Although landfilling of municipal solid waste (MSW) is banned in the EU and on decline in many parts of the world, the majority of waste is still deposited in landfills in multiple geographies. Waste in landfills typically decomposes in an anaerobic process into ‘landfill gas’ whereby primarily methane (40–60 % of landfill gas) and carbon dioxide is generated. The methane content is high enough to be used as fuel in combustion engines or gas turbines for power production.
If not captured properly, landfill gas poses serious challenges from a health, security and ecology point of view. Landfill gas is odorous, flammable as well as a potent greenhouse gas, with a heating effect more than 20 times higher than carbon dioxide.
Therefore, degasification and combustion of the landfill gas is mandatory e.g. in the European Union. If there is no direct use for the gas, such as power or heat production, the landfill gas is flared.
Use of landfill gas
Power and heat generation
Depending on multiple factors, a given landfill can generate more or less gas. As a very rough rule of thumb, a well run landfill that receives 100 000 tons of MSW annually (collected from around half a million inhabitants) and has 1 million tons of MSW in place, can generate enough gas to fuel a 1 MW gas engine. Capital expenditure for such investment is around $ 1.5 million and it can generate around 8 000 MWh of power per year. With a power tariff of e.g. $ 100 per MWh, this equals $ 0.8 million in annual revenues.
If power tariffs are high enough, the landfill gas is often used for power production in gas engines or turbines. The efficiency of turbines and engines is around 30 % and 40 % respectively and the rest of the energy released in the combustion is heat energy in the form of exhaust gases, typically with a temperature of around 600 °C. The most efficient use of landfill gas is therefore power production with heat recovery for e.g. district heating or where the heat can be used for evaporation of undesired water in the landfill area.
Evaporation of water
Landfills generally cause substantial ecological challenges apart from the landfill gas. Rainwater runs through the landfill and absorbs all possible toxicity from the landfill; so called ‘leachate water’. Leachate water, if not dealt with properly can migrate to ground water or rivers. Collection of leachate water is only one part of the solution whereas purification or otherwise treating the water is the second part. Often, leachate water is treated and purified with reversed osmosis, but this requires expensive equipment and large quantities of electricity therefore making it costly. Recovia has a solution whereby the heat from the exhaust gasses is used to evaporate the leachate in an extremely efficient process whereas the solids can be collected and safely deposited separately.
Landfill gas-to-power from A to Z
Recovia has extensive know-how to take a full perspective on landfill gas-to-power as integrator in all parts of the chain. We are seeking to use this know-how together with local partners in any market. Below is described the relevant parts of the value chain.
1. Technical assessment of a landfill
One of the most crucial aspects of a landfill gas-to-power project is to correctly assess the gas potential in a given landfill. For this end theoretical models are used where a multiple of factors are considered including amount of waste in place, amount of waste landfilled annually, landfilling practices, composition of the waste (if available) etcetera. However, the theoretical model is ideally combined with a test pumping procedure in commercial investment projects. Test pumping the landfill for a minimum of 2‐3 months is therefore desirable. In commercial projects for energy production purposes, Vireo also attracts a second opinion from leading landfill gas experts in order to be as accurate as possible.
2. Choice of gas extraction technology
Recovia has applied different philosophies for gas extraction systems and therefore has a realistic and objective view on choice of gas extraction technologies. We have named the two major philosophies ‘traditional’ and ‘new’.
Traditional gas extraction technology
The ‘traditional’ technology can be described as drilling holes (300‐1000 mm wide) into the landfill body with a distance of 20‐50 meter between wells. In the middle of the hole a perforated plastic pipe of around 160 mm width is inserted and the void between the pipe and the walls of the hole is filled with gravel. This is named ‘gas well’.
The gas well is connected with gas suction lines (often 110 mm wide) that generally are buried in the landfill body. The suction pipes are connected to one or several ‘collector stations’ (e.g. up to 20‐30 of them per station) which allows the system operator to measure the methane content in each well and regulate the gas flow accordingly, in order to maximize methane extraction long-term. If the methane content starts falling e.g. below 50 %, the gas well is closed somewhat until the methane content stabilizes around 50 %. Conversely, the flow is increased if the methane is above e.g. 50 %, until the methane content starts to fall again towards 50 %.
From the collector station(s), a single gas pipeline is connected which leads to a ‘gas station’ that most often is located adjacent to the landfill body, e.g. in the same area as the entrance to the landfill. The blower of the gas station creates the necessary vacuum that sucks out the gas from in each well and second blower boosts the gas pressure necessary to feed the gas combustion engine or turbine (see further under power generation).
