In our modern world, landfills are not simply repositories for unwanted waste. They also generate significant amounts of methane. Welcome to the fascinating world of biogas capture in landfills.
The Process of Biogas Production
When municipal solid waste (MSW) is first deposited in a landfill and compacted, it undergoes an initial aerobic (with oxygen) decomposition stage where little methane is generated. Typically, within less than a year, as the layers of waste are buried and compressed, the oxygen is fully consumed, and anaerobic conditions are established. Under these conditions, a new type of bacteria takes over decomposition activities and releases Biogas, or landfill gas (LFG), in the process.
LFG comprises 50 to 75 percent methane (the primary component of natural gas), with the balance being carbon dioxide (CO2) and a small amount of non-methane organic compounds. This gas can be collected in pipes and later burned to generate heat or electricity.
Gas Capture System Design for MSW Landfills
During the initial design stages of a gas collection system (GCS), it is imperative to consider the site conditions. Factors such as the landfill site's topography, the moisture content and composition of incoming waste, compaction rates and waste depths, and the permeability of cover layers and final cover, all significantly impact the ultimate design of the GCS. We can ensure an optimal and effective GCS design by considering these aspects from the outset.
Landfill sites with higher moisture content experience faster landfill gas generation, increasing peak production rates. To effectively manage this, it is crucial to have a gas capture system capable of handling the relatively higher flow. However, the elevated moisture content poses challenges for GCS systems, as they are required to work harder and may struggle to cover a larger area. In such cases, additional collectors might be necessary to effectively manage the site. Conversely, some operations may choose to introduce moisture to expedite gas production.
The effectiveness of a gas collection system can be significantly reduced by using permanent daily covers and cell covers that restrict liquid passage. The reduction appears because leachate cannot easily drain through the layers, and produced gas cannot flow readily to collection points. As a result, this situation poses significant health and safety issues and environmental hazards. If internal temperatures reach high levels, trapped pockets of methane have been known to spontaneously combust. Additionally, undrained leachate can escape during periods of heavy rain.
Removable daily covers are an excellent solution for reducing operational costs and addressing flow restrictions within waste masses. An impermeable layer is necessary to ensure an airtight seal for permanent intermediate and final landfill covers. These collectors employ pumps to create a vacuum and extract the gas. Additionally, impermeable covers prevent air and water infiltration from upper layers, enhancing pump efficiency. Ultimately, incorporating impermeable covers at critical intermediate and final stages significantly improves the overall performance of a gas collection system (GCS).
Environmental Conditions
To ensure optimal collection efficiency and economic return for landfill gas (LFG) energy projects, it is crucial to follow best practices when designing and installing landfill gas collection systems.
Among the key factors to consider, temperature and precipitation play a vital role. Accounting for temperature involves evaluating the response of gas collection system (GCS) components under different weather conditions, including freezing temperatures or heat waves. The design can accommodate and adapt to any situation by anticipating these extremes.
Furthermore, rain and snow add moisture to the waste mass, potentially overwhelming the collection system or hindering gas production. Conversely, low precipitation poses a different set of challenges. Therefore, it is imperative to account for these variables during the design phase, especially as precipitation patterns become increasingly unpredictable.
Configuring the Collectors
LFG collectors can be installed horizontally across the landfill or vertically through drilled borings. The vertical configuration offers a significant advantage, allowing the collectors to be operational immediately upon installation and providing more effective control of surface emissions than horizontal collectors. Furthermore, vertical wells can be adjusted as necessary throughout the landfill's lifespan, which is particularly important as the landfill matures, and gas production patterns change. However, in active landfills where new waste layers are added, constant compaction is required around the vertical wells, and the borehole must be regularly extended.
On the other hand, horizontal collector pipes are laid across the landfill's surface within trenches. They are surrounded by stones or other aggregate materials that allow gas and liquid flow while preventing the intrusion of solids. These collectors typically consist of perforated or slitted plastic pipes. Horizontal collectors are commonly installed in active areas of the landfill as they are placed at the surface and generally have less impact on landfill operations than vertical wells. However, they may ultimately be replaced by a vertical collector system once the landfill is completed.
Geosynthetic liners play a vital role in landfill gas capture systems. They act as barriers, preventing the release of harmful gases and leachate. Additionally, they create an airtight enclosure to effectively capture and direct these gases for collection.
In biogas systems, RPE geomembranes are durable, long-lasting floating covers. These covers rest on the surface of completed landfills, effectively trapping methane gases generated during waste decomposition, enabling safe and efficient collection and redirection of the gases for processing or flaring.
