August 2024
Special Focus: Plant Safety and Environment
Review steam system designs to accelerate chemical industry decarbonization
Industrial steam systems range from simple systems comprising a low-pressure single steam user to a complex system with multiple pressure levels, multiple generators and multiple users integrated with an onsite cogeneration facility. This article will present the typical CO2 emissions estimates for chemical and petrochemical facilities and describe options available to reduce these emissions through additional heat integration, fuel switching, and the use of heat pumps and electric boilers.
R. B. SINGH, Bechtel, India
Steam is an extremely efficient heating source widely used in the chemical industry, contributing 35%–40% of the onsite energy usage of a facility. Steam is generated by using a boiler to add energy to feedwater. In most industrial applications, the energy is released from the combustion of fossil fuels or from process waste heat. The use of fossil fuels generates carbon dioxide (CO2) emissions, which must be reduced as industry traverses the path toward decarbonization. In addition, one of the technically feasible paths to decarbonization of the chemical industry will require installing carbon capture and sequestration (CCS) technology. CCS typically involves solvent-based CO2 absorption. This technology requires significant steam for the regeneration of the solvent. Therefore, the path to decarbonization will involve not only reducing CO2 emissions from fossil-fuel-fired steam generation facilities, but also finding additional steam for application in CCS without increasing the CO2 footprint.
Industrial steam systems range from simple systems comprising a low-pressure (LP) single steam user to a complex system with multiple pressure levels, multiple generators and multiple users integrated with an onsite cogeneration facility. This article will present the typical CO2 emissions estimates for chemical and petrochemical facilities and describe options available to reduce these emissions through additional heat integration, fuel switching, and the use of heat pumps and electric boilers.
CO2 emissions from steam generation. The steam required to be generated by burning fossil fuel in a boiler will depend on the configuration of the facility. Each chemical plant/refinery is unique, and the overall steam and power balance depends on the feedstock, capacity, products being produced, waste heat sources, byproducts utilized as fuel, and the cost of purchased electricity and power. TABLE 1 presents the typical energy consumption needed to produce some key chemicals, as reported in literature.
CO2 emissions also vary depending on the fuel used to meet the energy requirements identified in TABLE 1. High heating values and CO2 emissions from different fuel types are presented in TABLE 2. As shown in TABLE 2, the fuel selection will have a significant impact on a facility’s CO2 emissions.
Steam system configuration. A typical steam network for a large refinery/chemical facility is shown in FIG. 1. It consists roughly of 3–4 main pressure levels. Steam at the high-pressure (HP) level is produced primarily by using steam boilers and recovering heat from process furnace flue gases. The other levels of pressure are fed by steam generated in heat exchange with process streams. The headers are connected by letdown valves and turbines, the latter are used in a direct-drive configuration to operate compressors and pumps. Facilities with onsite cogeneration will additionally have combined-cycle gas turbines with heat recovery steam generators and steam turbines. For simpler facilities, the steam system may be composed of only a few steam levels and steam boilers generating steam at LP or medium-pressure (MP) levels with no turbines.
FIG. 1. A schematic of a typical steam system in chemical plants.
Typically, the steam and power network are optimized utilizing site-specific fuel and power prices. This implies that a site where fuel and power prices are higher will have a more extensive exchanger network to recover waste heat. In addition, the relative cost of fuel and power plays an important role in decision-making regarding the configuration of turbomachinery drives. For example, complex steam systems in ethylene plants usually contain extraction–exhausting steam turbines, backpressure steam turbines and heat exchangers, which provide shaft power to the compressor and pump, along with heat to the stream.
Due to variations in different operating scenarios and to optimize for changes in fuel and electricity prices, some of the optimized systems also have pump services in 1 + 1 configurations, with one running on an electric motor and the other on a steam turbine drive. Depending on the cost deviations of steam and electricity for providing per unit shaft work, either a steam turbine or electrical motor is selected to drive a pump. Furthermore, letdown valves are used to regulate the pressure of different steam headers. In fact, when either the steam supply is low or electric power is cheap, the operation of a significant number of turbomachines can be switched to electrical motors, which can provide the plant with significant flexibility. More electrical motors are switched to run from standby, since the cost of electricity generation is cheaper than that of the additional steam generation.
Today, most facilities use the boilers’ fuel consumption and the electric power used for the electrically driven pumps/compressors as the main variables for optimization. Most complex steam networks will have a detailed model of steam streams and a complete layout of the steam network. In older facilities, CO2 emissions were not accounted for in such optimizations. Once CO2 emissions have been accounted for, the selection between a steam drive and a power drive for pumps/compressors will change depending on the carbon intensity of the power available at the site. Steam that is not produced utilizing waste heat must be reduced or eliminated by converting more drives to electric power and optimizing heat integration by utilizing heat pumps and low-carbon steam generation options, such as electric boilers.
