By Doug Heaton, P.Eng.
The environmental impact of landfills is an issue that has grown in importance in recent years. Such is the case in Edmonton where the remaining useful life of the Clover Bar Landfill Site is rapidly drawing to a close. Flammable, toxic and odorous gases generated by garbage decomposition at any landfill can raise concerns by environmental groups and nearby residents. This continues to be an ongoing issue following final closure, as escaping gas can kill trees and grass.
A major portion of the gas created by the decomposition of garbage is methane, the primary component of natural gas. The Clover Bar Generating Station, owned and operated by Eltec Inc. (Formerly Edmonton Power, Inc.) lays within three km of the Clover Bar Landfill Site. This station is a peaking plant fired on natural gas and can generate 660 MW at full load. Studies show that it would be both technically and economically feasible to use landfill gas at the Clover Bar Generating Station by adding it to the regular natural gas fuel supply.
Landfill Gas
Methane is the most abundant organic chemical in the Earth's atmosphere. According to estimates, landfills contribute approximately seven per cent to total worldwide methane emissions (1). While 75 per cent of methane in the atmosphere is naturally occurring and therefore difficult to control, landfills are responsible for 25 per cent of the controllable methane sources. Gas production rates from a landfill vary depending on garbage composition, moisture content, temperature and methane partial pressure. Gas production cannot be halted or contained as it gradually percolates through the garbage and escapes to the atmosphere by the path of least resistance. Problems can therefore arise if the issuing gas is not carefully controlled or flared. Gas can even migrate and accumulate in nearby buildings, increasing the risk of explosion. Completed landfill sites are usually made into golf courses or parks for safety reasons although escaping gas can still destroy grass and trees.
The composition of garbage in a landfill varies depending on the cultural habits of the society filling it. Typical contents are food and garden waste, paper products, plastics, rubber, textiles, wood and inert materials. These ingredients generate different quantities of methane due to differing chemical characteristics. In recent years, as environmental awareness has increased, the relative quantity of recyclable materials and heavy metals has declined significantly.
Table 1 shows a typical analysis of garbage presently generating gas and the volumes of methane gas that could be expected from each kilogram of a given component. Paper products are seen to be both the main constituent and the producer of the greater relative amounts of methane. This is a significant observation. As paper recycling becomes more prevalent, the methane production rates can be expected to drop. In the ruture, landfills may not be quite so attractive as methane sources as conservation practices take hold. The old part of the Clover Bar Landfill Site drilled for this project contains garbage buried prior to the advent of the modern environmental movement. It was expected to be a prolific methane gas producer but actual experience showed that due to its age this was not the case.
The biodegradable organic components of garbage break down first into amino acid intermediates and then to simpler fatty acids, predominantly acetic acid. Two main classes of decomposition mechanism can occur, aerobic and anaerobic. The route taken will depend on the presence or absence of oxygen. In the presence of air, aerobic bacteria causes the decomposition of these fatty acids to occur forming carbon dioxide and water.
This describes the activity in the domestic compost heap, provided it is frequently mixed and ventilated. Buried deep in a landfill, other bacteria are present that can exist only in the absence of oxygen.
These bacteria convert the fatty acids such as acetic acid anaerobically to methane and carbon dioxide.
CH3C00H-----> 2C02+H20
Aerobic decomposition of acetic acid
CH3C00H -----> 2CH4+C02
Anaerobic decomposition of acetic acid
Cellulose (paper) is the major producer of methane with an overall equation as follows;
C6H1005 + H20----> 3CH4+3C02
Anaerobic decomposition of cellulose
The mechanisms of garbage decomposition fall into five distinct phases as illustrated in Figure 1.
The first three phases are transitional and of short duration. Phase four is a long period of stable gas production. Phase five describes the reduction of chemical and biological activity to zero. Phase four determines when commercial landfill gas production could commence and has the potential for lasting 20 to 25 years. The gas produced will have a composition of about 50 to 60 per cent methane and 40 to 50 per cent carbon dioxide. During this phase, the methane, carbon dioxide and fatty acid concentrations remain in equilibrium.
The gas production rate depends to a large extent on the composition, moisture content, temperature and methane partial pressure in the location of a particular gas well. While the prediction of landfill gas production rates is a fairly mature science with very developed techniques, actual drilling and field testing are needed to confirm predictions.
The gas removal rate from a given well is important. If air is introduced into the landfill through excessive gas withdrawal, the biochemical balance will be destroyed and aerobic activity will again dominate. Landfills have a heterogeneous composition distribution reflecting seasonal variations and management practices. This will influence localized gas production rates. Contaminants will also affect gas production rates. The fatty acid levels, and hence methane gas production rates, will be inhibited by the presence of toxic components such as carbon tetrachloride and chloroform. Industrial wastes may contain concentrations of the common salts of potassium, magnesium, sodium, calcium and ammonium cations and the sulphide anion. These salts, or a pH outside the 7-8 range , will also have a toxic effect on methanogenic bacteria.
Landfill Gas Projects
Landfill gas has long been considered a plentiful and cheap fuel source. Many people have tried to capitalize on this phenomenon with varying degrees of success, from farmers converting manure to produce methane gas for barn heating, to small municipal power projects firing gas turbines. Worldwide there are estimated to be approximately 242 commercial landfill gas operations in 20 countries with the majority located in the USA, Britain and Germany.
Four years ago there were approximately 11,000 landfill sites in the USA of which over 6,000 were in active operation. About 140 of the larger landfills have been developed for landfill gas production in North America. Current garbage in existing U.S. Landfills could yield nearly 142 billion Nm3 of gas over the next 20 to 30 years. This is equivalent to 1.4 billion barrels of oil.
