How it works
How Waste-to-Energy Gasification Works
Gasification heats municipal trash with limited oxygen to make a synthetic gas, which is then burned to raise steam and generate electricity. Because that gas is ultimately combusted, European Union law defines these plants as waste incineration and applies incineration emission limits, including limits for dioxins and furans1 — a pollutant the International Agency for Research on Cancer classifies as a known human carcinogen8; independent technical reviewers classify municipal-waste gasification and pyrolysis as “high risk, low yield” and note they are regulated as incinerators in both the U.S. and the EU.14
This page explains, with every technical claim anchored to a peer-reviewed study or a primary regulatory record, what municipal-solid-waste (MSW) gasification is, the three stages the proposed Plainfield facility would use, why gasification and incineration converge at the point of combustion, what residue streams the process leaves behind, and what the scientific and federal-agency record documents about those residues and about the commercial track record of these plants. It describes the class of technology and the published evidence, not a prediction about any single site. The detailed pollutant hazard classifications and the fine-particulate (PM2.5) mortality evidence, including the dose-response chart, are set out on the Health page.
The technology
What Gasification Is
Incineration burns waste with abundant oxygen, fully oxidising it to heat, ash and flue gas. Gasification instead heats the waste in an oxygen-starved chamber so that, rather than burning outright, it breaks down into a combustible synthesis gas, or syngas; in a waste-to-energy configuration that syngas is then combusted for energy. Independent technical review describes the class plainly: these processes “attempt to convert solid waste into synthetic gas or oils, followed by combustion,” and in the U.S. and EU “they are regulated as waste incinerators.”14 The electricity comes from combustion at the end of the line, not from the gasification step itself.
The technology the developer proposes is set out in SMART Technology Systems’ own response to CT DEEP’s Materials Management Infrastructure Request for Information, filed on the state regulatory record: it describes replacing older “mass-burn” plants with a package that recycles metals and glass, prepares a refuse-derived fuel, uses “gasification technology” to convert that fuel into a synthesis gas burned in a steam cycle, adds an anaerobic digester for the separated organics, and claims carbon-capture and Class I renewable-energy benefits — the developer’s stated technology, in its own words on the DEEP record.2 As reported in the press, the developer describes an 81-acre facility in a residential zone using a gasification system, an anaerobic digester, a boiler and a steam turbine.22
The project’s scale figures are the developer’s own, stated in its filings with CT DEEP and reported in the public record: an official DEEP environmental-justice filing for the Norwich Road / Black Hill Road site places the proposed facility in Plainfield and under DEEP’s environmental-justice review,3 and the developer’s stated throughput, as reported, is roughly 1,800 tons of solid waste a day — up to about 468,000 tons a year, some 9,000 tons a week — generating about 45 megawatts of electricity, with operation no sooner than 2028.2223 These are the developer’s figures, not independent findings; the technical and environmental claims below rest on the peer-reviewed and agency sources cited.
As a waste-processing facility, the project would require a solid waste facility permit from CT DEEP under Conn. Gen. Stat. §22a-208a and is subject to the environmental-justice public-participation requirements of §22a-20a, a process DEEP has opened for this site;35 an electric-generating facility of this size may also fall under Connecticut Siting Council review under §16-50i et seq.5 It remains a proposal under active review by state agencies; no final permit decision has been issued.4
Step by step
The Three Stages
As described in the developer’s own DEEP filing and in public reporting, the Plainfield process would run in three stages.223 The chemistry of each stage — combustion of a syngas, and the residue streams it produces — is what the scientific sources further down address.
| Stage | What is described | Output stream |
|---|---|---|
| 1. Sorting | A bulk-handling system separates recyclable material and organic residue and removes hazardous material from the incoming trash, preparing a refuse-derived fuel. | Refuse-derived fuel (RDF); separated organics; rejected residue. |
| 2. Gasification | A gasification unit heats the prepared refuse-derived fuel with limited oxygen to convert it into a synthetic gas, which is then burned in a boiler to raise steam for a turbine. | Syngas (combusted for electricity); bottom ash and slag; fly ash captured in gas cleaning. |
| 3. Anaerobic digestion | The separated organic material is sent to an anaerobic digester, which captures biogas through fermentation of the organics. | Biogas; digestate; process wastewater. |
Two of the three stages end in combustion or biological breakdown, and each stage leaves a solid or liquid residue that must be managed. Those residues, not the electricity, are where the measurable environmental questions concentrate.
The distinction
How It Differs From Incineration — and Where They Converge
The developer’s framing stresses that gasification is not mass-burn incineration, and on the first step that is accurate: the initial thermal stage runs oxygen-starved and produces a gas rather than immediately burning the waste to ash.2 The distinction narrows to the point of the process where the syngas is combusted to produce energy.
European Union law makes the convergence explicit and is the strongest primary authority on the question. Under the Industrial Emissions Directive (2010/75/EU), the definition of a “waste incineration plant” expressly covers “the thermal treatment of waste … through the incineration by oxidation of waste as well as other thermal treatment processes, such as pyrolysis, gasification or plasma process, if the substances resulting from the treatment are subsequently incinerated.”1 Because a waste-to-energy gasifier burns its syngas to make electricity, it falls inside that legal definition, and the directive’s incineration emission limits — including limit values for dioxins and furans — apply.1
Those dioxin limits exist because the pollutant is extraordinarily potent. The International Agency for Research on Cancer classifies 2,3,7,8-TCDD — the reference dioxin — as Group 1, carcinogenic to humans, its highest certainty tier, and describes it as an unusually potent multi-site carcinogen.8 The U.S. EPA sets an oral reference dose for TCDD of 7 × 10−10 mg per kilogram of body weight per day — among the lowest, and therefore most potent, values in its entire Integrated Risk Information System.11 Because dioxins occur as mixtures, the World Health Organization weights them against that benchmark using a toxic-equivalency (TEQ) framework, the same system regulators apply.10 The same class of dioxin is already documented in municipal-waste ash, discussed in the residue section below; the full hazard classifications and the human-health evidence are on the Health page.
