Xavier Lombard, Bertrand Courcelle, Verdesis, Belgium,
Thierry Dhainaut, Thierry Lora Ronco, EDF R&D, France,
Giovanni Palmas, Antonio Bertolotto, Marcopolo Engineering, Italy
The first microturbine running on landfill gas in Europe has been installed in Italy. The project gave the developers a unique opportunity to directly compare microturbines with reciprocating engines in this challenging application.
In 1994, a landfill site in Piemonte, south of Turin, Italy, was closed after 15 years of operation. In 1995, reciprocating engines were installed to produce electricity from the landfill gas, and the site is expected to continue producing power for some years to come.
Over its 15-year life, the landfill was filled with 1 million m3 of municipal waste. Initially, biogas production from the site was expected to fall to uneconomic levels in 2002, but the installation of automated systems to manage the landfill gas wells and the expertise of a local operator have allowed the biogas production level to be maintained.
In 1995, Marcopolo Engineering equipped the landfill site with two reciprocating engines generating up to 800 kWe. The biogas production level has decreased over the years, however, and production now stands at around 120 Nm3/h with 50 per cent methane (CH4). A new engine of 250 kW was commissioned in January 2002. This engine is currently running at part-load (around 160 kW). Two blower fans ensure a stable depression in the landfill. The biogas is cooled down to 7°C and the condensates are automatically drained and treated in the nearby wastewater treatment.
With biogas production levels falling, Marcopolo Engineering and Verdesis embarked on a project to install a 30 kW microturbine at the site. The project gave the companies a unique opportunity to evaluate the relative advantages and drawbacks of reciprocating engines and microturbines in a landfill application on a commercial installation.
Verdesis led the project and supervised the overall installation of the microturbine. EDF R&D also actively participated in the project, bringing both the equipment (microturbine, compressor and biogas filter) and the expertise to monitor relevant parameters of the experiment.
The microturbine is a C30 model from Capstone Turbine Corporation. The compressor is a Copeland.
Engine efficiency of reciprocating engine and microturbine using landfill gas as fuel
The installation and commissioning of the microturbine were done in just three days. The microturbine arrived on a Monday and on the same day the three partners made all the electrical and piping connections. The commissioning was carried out on the following Wednesday and was a seamless process thanks to Capstone’s expertise in this field. Initially the compressor was tripping on low inlet pressure, but this was corrected by bypassing the protection not needed for this application.
A commercial comparison
Microturbine technology can have numerous advantages over other technology types when burning low quality biogas. Microturbine emissions are extremely low compared to reciprocating engines. Reciprocating engines offer low initial costs, but require frequent and expensive maintenance. Reliability with low quality biogas is also a concern.
In landfill applications, microturbines have a unique capability to maintain their efficiency even when the methane content within the biogas is decreasing. This, and the option of having several units installed on one site, allows the biogas profile curve to be followed, i.e. electricity can continue to be generated as the amount of biogas produced, and the level of methane within that gas, declines over time.
Reciprocating engines have been used for many years and their installation is not considered to be very difficult. Nevertheless, the complexity of the installation requires very skilled engineering and operational expertise as faults can arise in any of the various components. Microturbines, however, bring a high degree of sophistication but have fewer components.
Therefore, a microturbine connected to a compressor/chiller stage is an efficient system for generating electricity on a landfill site or a sewage plant. A microturbine-based operation also brings another financial gain on the exhaust treatment stage since the microturbine exhausts are 5 to 50 times cleaner than most typical engines (without post combustion). Other advantages of microturbine installations include:
- The synchronisation relay is embedded into the power electronics
- The air-cooling process for reciprocating engines is noisy, whereas microturbines do not need additional air-cooling; indeed, they are designed for outdoor installation and the level of noise is comparable to a containerised engine (70 db at 1 m).
In terms of the flexibility of the generating unit, tests performed at the landfill site on the 250 kW reciprocating engine have shown the following constraints:
- The efficiency dramatically decreases when the methane content falls below 50 per cent.
- Maintenance costs are the same whatever the power generation level – this is inconvenient if the engine can only run at part load
- Auxiliary losses remain practically constant, which penalizes the economics at part load
- Engine start-up is very sensitive to the biogas methane content and must be done manually.
The project has also highlighted several advantages of the microturbine:
- Easy to install or move to another site
- The ability to run with a low methane content: CH4 >35 per cent
The efficiency of the microturbine does not decrease as methane content falls
The presence of several microturbines on the same site allows the biogas generation curve to be followed, and the maintenance costs to be kept proportional to the kWh generated.
To analyse the economics of microturbines versus reciprocating engines, the following hypothesis has been used:
- One engine plus installation is paid with a credit at six per cent for five years
- Six microturbines plus auxiliaries and installation are paid with a credit at six per cent for eight years
- Maintenance is g5/h for the 250 kW engine
- Maintenance is g0.46/h per microturbine
- The biogas curve is decreasing by eight per cent per year
- Neither the price of biogas capture nor the amortisation of the biogas capture is included in the calculation.
