Power generation from landfill gas

Power generation from landfill gas - The Garraf landfill site Enough power to light up the entire city of Barcelona could - in theory - be provided from one Spanish landfill site, according to modelling of its biogas generation capacity. But risk assessment and continued government co-operation will be of vital importance if this capacity is to be fully utilized. Christopher Eden The controlled landfill site of Vall d'en Joan - Garraf - is the largest landfill site in Spain, and one of the largest in Europe. It is located in the centre of the Natural Park of Garraf on the outskirts of Barcelona. Since its opening in 1974, more than 22 million tonnes of refuse has been deposited there, and projections are that there will more than 25 million tonnes in place once it closes in 2004 or 2005. The landfill site occupies an area of around 64 hectares and, although the depth varies with the original contours of the landfill site, it is over 100 metres deep in places, filling a complex valley system.In May 2001, two tenders were issued by the owner of the landfill site, the Entidad Metropolitana de Serveis Hidràulics i Tractament de Residus (EMSHTR) - the department of the City of Barcelona responsible for the treatment of solid waste. The first of these tenders was for the physical restoration of the oldest part of the landfill, and the second for the control and use of the biogas that is being generated from the whole of the landfill site. The power generation contract was awarded to a temporary joint-venture company formed by CLP Envirogas S.L. and ECYR (Endesa Cogeneración y Renovables). The scope of the project awarded included the theoretical modelling of gas production from the landfill site, physical pumping trials to prove the models, the construction of a gas extraction system, and the installation of a power generation complex to produce and export energy to the Spanish National Grid.The project has been entirely financed by the developers, which have not only provided the technological know-how and experience, but also assume much of the risk for the project. In addition to this, the project pays a royalty to EMSHTR for using the gas. Legislative background The Spanish Government is committed to encouraging the production of renewable energy. In December 1998, a Royal Decree, or new legislation, was approved. This legislation obliges the main electricity supply companies to purchase part of their baseload supply from renewable energy sources, which in effect means that they must purchase all the energy produced by authorized renewable energy generators. In exchange for this, the renewable energy producer can choose between a fixed price and a price that varies according to the behaviour of the electricity 'pool'. In the latter case a subsidy is included that varies according to the technology employed to generate the renewable power.The document that governs the production of renewable energy in Spain is the above-mentioned Royal Decree, Real Decreto 2818/1998, published on 23 December 1998. The law is designed to encourage renewable energy production, and so comply with the various global and European treaties that attempt to reduce the effects of the emission of greenhouse gases. The target for the Spanish Government is to produce 12% of energy demand using renewable sources by the year 2010, one that is being met by a large push in the construction of wind energy projects. Landfill gas and other sources of biogas account for a smaller percentage of total renewable energy production in Spain, while solar energy remains largely a demonstration technology. View of the Garraf landfill site and power generation plant One of the main problems for renewable energy producers is that the premium paid for the production of renewable energy is subject to yearly assessment. This effectively means that the Spanish Government reserves the right to set the level of the premium paid for renewable energy. The tendency in the two years since the system has been operational has been for the premium to reduce slightly. However, given the fact that the pool price of energy is governed not only by the price of traditional fossil fuels but also by the input of energy from sources such as hydro electric schemes (projects that under normal conditions reduce their power output during the summer months, thus leading to an increase in the pool price), the price of electricity sold to the grid has been maintained at relatively stable levels, and has ensured a fragile commercial viability for small-scale renewables projects. It is evident, though, that until the Spanish Government adopts a system that enables renewable power generators to establish reliable long-term cash flow models, these small-scale projects will remain at the mercy of the changing fashions in the legislative environment. Risk assessment The risks of power generation from landfill gas are not only commercial - renewable energy production, by its very nature, is a risky business, as the availability of fuel can be an unknown quantity. Whether it is wind, sunshine or availability of organic material, the risk of a reliable source of fuel has to be carefully assessed before embarking on a renewable power generation project. This is especially true if using landfill gas as a fuel source. The business is - so to speak - littered with examples where an over-assessment of available fuel resulted in an engine, or several engines, standing idle, and this is potentially