Many companies have set ambitious carbon and energy reduction goals, and now are working to make those goals a reality.

For industrial plants, the largest single natural gas user is often their steam boilers.

Steam is integral to industrial heating operations and may be used directly, where the steam is directly injected into the process in some manner, or indirectly, where a heat exchanger is used to transfer heat from steam to the process.

A gas-fired boiler is then typically the first target in a decarbonization initiative, with the assumption being that it will be replaced with an electric boiler to eliminate natural gas use at the facility.

We can walk through an example of what this would look like:

In our example, a facility uses natural gas boilers to generate 15,000 LB/Hr of steam at 120 PSIG.  Based on steam tables, the enthalpy of the saturated steam is 1,193 BTU/LB.  If the DA tank is maintained at 5 PSIG, the feedwater enthalpy is 195 BTU/LB.  So, the energy required for steam generation is:

The steam generation efficiency represents the energy transferred to the boiler water to produce steam over the fuel energy imparted at the boiler.

Steam generation efficiency is useful as it incorporates losses from heat/moisture exiting the stack, blowdown, and radiant/convective losses from the boiler shell.

For an industrial boiler, steam generation efficiencies are generally between 80 to 85 percent.

Unless condensing heat recovery is incorporated, efficiencies above 90 percent are impossible due to the water content in the exhaust.

Our example boiler has an efficiency of 84 percent and operates continuously, so the natural gas input required for steam generation is:

For an average gas rate of $5/MMBTU, the annual fuel cost is:

On the surface, the most straightforward way to decarbonize this site appears to be the replacement of gas boilers with electric boilers.

Electric boilers do not have combustion losses and thus can have efficiencies on the order of 94 percent.  For an electrical rate of $0.075/kWh, the operating cost is:

Even for a company prioritizing carbon reduction, this increase in operational expenditure may be impractical.

In addition, there are often significant costs associated with increasing transformer capacity and with local grid constraints.  Replacing a gas-fired boiler with an electric boiler will not achieve a return on investment with the current utility rates

However, is it the right step in emission reduction?

For natural gas combustion, the CO₂ emission factor is about 118 LB CO₂/MMBTU [1].  For electricity generation, the CO₂ emission factor is about 0.86 LB CO₂/kWh [2].  This will vary by state depending on the energy source used to generate electricity.  However, this number is a general average for the United States.

So, the annual carbon dioxide emissions for a gas boiler are:

Compared to an electric boiler:

So, if this facility eliminates their direct (Scope 1) emissions using an electric boiler their operational cost will increase by almost 4x, and the indirect (Scope 2) emissions increase.

Renewable Energy Credits (RECs)

Many companies can purchase unbundled RECs, or renewable energy credits.  Unbundled RECs are sold separately from the electricity generated from a renewable source.  This allows facilities that do not have access to renewable energy to purchase the rights to emission-free energy.

Although RECs can be a tool in a decarbonization roadmap, they do not solve the issue of operational cost increases or the issue of the increased demand on the local grid when large industrial facilities electrify.

Is replacing a gas-fired boiler with an electric boiler an “energy efficiency project?

After all, based on the efficiency numbers above there is less energy input required at an electric boiler with an efficiency of 94 percent to generate the same amount of steam as a gas-fired boiler with an efficiency of 84 percent.

Replacing a gas boiler with electric is certainly considered electrification and a fuel-switch.

Whether or not it saves energy is highly dependent on the geographical location, and how electricity is produced.

About 60% of the electricity in the United States is generated with fossil fuels [2], and the average efficiencies for natural gas and coal-fired power plants are 45% and 32%, respectively [3].

Therefore, there may be energy savings at the boiler, but there is not a reduction in the primary energy source (natural gas in this case) on an average basis for the United States.

Electrification is necessary at some level in a decarbonization roadmap, and would ideally be coupled with updates to the grid which allow for clean electricity generation.

However, in the short term, industrial facilities must grapple with how to achieve their carbon reduction goals without significantly increasing operational costs and Scope 2 emissions.

Sources Cited:

[1] https://www.epa.gov/sites/default/files/2020-09/documents/1.4_natural_gas_combustion.pdf

[2] Frequently Asked Questions (FAQs) – U.S. Energy Information Administration (EIA)

[3] https://www.eia.gov/todayinenergy/detail.php?id=44436