HomeCoal FiredEquipmentRecovering low-quality gas in waste gas-to-power

Recovering low-quality gas in waste gas-to-power

Oxidizer technology can allow low-quality gas from landfill as well as oil and gas sites to generate on-site power rather than being flared off, write Alain Castro and John Millard

Conventional reciprocating engines and gas turbines have problems combusting landfill gas that is below 30%-40% methane. So, when a landfill becomes inactive, the quality of the emitted landfill gas reduces. And as the landfill gas methane content falls below 30%, the traditional decision is to decommission the power generation equipment, at which time the landfill will stop generating revenues from the sale of energy.

However, what many people don’t realize is that a landfill will typically continue to emit low-quality methane gas for another 65 years after it has become inactive.

The owners and operators of an inactive landfill must comply with laws for how they manage these gas emissions, after the power generation equipment has been decommissioned and removed. In most gases, the local laws will require the owner of the inactive landfill to continue flaring the gas for as long as there are gases being emitted. But, after a landfill has been closed for many years, the methane content will eventually drop to levels that are 20% and lower. At this stage in the life cycle of a landfill, the low-level methane gas is too weak to burn with a flare system.

However, the landfill must still comply with the laws. And hence, many older landfill sites must use and pay for a supplemental fuel, just to destroy the landfill methane. This is a real operational cost issue, as landfills will continue to emit methane gases for many decades after they have been closed, but the methane content of these gases is simply too low to be utilized in any useful manner with traditional equipment.

Attero, one of the largest producers of biogas in the Netherlands, was facing this problem at its Schinnen landfill. The original landfill gas-to-electricity project, commissioned in 1995, used three 825 kW engines (2.475 MW total capacity). After the landfill was closed, the gas energy density decreased to the point where the engines were struggling to operate.

Attero actively managed the landfill gas collection to optimize the gas quality by separating the bad producing wells from the high quality ones. This extended the operational life of the engines by several years, but eventually the engines had to be decommissioned as they simply could not operate on such low-quality gases.

By 2012 the landfill gas calorific value had dropped to the point where only one reciprocating engine would still run at 50% power, but only for about three to four days. The landfill gas had to be flared for the rest of the week.

Seeking alternatives

Attero decided to seek alternatives for reducing its flaring costs, with the restriction that the landfill gas quality was going to be below 30% methane (<300 Btu/scf or <12 MJ/Nm3). The firm researched technologies from all over the world, and ultimately decided to replace its last struggling reciprocating engine with a 250 kW Power Oxidation Powerstation (supplied by Ener-Core, Inc).

There are two distinct advantages of the Power Oxidizer Powerstation. First, the system is not limited by the physics of the combustion process, and hence has the ability to operate on gases that are as low as 5% methane. This enables Attero to continue generating power (and revenue) from its landfills for many decades after they have been closed and their methane quality levels have fallen.

Second, the system operates with very low NOx exhaust emission of below 1 ppm. In fact, the level of NOx are likely the lowest of any waste-gas-to-power technology in the world.

The oxidizer technology replaces the combustor in a gas turbine. Modern combustors are designed to run on high-quality gases (typically gases which are close to pipeline specifications). Hence, the lower-quality gases that exhibit characteristics that are far outside the pipeline specifications in terms of energy density or purity (level of contaminants within the gas) are not easily used by any combustion-based technology.

This is particularly true in areas where the air emissions regulations are strict.

Difficult gases

The Power Oxidizer allows the use of difficult and often unusable industrial waste gases from oil and gas fields, coal mines, landfills and industrial plants.

Where crude oil is extracted there is always a gas component. Like a fizzy drink has CO2 dissolved in it that is released with the pressure drop of opening the bottle, so is associated gas ‘released’ from the oil when it is brought to the surface. The associated gas is often very weak in terms of hydrocarbon content and is a mix of hydrocarbons, as well as a large percentage of CO2. In many cases these gases are also sour – that is they contain sulphurous compounds – mainly H2S and Mercaptans. All this makes the associated gas low value – as it is expensive to clean up to the standard of pipeline natural gas.

These gases are often difficult to use for energy generation because of the complex mix of hydrocarbons, and the corrosive nature of the sulphur compounds – as well as the detrimental affect of CO2 on the combustion process. The result – these gases are often flared, or mixed with more valuable gas to bring them up to a quality level where they can be used in gas engines.

