Waste not, want not

The global natural gas industry expects to play a key role in achieving climate targets under the Paris Agreement, even as demand is projected to increase at least through the mid-2030s, with LNG experiencing a runway of growth until the mid-2040s.

While the climate benefits of using natural gas in power generation as opposed to coal are well known, the natural gas industry needs to reduce its own carbon footprint, and one of the easiest ways to do this is by capturing the vast amounts of energy lost through any number of upstream, midstream and downstream processes.

Advertisement: The National Gas Company of Trinidad and Tobago Limited (NGC) NGC’s HSSE strategy is reflective and supportive of the organisational vision to become a leader in the global energy business. ngc.co.tt

According to a recent white paper from Exergy, which specialises in developing waste heat recovery systems for the oil and gas sector and other industries, fully one-third of the energy utilised by the oil and gas sector is dissipated as thermal losses, either directly into the environment or through cooling systems.

And in the midstream and downstream, recovery of wasted energy offers a viable opportunity to reduce Scope 2 CO₂ emissions.

According to the Knowledge Center on Organic Rankine Cycle (KCORC) technology, the European natural gas supply chain holds technical potential to recover some 18.4 TWh of electrical energy by installing 3.6 GW of capacity to capture waste heat in the gas supply industry, including LNG and gas-to-liquids (GTL) liquefaction plants and LNG regasification plants.

In the US, the potential to generate electricity from waste heat has been estimated at some 1,102 MW from 1,300 sites – mainly compressor stations – in the natural gas pipeline transmission grid, while in Europe, 1,300 MW of gross power generating capacity could be installed at compressor stations, generating up to 10.43 TWh/year of electricity while avoiding 3.7mn tonnes/year of greenhouse gas emissions and cutting energy costs by up to €934mn annually.

Organic Rankine Cycle (ORC) systems are ideally suited to capture waste heat from compressors, flares, LNG regasification and natural gas pressure reduction stations because they are designed to convert low-temperature thermal energy – in the 80-400°C range – to electric energy in a closed thermodynamic cycle.

“These waste heat sources can be efficiently harnessed using Organic Rankine Cycle waste heat recovery systems and turbo expanders,” Exergy’s white paper notes. “The electricity generated can provide revenue and contribute to an overall reduction in carbon emissions.”

ORC power generation is essentially the same as generating energy from steam turbines except ORC systems use organic substances (hydrocarbons or refrigerants) instead of water converted to steam as a working fluid.

Advantages of ORC systems include:

• Enhanced cycle efficiency, particularly useful at partial load operations found in gas compressor stations.
• Compact design.
• Prevention of turbine blade erosion issues thanks to the dry expansion of hydrocarbons.
• Elimination of water consumption, unlocking remote area installations where access to water may be restricted.
• Automated operation requires no on-site personnel.
• Fast startup and shutdown.
• Reduced maintenance costs.

Use Cases

Across the US there are about 1,650 natural gas pipeline compressor stations, Exergy notes, with another 20,000 across Europe, and with demand for natural gas expected to increase, there will be a corresponding increase in capacity requirements.

Compressor stations are driven either by gas turbines, steam turbines or internal combustion engines, although a growing number of pipeline operators are turning to grid power, where available, to drive compressors.

“Gas turbines release most of their heat through hot exhaust gases and operate with 200-300% excess air, resulting in higher exhaust temperatures (ranging from 450-550°C) and greater exhaust flow rates per horsepower output,” the white paper says. “Consequently, they present the greatest potential for feasible heat recovery projects.”

To ensure enough power to drive compressor units, typical installations in gas compressor stations consist of multiple gas turbine units ranging from 6-25 MWh of capacity. One ORC waste heat recovery system can recover exhaust gas from multiple gas turbines, increasing operational efficiency by up to 40%.

“Payback terms can range from three to six years, depending on required electrical output of the ORC system, plant configuration, electricity selling prices, incentives and a favourable regulatory framework that could include carbon credits, green certificates and any additional premiums for saved CO₂ emissions,” the white paper says.

Exergy provides an example of a solution proposed for a project in Louisiana, an ORC system at a compressor station with three gas turbines totaling an average heat availability of 43,290 kWth. The ORC system can generate 12 MW of power, resulting in annual average electricity production of more than 82,000 MWh.

Beyond the recovery of waste energy at compressor stations, Exergy says, ORC systems can be installed at LNG regasification terminals, where heat energy is recovered from the turbines used to regasify the natural gas, and also offer cold energy recovery opportunities.

“Through the strategic implementation of ORC systems…not only can wasted energy be captured and utilised, but the associated reduction in greenhouse gas emissions helps to align with sustainability goals,” the white paper concludes, noting that capturing waste heat to generate power optimises energy resources and enhances competitiveness.

“Embracing these advancements not only ensures a more sustainable and efficient gas value chain but also underscores a commitment to innovation and responsible resource management in the broader energy landscape.”

Join us on June 25, 2024, at 03:00 PM CET for a dynamic webinar exploring innovative technical solutions to unlock the untapped energy recovery potential within the gas value chain, paving the way towards greater energy efficiency and sustainability. Register now to secure your spot!