a commercial 'nightmare scenario'. Even if all methods to assess fuel availability within any landfill site are rigorously followed, there will still remain a question of doubt until engines are commissioned and power is exported to the electricity grid. Reliability of the fuel source has to be assessed before embarking on a project There are several methods for assessing fuel risk, ranging from computer modelling of waste decay rates versus the amount of waste in place in a landfill site, to physical methods such as pumping trials. All of these are subject to many variables, the nature of which is hard to define. Lack of humidity in the waste, high leachate levels, permeable underlying strata, and the evolving nature of the gas as waste degradation progresses are all factors that - although they have been described and scientifically modelled - remain unknown and mostly unquantifiable. In the end, a combination of methods is usually employed to minimize risk and maximize the possibility of project success. Experience, and indeed failure in the construction of this type of project in many landfills, helps to ensure that the operators have a higher chance of correctly defining the level of capacity to install at the beginning of a project. The most efficient method is to estimate below the theoretical maximum, leaving an 'absorption zone' to account for unknown factors. Once the engines are operational, ongoing flaring of gas will provide more 'live' data as to additional potential capacity. View of the engines together with the high-temperatureflare stack, showing the elongated exhaust pipes usedto avoid accumulation of heat above the radiatorsOther factors to consider in whether a project will be viable or not include: location of the landfill site cost of the electricity export line active life of the landfill site level of engineering of the site the type of restoration whether or not there will be disruption to the gas extraction system during ongoing site operations In other words, there are a number of factors to consider when deciding whether or not a landfill gas-powered generation system is commercially viable. The commercial risk associated with a possible reduction in subsidy is one that could and should be removed from the equation. What is fundamental in any subsidy programme for the promotion of renewable energy is that it should consider not only the financial aspects, but also the level of technological risk that the operators have to assume, given the uncertainty attached to the availability of the fuel source. The Garraf landfill site Data provided by the owner of the landfill were introduced into a computer model. Since landfilling at the site began in 1974 more than 22 million tonnes of refuse has been deposited. As indicated in Figure 1, the site is modelled to produce more than 13,000 Nm3/hour of biogas from 2001 (Nm3 being the 'normalized' volume of gas). This will reduce to around 6000 Nm3 by the year 2015. Theoretically, the former figure is sufficient to produce up to 20 MW of power. However, because of the uncertainties of restoration, capping and so on, a significantly reduced level of 12 MW was selected. Should gas levels remain consistent with the models, it will be possible to install more engines, as conditions of operation on the site become more stable and better defined. FIGURE 1. Model for biogas production from the Garraf landfill site in Barcelona,with optimistic, pessimistic and average models, estimated with 50% CH4 and70% collection efficiency. The 10 MW, 12 MW and 15 MW figures relate toplanned capacity, based on the modellingThe financial models for the project were constructed on the basis of 12 engines with a gross capacity of 1048 kWe each or 12,576 kWe in total. This level of power generation is projected to remain the same during the first six years of the project and reduce thereafter to around 9 MW by the end of the concession in 2013. Project details The project was part of a threefold execution of projects being carried out on the site. In parallel with the construction of the gas extraction system, a thoroughgoing upgrade of part of the site has been carried out. This upgrading includes the installation of an impermeable liner and the movement of several thousand tonnes of material for capping and restoration. As a result of this work, along with the ongoing site operations and waste input, much of the site was unavailable for the installation of gas extraction facilities. Table 1 lists some of the details of the landfill gas power plant installed. TABLE 1. Main technical details of the Garraf landfill gas power plant Number of extraction wells Approximately 240 Well dimensions 500 mm diameter 20 metres deep on average wellscreen of 160 mm external diameter Booster and gas collection system Three boosters, supplied by Organics Ltd, each with a capacity of 3000 Nm3/hour Flaring facilities Up to 3000 Nm3/hour designed to ignite automatically in the event of engine failure Flow rate to the engines Each engine has a flow rate of around 560 Nm3/hour at a methane content of 50% Engines 12 engines mounted in acoustic containers acoustic conditioning: 60 dBA 1048 kWe each electrical efficiency: 37.1% Sub-station transformer power: 18 MVA export tension: 66 kV Electricity line double circuit of 66 kV one circuit of 25 kV length: 2.1 km, following the trace of an old electricity line from the landfill site The gas extraction system consists of over 240 extraction wells connected to a network