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Gradual Oxidation of Waste Gas
INSTEAD OF VENTING OR FLARING, IT IS POSSIBLE TO OXIDIZE WASTE GAS AND CONVERT IT INTO GENERATED POWER
The U.S. Environmental Protection Agency (EPA) estimated that industrial sources (agricultural, waste, and energy) emitted 342 million metric tons of methane into the atmosphere in 2010 alone. Methane is both a powerful greenhouse gas and a highly energy dense component of waste gases.
Releasing large quantities of methane into the atmosphere is not only detrimental to the environment, but also a wasteful solution for an energy dense fuel. For this reason, governments are increasingly regulating methane emissions. Producers have therefore begun to push the limits of using low methane gases as a renewable source of energy.
The decision to dispose of or use a waste gas stream is typically determined by evaluating criteria such as heating value, cleanliness, corrosiveness and availability. The heating value of the gas is a measure of the fuel’s energy density (Figure 1).
Figure 1: Operating ranges and typical waste gas energy densities[/caption]
It determines whether the fuel can operate conventional technology without supplementing it with a more energy dense fuel. In addition, the heating value determines flow requirements for the machine. The heating value limit is set at 300 BTU/scf and above for current reciprocating engines and gas turbines technologies.
Cleanliness refers to the presence of particulate such as siloxanes, organic and sulfur compounds in the waste gas stream that require removal before use. The removal of these compounds prior to the prime mover ensures proper combustion.
Corrosion occurs primarily in the hot section of the engine. Heavy metals in the gas stream are major contributors to corrosion. Specific treatment and removal of these metals is a necessity in order to ensure a viable life for the machine.
Fuel availability, specifically seasonal variations and future reserves, are of particular importance when evaluating a waste gas utilization project. In most cases, a waste gas project will not be financially beneficial if it requires continual supplementing of fuel.
Such criteria help waste gas producers evaluate the economic benefits of waste gases and the potential for their exploitation. Energy producers should carefully consider waste gas characteristics as part of the financial feasibility process.
Typical waste gas producers include biomass, landfills, coal mines and associated petroleum gases (APG) from oil & gas operations. The gases from these processes usually have heating values below that which current technology can support (300 BTU/scf). Because current waste gas utilization projects use conventional reciprocating engines or gas turbines as their prime mover, many of these projects remain uneconomical.
In these cases, producers dispose of the gas either through venting or flaring. Venting is detrimental to the environment, with the EPA estimating that methane is over 20 times more harmful to the environment than CO2.
For this reason, most countries require that waste gases are flared rather than vented. While flaring reduces the release of emissions, it is a financial burden to the producer because it often requires a supplemental gas stream to facilitate combustion due to the low heating value of waste gas streams.
The Texas Commission on Environmental Quality (TCEQ) found that above 300 BTU/scf, flare efficiency meets the standard of 98% destruction. Below this level, unburnt emissions are about 20% and head to wards 40% as you go lower in heating value. The Global Gas Flaring Reduction partnership, led by the World Bank, estimated that 140 bcm of gas was flared in 2011.
Gradual Oxidation
Gradual Oxidation technology was engineered as an alternative to conventional combustion to allow gas turbine and reciprocating engine manufacturers to produce power from previously unusable low-energy sources. The gradual oxidation reaction is fundamentally different from combustion in that it occurs at a lower temperature (2,200°F), lasts longer (seconds) and is flameless. It offers flexible fuel capabilities, lower emissions and the destruction of volatile organic compounds (VOC).
Figure 2: Gradual oxidation reaction cycle[/caption]
Gradual Oxidation be gins with the mixing of fuel gas with air to a 1.5% fuel-to-air ratio. This mixture is pressurized and introduced into the gradual oxidizer, which is heated above the auto-ignition temperature of the fuel. At this temperature, a chemical reaction releases heat ener gy. A prime mover can harness that energy to turn a generator (Figures 2 & 3).
Figure 3: Flow diagram for power production using gradual oxidation[/caption]
A 1.85 MW pilot project is ongoing that pairs Gradual Oxidation technology with Dresser-Rand’s KG2-3G gas turbine. The KG2-3G has improved performance and lower emissions than previous generations of the KG2, as well as 26% efficiency and less than 25 ppm NOx using a dry low NOx combustor. An externally fired version was implemented by diverting the airflow from the compressor to an external combustor and returning it to the gas turbine.
The gradual oxidizer is a further step in this evolution as it is a one-for-one replacement for the external combustor. The gradual oxidizer adds heat to the air from the compressor prior to expansion through the turbine (Figure 4).
Figure 4: KG2-3G turbine modified for external combustion[/caption]
Fuel flexibility
The current heating value threshold for reciprocating engines and turbines is around 30% methane (natural gas has a methane content of between 70% and 90%). If a gas stream has below 30% methane or 300 Btu/scf (Higher Heating Value or HHV), reciprocating engine and turbine manufacturers suggest supplementing the gas stream to achieve rated outputs.
Gradual oxidation enables the production of energy from low energy density gases that combustion technologies must supplement with higher calorific value gases. In order for the gradual oxidizer to use the low energy gas, there must be a high enough flow rate to sustain the oxidation reaction and maintain the energy input requirements of the machine.
The flow rate and energy density of the waste gases are inversely proportional and equal to the total energy input of the machine. The energy input for the 1.85 MW KG2-3G w/GO system is about 18.1 MMBTU/hr Lower Heating Value (LHV). For example, if the waste gas has 20% methane content (calorific value of 200 BTU/scf) then the flow rate of waste gas required to produce 1.85 MW of continuous power is 1,670 scfm. However, if the gas has a methane content of 30% then it only requires 1,113 scfm. These figures assume standard ambient temperature and pressure.
On the emissions front, the formation of NOx is largely attributed to high combustion temperatures, whereas CO formation is due to incomplete combustion. Gradual oxidation is a flameless chemical reaction quite different from combustion that does not require ancillary equipment to control pollution. Its reaction occurs at a relatively low temperature, well below the formation temperature of NOx and it lasts long enough to avoid the release of CO.
Additionally, as it is a longer lasting reaction than combustion, it can destroy VOCs without the high temperatures necessary for most thermal oxidizers (which lead to further NOx creation).
Using low energy gases for power production presents a large potential market for turbine manufacturers. After all, 140 billion cubic meters of waste gas were flared worldwide in 2011, which could be translated to 12,000 MW of power production.