Heat integration. Heat integration is an important option for increasing energy efficiency in chemical process plants and oil refineries, helping to reduce steam generation from fossil fuel firing. The feasibility and costs of heat integration depend on the temperature approach between the waste heat and the required steam, plant layout and cost of fuel. Payback periods of < 5 yr have been typically utilized for assessing the feasibility of heat integration. Decarbonization requires revisiting the original basis for not considering some of the heat integration options due to longer payback considerations. These decisions are complex and sometimes counterintuitive and require a complete model of the steam system to understand how a steam-saving project would influence fuel, electricity and CO2 balances at the site.
This is illustrated in the heat integration modifications shown in FIGS. 2 and 3. The aim of one modification is to reduce the amount of fuel gas consumed by the process furnace by using preheated steam in the reactor feed stream. The other modification involves the heat needed for a distillation tower reboiler, which is provided by LP steam. The LP steam is replaced by internal heat exchange. The second modification will decrease LP steam consumption, and this LP steam can be used in the first modification to reduce fuel firing in the process heater, which, in turn, will decrease associated CO2 emissions. All steam uses must be analyzed after building in the cost for CO2 abatement to find opportunities.
FIG. 2. A retrofit of reactor effluent heat integration to reduce fuel consumption by utilizing LP steam.
FIG. 3. A retrofit of a distillation column reboiler to reduce LP steam consumption.
Electrification—Heat pumps. A heat pump is a device that increases the temperature of a waste-heat source to a temperature where the waste heat becomes useful. By using heat pumps, it is possible to replace steam with another waste-heat source, which helps to reduce energy costs and associated CO2 emissions.
Heat pumps convert lower-temperature waste heat into useful, higher-temperature heat. The work required to drive a heat pump depends on how much the temperature of the waste heat is increased.
There are two types of heat pumps that utilize electrical energy:
- Closed-cycle mechanical heat pumps that use mechanical compression of a working fluid to achieve temperature lift
- Open-cycle mechanical vapor compression heat pumps that use a mechanical compressor to increase the pressure of waste vapor.
FIG. 4 depicts open-cycle and closed-cycle heat pump systems. Open-cycle vapor compression is typically applied in distillation systems such as propane/propylene separation.
FIG. 4. Schematics of open-cycle and closed-cycle heat pumps.
A key parameter for a heat pump is temperature lift, which is the difference between the evaporator and condenser temperatures. Temperature lift is directly related to the temperature at which waste heat is available and the temperature at which it is to be utilized. For decarbonization projects, the basic goal is the reduction in CO2 emissions by virtue of saving steam and associated fuel firing. For such projects, the electricity used for a heat pump must be from a source with low carbon intensity.
The relationships between work input, temperature lift and heat output are determined by using a parameter known as the heat pump coefficient of performance (COPHP) (Eqs. 1 and 2):
COPHP = Qout/Win (1)
COPHP = Tout/(Tout – Tin) (2)
where Tin and Tout are the temperatures (measured in Kelvin) at which the heat pump receives and delivers heat, respectively. The COPHP of an actual machine will be 65%–75% of that for an ideal machine. TABLE 3 presents a sample calculation for two different temperature lifts and two different natural gas prices for 1 MMBtu/hr of waste heat.
The method presented above helps determine if there are sufficient savings in operating cost and CO2 emissions to pursue a heat pump application. Although simplified, this method provides a reasonable idea of the economics of installing a mechanical heat pump.
Electric boilers. Electric boilers utilize the conductive and resistive properties of water to carry electric currents and generate steam. The electric current flows from an electrode of one phase to the grounded counter electrode, using the water as the conductor. Since the water has electrical resistance, the current flows generate heat directly in the water itself. Nearly 100% of the electrical energy is converted into heat with no stack or heat transfer losses.
Electric boilers have been used in specialty applications. These applications were limited due to the high operating cost of electric boilers, as compared to fossil fuel-fired boilers, purely on economic considerations. Considering the CO2 emissions associated with fuel-fired boilers and the requirements for decarbonization, electric boilers must be included as one of the solutions for lowering emissions. On average, the capital cost of an electric boiler is lower than that of an equivalent natural gas-fired boiler. However, electricity prices have historically been at least three times natural gas prices on an energy content basis (per MMBtu). When accounting for the relative efficiency of each type of boiler, the hourly fuel cost of an electric boiler could be between 2.5 and 3.7 times that of a natural gas boiler. TABLE 4 presents a sample calculation for three different natural gas and electricity prices. It also presents the estimated CO2 emissions reduction when electricity is sourced from a low-CO2 source. The calculation basis is a fuel boiler with 85% efficiency, an electric boiler with 99.5% efficiency, electricity sourced from renewables and a CO2 price of $90/t.
Takeaway. The decarbonization of steam systems will require revisiting the designs that were typically optimized for the economics of fuel and power costs. Introducing the cost of CO2 emissions will lead to a different optimum and will involve the evaluation of switching pump/compressor drives, as well as the inclusion of additional heat integration and the use of heat pumps and electric boilers. The objective will be to minimize or eliminate any steam generation using fossil fuel firing, and to source additional LP steam for any CCS application.
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