Typical of landfill gas recovery and utilization plants of significant size presently in operation in the USA include Puente Hills in Los Angeles. This is the world's largest landfill gas facility generating 50 MW of electricity. Mount Trashmore at Virginia Beach, Virginia was the worldıs first combined cycle plant generating 10 MW of electricity. In Canada, about 17 million tonnes of municipal solid waste is landfilled in over 200 sites of varying sizes. Total methane emissions in Canada in 1990 was estimated at about 990,000 tonnes. This is projected to increase to 1,290,000 tonnes a year by the year 2020. Ontario accounts for about 35 percent of the total emissions, with Quebec and British Columbia not far behind.
Small landfills containing less than 2 million tonnes of municipal solid waste accounted for 28 percent of methane emissions in 1990, followed by 42 percent for medium sized landfills containing 28 million tonnes of municipal solid waste. Large landfills account for about 30 percent emissions.
In 1990, it was estimated that 623,000 tonnes of methane were ultimately recoverable, which equates to 17,124 barrels of oil. This is now expected to rise to 827,000 tonnes of methane by 2020. Canada has been slow to develop landfill gas collection systems due to size and location of markets for the gas; long term availability of gas; high collection cost and low revenues; regulatory difficulties and high risk.
Several projects have been in place for some time. Brock West landfill in Pickering, Ontario, for example, is one of the largest.. It has been in operation collecting gas since 1986 and generating electricity since 1991. Approximately 408,000 m3 per day of gas is collected from 19 million tonnes of municipal solid waste to generate 23 MW of electricity. The Clover Bar landfill in Edmonton, Alberta has collected landfill gas at a rate of 70,000 m3 per day from 14 million tonnes of municipal solid waste and used to generate 2.7 MW of electricity.
The national and international agreement to reduce greenhouse gas emissions has stimulated landfill gas collection activities. There were 24 projects in Canada projected to be completed or under construction by 1995 for gas control or energy recovery.
Unfortunately, several energy recovery projects have since been delayed or put on hold. Further development of presently untapped landfills depends on tax and other financial incentives, changes in regulations and better information.
Clover Bar Project
Edmonton is fortunate to have a large landfill site so close to the natural gas-fired Clover Bar Generating Station. In the winter of 1989, drilling at the Clover Bar Landfill by Environmental Technologies Inc. of Calgary yielded promising gas production projections. The City of Edmonton Public Works Department was approached for drilling rights to the gas and natural gas consumers near the landfill site were contacted as possible purchasers.
In August 1990, Edmonton Power (now Eltec) entered into discussions with Environmental Technologies Inc. as a possible user of landfill gas or its derivatives. An initial feasibility study was carried out. It was determined that the methane/carbon dioxide mixture (with trace impurities removed) would be an acceptable fuel supplement for Edmonton Power's Clover Bar Generating Station. Unlike many projects where landfill gas is used in its raw condition, it was agreed that landfill gas at Clover Bar would be filtered, compressed, scrubbed and dehydrated to meet a set of negotiated specifications.
A landfill gas supply contract was signed in June 1991 between Edmonton Power and Environmental Technologies Inc. In a second contract, Environmental Technologies Inc. agreed to pay a five per cent royalty to the City of Edmonton Public Works Department, the owners of the landfill. In January 1996, Eltec Inc., the non-regulated subsidiary of Edmonton Power Corporation, purchased the Clover Bar landfill gas collection, purification and distribution system.
Production Rates
The original plan for Clover Bar called for a gradual landfill gas production build-up. In the first eighteen months, landfill gas production operated at rates of 50,000 Nm3 per day increasing to 70,000 Nm3 per day by 1994. Landfill gas capture is now planned to increase to at least 100,000 Nm3 per day-1 by 2000. This landfill gas rate is projected to generate 6.6 MW of electrical energy. Between startup in April 1992 and February 1, 1996, over 65 million Nm3 of landfill gas have been processed and used to generate 116 gigawatt-hours of electricity. Gas is expected to be commercially available until 2011.
Collection and Purification
Initially landfill gas was collected from 63 wells joined to a collection system operating at a slight vacuum. By December 1994 this number had increased to 106 wells and is projected to reach 124 wells by 1998. Typical average generation from a well is 1,300 Nm3 per day although many wells could reach rates as high as 2,000 Nm3 per day, particularly in areas where the garbage is relatively new. The present rate of active wells at Clover Bar is in the order of 1,620 Nm3 per day per well.
Well clusters comprising of four vertical wells, were drilled to a depth of approximately 20 metres. A well is constructed using perforated flexible high density polyethylene pipe so shifting of the landfill mass can be tolerated. The total collection system will ultimately use about 14,000 metres of pipe to connect about 124 wells.
The wells are connected by a 75 mm diameter plastic pipe to a central collection point. The collection points are in turn joined by 150 mm diameter laterals to a 300 mm diameter collection header. Each header is tied into a 355 mm diameter main header for transport to the purification plant. Following treatment, the landfill gas is delivered to the Clover Bar Generating Station through a 200 mm polyethylene pipe at a pressure of 690 kPa.
The gas wells are adjusted frequently to ensure maximum production and steady net heating value for the combined gas flow entering the purification plant. Raw landfill gas is drawn into the purification plant using slight suction provided by blowers running in reverse mode.
After passage through a knockout drum, the landfill gas is scrubbed with Selexol in a packed column to remove impurities. The gas is compressed to 690 kPa (g) using a Joy reciprocal compressor and dehydrated with Selexol (c) in a bubble cap column. Selexol is a homologue of polyethylene glycol dimethyl ether.
The gas is passed through a Pall filter to remove entrained Selexol drop>