Mobirise

BIOGAS ELECTRICITY GENERATION

Lesson 6.

OPTION 1.


Various technologies to generate electricity from biogas on a household level are available. In principle, the chemical energy of the combustible gases is converted to mechanical energy in a controlled combustion system by a heat engine. This mechanical energy then activates a generator to produce electrical power. The most common heat engines used in for biogas energy conversion are gas turbines and combustion engines. Combustion engines can be either internal combustion engine (e.g. reciprocating engine) or external combustion engine (e.g. Stirling engine).


For small-size heat engines, combustion engines are popular as they are more efficient and less expensive than small gas turbines. However, gas turbines may be more efficient when operating in a cogeneration cycle producing heat and electricity. Cogeneration or combined heat and power (CHP) describe the simultaneous generation of both electricity and useful heat. Heat engines (also thermal power plants) in general do not convert all of their thermal energy into electricity. In most cases, a bit more than half is lost as excess heat. By capturing the excess heat, CHP use heat that would be wasted in a conventional power plant, potentially reaching an efficiency of up to 89%, compared with 55% for the best conventional plants. This means that less fuel needs to be consumed to produce the same amount of useful energy. By-product heat at moderate temperatures (100-180°C) can also be used in absorption chillers for cooling. A plant producing electricity, heat and cold is sometimes called trigeneration or more generally a polygeneration plant.


Micro cogeneration is a so-called distributed energy resource (DER). Biogas is burned for running a generator (e.g. micro turbine). The installation is usually less than 5 kWe (Kilowatts-electrical). Instead of burning fuel to merely heat space or water, some of the energy is converted to electricity in addition to heat. This electricity can be used within the home or business or, if permitted by the grid management, sold back into the electric power grid.


Mini cogeneration is a DER producing usually more than 5 kWe and less than 500 kWe and the excess energy is generally fed into the electricity grid. To be viable a good base load for electrical demand and heat demand must exist.


Current Micro- and Mini CHP installations use five different technologies: micro turbines, internal combustion engines, external combustion engines (stirling engines), steam engines and fuel cells.


Biogas systems are an environmental friendly way of energy production and have a positive impact on climate change. In fact, the contribution of a methane molecule (CH4) to the greenhouse effect is 21 times greater than that of a carbon dioxide molecule. Therefore burning methane, even though producing CO2, reduces its impact on the environment.


Applicability

The technology is easily adaptable and can be applied at household or community level. To minimise distribution losses, the reactors should be installed close to the CHP where the gas can be used.

Micro cogeneration is a so-called distributed energy resource (DER) useful for a single house or small business because of the low power output. This electricity can be used within the home or business or, if permitted by the grid management, sold back into the electric power grid.

Mini cogeneration DERs supplies electricity for more than one household and if the excess energy can be sold, these installations are generally more viable from an economic point of view. Thus, mini CHPs have a large role to play in the field of carbon reduction in buildings where more than 14% of carbon can be saved by using mini CHPs.

Biogas cogeneration is extensively used and disseminated in rural China, Nepal, Vietnam, rural Costa Rica, Colombia, Rwanda, and other regions of the world where waste management and industry closely interface.


Advantages

  • Generation of renewable, green electricity
  • Low operating costs
  • Underground construction minimizes land use
  • Long life span
  • Reduces greenhouse gases
  • Increases family income by selling back electric energy to the electric power grid
  • On site use of heat


Disadvantages

  • Requires expert design, skilled construction and expert maintenance required
  • Biogas production below 15°C, is no longer economically feasible 
  • High capital costs


Combined heat and power (CHP) unit “micro size” in Germany.
Source: GTZ

Schematic of a biogas plant used for power generation 


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OPTION 2.

Theoretically, biogas can be converted directly into electricity by using a fuel cell. However, this process requires very clean gas and expensive fuel cells. Therefore, this option is still a matter for research and is not currently a practical option. The conversion of biogas to electric power by a generator set is much more practical. In contrast to natural gas, biogas is characterized by a high knock resistance and hence can be used in combustion motors with high compression rates. 