Calculations show that the reciprocating engine is more economic at full power, but after a power output reduction of 20 per cent, the microturbines bring a better price per kWh. In addition, the simulation showed that by 2006, the reciprocating engine would become uneconomic due to the reduced biogas production.
The project highlighted a number of advantages of microturbines over reciprocating engines, including the ability to run on gas with a low methane content, and ease of installation
Investment and maintenance costs of reciprocating engine and microturbines at the Piemonte landfill project in Italy
In addition, seven microturbines commissioned in 2002 would bring a better return on investment compared to a 250 kW reciprocating engine. In 2009, four microturbines will still be running at full power at the site, and the remaining three microturbines can be redeployed at other sites.
The biogas-recovering unit at the landfill site in Italy was stopped for three days in July 2002 to change the main transformer from a 1 MW to a 400 kW transformer. After the transformer exchange, the microturbine was commissioned. The electrical power production capability of the site was increased by 20 per cent as the biogas quality required by the microturbine is poor – about 35 per cent minimum.
Measurement of the gas flow and the electrical generation has been carried out over several months by EDF R&D and Marcopolo Engineering. The microturbine efficiency assessment is based on results accumulated during EDF R&D laboratory testing for different values of methane content and throughout the on-site experiment in Italy.
This measurement has to be considered as a snapshot because many parameters can influence the final results such as air inlet temperature and pressure, biogas inlet pressure, biogas methane content and altitude.
The microturbine has a complex control system that allows it to achieve the best efficiency and the best emission reductions. The microturbine regulates the rotational speed of the engine core from 50 000 r/min to 90 000 r/min. The frequency of the alternator is therefore moving from 1600 Hz to 3000 Hz. Power electronics using IGBT converts this variable frequency voltage in DC current into a clean 50 Hz current source.
The power electronics embedded in the microturbine package does not require any synchronisation relay. In addition, the power electronics has a very rapid reaction time in case of voltage drop on the grid side. This is why the short circuit capability of the microturbine is lower than 1.5 times the nominal power.
The microturbine control command has several protection functions embedded in it to meet the requirements of Enel. These protections are over/under voltage and over/under frequency.
If the grid voltage drops suddenly, or if there is significant frequency variation, the microturbine will automatically disconnect itself from the grid in order to prevent any current feedback onto the grid. The synchronous generators of recip engines can also have the ability to run unexpectedly in stand-alone mode during a grid outage. The microturbine formatted in grid mode does not have the capability to run in stand-alone mode.
Test bed behaviour
Part of EDF R&D’s role in the project was to develop a specific test bed to characterize the behaviour of the microturbine at different biogas methane content levels. It achieved this by mixing methane with carbon dioxide.
The test results show that the microturbine can run with the same parameter setting on the whole range of methane variation (between 40 and 67 per cent CH4 for a 55 per cent CH4 nominal setting for instance), without any significant efficiency change.
Emissions: Emissions have been monitored during the lab tests for different methane contents in the synthetic biogas, with the same reference setting of 55 per cent CH4 content. A very low level of NOx (less than 9 ppm) was measured for each operating point. On the other hand, carbon monoxide (CO) levels increased as methane content decreased (up to 60 ppm for 47 per cent CH4 content). This second test shows that some combustion adjustment is still required to overcome the pollutant generation over the full range of operation of the microturbine. Capstone has noticed the same issue on a US landfill site with a methane content below 45 per cent and has made adjustments to the combustion software to reduce CO levels.
Measurements done on the Italian site have confirmed both these preliminary observations, i.e. very low NOx levels and a higher level of CO.
Auxiliary: The microturbines have proven in the USA to be very reliable in biogas applications, confirming the very low maintenance concept. Reciprocating engines need an oil change every 500-1000 hours, while microturbines require no oil changes but do need two annual visits to change several filters.
The auxiliary is the biggest challenge facing the integrator of a microturbine in a biogas application such as landfill, sewage plant or industrial gas. There are many compressor brands in Europe but the experience on corrosive biogas is still limited. Besides, in order to maintain the microturbine operating flexibility, compressors need to be able to deal with different flow for various prime-over or different methane contents. This flexibility has to be reached but not at the expense of fixed losses or cost. Further to this project, Verdesis has developed a new concept integrating on one skid the biogas treatment including the compressor, and biogas dehumidification.
A versatile solution
The first microturbine was installed in 2002 and in October 2003 the scale of the project will be increased through the addition of further microturbine units. The first microturbine was carefully monitored in order to make a fair comparison with 250 kW engines. These tests have demonstrated the effective operating versatility of microturbines with numerous advantages such as running with CH4 at less than 35 per cent, no need for post-combustion to fulfil emission limits, the possibility to put the microturbines in a container with a noise level lower than 40-50 db.
The economics of the microturbine project is increased by its flexibility to generate electricity in line with the biogas curve. This leads to maintenance costs being proportional to the electricity being generated. These tests have also confirmed the economical advantage of the microturbine in landfill applications with a biogas production lower than 250 Nm3/h and a decrease in the biogas production of 8-12 per cent per year. Several projects led by Verdesis and Verdesis Suisse on larger scale are now scheduled to start with multiple microturbine installations in Italy, Belgium, Germany, Switzerland and France.