April 2024

Power and Utilities

Decarbonization opportunities in refinery and petrochemical complex-organic Rankine cycles

In an organization’s pursuit of decarbonization, significant opportunities exist in recovering waste heat from high-temperature streams instead of rejecting it to heat sinks. This article analyzes the economics of using the organic Rankine cycle (ORC) to recover waste heat and generate electricity.

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In an organization’s pursuit of decarbonization, significant opportunities exist in recovering waste heat from high-temperature streams instead of rejecting it to heat sinks. This article analyzes the economics of using the organic Rankine cycle (ORC) to recover waste heat and generate electricity. Although there have been significant efforts to optimize heat integration over the years, based on a report by the U.S. Department of Energy (DOE), the industrial manufacturing sector consumed 30% of all energy in the U.S. in 2009, and about one-third of the energy was discharged as waste heat to the atmosphere.

Most heat discharged to the atmosphere is low-quality heat, which is difficult to recover conventionally, unlike heat integration between process streams. The ORC provides a pathway to recover this low-quality heat and convert it into electricity. Electricity generated using the ORC is equivalent to green power because no greenhouse gas (GHG) emissions exist. This leads to decreased power imports, thus incentivizing organizations to earn carbon credits. This case study explores replacing an air cooler in an existing operating facility with an ORC block, forming the basis for economic evaluation (FIG. 1).

FIG. 1. Existing vs. proposed schematics.

The ORC. 

The Rankine cycle forms the thermodynamic base for all power plants where water evaporates to make superheated steam and generate electricity through turbines.

In the case of the ORC, organic fluid with a low vaporization temperature at high pressure extracts heat from low-temperature heat sources. Since selecting an organic fluid influences the ORC’s performance, significant research has been conducted to analyze the effects of working fluid on the ORC performance. It was inferred that the selection of working fluid is governed by the heat source temperature and the properties of the working fluids, mainly the critical temperature, specific heat and latent heat of vaporization.¹

Generating electricity from waste heat has many advantages, such as:

1. Waste heat power is generated onsite and does not require a grid connection
2. The electricity generated is on par with renewable power because there are no associated carbon emissions
3. Significant energy savings by eliminating air coolers used to mitigate discharged low heat into the atmosphere
4. Environmental benefits
5. It is already applied in geothermal energy and is not a novel concept.

The barrier to using waste heat for power generation is the lack of experience in designing and operating plants in industries with available low-heat sources. In addition, it will require interfacing with different parties like technology licensors; engineering, procurement and construction (EPC) contractors; and vendors to align in the conceptual design stage.

Modeling the ORC system. 

This study evaluates the economics of converting heat from a high-temperature stream in a refinery unit and cooling it from 178°C to approximately 65°C using an air cooler. I-butane was chosen as the working fluid because it resulted in the highest efficiency.¹ A simple ORC system was modeled using a proprietary simulation tool^a using the Soave-Redlich-Kwong (SRK) equation as a thermodynamic package for the ORC fluid and hot process stream. The backpressure was selected so the saturation and cooling water return design temperatures had a sufficient approach in the condenser (FIG. 2).

FIG. 2. ORC flow scheme.

 

For ORC scheme efficiency estimations, the following was considered in line with industry practices:

1. Expander polytropic efficiency of 85%
2. Pump and motor efficiency of 75% and 95%, respectively
3. Generator loss of 10% of the isentropic turbine work
4. For an existing air cooler, two bays with four motors were considered. The power for each motor was 22 kilowatts (kW) based on the air cooler vendor input
5. The pressure drop across exchangers was 14 kilopascals (kPa), but the pressure drop in piping and its components was negligible (TABLES 1 and 2).

Economic analysis. 

An economic analysis evaluates the return on investment (ROI), considering the financial incentives associated with organizations reducing their carbon footprint. Economic incentives are considered a monetary value to renewable power with 24-hr availability. The direct emissions are summarized in TABLE 3. However, carbon trading prices under various emissions trading systems (ETSs) vary globally (FIG. 3).

 

 

The Network for Greening the Financial System’s Net-Zero 2050 scenario (NGFS 2021) estimates carbon prices between $100/t−$200/t of carbon dioxide (CO₂) in 2030, rising sharply until 2050.⁴ Earning carbon credits is an incentive for economic analysis. According to a report by the World Bank, carbon prices are expected to increase to $122/t by 2030.³ For conservative economic analysis, $85/t was used.

