In the first part of this article, published in June 2021, we discussed about the importance of finding the weak energy efficiency spots in a crude oil refinery. In this second part, we will explore how these challenges can be resolved further by adapting different approaches, comment Ekaterina Kalinenko and Miro Cavkov
Typically, the refinery heat sources are powered by fuel, or diesel oil, in a direct burning process. An eco-friendly approach and opportunity to comply with the global emissions regulations is to consider investing in an exhaust flue gas treatment complex – scrubbing units for the SNOX emissions, carbon capture unit and filters for fine particles.
Main energy consumers at a refinery are fuel (heaters) – 74%-78% of total refinery consumption; steam generation (HpS, MpS and LpS) – 18%-20% of total refinery consumption; and electricity (motors, tools, lighting) – 4%-6% of total refinery consumption. For example, increased plant capacity implies a proportional increase in raw material and resource consumption for production, but certain methods have resulted in a 10%-20% reduction in costs compared to the linear growth option (depending on the cost item).
To improve energy efficiency of technological processes, the project included: heating of fuel gas before feeding it into the furnaces, in order to reduce fuel consumption; using heat from flue gases to pre-heat raw materials in the convective part of the furnace; use of heat from visbreaking products to pre-heat raw materials and fuel gas, and to generate low-pressure steam; and speed-controlled electric motors for air-cooling units.
The use of secondary energy resources (steam, or heat of flue gases) is the criterion, which characterises the used technology as energy-saving. A plant’s energy efficiency is determined by a rational conduct of the technological process, which is carried out on the basis of the modern technology with use of instrumentation and control devices, centralised management, and high qualification of the operating personnel.
The principle is that the unit using the heat of the cracking residue produces low-pressure steam that is superheated in the furnace super heater at the expense of the heat of flue gases and is further used to evaporate hydrogen sulphide from the cracking residue, and the excess of low pressure steam is transferred to the plant network. Reduced energy costs are achieved by controlling energy consumption, and improving the power factor.
The energy efficiency calculations for a fired heater are very similar to that of a steam boiler. Since many processes uses some type of a fired heater, they present many opportunities for optimisation over an entire process facility.
Factors that can impact boiler efficiency
There are many factors that can impact boiler efficiency. Most of these factors also apply to fired heaters: (i) flue gas temperature – function of fuel type, lower flue gas temperature means higher efficiency, dew point can be a limitation; (ii) excess air – too much air will require more heat, typical burner setup –> 10% for gas and 15% for fuel oil; (iii) radiation and convection – function of ambient conditions and insulation condition; (iv) boiler blowdown – excessive blowdown is a major contributor inefficiency, normal blowdown energy loss 3%-5%, up to 80% heat can be recovered from blowdown; (v) ambient temperature – combustion air entering the boiler, 4°C change –> 1% boiler efficiency; (vi) fuel specification – some of the hydrogen content of fuel goes to water consuming energy to vaporise the water during combustion, C:H ratio of fuel gas is important, increase in H2 content –> increase in water –> lower fuel efficiency, methane: CH4+2O2 –> CO2+2H2O+heat, ethane: 2C2H6+7O2 –> 4CO2+6H2O+heat; and (vii) other boiler/furnace losses include – radiation losses from openings (furnace inspection doors), cold air infiltration into the furnace (through small openings, faulty gaskets), tube scaling (dissolved solids causing scaling on water tubes, frequent conductivity testing of boiler water required to reduce scaling potential, effective boiler blowdown), lack, or badly insulated water, steam and gas piping.
Improved energy efficiency through heat regeneration
Another available option for improved energy efficiency is heat regeneration. Regeneration uses a pair of burners, which cycle to alternately heat the combustion air, or recover and store the heat from the furnace exhaust gases.
When one regenerative burner is firing, the other is exhausting the furnace gases. Exhaust gases pass through the regenerative burner body and into a media case, which contains refractory material. The refractory media is heated by the exhaust gases, thus recovering and storing energy from the flue products. When the media bed is fully heated, the firing regenerative burner is turned off and begins to exhaust the flue products.
