On the polluting pitfalls of burning wood for energy

By David Bezanson, Ph.D.

Many airborne toxic co-pollutants from biomass combustion are also greenhouse gasses (GHGs). At the federal level, the most commonly used lists of airborne toxics are those created by the EPA:

  1. Hazardous Air Pollutants
  2. Criteria Air Pollutants (PDF).

In California, the most commonly used list, Toxic Air Contaminants, was adopted by the California Air Resources Board (CARB) to replicate the HAP list. Because there are thousands of toxics that are not in any of these lists, this article does not limit its scope to any list. Better protection of public health would be achieved by expanding these lists and setting more stringent regulations. Toxic means any substance that has a toxic effect on biologic life. Per Merriam Webster Thesaurus, toxic means “containing or contaminated with a substance capable of injuring or killing a living thing.”

Let’s focus on one form of biomass energy: woody biomass. Wood is transformed into electricity—Voila! It might sound neat, but it’s actually dirty. In addition to dozens of toxic emissions, this combustion emits an amount of carbon dioxide (CO2) that equals or exceeds the amount of CO2/kWh from coal combustion. Most of the toxics in each table have no threshold, i.e., any amount of exposure causes harm.

Let’s take a look at what’s getting spewed into the air when woody biomass gets burnt at a biomass powerplant to make electricity:

Table 1. Toxic Emissions from Combustion of Woody Biomass

Toxic emissions and their 20 Year Global Warming Potential (GWP¹)

  • CO (carbon monoxide): 3 – 10
  • NOx (nitrogen oxides): 30 – 33
  • O3 (tropospheric ozone): 62 – 69 
  • CH4 (methane): 84
  • Brown carbon (part of PM²): up to 1,000
  • Black carbon  (part of PM): 2,500-4,400
  • VOCs (Volatile Organic Compounds): up to 84
  • Polycyclic Aromatic Hydrocarbons: N.A.
  • Formaldehyde, benzene, heavy metals (lead, mercury, etc.): N.A.³

Now, let’s look at the emissions released during the actual wood pellet manufacturing process.

Table 2. Toxic Emissions from Wood Pellet Manufacturing

Toxic emissions and their GWP⁴

  • CO: 3 – 10
  • Methyl chloride: 6
  • Methylene chloride: 11 
  • NOx: 30 – 33
  • O3 (tropospheric ozone): 62 – 69 
  • CH4: 84
  • Brown carbon (part of PM): up to  1,000
  • Black carbon (part of PM): 2,500 – 4,400
  • Volatile Organic Compounds: up to 84
  • Dust: extreme variance
  • Polycyclic Aromatic Hydrocarbons, formaldehyde, benzene, acrolein, methanol, dioxin, acetaldehyde, phenol, propionaldehyde, hydrochloric acid: All N.A.⁵

As you can see, each toxic with a known 20-year GWP listed in the tables above has a GWP that is far higher than that of CO2. You’ll also notice the extensive overlap between the mill toxics and the power plant toxics. Recently harvested woody biomass is delivered to mills and ordinarily left in piles outside. Precipitation collects within the piles and aerobic decomposition emits CH4. Nearly all toxics from the power plant table are also emitted from combustion of fossil fuels.

Well-known interactions occur amongst toxics that lead to the formation of other toxics or GHGs. For example, VOCs form O3 and secondary toxics. Little is known about the synergistic effects of simultaneous exposure to many toxics. Health impacts and GHG effects of each of the above toxics should be included in benefit/cost  and life cycle analyses. Research is warranted to determine the GHG impact of the sum of toxic co-pollutants separately from the GHG impact of the sum of CO2 emissions⁶.

Residents near mills report noticing dust within a half mile radius. Dust, composed of visibility reducing particles, often contains a wide diversity of toxics (such as pathogenic microbes, pesticides and heavy metals). The smallest particles are the most difficult to detect and the most deadly. Local residents also complain of noise pollution from the mill and trucks that serve it. It is likely that the trucks have combustion engines, which spew numerous toxins.

