Greenhouse gas (GHG) emissions and their possible effects on climate change are at the top of the environmental agenda. Although the subject is one of continuing debate, there is a growing concern that rising COemissions have an anthropogenic source.

According to the International Energy Agency, one of the most effective ways to reduce the environmental impact of human activity is to promote energy efficiency. Efficiency improvements can reduce the need for investment in energy infrastructure, cut fuel costs, increase competitiveness, and improve consumer welfare. Energy security also can profit from improved energy efficiency by decreasing reliance on imported fossil fuels.

Environmental performance

The oil and gas industry is accountable for its contributions to GHG emissions when it consumes oil to produce oil. Although the E&P business is not an energy-intensive industry, there are many opportunities for reducing emissions from process operations. The importance of energy consumption in E&P operations can be usefully expressed by using the Energy Index (EI) – the energy consumption per unit of production. During recent years the mean EI has risen to 3% to 4%, meaning oil and gas companies currently are consuming 3 bbl to 4 bbl/oil for each 100 bbl produced.

According to data presented in the “Environmental performance in the E&P industry – 2009,” issued in October 2010 by the International Association of Oil & Gas Producers, the majority of energy requirements were met by combustion of fuels on site (about 80%). No breakdown was specified by E&P companies for about 16% of the total, but where a breakdown was provided, emissions from onsite combustion accounted for 95% of the total.

The next step on the path toward a more sustainable business is to address the environmental issues focusing on the energy-intensive operations of E&P activities. Strong investment in energy efficiency and the promotion of energy saving initiatives are the most effective ways to reduce energy consumption with low investment costs.

Currently, oil and gas companies are consuming 3 bbl to 4 bbl/oil for each 100 bbl produced. (Image courtesy of Eni)

Energy efficiency opportunity

Energy recovery from waste heat is one of the most promising of the initiatives that could be applied to reduce the environmental impact of oil and gas operations. Waste heat is energy that is discarded as part of a larger process. Typically it has medium to low exergetic value, being available at relatively low temperatures such as flue gases in gas turbines applications. For this reason, it often is difficult and unsuited for recovery; however, some interesting solutions already exist for certain conditions.

Organic Rankine Cycle (ORC) technology is a proven solution that has been applied to produce thermal and electric power from low-temperature energy sources such as biomass and geothermal heat.

The technology is based on the known thermodynamic Rankine Cycle, which converts heat into work, commonly using steam as a process fluid. The working fluid is pumped into a heat exchanger, where it is evaporated, passes through a turbine generating mechanical/electrical power unit, and then is recondensed. ORC uses organic fluids (such as pentane, Freon, or Siloxanes) in place of water, with selection based on the heat source temperature. Fluids’ higher molecular weight and lower vaporization temperature allow maximizing energy recovery and power generation with a 20% thermal-to-electrical conversion efficiency. The package also can deliver a hot water stream that, if exploited, raises overall recovery efficiency to 98%.

Oil and gas facilities commonly are equipped with gas turbines both for power generation and as mechanical drive compressors. The number and size of these facilities depend on the amount of production the plants manage. More than 85% of the installed turbines used in mechanical drive applications are simple cycle, and their size is unsuited for economical coupling with a heat recovery steam generator (HRSG) to produce steam and so being retrofitted to common combined cycles. This means gas streams from turbine expansion, at about 300°C to 400°C (572°F to 752°F), historically have been wasted.

ORC pilot plant

Eni’s R&D department has engineered the transfer of the ORC technology from its original field of application to an upstream plant, where it can recover energy from the exhausted gases of gas turbines.

The Fano gas plant has two 10 MW open-cycle gas turbines that are operating in mechanical drive for compressing a low-pressure gas stream coming from offshore installations in the Adriatic Sea. These turbines allow the compression of gas from 5.5 bar to the network pressure of 55 bar, producing a stream of exhaust gases at about 450°C (842°F).

Approximately 12 thermal-MW can be recovered from this energy loss by means of a heat recovery unit (HRU) composed of a chimney that allows the heating of a diathermic oil to a temperature of about 310°C (590°F). The diathermic oil loop functions as an energy carrier, routing recovered heat to the ORC package. So nearly 3 MW can be produced, which is roughly equivalent to 15,000 cm/d of natural gas – about 1% of the plant’s daily gas production –improving efficiency from 30% to 42%.

Produced power will be sold to the national electrical grid, increasing the company’s revenues and avoiding the emission of 12,000 metric tons/yr of CO.

Operationally, ORC technology presents interesting characteristics. It is a skid package that is easy to install, and it is fully automatic. The use of an organic fluid leads to low mechanical stress for the turbine, which means lower maintenance costs. The package does not need an HRSG to recover waste heat from turbines as in common steam combined cycles. A simple HRU with a diathermic oil loop is enough to transfer a low-temperature waste energy stream to the package. Flexible system architecture and the reduced footprint allow the technology to be easily retrofitted into existing plants.

Economic analysis shows technology cost is in the range of US $2,000 to $2,500/kW, and the total project investment can be recovered in about seven to eight years.

The ORC pilot plant has two 10 MW open-cycle gas turbines that will compress low-pressure gas from offshore installations in the Adriatic Sea.

The pilot plant is under construction – the first installation in the oil and gas upstream business. Its final goal is to validate the technology and evaluate its reliability and performance in an upstream setting. Startup is scheduled for early 2013.

While work continues at a rapid pace, the company is involved in a worldwide evaluation of similar energy consumption and waste in oil and gas production fields. Preliminary evaluations show huge potential for increasing the energy efficiency of installed facilities with great opportunities to optimize consumption, reduce operating costs, and diminish GHG emissions.

Acknowledgements

The authors would like to thank Alberto Delbianco and Paolo Prada, Eni E&P, and Marco Ferrari, Tecnomare