The regenerative burner with the hot media bed then begins firing. Combustion air passes through the media bed and is heated by the hot refractory. Air pre-heat temperatures within 150°C-260°C of the furnace are achieved resulting in exceptionally high thermal efficiency.
Compared to a standard furnace, fuel efficiency will increase by about 30%. At the same time, it is possible to achieve near 50% reduction in NOx emissions. The advantages are: combustion efficiency improvement; increased production from existing facilities; reduced CO2 and CO emissions; lower NOx emissions; and smaller furnaces in new installations.
Regenerative burners offer various advantages in energy efficiency. The combustion efficiency is vastly improved due to the increased air temperature. It also lowers the combustion temperature, leading to lower NOx production, which is good for the environment. The higher air inlet temperature ensures complete combustion. Therefore, the CO and CO2 emissions in the flue gas are reduced.
Finally, the higher efficiency burners allow for smaller furnaces in new installations. Requirements are: even distribution of the heat input – numerous parallel process passes (heat supply to any pass must be guaranteed), avoiding hot spots (overheated coil areas reduce furnace run length); heat input efficiency, environmental constraints – NOx, CO; operability – remote controlled operation (switches from/to hot steam standby, controlled shutdown, high turndown ratio required).
Return on capital investments in energy efficiency
At present, basic energy prices in certain regions are still below European levels, which explains the low return on capital investments in energy efficiency. For example, it is estimated that in Europe it makes sense to replace with a more efficient furnace at around 75% efficiency, while in Russia the break-even point for investments is at around 65% efficiency.
In case the above configurations are not giving the desired results, then the solution could be more drastic – to revamp the heaters to run on cleaner fuels. One of the alternative cleaner fuels available is natural gas, which is gaining popularity in refineries in the form of LNG deliveries and local regasification stations. The configuration natural gas + scrubbers + CCS + fine particles filters is one of the available routes for increased refinery decarbonisation through energy efficiency.
Last resort is to fully electrify the fired heaters, in order to revamp them to electric heaters. In terms of emissions, this is the most promising way to decarbonise the refinery heat production. However, here comes the question: Does this solve the environmental issue? Locally, definitely yes – no flue gases around the refinery, less emissions. On a global scale – it depends on the electricity power source.
If a particular refinery gets its energy supplies from an ERP, which is transmitting energy generated by a coal-fed power plant, this should not be considered as a complete decarbonisation route.
On the other hand, if the particular power plant has solved its environmental issues by investing in the same type of equipment – scrubbers, CCS and filters – then the overall emission equations can be considered better for the environment, then such configurations can work in our favour. As the fired heaters, the power plants can also be revamped to work on cleaner fuels such as LNG. This is a true example of the current energy transformation and energy transition possibilities.
Reducing energy losses
A fundamental part on reducing energy losses is through the proper insulation of the units and piping. Sometimes in a refinery, the haze is visible with a bare eye, but the silent losses are not. Good news is that we live in an advanced technological world, so the modern tools such as thermal cameras and ultrasonic non-destructive methods are available to help the refinery staff to identify precisely from where the losses are coming from.
Considering that the above mentioned conditions for improvement are achieved, it is time for process technology improvements – better production technology means shorter reaction time, which by itself means reduced energy consumption, therefore lesser overall emissions.
Additional power generation and heat recovery can be achieved in the following ways: (i) open cycle power plants (OCPP); (ii) cogeneration – combined cycle power plants (CCPP), combined heat and power plants (CHP); and (iii) trigeneration – combined heat, power and refrigeration plants.
It is important to ensure that the efficiency of power generation units are as best as can be economically achieved within the specific application considering the cost of fuel, the availability of utilities, the possibility of providing heat, or even refrigeration to adjacent facilities.
