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Conversion of biogas into LNG becomes reality

by Dan Clarkson

Landfills and wastewater treatment plants emit biogas from decaying waste. To date, the waste industry has focused on controlling these emissions to our environment and, in some cases, tapping this potential source of fuel to power gas turbines, thus generating electricity.

The options now look set to increase following the introduction of technology from US-based Prometheus Energy Company to convert biogas to methane on a commercial scale. In its newly installed California plant, Prometheus converts landfill gas to liquid natural gas (LNG), which is now being used as a diesel substitute in Orange County Transit Authority buses in Southern California.

This article outlines the story behind this development, and looks at the potential market for this technology.

What is LNG?

LNG is simply liquid fuel formed by cooling natural gas to as low as -162°C. LNG is a form of methane - the form that is most useful for use as a vehicle fuel. Methane can also be in the form of compressed natural gas (CNG), liquid compressed natural gas (LCNG) or left as pipeline-quality gas. For the purposes of this article, the terms LNG and methane can be used interchangeably.

Liquefaction enables transportability and storability while transforming the energy source into a superior form of fuel. Moreover, LNG can hold the same energy content in approximately 1/600th of the volume required of methane at room temperature and pressure, and one-sixth of compressed natural gas (CNG). This means longer-distance travel is possible, with smaller fuel tanks on vehicles - cutting down on weight and therefore wear and tear on tyres, brakes and other vehicle parts.

The Frank R. Bowerman Landfill in Southern California - site of the world’s first commercial-scale landfill gas-to-LNG plant - will produce 5000 gallons (18,900 litres) of LNG per day Click here to enlarge image

In addition, the most compelling reason for using natural gas fuel grows from an increasing global desire for a cleaner environment. The use of natural gas as a vehicle fuel produces less pollution than gasoline or diesel, biodiesel or ethanol. Natural gas vehicles reduce smog-causing NOx emissions by 50% or greater and particulate matter (soot) by up to 70% compared with diesel fuel. These vehicles produce fewer greenhouse gases, and in many areas (such as Southern California, where Prometheus’ first plant is located, and in all of the United States by 2010) regulations are forcing alternatives to traditional diesel-powered vehicles.

Processing the landfill gas

Landfill gas is a gas mixture generally recovered at 1-2 psig from a set of collection wells installed at the landfill.

The primary components of landfill gas are methane (CH4), carbon dioxide (CO2), and nitrogen (N2). (The average concentration of methane specified in the LFG is ~45%, CO2 is ~36% and nitrogen is ~18%.) Other components in the gas are oxygen (O2), water vapour and trace amounts of a wide range of non-methane organic compounds (NMOCs). Overall, landfill gas is a relatively poor-quality gas that is of limited use in its unprocessed state.

The small concentrations of O2 in landfill gas are removed from the process stream by conversion to CO2 and H2O. Essentially all the CO2, H2O, NMOCs, and most of the N2 are rejected during the production of LNG. The trace concentrations of NMOCs in the gas are chemically absorbed, removed as condensates or physically adsorbed in the pre-purifier module.

Prometheus’ California plant uses a combination of purification and cryogenic liquefaction technologies. The proposed process utilizes filters, phase separators, selective reactions, physi-adsorption, freezing and cryogenic refrigeration techniques to purify and liquefy the landfill gas feed stream to produce LNG that has > 97% CH4. While the basic technologies are well understood and have been successfully used commercially for many years, their unique application in this context is the innovation of the company and its key personnel.

The process design incorporates a systematic approach to landfill gas purification prior to and while cooling and liquefying the feed gas mixture. The process also uses a novel method to reject N2 from the LNG. The optimal recapture of work and thermal energy increases the overall thermal efficiency of the system while minimizing its capital costs.

There are no regulated emissions from the landfill gas-to-LNG process except those produced by the natural gas generator for power production. The residual CH4 and other materials contained in the feed gas (N2, CO2, and trace amounts of NMOCs) will be piped to the generator and used as fuel for the plant. Any further remaining CO2 is vented back to the existing flare.

Potential impact of methane

Two main factors set the company’s technology and processes apart from current conventional and alternative fuels, according to Kirt Montague, the company’s Chief Executive Officer.

‘First, the company exploits low-cost, waste, and stranded gas resources that provide feedstock gas at costs significantly less than the market and its competitors,’ he says. In most other biofuels, the feedstock comes from products that already have an economic value and are used to feed people and livestock - corn, soybeans, and various vegetable oils. Even current suppliers of LNG vehicle fuel create the fuel from pipeline gas, a commodity used to heat homes and cook food. And, since pipeline gas is sold at index costs, the cost of the resulting fuel varies with the index cost of its feedstock. By using waste gases for feedstock, Prometheus expects to lower the cost of LNG to the end-user and in some cases can provide fuel in long-term contracts not tied to the index.

LEFT TO RIGHT California’s Orange County Transit Authority bus fleet is powered by landfill gas-derived LNG • An LNG vehicle being filled up Click here to enlarge image

‘Secondly,’ Montague continues, ‘Prometheus creates what we call “distributed fuel” - that is, fuel created as close as possible to the end-user. This is enabled by processing methane at plant sizes significantly below the capability of existing technology.’ Every populated region has its landfill and/or wastewater treatment plant; rural and other areas may have sources such as dairy and farm waste gases, coal or stranded well gases, from which the raw energy is derived. Globally, Prometheus envisions future local production of fuel for local fleets at each of these sources of waste and stranded sources of biogas.

