Clean Power Professionals Group

This special interest group is where you can bring thoughts and ideas on renewable energy. 

40,206 Members


The Path to Net-Zero – Part 1


I usually post a short paper (hopefully less than 3,000 words) every week, and these are intended to educate readers that are interested in the various subjects where I feel qualified to write these papers. All of these papers are related to energy in some way.

When a subject is too deep to cover with a single paper of the preferred length, I break it up into two or more papers, which I call a series. This paper is first in a series of two papers on what I feel our future holds when it comes to achieving a sustainable reduction in greenhouse gas (GHG) emissions. This paper explores carbon dioxide an overview of GHG emissions and in depth: (CO2) emissions and the steps we might take to reduce them. The next paper will deal in depth with methane emissions and financial incentives that will drive down GHG reductions.

I recently posted a three-paper series on the likely impacts of climate change on electric, gas and water utilities. Links to these are below.

This series will cover the problem, each major GHG source, and suggested solutions.

2.The Problem

The cause of climate change is the release of GHGs by humans into the atmosphere. This is a scientific fact that I discussed in detail in the prior series ("…part-1-te future", linked above). The primary GHGs are carbon dioxide and methane

Approximately 40 gigatons (1 gigaton = 1 billion tons, where a ton is 2,000 pounds, hereafter abbreviated Gt) of CO2 were emitted globally in 2014.[1] Specific sources of these emissions and approximate contributions include:[2]

  • Electric production (16 Gt)
  • Mobility (11 Gt)
  • Iron & steel production (3 Gt)
  • Cement production (2 Gt)
  • Other industries (4 Gt)
  • Residential & commercial (4 Gt)

Human-caused methane emissions reporting by the EPA is no longer credible, thus I will use the reference estimate of over 14 million tons for the U.S. or 0.35 Gt of CO2 equivalent (GtCO2eq).[3] World emissions of methane are estimated at 7.5 GtCO2eq in 2010. These come mainly from the following sources: [4]

  • Digestive fermentation, mainly by ruminant animals (2.2 GtCO2eq)
  • Agriculture (1.6 GtCO2eq)
  • Oil and gas production and distribution (1.5 GtCO2eq)
  • Landfills (0.8 GtCO2eq)
  • Wastewater (0.7 GtCO2eq)
  • Coal mining (0.5 GtCO2eq)
  • Other sources (0.2  GtCO2eq)

Methane has a global warming potential 104 times greater than CO2 in a 20-year time-frame, but only 28 times in a 100-year time-frame. As methane degrades it forms CO2.

3.Reducing Carbon Dioxide Emissions

Given that methane is a much stronger greenhouse gas, one would think that we would cover it first. Instead, we will look at CO2 first because (1) it is the primary GHG by effect, (2) we are much closer to solutions for the major sources of CO2, and (3) my last paper ("I Like Smoke and Lightning…") covered the metals industry, of which the iron and steel industry is the largest sector, and also one of the largest sources of CO2. We will start with some smoke and lightning (I promise, no more rock puns).

3.1.Iron and Steel Industry

Several decades ago we would be in deep trouble, because the only process for producing steel was with integrated mills. Fortunately, two new technologies have emerged since then, plus one old technology has reemerged. The two new technologies are mini-mills, and directly reduced iron refining. The old technology is (believe it or not) charcoal. These are each covered in the following subsections. For more details on this industry see the paper linked below.


Mini mills use electric arc furnaces to melt, refine and alloy scrap steel and directly reduced iron (next subsection). Electricity that is renewably produced will allow these facilities to operate with very low CO2 emissions compared to integrated mills. Processing is required for the gasses and particulates that are emitted, and some of these are GHG (mainly nitrogen oxides). These processing techniques are currently used to meet air quality standards. Mini-mills can produce all types of iron and steel.

3.1.2.Directly Reduced Iron (DRI)

Directly reduced iron processing consists of heating iron ore to 800°C - 1,200°C in an atmosphere of carbon monoxide and/or hydrogen. These gasses can be produced from syngas which can be renewably obtained from the pyrolysis of biomass (pyrolysis does not emit significant GHG). The output gas for a hydrogen atmosphere is water vapor, and the output gas for carbon monoxide is CO2. Any output CO2 would need to be captured and sequestered for a truly CO2-free process. Other pollutants are produced (and hopefully mitigated), and these are similar to those seen from mini-mills. The iron output is typically called sponge iron, and has a purity of greater than 90%.