In most cases, biogas is used as fuel for combustion engines, which convert it to mechanical energy, powering an electric generator to produce electricity. The design of an electric generator is similar to the design of an electric motor. Most generators produce alternating AC electricity; they are therefore also called alternators or dynamos. Appropriate electric generators are available in virtually all countries and in all sizes. The technology is well known and maintenance is simple. In most cases, even universally available 3-phase electric motors can be converted into generators. Technologically far more challenging is the first stage of the generator set: the combustion engine using the biogas as fuel. In theory, biogas can be used as fuel in nearly all types of combustion engines, such as gas engines (Otto motor), diesel engines, gas turbines and Stirling motors etc. 


Appropriate Combustion Engine 


External Combustion Engines (EC Engines) 

Stirling Motors: In such motors, biogas is combusted externally, which in turn heats the stirling motor through a heat exchanger. The gas in the stirling motor hence expands and thereby moves the mechanism of the engine. The resulting work is used to generate electricity. Stirling motors have the advantage of being tolerant of fuel composition and quality. They are, however, relatively expensive and characterised by low efficiency. Their use is therefore limited to a number of very specific applications. 


In most commercially run biogas power plants today, internal combustion motors have become the standard technology either as gas or diesel motors. 


Internal Combustion Engines 

Diesel Engines operate on biogas only in dual fuel mode. To facilitate the ignition of the biogas, a small amount of ignition gas is injected together with the biogas. Modern pilot injection gas engines (“Zündstrahlmotoren”) need about 2% additional ignition oil. Almost every diesel engine can be converted into a pilot injection gas engine. These motors running in dual fuel mode have the advantage that they can also use gas with low heating value. But in that case, they consume a considerable amount of diesel. Up to engine sizes of about 200kW the pilot injection engines seem to have advantages against gas motors due to slightly higher efficiency (3-4% higher) and lower investment costs. 

Gas Motors with spark ignition (Otto system) can operate on biogas alone. In practice, a small amount of petrol (gasoline) is often used to start the engine. This technology is used for very small generator sets [(as option 1. above) (~ 0.5-10 kW)] as well as for large power plants. Especially in Germany, these engines have advantages as they do not need additional fossil fuels that would lead to lower feed-in tariffs according to the Renewable Energy Law (EEG). 

Gas Turbines are occasionally used as biogas engines especially in the US. They are very small and can meet the strict exhaust emissions requirements of the California Air Resources Board (CARB) for operation on landfill and digester gases. Small biogas turbines with power outputs of 30-75 kW are available in the market. However, they are rarely used for small-scale applications in developing countries. They are expensive and due to their spinning at very high speeds and the high operating temperatures, the design and manufacturing of gas turbines is a challenging issue from both the engineering and material point of view. Maintenance of such a turbine is very different from well-known maintenance of a truck engine and therefore requires specific skills. 


Today, experience of the use of combustion motors to produce electricity from biogas is extensive; this can be regarded as a proven standard technology. Over 4,000 biogas plants with internal combustion motors are in operation in Germany. 


However, it has taken lengthy and determined effort to make this technology as durable and reliable as it is today. Internal combustion motors have high requirements in terms of fuel quality. Harmful components - especially hydrogen sulphide - in the gas can shorten the lifetime of a motor considerably and cause serious damage. 


This must be addressed in two ways : 

  1. Production of clean biogas; and 
  2. Use of appropriate and robust motors and components. 


In theory, most engines originally intended for cars, trucks, ships or stationary use can run on biogas as fuel and are available almost everywhere within a power range between 10 and 500 kW. This holds true especially in the case of dual fuel use. Robust engines with a certain sulphur resistance are mostly free of non-ferrous metal (German: “Buntmetalle”), as these materials are highly prone to damage through sulphur-rich biogas. 


 Appropriate Gas Quality 

For use in gas or diesel engines, the gas must fulfil certain requirements : 

  1. The methane content should be as high as possible as this is the main combustible part of the gas; 
  2. The water vapour and CO2 content should be as low as possible, mainly because they lead to a low calorific value of the gas; 
  3. The sulphur content in particular, mainly in form of H2S, must be low, as it is converted to corrosion-causing acids by condensation and combustion. 


The water vapour content can be reduced by condensation in the gas storage or on the way to the engine. 