Based on a report by McKinsey & Company, power purchase agreements with long duration energy storage (LDES) technology improvements are expected to reduce in coming years.² However, today, they range from $100−$200, depending on the region. For conservative economic analysis, $100 was used.

Additional considerations for economic analysis included:

1. The plant operates for 8,400 hr/yr
2. 500 g/m² of CO₂ emissions per kWhr of electricity produced using natural gas⁵
3. One carbon credit is equivalent to 1 metric t of CO₂ emissions
4. Power consumption per air cooler fan is considered 22 kW as per vendor input.

Based on TABLE 2, 841.5 kW of electrical power is generated per hour. This equates to the following amount of electricity generated per yr (Eq. 1):

0.8415 × 8,400 = 7,068.6 MW           (1)

Based on TABLE 3, renewable electricity payback cost is $100/MWhr of electricity. This equates to the following earnings via renewable power generation (Eq. 2):

7,068.6 × 100 = $706,860/yr             (2)

Based on TABLE 2, the electrical power consumption in the pump is 107 kW, and the total power consumed in four air cooler fans is 88 kW (22 kW per fan). The net increase in power consumption due to the pump’s installation and air cooler’s removal is approximately (Eq. 3):

107 kW – 88 kW = 19 kW                   (3)

Based on the carbon price in TABLE 3, the carbon credit earned on CO₂ emissions is the following (Eq. 4):

(841.5-19) × 8,400 × 500/1,000,000 × 85 = $293,632/yr           (4)

The total generated per year with carbon credits and power generation equals $1 MM.

Total installed cost (TIC) estimation. 

Sizing is performed for equipment in the ORC loop. Sizing and cost estimates based on the author’s company’s database, including vendor input, are provided in TABLE 4.

ROI. 

A simple payback period concept calculates the ROI, dividing the total investment return by the return received every year until the TIC is recovered. A simple payback period concept is used for high-level return estimations:

1. Total equipment cost: $3.259 MM
2. TIC: $8.15 MM
3. Cash generated/yr: $1 MM
4. Payback period: Approximately 8.2 yr (FIG. 4).

FIG. 4. Cash flow diagram.

Takeaway. 

Carbon credit prices are expected to trade higher in the future with government mandates being enacted to curb organizations’ carbon emissions. In addition, with the economics of generating continuous renewable power still not very attractive because it requires significant investment to have LDES, using waste heat recovery to generate electricity offers an economically attractive option, as well as a strategic investment.

Implementing an ORC scheme utilizing process heat can pose operational challenges and may upset the main process in case of any malfunction and/or mis-operation in the ORC circuit. The ORC circuit must be very reliable to offset these challenges. The ORC turbines have a very high reliability and the circulating pumps’ reliability can be increased. These challenges can be overcome by adequate interlocks and control schemes, and all these aspects can be analyzed during hazard and operability (HAZOP) and safety integrity level (SIL) assessments. HP

NOTE

^a AVEVA Pro/II™ Simulation

LITERATURE CITED

1. Jung, H., Krumdieck, S., Vranjes, T., et al., “Feasibility assessment of refinery waste heat-to-power conversion using an organic Rankine cycle,” ResearchGate, January 2014, online: https://www.researchgate.net/publication/273826982_Feasibility_assessment_of_refinery_waste_heat-to-power_conversion_using_an_organic_Rankine_cycle
2. McKinsey & Company, “Decarbonizing the grid with 24/7 clean power purchase agreements,” May 2022, online: https://www.mckinsey.com/industries/electric-power-and-natural-gas/our-insights/decarbonizing-the-grid-with-24-7-clean-power-purchase-agreements
3. The World Bank, “State and trends of carbon pricing 2023,” May 2023, online: https://openknowledge.worldbank.org/entities/publication/58f2a409-9bb7-4ee6-899d-be47835c838f
4. NGFS, “NGFS climate scenarios for central banks and supervisors,” June 2021, online: https://www.ngfs.net/sites/default/files/media/2021/08/27/ngfs_climate_scenarios_phase2_june2021.pdf
5. U.S. Environmental Protection Agency, “Emissions & generation resource integrated database (eGRID),” online: https://www.epa.gov/egrid

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