The Ener-Core Power Oxidizer on the other hand can use these gases as fuel without any pre-cleaning or pre-mixing with valuable natural gas – even with a methane content below 15% and CO2 levels of 35% or more – something that simply cannot be done with a normal combustion approach. The Ener-Core system is thus able to permit the efficient use of this weak gas – converting it to both power and heat for on-site use or export – increasing the energy efficiency of the operation, reducing the site-related energy purchasing expenses, providing a revenue stream from a gas that would otherwise be incurring operating ‘costs’ due to its cleanup and destruction, and avoiding the use of valuable gas that can be sold at its full market value.

In today’s energy markets, squeezing the last drop of system efficiency and reducing the costs of operation are foremost in the minds of the plant operators.

The secondary effect of this is, of course, environmental. By employing an Ener-Core system not only does this develop a positive CO2 balance (avoided use of natural gas, efficient use of an otherwise waste gas) the system works flamelessly, avoiding NOx emissions almost entirely.

This same approach can be used at gas processing plants, where gas is cleaned and prepared for injection into natural gas supply pipelines. The extracted gas is cleaned of CO2, N2, sulphur compounds and longer chain hydrocarbons such that it conforms to an agreed standard composition for natural gas.

In doing so, many of the components are separated as liquids and sold as valuable products (e.g., propane, butane etc), but such extraction systems are not 100% efficient, and this results in a tail gas that is often difficult to use.

Once again this ‘waste gas stream’ is nectar to the Power Oxidizer and it can be used efficiently on site for power and heat generation – thus increasing the efficiencies of the overall operation. Our 2 MW system paired with the Siemens (Dresser-Rand) KG2-3G gas turbine allows us to work with the bigger gas streams that are present at such locations.

The strength of the technology extends beyond its ability to run on gas mixtures that would cause major issues with normal combustion systems, but it is also very resistant to changes in the gas compositions – meaning that the system does not have to be re-tuned continuously, and even continuous variations on a day-to-day and hour-to-hour basis can be taken in its stride. The Power Oxidizer itself works a little like a thermal flywheel, so even several minutes of zero fuel will not stop the system.

As shown in the figure on p.16, Ener-Core’s process accelerates the naturally occurring chemical oxidation reaction that occurs between hydrocarbons and air, such that the oxidation reaction takes place in one to two seconds (the same reaction typically takes 10 to 20 years if the gases are released to the atmosphere). The reaction is contained within a vessel, so a precise amount of heat energy at pressure is released. The gas turbine prime mover can then harness that pressurized heat energy to turn the generator and feed it back to the facility or to the grid.

An opportunity for savings

In the case of Attero and its weakening landfill gas problem, this unique capability of productively utilizing the low energy density gas (often below 30% methane) presented Attero with an opportunity to continue enjoying revenues through the sales of renewable power long after a landfill became inactive. It also represented an opportunity for the company to save on the costs of flaring these gases. The 250 kW power station was delivered to the site in early 2014, with the installation completed by June 2014.

As the landfill methane production from the inactive landfill continues to reduce over time, the reciprocating engine operation becomes more limited due to the lack of landfill gas above 30% methane. Table 1 shows the expected annual power generation for the smaller 250 kW powerstation in comparison to the limited intermittent operation of the reciprocating engine operating at 400 kW.

Other sources

Closed landfills are not the only source of low energy density gas. This type of problematic gas with low hydrocarbon content also exists in oil and gas fields all over the world.

In August 2014, Ener-Core completed the pilot phase of a methane emissions reduction project for a major Canadian integrated oil company. The pilot phase involved testing the operation and measuring the exhaust emissions of the methane gas (typically 6%-8% methane) that gets emitted from oil drilling sites.

Ener-Core was able to demonstrate that its 250 kW Powerstation could generate continuous energy from this low-quality gas that typically gets flared or vented from oil drilling sites. Table 2 shows the low energy density gas tested, as well as the exhaust emissions. These results were used by the oil company for the initial project permitting with their environmental regulators.

Enabling conversion to clean power

Today, a wide range of industries produce low-quality waste gases which are not suitable for combustion, and hence not suitable for power generation using traditional technologies. These industries include oil and gas drilling, oil and gas refining, steel mills, coal mines, petrochemical plants, food processing plants, alcohol distilleries, etc.

As the traditional power technologies often cannot operate on the low-quality waste gases that are emitted by these industries, they have historically resorted to deploying emissions destruction equipment, such as thermal oxidizers and scrubbers. However, the scrubbing, flaring and other emissions destruction processes offer no return on investment.

The best way to assist these traditional industries in becoming more environmentally sustainable is to provide them with tools that enable them to productively convert their waste gases into clean power. In most cases, this clean power can be used and/or sold in a financially profitable manner, thereby making it even more attractive to become environmentally sustainable.

Alain Castro is CEO and John Millard is European Commercial Director at Ener-Core Inc www.ener-core.com