In both the suction lines as well as before the collector stations and the gas station there is dewatering equipment in order to put less stress on the generators.
New gas extraction technology
Over the past ten years a new landfill gas extraction technology has emerged, developed and patented by the Dutch company Multriwell. The technology differs from traditional systems and is based on gas permeable filters that are pushed down in the landfill in a dense grid of 3×3 meters. Each filter is connected with a horizontal transportation filter that is laid down on a sand layer on top of the landfill. The filters are connected to gas collection pipes which are connected to larger transportation pipes that deliver the collected landfill gas to a blower.
Comparing traditional systems and the Multriwell technology provides the following insights:
The traditional technology requires less preparation of the landfill whereas the Multriwell system requires the landfill surface to be levelled and covered with an impermeable layer (a robust final cover or a temporary more simple cover). Both systems allow for continuing landfill operations on top of the extraction system; in both cases some precautions will be necessary for landfill operators to prevent damaging the systems.
The Multriwell system has a higher recovery rate for the landfill gas and also improves the gas production inside the landfill. Therefore, the gas yield of a Multriwell system is considerably higher than traditional systems. In terms of construction costs, the comparison largely depends on the scope of the traditional system applied. In case a top cover is part of the traditional extraction system, the costs of traditional systems and the Multriwell system are comparable. If the traditional system is applied without top cover, the traditional system will be cheaper, but also deliver much less gas from the landfill.
In the end, one has to take a holistic perspective before choosing a specific gas extraction technology and base it on several factors to get the most economical solution. Multriwell in many cases proves to be the favourable option.
3. Choice of power generation technology
Power from landfill gas can be generated both in combustion engines with an efficiency rate of around 40 % and gas turbines with an efficiency rate of up to 30 %. Gas turbines traditionally have been applied in larger projects with 3 MW power or more whereas engines are more versatile. For example, our smallest engine in Belarus has an effect of only 170 KW.
Recovia has experience from the leading manufacturers of gas engines; Caterpillar (MWM) and Jenbacher.
Gas turbines have traditionally been reserved for larger project (3 MW and upwards) due to relatively higher investment costs per installed effect. However, Vireo has signed an agreement with a very promising Russian turbine company (in2rbo) that is about to launch turbines in the 1 MW range. Due to significantly lower operational costs for turbines, this can become a revolution for power generation from biogas and landfill gas and we plan to install the first turbine in Russia mid 2019.
4. Connection to the power grid
Connecting a small-scale power generating unit can often be a challenge from an administrative point of view depending on local law and regulations. Recovia has extensive experience from dealing with local power grids and has successfully connected multiple projects to the grid in Russia, Poland and Belarus.
5. Business development and legal
The universe surrounding a landfill gas-to-power project is often complex and the technical providers aside, it includes landfill owners and landfill managers, municipal structures and other permitting bodies. In order to reduce the administrative risks, it is important that the contractual relationships are negotiated well and formalized in watertight agreements. Vireo has an extensive database of agreements negotiated over the years, which provide a solid base for each individual case.
6. Operations of gas extraction systems and engines
Recovia has a very advanced technical team that runs the installations in Belarus and is capable of educating local operations teams either on the location or at Recovia’s premises in Belarus.
Gas extraction system
When the landfill gas-to-power system is built, it will very soon become inefficient if not actively run properly, regardless of how well it is built. The main activities for keeping the gas extraction system in good shape includes regulation of each gas well carefully in order not to overload a given well or area, which otherwise may reduce the gas generating capacity of the waste permanently. In the case where landfilling operation continues after the gas system is installed, operations of the gas system requires a careful adjustment of the gas extraction system since continued landfill generates more gas. Professional operations of the gas system well, also involve a careful removal of condensate water in the system as well as repairing and, if applicable, prolonging vertical gas wells to capture more gas. From a security perspective, a careful monitoring of e.g. CO (carbon monoxide) is essential since high content of CO may indicate internal fires in the landfill, in which case the gas flow must be shut down or at least reduced.
Operations of engines
Gas engines require planned maintenance and unplanned maintenance and repair. The more diligently the technical team carries out preventive care, the less unplanned maintenance and repair is needed. Recovia in Belarus reaches around 99 % uptime (i.e. the engines are running 99 % of the time) thanks to extraordinary discipline in preventive care and repair as well as highly efficient maintenance procedures. More and more of the maintenance is carried out by the internal team, whereas only certain more complicated services are carried out by the engine providers. All in all, Recovia has a cost efficient in-house operations team and the capacity to train new engineers and operators.