3.1.3.Charcoal with Carbon Capture and Sequestration

Coal is used to make coke, and most integrated mills use coke to reduce iron ore to pig-iron. Although somewhat less efficient, charcoal can be substituted for coal in this process. In fact Brazil uses charcoal in this application because they have limited coal. Brazil produces some 23-36 million m3 of biological charcoal each year to make iron and steel.[5] Some of it is from eucalyptus plantations on a 7-year rotation but most is from old growth forests. In the prior "…mitigating-climate change" paper linked in the Intro, we discussed the idea of planting large forests of fast-growing wood (like eucalyptus). This wood could be harvested and combusted for process heat, with carbon capture and sequestration (CCS) for mitigation of CO2 in the atmosphere. Charcoal-fired integrated steel mills with CCS could fit into this application-set.

3.2.Electricity Production

Many of my papers have been focused on this subject, mainly because this function is undergoing an explosion of technology that makes present-days one of the most exciting periods in history (ditto for the next subsection, and they're related). Thus this subsection will be brief.

Electricity will evolve to 100% renewables (including carbon sequestering generation) before any other source of energy. Thus this will become the source for replacing other energy that is not as clean. This will be a boon for electric utilities, facilities and energy equipment manufacturers.

Currently in most areas of the U.S. generation is evolving to natural gas fired combined cycle or peakers, renewables, and a few battery energy storage systems (BESS). Currently (mid 2018), California generates about 75% of our electricity with natural gas or non-GHG emitting sources (including renewables). In California utilities will add more renewables and BESS. Peakers and older combined cycle plants will be retired. Newer combined cycle plants might add CO2 sequestration technology to preserve diversity in the generation mix.

Solar plus storage is extremely cost-effective, dispatchable and very scalable. Thus facilities that can justify on-site generation will mostly rely on these technologies (it even works for residences).


Before lithium-ion (LiIon) batteries became dominant in BESS, they formed a rapidly growing market in cars. Furthermore there are many orders of magnitude more cars being sold than BESS deployed. There were over 600,000 global car sales in the first six months of 2018. The price of LiIon batteries came down, and the rest is history.

Now BESS is returning the favor. Both Gigafactories as well as gigasized BESS are pushing LiIon pricing down more rapidly. I covered many large BESS project in a paper posted in late June and linked below:

Also PG&E is planning several major projects per the recent article linked below.

Recent estimates of future light vehicle penetration can be seen in the chart below.

Most heavy-duty truck manufacturers (and Tesla) are offering or planning to offer full ranges of medium and heavy duty vehicles (including busses). See the paper I posted in February and linked below for more information.

If the emerging electric vehicles are charged with renewable electricity, this problem is moving towards a solution (in a few decades).

Additional challenges will come from the following sectors:

Air and express delivery services (a.k.a. expedited delivery services): In the short-term bio-fuels may provide a bridging low-carbon solution for jet aircraft. These already exists, and should reduce the carbon content and cost over time and with increased volume. These fuels are called Alcohol to Jet Synthetic Paraffinic Kerosene (ATJ-SPK). See the article linked below for more information.

In the longer-term (assuming no other breakthrough technology emerges), liquid hydrogen may hold the most promise. The U.S. military is already using this for long-duration drones. This will require at least a redesign of aircraft power plants, and probably the entire aircraft.

Passenger air transport: See comments in the above two paragraphs and link.

Freight (and passenger to a limited extent) rail: The solution here is simple, rail electrification, with no new technologies required. However the U.S. has about 140,000 route-miles of track, almost none of it currently electrified, this upgrade will probably require several $Trillian over many decades. Excluding light-rail commuter systems, there are only many fragmented segments on the east coast. Many of these segments are in long tunnels where diesels pollute too much to be safe. These lines require electro-diesel locomotives that can operate using either propulsion method. There are basically three variants of these, primarily electric, primarily diesel and full dual mode. In the U.S. these seem to be primarily diesel which operate with reduced performance on short electrified segments. [6]

The US is well behind other developed countries in electrifying their rail system and deploying high-speed rail (which requires electrification). See the figure below showing miles of high-speed rail track in operation, by country in 2018. [7]

The only substantial high-speed rail system under construction is the California High Speed Rail system. Phase 1 of this (see figure below) is currently scheduled to be completed in 2033. This initial phase will be more than 500 miles long. When the full system is completed, it will be approximately 800 miles long.7

Maritime: There is already an emerging technology to produce large boats with liquid or pressurized hydrogen feeding fuel cells. Go through the link below.

3.4.Cement Production

I will address this industry in a more thorough paper in the future. The following is a summary.

About 0.8 Gt of CO2 is emitted in creating process heat used in manufacturing, and 1.2 Gt is released by the chemical reaction that converts the limestone, clay, and other feed materials to clinker (basic Portland cement material).

