Biogas flare & flare gas

Throughout human history, improvements in the quality of life have often come with detrimental side effects. Oral health, for example, made great strides because of the fluoridation of water. At the same time, there are indications that too much fluoride contributes to the weakening of bones and ligaments, and might exacerbate psychological pathologies. Scientific research responds to issues such as these by searching for ways to retain the benefits while mitigating the side-effects. This sort of investigation takes place across the broad spectrum of natural and applied sciences. An excellent by-product of such discovery is the use of flaring in the recovery and processing of fuels.

What Is Flaring?

When petroleum, natural gas or other energy sources are recovered and refined, there are associated gases that do not necessarily help — and may even hinder — the production of fuel in terms of safety and efficiency. Flaring simply refers to the burning of gas that is superfluous to developing the energy source. Utilizing a stack or boom through which the flare gas will travel, technicians drive the unwanted compounds to the tip where they react with the air. This reaction is recognizable to many who may see a flame atop a tower at oil fields or fuel processing facilities.

Is Flaring Necessary?

Flaring oil and gas is undertaken for the sake of the health and safety of workers. One glaring hazard addressed by flaring is the dangerous build-up of pressure in pipes and tanks. Such an occurrence could lead to an explosion. Another beneficiary of flaring is the atmosphere: combustion at the flare tip releases water vapor and carbon dioxide (CO2) but it also neutralizes the effects of sulfur dioxide and other volatile organic compounds. According to the U.S. Environmental Protection Agency (EPA), “Methane emissions reductions of 2,000 Mcf (million cubic feet) per year apply to a single flare with a single pilot.”

How Does Flaring Work?

Gas flare systems begin with an inlet flow to regulate the pressure of incoming natural gas or crude oil. The flow proceeds to a knockout drum where liquid and vapor part ways. From there, the gas flows to a flashback seal drum. There is a safeguard to this transition, however: pressure must fall within a certain range before the flashback seal drum will open. Purge gas is pumped into the drum to expel and vestiges of combustible gas. In so doing, the system forbids the flame atop the flare stack of coming down and wreaking damage. Only then can the gas enter the gas flare stack.

Is Flaring the Same as Venting?

Flaring and venting are often confused for one another. In fact, venting is the regulated release of gases into the atmosphere during the production phase of the energy extraction process. No combustion occurs in venting whereas flaring is the controlled burning off of gases during both production and processing phases.

Are There Drawbacks to Flaring?

Although flaring ostensibly protects the atmosphere from toxic and volatile organic compounds, it nevertheless is responsible for the release of CO2 and methane (CH4). That these compounds are significant contributors to global warming is well-testified. In fact, countless public and private studies focus on CH4 emissions alone: some of the estimates from targeted natural gas facilities reach 10 percent of the total discharge.

Yet CH4 is not the only consequence of natural gas flaring. Another component of this burn-off is something known as black carbon, i.e. fine particulate air pollution. The layperson knows it as soot. The result of incomplete combustion of oil, natural gas or wood, among other fuel sources. While black carbon has a short shelf-life as an atmospheric threat, it can do much damage in that brief time. It is known to induce a detrimental effect on crops, soil and the cryosphere, i.e. ice and snow, as well as human health and wellness. Reducing black carbon even modestly is shown to mitigate these problems.

Solutions to Flaring Problems

Finding remedies to the negative side-effects of flaring is an ongoing pursuit. In the case of natural gas, flaring is not an urgent necessity if there is immediate pipeline access through which the gas could be transported to a treatment plant for purification. Absent that infrastructure, at any rate, there is other technology that points to flare gas recovery. In Taiwan, for instance, compressors are now fitted to refinery flare systems — by law — so that exhaust is directed to a ground flare, one that is completely enclosed during combustion. Only when processing volume passes a certain threshold will the flare stack design accommodate excess exhaust. This flare gas recovery system offers a more eco-friendly alternative.

Flaring and Biogas

What is Biogas?

Sometimes referred to as renewable natural gas, biogas is methane derived from organic material. Dubbed substrate in biogas production, the organic matter runs the gamut — decaying foliage and animal carcasses; food waste and compost; wildlife droppings and livestock manure; sewage and wastewater, to give a sampling. When such substances are bereaved of oxygen, anaerobic digestion takes place whereby the material is chemically disintegrated by bacteria over several stages. When the final reactions take place, both CH4 and CO2 are released from the substrate. Anaerobic digester technology now allows substrate to be processed in large amounts and the biogas to be captured and purified for use as vehicle fuel and for conversion into electricity.

How Is Biogas Captured and Refined?

Contingent on the type of organic matter serving as substrate, it may first be pulverized into a more homogenous texture. From this point, the substrate enters the digester tank where it is heated, often to over 100 degrees Fahrenheit. When this fermentation period — 20 days or more — is complete, between 40 and 70 percent of the gaseous yield is methane. The rest is mainly CO2 and some trace amounts of contaminant compounds. Many means are available for separating out the CH4, including absorption, adsorption and membrane filtering. The substrate remainder, now called digestate, serves as a potent fertilizer for agricultural crops.

What Gases Hinder the Performance of Biogas?

Among the compounds that compromise biogas performance or otherwise threaten health and efficiency include:

  • Water vapor
  • Hydrogen sulfide
  • Halogen compounds
  • Ammonia
  • Siloxanes
  • Volatile organic compounds

Particulate matter akin to black carbon may also be present in raw biogas. Not only do impurities like these inhibit fuel efficiency, they can also cause corrosive damage to containers and conduits through which the biogas is transported.

How Are the Contaminant Gases Removed

Common methods of removal have demonstrated effectiveness in purifying methane for energy use.

1. Absorption — One way to upgrade is to pass the biogas through a column of sodium hydroxide (NaOH) solution, which absorbs the contaminants as the H4 passes through.

2. Adsorption — This involves causing the biogas to have contact with a solid surface made from iron oxide, activated carbon, silica gel and/or other compounds that cause the unwanted gas molecules to adhere to the surface.

3. Membranes — Porous materials are used through which the impure gas molecules pass while methane molecules are trapped.

Can Flaring Eliminate Harmful Compounds?

Often, the biomass found in sewage plants or landfills is high in contaminating gaseous compounds. Some of these facilities opt for a biogas flaring system. Landfill gas is frequently burned in an enclosed biogas flare. Flaring might be constant or it could be done intermittently. The former diminshes the amount of methane released into the atmosphere while the latter takes into account the condition of the energy conversion equipment. It is worth noting that biogas is carbon neutral so any emission of CO2 does not count as added greenhouse gas.

Using biogas for energy by definition limits the volume of methane rising in the atmosphere whether flaring is employed or not. Thus, flaring causes fewer environmental problems when burning biogas than natural gas from geological deposits. Needless to say, enclosed flaring is even more eco-friendly. Whether to flare or use another technique to eliminate the harmful gases is often dependent on the substrate that gives birth to the biogas.

In Summary

Energy professionals utilize flaring to burn exhaust gases when producing and refining natural gas and petroleum. Whereas venting releases these gases directly into the atmosphere, flaring, i.e. burning the gases, releases less and destroys more. There are, nonetheless, ecological negatives related to flare recovery systems. Methane, a primary greenhouse gas, is still emitted from tthe flare stack. In addition, black carbon particulate is also a consequence of the flare. Biogas is also flared in some cases. The undesireable consequences of flaring with fossil fuels are somewhat mitigated with biogas.


AirFacts Magazine