of pipes that range in size from 355 mm, with connections to individual extraction wells being up to 63 mm diameter. The total length of the gas extraction system is more than 10 km.An automatic control system that would have controlled each of the extraction wells individually according to pressure and methane content was rejected on the grounds of additional complexity, cost and difficulty in long-term operation. Additional gas extraction wells were installed in its place. These are operated and maintained by fully trained operators who not only optimize gas extraction from individual wells but, because gas extraction systems on landfill sites are high-maintenance installations, also carry out regular visual inspection and repair of all the components of the gas extraction system.Drilling of the extraction wells commenced in early 2002, and was carried out on several artificial terraces that were constructed as part of the restoration programme. As gas that is generated within a landfill site is supersaturated with water, it tends to condense on the walls of the connecting pipelines once it is removed from the humid conditions that operate at depth within the landfill site. In order to minimize accumulation of this water (or condensate) and subsequent pipeline blockage, connection of the extraction wells was carried out by ensuring that the pipeline was laid with a significant gradient. This configuration takes advantage of the effects of gravity and diverts condensate to low points in the system where it can be safely removed. Because of the way the landfill has been built - following the contours of the original valley system - the site has very steep slopes, a feature that assists in dewatering the system. View from the booster station platform showing the three boosters each with a capacity of 3000 Nm3 Boreholes being drilled in the waste for extraction wells, to a depth of 20-25 metres The average depth of the extraction wells is around 20 metres. Given the V-shape of the original surface below the waste, drilling was deeper in the centre of the site than on the edges. The final depth was limited due to the presence of leachate within the landfill at differing horizons.The gas field is kept under constant pressure by three booster sets supplied by the company Organics Ltd that are capable of a maximum flow rate of 3000 Nm3 each. This means that there is an effective ceiling of gas extraction of 9000 Nm3/hour, enough for around 15 MW capacity. A high-temperature ground flare, with a capacity to incinerate 3000 Nm3/hour of landfill gas at a temperature of more than 1000ºC, and with a residence time of 0.3 seconds, is installed before the engine complex to provide emergency back-up. Under normal circumstances the flare will be switched off, although as the project progresses, it will be used to confirm whether there is sufficient gas to increase electrical output. During the construction phase of the gas collection system, the flare was used to confirm the results of the theoretical calculations that form the basis of the original proposal.The site uses 12 Jenbacher JMS 320 engines, each with a capacity of 1048 kW. They have been supplied in containers that, because of the environmental requirements, have a noise specification of 60 dBA (acoustic decibels) at one metre. One of the engines has equipment to continually monitor the rate of carbon monoxide and oxygen production, ensuring that the stringent environmental requirements related to emissions, as stipulated by both European and Catalonian legislation, are complied with. Inside of one of the 12 containers, showing one of the Jenbacher JMS320 engines View of the engine containers, together with the high-temperature flare stack Although the engines produce energy at 6.3 kV, the power station is equipped with transformers that supply energy at 0.4 kV to the on-site facilities and, on a separate platform located some 100 metres from the engines, the export transformer 6.3/66 kV has a total power capacity of 18 MW, which ensures that there is room for expansion should ongoing gas trials prove positive. Environmental statistics It is well known that uncontrolled emission of landfill gas contributes significantly to the global production of greenhouse gases. Methane, one of the main components of landfill gas, is some 21 times more effective as a greenhouse gas than carbon dioxide.Annually, the engines at Garraf will burn 50 million m3 of methane. This equates to the removal of around 600,000 tonnes of CO2 annually. Garraf burns 50 million m3 of methane a year, equivalent to 600,000 tonnes CO2 The Garraf power generation plant will produce around 100 million kWh of energy per year. This means that there will be a considerable displacement of CO2 that would otherwise be produced from conventional fossil fuel energy plant. Effectively, it amounts to a saving of around 50,000-150,000 tonnes of CO2 per year.Altogether, the power plant at Garraf will result in a saving of up to 750,000 tonnes of CO2 being emitted to our atmosphere. This is equivalent to the planting of around 17,000 hectares of new forest. This becomes significant when one considers that the whole size of the Natural Park of Garraf covers an area of some 12,820 hectares.Perhaps a more concrete comparison is that the 100 million kWh of power that will be generated by the Garraf power plant is enough to supply electricity to the entire public lighting system in the City of Barcelona, which uses around 97 million kWh a year.