Yes to ‘Landfill Gas-to-Energy’?

The Environmental Protection Agency (EPA) stated that Municipal Solid Waste (MSW) landfills in the United Stated are responsible for 18 percent of methane emission; making it landfills the third-largest source of methane emission. According to the Intergovernmental Panel on Climate Change, an environmental organisation supported by the United Nations, identified methane to be the second most prevalent greenhouse gas emitted in the U.S. which has a global warming potential around 30 times higher than carbon dioxide over a 100 year span.

MSW are considered an important environmental problem due to the increase of population as well as the rapid industrialization. LFG is produced from a series of physical, chemical, and bacterial processes that occur in the MSW, anaerobic decomposition is the most important of the processes. Anaerobic decomposition is a process by which biodegradable materials are broken down by microorganisms with the absence of oxygen. In landfills, this process produces 50–55 percent of methane and 40–45 percent of carbon dioxide.

Approximately, 1.5 billion tonnes of methane is produced from landfills every year but only about 10 % of this is captured. Since the LFG is very rich in methane, an essential component of natural gas and an important energy source, it can be captured to provide energy and environmental benefits. Laura Truettner, Manager for Freshkills Park Development, believes that using LFG as a source of energy “is a great system and the economics work well at a place like Freshkills, where the characteristics of the municipal solid waste are such that there is an economically viable amount of methane in the landfill gas. But it is all about economics because its expensive to install the collection and treatment systems necessary to turn landfill gas into a source of energy. And landfill gas generation declines with time- so ten years ago, Freshkills generated enough methane to heat 22,000 homes, now it only generated enough methane to heat 10,000 homes- but the costs of maintaining that collection and treatment system have remained the same so how do we calculate the economic benefits?”

Initiatives have been starting to evolve globally, such as Finland. In 2012, a working group called “Bioenergy from Waste” proposed 13 waste-to-energy actions. One action in particular focuses on reducing greenhouse gas emissions from landfills promoting the efficient recovery of LFG. Although the amount of methane generated reduces, studies shows that LFG can reduce the challenging long-term after-care effects of landfills and extracting and recovering organic material for energy would be easier (as economic drivers). Thus, waste utilization could also give greenhouse gas emission savings due to the replaced fossil fuels.

LFGs are captured through an extraction system which consist of perforated pipes, channels, and possibly a membrane. Careful implementation of this system is necessary in order to ensure efficiency. The perforated pipes can either be horizontal or vertical. Both systems extract the same amount of LFG, however, the horizontal system can be fixed from the beginning resulting in a higher totally LFG extraction. One of the issues faced with the horizontal system is the possibility of leachate (liquid containing collected soluble landfill constituents) entering the pipe. On the other hand, the vertical system can be easily installed after the landfills are finished. In either system, LFG is extracted by a gas pump and connected to main collection pipe or various pipe.

The number of horizontal pipes installed have increased in the U.S. in recent years due to the LFG collection regulations. The EPA, U.S regulations require the collection of LFG within 5 years of initial waste dumping or 2 years after complete closure of the facility.

However, for a bioreactor landfill, the rules are a little different. Bioreactor landfills are a new technology that have been recently implements which aims to increase the production of methane, reduce the amount of leachate and finally reduce the amount of land required for landfills. The EPA requires bioreactor landfills to have a gas collection system in place before liquids are added. Additionally, 180 days after the liquids are added (or when the waste reached 40% moisture) gas collection must begin.

Once the LFG is captured, it can be flared, combusted, or upgraded to biomethane. Flaring LFG is simplest way of getting rid of methane: it is combusted producing carbon dioxide and water [CH4 + 2O2 = CO2 + 2H2O]. Since carbon dioxide has a much lower global warming potential that methane, flaring methane rather than releasing it as it is into the atmosphere is the logical approach.

LFG can also generate energy through combustion, however this requires the gas to go through a ‘cleaning phase’. This step is important to ensure the removal of the contaminants, such as siloxanes, that cause chemical corrosion of the engine resulting in the emission of acidic gases.

The gas has to pass through a renewable energy facility compression system where the LFG will be dewatered, pressurized and filtered in order to be used as a renewable energy fuel. Firstly, filtering take place in order to remove any large that pieces of debris and liquid in the gas. The filtered gas then enters the compressor, where the pressure is increased high enough for the gas to be used as a fuel. However, this process increases the gas temperature so high that it needs to be cooled down. Thus, the gas is passed through an after-cooler without loosing the moisture in the gas. The gas has to be filtered again to remove the condensed moisture from the after-cooler. Finally, the gas is reheated to ensure and prevent any condensation in the fuel. It is important to note that this process takes a couple seconds; it is a fast process even with the various steps.