3.5.Other Industries

The main industrial activities contributing CO2 include:

  • Non-energy use of fuels (mainly by chemical and petroleum industries)
  • Natural gas production
  • Municipal solid waste combustion
  • Lime manufacture

For other sources go through the link below.

In general each of these industries will need to either transition to processes that produce (much) less CO2, and/or sequester CO2 that is produced.

3.6.Residential and Commercial

These sectors are fairly easy to decarbonize over time (compared to industrial sectors). The main CO2 sources are related to comfort heating via heating oil or natural gas. In the short term biomethane or liquid fuel derived from bio-mass (or direct biomass combustion) can be used for heating. These provide a low-carbon alternative to ordinary natural gas and heating oil. Given several decades, heaters are reasonably easy to convert to electrically powered heat via heat pumps, resistance-heating and architectural improvements (very high insulation and high efficiency glazing to provide passive solar heating).

3.7.California SB-100

The subject bill, "The 100 Percent Clean Energy Act of 2017", was introduced, but not passed last year. This year it looks like it has a reasonable chance of passing. There is a link below to the full bill, but below is a brief summary.

Currently California's Renewables Portfolio Standard (RPS), requires all electric utilities to deliver 50% eligible renewable energy resources by the end of 2030. SB100 changes the 2030 goal to 60% renewables, and adds a RPS goal of 100% renewables by the end of 2045.

Note that "eligible renewable energy resources" does not necessary mean zero-carbon. The California Energy Commission's Renewables Portfolio Standard Eligibility Guidebook (linked below) provides that combustion of renewable fuels is eligible, and this is without CO2 sequestration. The most and largest eligible energy sources today and (especially) in the future (solar, wind and hydro) are near-zero-carbon. In 2017 34% of California's energy came from these three sources, and only 2% came from biomass combustion. Note that biomass combustion is also currently low carbon, and should evolve to near-zero-carbon in the future.

It should also be noted that the above linked guidebook, does not include large hydroelectric sources as eligible (> 30 MW per unit for most units, and 40 MW per unit for units "…operated as part of a water supply or conveyance system."). Incremental efficiency improvements in large hydro units can be eligible under certain conditions.

This guidebook also does not recognize nuclear generation's eligibility under any condition. There is only a single nuclear power plant in California (PG&E's Diablo Canyon Nuclear Power Plant), and it is scheduled for closure in 2025. This plant currently provides about 9% of California's power (2017).


[1] Tom Boden, Bob Andres, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory,

[2] Steven J. Davis, et al, Science, "Net-zero emissions energy systems", June 29, 2018,

[3] Environmental Defense Fund, " Major studies reveal 60 percent more methane emissions",

[4] Global Methane Initiative, "Global Methane Emissions and Mitigation Opportunities,

[5] Jeanette Fitzsimons. Coal Action Network Aotearoa, "Can We Make Steel Without Coal?",

[6] Wikipedia article on "Electro-diesel locomotive",

[7] California High Speed Rail Authority, 2018 Business Plan, June 1, 2018,

John Benson's picture

Thank John for the Post!

Energy Central contributors share their experience and insights for the benefit of other Members (like you). Please show them your appreciation by leaving a comment, 'liking' this post, or following this Member.


Stephane Bilodeau's picture
Stephane Bilodeau on July 16, 2018

Really Great Post and insights.  I definetely look forward for Part 2!

Robert Hargraves's picture
Robert Hargraves on July 17, 2018

John, I particularly like your concise, numeric summary of the problem. Transportation and metal-making technologies that emit little CO2 are also nicely summarized.  The idea of consuming wood (charcoal) for iron-making deforested England. We need to end the fantasy that buring biomass is CO2 neutral; growing forests is about the only economic method of removing CO2 from the atmosphere. A ten-ton tree has absorbed roughly ten tons of CO2.

Nuclear power is a reliable zero-emission source of elecdtric power. New liquid fission technologies mean unsubsidized fission power will be cheaper than untaxed fossil fuel power. Despite the hype about battery costs decreasing, the costs are an order of magnitude too high to compensate for intermittent sources such as wind and solar.



Get Published - Build a Following

The Energy Central Power Industry Network is based on one core idea - power industry professionals helping each other and advancing the industry by sharing and learning from each other.

If you have an experience or insight to share or have learned something from a conference or seminar, your peers and colleagues on Energy Central want to hear about it. It's also easy to share a link to an article you've liked or an industry resource that you think would be helpful.

                 Learn more about posting on Energy Central »