For biogas projects, two main technologies are used to generate electricity (or, for added efficiency, electricity plus heat): gas reciprocating engines or gas turbines. However, evaluating the two technologies can often be like the proverbial apples and oranges comparison. Usually, the only two criteria used to measure value are equipment cost ($ per kW) and electrical efficiency. It is critical, however, to dive deeper and ask further questions before determining the right solution for your project. This is especially the case when it comes to utilising biogas, which brings its own challenges, such as regular changes in gas quality, variable volumes depending on seasons, hydrogen sulphide (H2S) and siloxane contamination, etc. Simply put, while cost and efficiency are important factors, they should be the last two criteria compared.
What is Biogas?
As defined by the International Energy Agency (IEA), biogas is “a mixture of methane, CO2 and small quantities of other gases produced by anaerobic digestion of organic matter in an oxygen-free environment”. The composition and quality of biogas will differ case by case based on the feedstock utilised and the production method.
Example of a biogas plant
Biogas can be produced in two different ways: Covered Anaerobic Lagoons (CALs) – a type of anaerobic lagoon that is covered with a flexible, gas-tight cover, and Anaerobic Digestors (ADs) – sealed, oxygen-free tanks designed for the anaerobic digestion of organic waste by microorganisms.
According to a report prepared by ENEA Consulting for Bioenergy Australia, the feedstock that can be used to generate biogas can include:
- Industrial waste such as waste from food and beverage processing, and dairy, sugar, meat, pulp and paper industries.
- Agricultural waste as animal by-products and crop residues.
- Energy crops as maize, silage, grass, sorghum, cereals and sugar beet.
- Sludge from a wastewater treatment plant (WWTP)
- Biowaste from households, communities or small-scale commercial and industrial activities.
How do Microturbines and Gas Engines Work?
When it comes to generating power and heat using biogas, there are two technologies to choose from: microturbines or reciprocating gas engines.
Microturbines are compact gas turbines that operate by combusting methane in combination with compressed air. The resulting hot, pressurised gases are expelled from the combustion chamber and directed through a turbine wheel, driving its rotation and thereby powering the compressor and electrical generator. Most microturbine systems are equipped with a compressor and turbine, a recuperator (a gas-to-gas heat exchanger that recovers heat from the exhaust to preheat the incoming air and improve efficiency) and various components for managing combustion and electrical output conversion.
Inside of a microturbine
Reciprocating gas engines, by contrast, are essentially conventional gas engines that have been adapted to accommodate higher fuel volumes (accounting for the presence of CO₂ in biogas) and engineered to tolerate increased levels of contaminants in the intake air, in contrast to the relatively consistent quality of conventional natural gas.
Typical reciprocating engine
Microturbines vs Gas Engines for Biogas
Many engineering, procurement and operations specialists are very familiar with reciprocating gas engines, and this, as well as other factors such as gas engines having a high rated efficiency at full load, low capital costs, etc., leads to them selecting gas engines for most biogas projects.
Microturbines, in comparison, are relatively new when it comes to biogas applications, but the technology is proving itself in this space due to the various benefits it offers over traditional reciprocating gas engine generators.
- Firstly, microturbines have a wider tolerance for changes in both Wobbe Index and calorific value, which can be thought of as the energy density per unit of volume of gas. In other words, as the gas quality changes (very common in the production of biogas), gas turbines can continue running with much more variability.
- Microturbines can also operate down to very low load points (even zero load). Unlike many gas generators, this means that when demand falls or biogas yields drop, they can continue to operate without causing equipment damage and premature wear. This is another important factor, as many sites we see reduced biogas outputs seasonally or due to other supply interruptions.
- While gas engines have a high apparent efficiency at 100% capacity, with reduced capacity, they have a rapidly diminishing efficiency until around 55% capacity, at which point, in many instances, they cannot run at all. In this scenario, uptime becomes more valuable than efficiency, especially for operations that need uninterrupted, continuous power.