In addition to its ability to be flared and combusted, LFG can be upgraded to biomethane through three different technologies. Pressure Swing Adsorption (PSA), the separation of the gases takes place under pressures far greater than atmospheric, then the pressure is decreased in order to regenerate the absorber. Vacuum swing absorption (VSA) takes place under ambient (or near ambient) pressure, and a blower is sued to regenerate the absorber in this case. Finally, Vacuum Pressure Swing Absorption (VPSA) uses both the high pressure during the absorption phase and the vacuum pump for the regeneration phase.

PSA, VSA, and VPSA all produce biomethane, the type of technology used however depends on factors such as initial capital investment, scale, and etc. The biomethane can be locally in service stations, or it can be transported by a truck to a centralized facility (located on one of the sites which is easier, but it could also be transported by a truck) and injected into a gas grid.

The environmental and economic benefits of methane capture is an important factor to consider when deciding what to do with LFG. From an environmental point of view, releasing landfills methane into the atmosphere has a lot of environmental implications as mentioned earlier (global warming and depletion of the ozone layer). From an economical point of view, however, if the methane capture was used in any renewable energy scheme (Clean Development Mechanism project), could provide a revenue of 85 million US dollars from carbon credit of carbon reduction (in 2010 based on US$13.20/tons of Carbon Dioxide). Truettner points out that “our understanding of the impacts of landfills on the groundwater and air is now pretty sophisticated and so we are in a position to design landfills that have far less impact on the environment- we are much better at controlling landfill gas emissions and potential groundwater contamination. That means that new landfills are designed to avoid environmental impacts, and that current closure of once active landfills (like Freshkills) are designed to prevent future impacts. But there is still an economic issue- smaller and/or older landfills operated by less fiscally responsible entities or in less well regulated areas will remain a problem.”

The government plays a very important role in promoting the use of LFG and the simplest way to do so would be through the establishment of certain policies and granting incentives. Recently, the increase of energy costs, technology improvement, policies and regulations to promote renewable energy have increased LFG to energy projects increased (113 to 560 operating projects over the past twenty years). For example, the Florida Energy, Climate Change, and Economic Security Act of 2008 stated that the statewide diversion ration should reach 0.75 of generated MSW (by weight) by 2020. Through this policy, changes and certain findings had to be made, which lead to possible solutions, one being banning yard wastes from landfills. Additionally, selling Renewable Energy Credits or receiving production inventive has also played an important role.

Landfills have evolved over the years and implemented new technologies to improve its efficiency. Even though its over all structure has been changing over the years, it has nevertheless, been changing to the better. This however is not the case for Truettner, “We are definitely better informed about designing landfills so that new ones have less impact on the environment. But no system is full proof- and failures happen. And its not just about technology its about the best use of space. Landfills take up a lot of space that could possibly be put to a better use. Freshkills Park is a great example- one can have 2200 acres of open space or one can have 2200 acres of active landfilling which even with the best technology is expensive, and creates environmental burdens of truck traffic, wind blown debris and odor. Not to mention the stigma of a landfill on property values and peoples perception of place.”

When looking at particular aspects of the landfills, we see improvement. That is evident in the percentage of methane recovery rates based of on the different recovery system types. Having an operating cell with an active LFG recovery system recovers 35 percent of the methane. A temporarily covered cell with an active LFG recovery system recovers 65 percent of the methane. A cell with a final clay cover and an active LFG recovery system recovers 85 percent of the methane. Finally, the most recent developed cell with a geomembrane final cover and an active LFG recovery system recovers 90 percent of the methane. Due to the low cost, the Malaysian government encouraged the use of landfills, however, Turettner believes that “we should use whatever existing capacity we have in the well operated and regulated landfills but should be aware of the short and long term costs of doing so and should seek ways of reducing the amount of solid waste we send to landfills in order to eliminate landfills. Landfills are expensive to maintain and operate and are, with some exceptions (like reclaimed mining lands) not the best use of land. NYC pays a premium to often low income communities for disposing of its municipal waste- and that’s not really a win win for anyone”. The question now should be if it is not a ‘win win for anyone’ then why are we still using landfills as our primary mode of MSW disposal in the 21st century?