‘The company also achieves transportation cost savings as a result of its distributed-scale facilities,’ adds Montague. ‘In the United States, for example, LNG that is currently servicing the transportation market is shipped from large-scale liquefiers that are up to 1000 miles (1600 km) from end-consumers. Currently transportation costs, reflected in the final cost of a gallon of fuel, can be as high as $0.45 per gallon ($0.12 per litre). Under the distributed fuel model, fuel transportation costs are as low as $0.05-0.08 per gallon ($0.01-0.02 per litre), lowering the end price of fuel to the consumer. Again, when the end-user of the LNG is the landfill owner, the price savings add up.’

First in the world

The company has installed the world’s first commercial-scale landfill gas-to-LNG plant on the Frank R. Bowerman Landfill in Southern California. This small-scale liquefaction plant is designed to produce 5000 gallons (19 m3) of LNG per day, with the fuel to power the Orange County Transit Authority bus fleet.

Landfill gas-derived LNG being transported by tanker truck Click here to enlarge image

The Bowerman plant was installed in late 2006 and is currently being commissioned. At present, the Bowerman Landfill flares 10 million standard cubic feet (approximately 283,000 m3) per day of gas, the equivalent of over 40,000 gallons (over 150 m3) of LNG per day, and this amount is increasing each year. Permitted for 150 m3 per day, the current 19 m3 per day plant is the first of three phases the company is planning at the landfill.

The company regards the project as part of its global initiative to produce transportation fuel from waste energy sources, and to partner with communities to provide transportation fuelling alternatives that have a positive impact on the environment. At a production level of 19 m3 per day, the project will reduce carbon dioxide output by the equivalent of nearly 10,000 tonnes per year.

A schematic of Prometheus’ technology Click here to enlarge image

While the Bowerman Landfill is quite large, the technology can be employed on any landfill down to a tenth its size, as long as it is plumbed for gas collection. Deploying this technology produces an additional high-value revenue stream at a landfill, while further solving landfill flaring problems.

The company is also involved with growing a fleet of LNG tanker trucks to haul the finished product to market, installing the re-fuelling stations and assisting in initiatives to convert vehicles from diesel to clean, LNG vehicles.

The market for LNG created from landfill biogas

The LNG market as a vehicle fuel is growing rapidly. LNG is particularly well suited for heavy-duty vehicles - such as refuse haulers, transit authority buses, ferries, rail yard engines, mining trucks and cement mixers - that refuel at centralized depots because of their low mileage per gallon economy and because of LNG’s high energy density and relatively clean burning characteristics. There are particular opportunities for the use of LNG at airports, for mass transit agencies, in ports, for regional trucking, taxis and government car fleets.

The initial target market for Prometheus in North America has been the clean fuel transportation market. The major drivers behind the growth in the US are tightening regulatory emissions standards and economic incentives in the form of tax breaks or mandates requiring the purchase of alternative-fuelled fleet vehicles. Over the last nine years, LNG and CNG vehicle fuel demand in the US has grown by nearly 20% per annum. Worldwide, this figure is 21%.

Prometheus is fortunate to be part of a global effort to clear the air of both localized air pollution and greenhouse gas emissions. Many of the company’s projects are enhanced by government regulations and incentives directed both at the sources of raw gas for its projects and at the end-user. The European Commission regards alternative fuels as an integral element of its policy framework that aims to secure energy supply and reduce GHG emissions.

Carbon credits are also a potential direct revenue stream and/or a strong driver for project implementation and market growth. The value of these carbon credits can be shared among equity owners in the projects and, as such, can form an additional value stream for a landfill project where the landfill owner is an equity partner.

The carbon market grew in value to an estimated US$21.5 billion in the first three quarters of 2006, more than doubling in value over the previous year. The market was dominated by activity through the EU Emissions Trading Scheme and the European Climate Exchange (ECX), which was valued at about $19 billion.

Dan Clarkson is Vice President of Prometheus Energy Company. e-mail: dclarkson@prometheus-energy.com

The potential for replication

The Bowerman plant is composed of various purification and liquefaction modules mounted on eight skids, along with a 35,000 US gallon (130 m3) tank, a 1 MW generator, a truck scale for measuring the product and a paved turn-around for the tanker trucks. Future landfill gas-to-LNG systems will consist of several integrated modules packaged as skid-mounted iso-containers, each with standard outside dimensions of 8 feet wide by 10 feet high by 20 feet long (2.4 x 3.0 x 6.1 metres). Containerization is part of the effort to replicate these projects quickly throughout the US and Europe so as to have the greatest impact on reducing greenhouse gases and pollution. Approximately 9000 square feet (836 m2) are needed for the plant.

Currently, most landfills in the United States are required to plumb and capture the landfill gas and bring it to a single point source for disposal, which at a minimum requires that the landfill flare the gas. In order to replicate Prometheus’ process, landfills elsewhere would need to be similarly plumbed to provide easy access to the raw landfill gas.

In addition to landfill gas applications, the company has large coal mine methane projects in the works in Poland and also turns all other forms of flared or wasted methane gas into LNG, including digested dairy and farm waste gas and gas from stranded wells.

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