide from biogas:

1. Binding the sulphur to iron salts in the digestion process: A method frequently employed to bind sulphur in biomass digesters is the use of iron salts (iron-II-sulphate, iron-II-chloride), which are mixed into the biomass in solid or liquid form. Here, the iron performs the function of binding the sulphur, thereby preventing the H2S from escaping into the biogas. The following chemical reaction takes place:
Fe2+ + H2S gFeS + 2 H+

The resulting iron sulphide salt and the elemental sulphur are present in insoluble form in the fermentation substrate and are removed along with the putrefied material. In biogas plants the iron salt can be mixed into the organic substrate or liquid manure or even measured into the digester, in which case it is vitally important to mix it evenly through, in order to prevent the release of H2S, as far as possible. The operational costs depend on the H2S content in the gas and this method is therefore not recommended in installations with high sulphur contents, like biogas from spent wash.

2. Biological desulphurisation in the digestion process: The exhalation of H2S is often prevented in biomass digesters by introducing (injecting) a controlled amount of air directly into the digester. This causes the autonomous growth of a certain number of micro-organisms which degrade H2S. In terms of plant technology, this process is very straightforward but has to be followed precisely. It reduces the methane yield as methane only forms well in anaerobic conditions (without oxygen). Individual cases have also been reported in which sulphuric-acid-forming micro-organisms occur and cause acid corrosion in the digester.

3. Biological desulphurisation in the downstream reactor: The biological degradation of H2S described in can also be produced in a separate reactor. The biogas is conducted from the digester through a solid bed reactor with tower packing. The tower packing is sprinkled with a nutrient solution.

The controlled injection of air results in the autonomous growth of a population of H2S-degrading micro-organisms. The pH value must be controlled. The advantage is that methane production is not impeded by oxygen. The disadvantage is that a typical reactor has to be built to a considerable size. This technology can be applied to very high contents of H2S, even the levels typical for anaerobic digestion of distillers wash (~30,000 ppm) can be covered.

Gas engine solution vs conventional configurations

Biogas produced from spent wash by a 60 tpd distillery can be utilised for fuelling a reciprocating engine based genset for generation of 2 MW electricity besides 1.3 tph of steam for meeting partial process steam requirements. A conversion efficiency of 40 per cent for electricity and another 30-35 per cent into heat can be obtained from this application. About 0.6 MW of the generated power can be utilised for distillery operations and a surplus of about 1.4 MW power can be exported to the grid. (Figure 3: Gas Engine Solution).

Power can also be generated from the available biogas through a steam turbine and boiler configuration, however, this will generate only 1 MW electricity instead of 2 MW generated through reciprocating engines. In this case, however, all the steam generated will be used for power generation as compared to about additional 1.3 tonne per hour steam recovered from engine waste heat. This clearly indicates that conversion through steam turbine route is less efficient with about 20 per cent conversion into electricity.

Economical dimension

From an international perspective, the Kyoto Protocol also plays an important role in the construction of biogas plants. More and more countries now recognise that the benefits of ecological and autonomous power supply are a huge advantage. Biogas to energy facilities provide base load power, and in combination with heat utilisation at site, the greenhouse gas reduction potential is outstanding. With India's increasing demand for power, exploring alternative energy resources such as biomass is critical to meet this demand.

Case of KCI Ankleshwar

Besides hundreds of European installations based on agricultural organic materials, there were already engines installed in India running on biogas from spent wash in the late nineties of the last century. One of the first installations was done by Kanoria Chemicals and Industries (KCI). The company has installed a 2 MW power generation plant using biogas obtained through biomethanation process of spent wash starting with the first 1 MW in 1998. Earlier, biogas was used in the existing boilers to produce steam but because of much more attractive economic viability, it is used directly in  Jenbacher purpose-designed IC engines (specifically developed for gas application) to produce power. The exhaust flue gases from the engine are used for steam generation required in the process. This step has not only helped KCI in meeting environmental norms but has also helped in making valuable contribution to enhance their profitability. Clarke Energy India has been involved with the gas genset installation since inception. The sets were sold and installed by Clarke Energy India (erstwhile Cogen India Engineering). The sets have been under an annual maintenance contract. The major overhaul on the sets (long block replacement) was also carried out by Clarke Energy India.

GE's experience in biogas operations

GE's Jenbacher gas engines business has been supplying biogas-fuelled gas engines all over the world for more than 30 years. Jenbacher biogas engines operate in varied climatic conditions and are characterised by fuel flexibility—natural gas as well as a broad variety of other gases such as biogas, landfill gas, associated petroleum gas, industrial waste gases as well as syn-gas.
The Jenbacher product programme for gas engines presently comprises four constructional series with a total of 11 engines graduated according to cubic capacity and number of cylinders. The programme for biogas covers a performance spectrum of 250-2,737 KW electrical and 294-2,880 KW in thermal energy (Figure 6). Various versions for fuels such as natural gas, propane and biogases like sewage and landfill gas are available. Jenbacher cogeneration systems achieve a total efficiency of more than 90 per cent under strict observance of all international emission regulations.

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