- Furthermore, unlike gas engines, a microturbine solution is modular, with a 1 MW installation comprising 5 x 200 kW turbine modules. Each of these can operate independently of the other, remaining online during servicing. It is therefore not a binary on/off impact on availability.
- The turbines are very tolerant to hydrogen sulphide (H2S) and can accept up to 5,000 ppm. This is a major cause of equipment failure and poor availability in many projects, increased service needs and shortened lifespan for a traditional gas engine.
- Moreover, the turbine is air-cooled and air lubricated. This means that we do not risk over temperature trips on hot days, which can occur with traditional generators with liquid cooling systems (radiators). The turbines having no lubricants or coolants also means that there is no contaminated waste, such as spent lube oil, coolant, hoses, etc., that need to be managed. Over a 10-year life, a microturbine will have 75% less mass to dispose of or recycle in comparison to a gas engine.
- When it comes to emissions, while the gas engines produce slightly less CO2 per kW of generated electricity, microturbines emit very low emissions of nitrogen oxides (NOx), volatile organic compounds (VOCs) and carbon monoxide (CO), due to their clean combustion and other factors.
While there are several individual benefits to microturbine technology when it comes to biogas, the overarching value proposition can be summarised in two words as “risk mitigation”.
End Uses for Power and Heat Generated from Biogas
The power generated from biogas can be utilised for site operations and/or exported to the electricity grid.
Steam recovery for food applications such as organic drink production
The heat recovered from power generation also has multiple uses. One of them is producing hot water using a heat recovery module (HRM) to be used for heating the anaerobic digester, thus optimising biogas production and reducing the need for other equipment and fuel to do the same job. Some other uses for the turbine heat include:
- Direct drying applications from turbine exhaust, including timber treatment, spray drying, paint drying, etc.
- Hot water from HRM for process heat and as feed water for steam boilers
- Steam recovery for food applications such as cured meat, frozen meals, and organic drink production
- Turbine heat, which can be converted into chilled water using an absorption chiller
- The 18% oxygen content in the turbine exhaust enables its use as combustion air for burners, significantly increasing system efficiencies and reducing the need for additional gas in the production of process heat
Optimal Group’s Experience
Optimal Group has had extensive experience with projects utilising biogas for the generation of power and heat for our clients.
A great example of such a project is McCain Foods’ Ballarat manufacturing facility, where we delivered a combined heat and power (CHP) system based on 2 Capstone C600 microturbines, with the exhaust being directed to a cofired burner. At a combined 1.2 MW of power generation, the installation has been sized to utilise the maximum biogas production available. Because the turbine exhaust is much hotter than ambient air (>280°C), the burner needs to add less fuel (natural gas in this case) to produce the same amount of steam. With 18% oxygen, the turbine exhaust can be used directly for combustion or mixed with fresh air, where not all turbines are operating. Using the exhaust heat directly as combustion air harnesses 100% of the available thermal energy, which results in total system fuel efficiency above 90%.
Turbines installed at McCain Foods Ballarat
The high-efficiency CHP installation provides a CO2 emissions reduction equal to the 7 MW of PV also deployed at the site, demonstrating the unique potential of biogas to reduce emissions for the food industry.
Sources
Emerson, H., & Wiltsee, G. (2004, February 15). Clean Power From Microturbines Using Biogas. https://www.biocycle.net/clean-power-from-microturbines-using-biogas/
ENEA Consulting. (2019, March 1). Biogas opportunities for Australia. Bioenergy Australia. https://www.energynetworks.com.au/resources/reports/biogas-opportunities-for-australia-enea-consulting/
Goldstein, N. (2006, September 20). Microturbines, Gas Engines Link Biogas To The Grid. https://www.biocycle.net/microturbines-gas-engines-link-biogas-to-the-grid/
IEA. (n.d.). An introduction to biogas and biomethane. https://www.iea.org/reports/outlook-for-biogas-and-biomethane-prospects-for-organic-growth/an-introduction-to-biogas-and-biomethane