Wind-driven toxics may be carried many miles. This has been studied most extensively for PM, which can travel many hundreds of miles, cover glaciers and snow, and eradicate the albedo effect of frozen water. PM has been documented to cause more premature mortality than any other airborne pollutant⁷. 

Toxics are not merely an annoyance that makes the air look dirty or the eyes irritated. With sufficient exposure, they kill botanicals and animals while retarding the growth of plants. Some, like heavy metals and PM, can accumulate in human blood and organs—causing chronic daily damage for life. There is no safe way to remove PM. Many of the above toxics cause or exacerbate major chronic disorders that are the most common causes of death (asthma, cancer, and cardiovascular disorders). Damage from exposure often begins with congenital abnormalities, preterm births, and developmental disorders. 

CCS (carbon capture equipment) does not remove toxics from the atmosphere or smokestacks. Equipment has been invented to trap only a few kinds of toxics. The only kind that is commercially available, to my knowledge, traps some sizes of PM. Collectively, industrial smokestacks emit thousands of kinds of toxics. Instead of inventing thousands of kinds of capture equipment, it is more cost-effective to improve mitigation policies. 

  • David Bezanson, Ph.D. is a member of Climate Action California’s Energy Team.
THE WOOD FOR THE TREES

Read all about a proposed biomass energy initiative in California—and the efforts underway to stop it.

  • Wood Pellet Giant Drax Targets California Forests (DeSmog ~ Mar 2024): “Plans for two industrial pellet plants would increase carbon emissions and hurt the health of rural communities, campaigners warn.”
  • An environmental tragedy is unfolding 50 miles south of Sacramento. There’s time to act (Sacramento Bee ~ Aug 2024): “An environmental justice tragedy is unfolding in South Stockton and its historically underserved communities of color, 50 miles south of Sacramento along the San Joaquin River. This story has all the classic features: corporate greenwashing, sham community engagement and a dubious industry poised to make a lot of cold hard cash. But what’s unique about the situation? There is still time to act.”
  • THE ECONEWS REPORT: Proposed Wood Pellet Biofuel Project Draws Criticism (Lost Coast Outpost ~ Nov 2024): “Should we turn California trees into wood pellets to be burned in foreign power plants? That’s the proposal being brought forward by Golden State Natural Resources (GSNR), a nonprofit organization formed by Rural County Representatives of California and Golden State Finance Authority. GSNR has just released their draft environmental impact report for the project, which proposed two wood pellet factories (one in Lassen County and another in Tuolumne) that will draw biomass from roughly a 100 mile radius around the plant. Those factories will turn woody biomass into pellets, which will be shipped by rail to Stockton where the pellets will be loaded onto ocean-going ships to be delivered, likely to foreign power plants where they will be burned for energy.”
  • Momentum builds to stop big biomass in California (NRDC ~ Jan 2025): “Nearly 50,000 people, elected officials, and over 185 organizations call on California to reject GSNR’s wood pellet production and export project.”
  1. The Global Warming Potential (GWP) of CO2, the most prevalent GHG, is 1.

2. PM = particulate matter. For more, see The Particulars of PM 2.5 (NRDC): “Why particulate matter…matters.”

3. N.A. means data not available or the atmospheric life is too brief to be considered a significant GHG. Those marked N.A. may have a significant radiative forcing effect, but this may last for only a few days. If emissions are occurring daily or at least five days per week, this could collectively sustain significant warming impacts. This is of great concern given that biomass power plants and mills ordinarily operate on a daily basis.

4. All these figures are for a GWP over 20 years with the exception of methyl chloride and methylene chloride where the GWP is for 100 years.

5. See footnote 3 above.

6. For more on this see:

7. For more on this see Particulate Matter and Premature Mortality: A Bayesian Meta-Analysis – PMC and the World Health Organization’s Air pollution is responsible for 6.7 million premature deaths every year.