In terms of energy efficiency and the cost of operation, it is also becoming important to consider the amount of CO2 being released to atmosphere. We will discuss open cycle power plants, consider cogeneration plants where heat is recovered from the exhaust gases, either as power, or steam, that can be fed to steam users and also trigeneration facilities where the heat remaining in the exhaust gases after the cogeneration step is used in absorption cycle refrigeration cycles to provide for example chilled water for air conditioning, or process duties.
The simplest and lowest capital option is to install an open cycle gas turbine, or gas engine to drive a turbine and generate power. In this configuration, the hot exhaust gases are directly vented to atmosphere normally at around 600°C. All the energy required to heat the gas to 600°C is therefore lost. In this type of configuration, the thermal efficiency is only around 35%. In some more modern turbines, it is possible to achieve somewhat higher open cycle efficiencies. The advantage of this configuration is that the plant can start-up very quickly and can operate automatically with minimal operator attendance. Open cycle power plants are used in remote location where utilities like water, steam and cooling water is not available. Open cycle power plant loads can also be increased and decreased relatively quickly as the power demand varies.
In comparison with cogeneration and trigeneration units, open cycle plants have the lowest capital cost but the highest fuel consumption per unit of power produced.
Heat recovery steam generator
For a higher efficiency closed cycle power plants can be considered. In these plants, the exhaust gas at 600°C is used to generate steam at high pressure while the gas is cooled typically to 200°C. The steam is generated in a Heat Recovery Steam Generator (HRSG) and expanded through a steam turbine to generate additional power. The outlet steam is condensed, normally in air coolers and recycled to the HRSG. These plants are much more complex to operate because of the additional steam and condensate systems. The load on these plants also cannot be changed rapidly as the temperature variations causes stresses in the HRSG and this could lead to early failure of these units. Compared to open cycle plants with an efficiency of 35% closed cycle plants can achieve efficiencies of 60%-65%. In general, the increased capital costs is easily rewarded by the fuel savings. In base load power plants (steady operation) closed cycle power plants are the norm.
If steam is required at a nearby facility, it may be more economical to supply the steam generated by the HRSG to such a facility. The operation of the plant is slightly more complex than a closed cycle plant because the steam requirements from the adjacent facility needs to be taken into account. There may for example be periods when there is no demand for steam, but power still needs to be generated. The design of the plant needs to consider such eventualities. Because the steam is directly used rather than expanded in a steam turbine, the overall thermal efficiency of the plant is greater. This type of plant is called a CHP plant.
The overall efficiency of a power plant can be further increased by generating chilled water, or another cold utility by installing an absorption refrigeration loop downstream of the HRSG. In such a case, the overall thermal efficiency can be increased to above 90%. The capital cost of such a plant can become prohibitive and careful analysis of the business case is required to ensure that such a configuration will be economical. The plant operation is even more complex as the demand from three independent systems needs to be managed.
A comprehensive operational improvement programme at a refinery is formed in three main stages: (i) Identifying the lagging performance of the best refineries and identifying the main areas for improvement. Typically, three to four areas of operational improvement account for about 80% of a refinery's financial potential. (ii) Identify the main levers for closing the gap with the best refineries. A complete set of improvement actions should be identified for each priority area. (iii) Formulate a detailed list of technical actions (including economic impact assessment, timing and who is responsible for implementation).
In conclusion
An important outcome of implementing a holistic (integrated) approach is sustainable transformation. When ‘tangible’ results are achieved, employees will have an incentive to identify opportunities for further improvement on a continuous basis. An energy efficient scenario in the oil and gas industry would significantly improve operational and financial efficiency.
Medium-sized companies will be able to save up to $50-$70mn per year through resource-efficient technologies and will create a holistic database through benchmarking and case studies so that the experience can then be applied to other assets within the company. Improving efficiency should be seen as an ongoing challenge for the company. If this approach is taken, companies will have the opportunity to maintain and strengthen their own position in the markets.
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