The reduction of the hydrogen sulphide (H2S) content in the biogas can be addressed via a range of technical methods. These can be classified as chemical, biological, or physical and divided into internal and external methods. Much experimentation has been carried out in the last two decades.


However, as complete elimination is unnecessary for use in robust engines, the following simple methods have generally established themselves as standard: 

  • An optimised steady fermentation process with continuous availability of appropriate feedstock is important to produce a gas of homogenous quality.


  • The injection of a small amount of oxygen (air) into the headspace of the storage fermenter leads to oxidation of H2S by microorganisms and hence the elimination of a considerable part of the sulphur from the gaseous phase. This is the most frequently used method for desulphurisation. It is cheap and can eliminate up to 95% of the sulphur content in the biogas. However, the right proportioning of air still seems to be a challenge.
  • Another option is external chemical treatment in a filter. The active material may be:


  1. Iron-hydroxide: Fe (OH)2 + H2S -> FeS + 2 H2O. This process is reversible and the filter can be regenerated by adding oxygen. Adsorption material may be iron-rich soils, waste material from steel or aluminium production ;
  2. Activated carbon: Certain companies provide activated carbon filters as a standard component in their gensets. 


Standard quality sulphur filters and filter material can be purchased on the market.                                          



Energy requirement for heating the slurry 


Energy required for heating the slurry in digester can be calculated by using the formulae below. 


QT = m×c× (T2-T1) ---- equation 1 

Where 

 QT is the total heat (Energy required for heating the slurry) and is expressed in Kilo-joule(Kj). 

m is the mass of the slurry and is expressed in Kilo-gram(Kg). 

c is the specific heat of slurry and is expressed in Kj/Kg°C. 

T2 is the desired temperature of slurry and is expressed in °C. 

T1 is the current temperature of slurry and is expressed in °C. 


mass of slurry = volume of digester (V) × density of slurry(ρ) and is expressed in Kg. ---- equation 2 


Where V is the volume of digester, expressed in m3 and ρ is the density of slurry, expressed in Kg/m3 . 


Density of slurry (ρ) = density of water + density of cow dung ---- equation 3 


Density of water is 1000 Kg/m3 

Density of cow dung is 0.13 Kg/m3 


Putting these values in equation 2 


Density of slurry (ρ) = (1000+0.13) Kgm-3/2 ≈ 500 Kg/m3 


From equation 1 we can get the mass of slurry by multiplying volume of digester with density of slurry calculated above. 


Specific heat of slurry = {specific heat of water (4.2Kj/Kg°c) + specific heat of cow dung (2.8Kj/Kg°c)}/2 

= 3.5Kg/Kg°c 


Putting these values in equation one we can know the energy required for heating the slurry. The unit of energy is Kj(Kilo joule)[3]. 



Necessary Framework Conditions 


In Germany, power generation from biogas is only profitable due to grid connection and sup-porting feed-in tariffs. By contrast, power generation in most developing countries seems to be especially profitable in settings far away from the national grid and other energy sources, as the legal framework conditions and the lack of appropriate feed-in tariffs do not support feeding into the grid. However, there are the first signs of financial and legal support for feeding in electricity from biogas power plants in countries such as Brazil. Output-oriented support schemes (such as the German EEG) have proved to be more successful than investment-oriented financial support. 


Direct subsidies and public financial contributions to installation costs have been crucial for the installation of some pilot plants. However, they have not provided incentives for proper and efficient operation. By contrast, the establishment of appropriate feed-in tariffs stimulates the construction of efficient plants and their continuous and efficient operation. 


Through its projects and programmes, GTZ therefore recommends the establishment of guaranteed feed-in price schemes similar to the one in Germany. 


However, besides price considerations, there remain many barriers to market penetration and development of the biogas sector: 

  • Lack of awareness of biogas opportunities 
  • High upfront costs for potential assessments and feasibility studies 
  • Lack of access to finance 
  • Lack of local capacity for project design, construction, operation and maintenance 
  • Legal framework conditions that complicate alternative energy production and commercialisation: for example, the right to sell electricity at local level has to be in place. 


As long as the national framework conditions are not favourable, electricity generation from biogas will remain limited to a few pilot applications. 


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