Combustion of the syngas also releases carbon dioxide. By the U.S. EPA’s own accounting for municipal-solid-waste combustion, roughly 47 percent of the energy produced is fossil-derived (about 53 percent biogenic), corresponding to a fossil emission factor near 1,016 pounds of CO2 per megawatt-hour — which is why a plant of this type is partly a fossil-fuel source, not a fully renewable one.12
An independent technical review by the Global Alliance for Incinerator Alternatives reaches the same practical conclusion from the operating record: it describes waste gasification and pyrolysis as processes “followed by combustion” that are “regulated as waste incinerators” in the U.S. and EU, and titles the technology “high risk, low yield.”14 The point is not that gasification and incineration are identical in every step, but that the “it’s not incineration” framing does not, by itself, resolve the emission and residue questions, because the syngas is burned.
The residues
What Comes Out: Ash, Slag, Flue Gas and Process Water
Thermal treatment of mixed municipal waste produces residue streams beyond the electricity: fly ash captured from the gas, bottom ash and slag from the chamber, flue gas out the stack, and process wastewater from gas cleaning and the digester. The peer-reviewed literature documents, with measured values, what these streams typically carry — and, for PFAS “forever chemicals,” that combustion does not eliminate them.
| Residue stream | What the peer-reviewed record documents |
|---|---|
| Fly ash | Peer-reviewed leachate testing of MSW-incineration fly ash measured leachable heavy metals — cadmium, chromium, lead, copper and zinc — and found fly ash “often considered hazardous because of its generally higher concentrations of heavy metals,” with cadmium and chromium exceeding regulatory limits in the tested samples.15 PFAS is not fully eliminated: a 2021 study of waste-incineration plants measured residual PFAS in fly ash at a mean of 16.4 nanograms per gram.16 |
| Bottom ash / slag | The same peer-reviewed study measured leachable heavy metals in bottom-ash leachate as well as fly ash.15 Residual PFAS was measured in bottom ash at a mean of 14.6 nanograms per gram.16 The heavy metals that recur in this ash — arsenic, cadmium, chromium, lead — are the same ones documented across the national coal-ash monitoring record.20 |
| Flue gas (stack) | A 2023 study provided the first confirmed measurement of PFAS in the flue gas of a waste-to-energy plant: 4.0 to 5.6 nanograms per cubic metre under normal operation, rising to about 27 with sludge co-firing, with total plant releases of roughly 7 to 20 grams per year (up to 56 with sludge). PFAS was also found in bottom ash, air-pollution-control residue and treated process water — meaning it leaves via the stack, not only in ash and water.17 |
| Process wastewater / leachate | The 2021 incineration study found PFAS concentrated in leachate at a mean of about 215 nanograms per millilitre — more than ten thousand times the per-gram level in the ash — meaning PFAS partitions into water and is released rather than destroyed.16 Separately, wastewater from coal-gasification processes is documented in the peer-reviewed record to carry phenols, benzene and other BTEX compounds, ammonia, cyanide, arsenic and polycyclic aromatic hydrocarbons (PAHs).19 |
Whether combustion destroys PFAS at all depends on temperature. A 2022 critical review found that PFOA and PFOS begin to break down at 350–450 °C but that full mineralization requires at least about 1,000 °C; below that threshold, products of incomplete combustion of unknown toxicity form, and PFAS and those byproducts have been recovered from combustor ash. Typical municipal-waste energy-recovery furnaces operate below 1,000 °C.18 Taken with the flue-gas and leachate measurements, this is why the peer-reviewed record does not support a claim that thermal treatment eliminates PFAS: it redistributes the chemicals into ash, stack gas and water.
The hazard of the heavy metals is not in dispute at the agency level: IARC classifies arsenic and cadmium, and their compounds, as Group 1, carcinogenic to humans — the same top tier as dioxin.9 The common thread across every measurement is that the pollutants of concern are not consumed by the process; they are concentrated and shifted into solids, stack gas and water that must then be stored, treated or landfilled. Where those residues are not fully contained, the federal record below shows they can migrate to soil and groundwater. Plainfield’s specific water setting, and where ash from Connecticut trash plants is disposed, are covered on the Water & Land page; the detailed toxicology and the PM2.5 health chart are on the Health page.
The record
What the Federal and Scientific Record Shows
Coal-gasification and manufactured-gas plants are the longest-running real-world examples of gasifying carbon-rich feedstock, and their federal Superfund files document how persistent the residues can be — in two cases, more than seventy years after the plants closed.
| Site | Operated | What the federal record documents |
|---|---|---|
| Waterloo, Iowa — coal gasification plant | 1901–1956 | Coal tar, PAHs, benzene and other BTEX, cyanide, arsenic, phenols and metals in soil and groundwater. EPA “determined that it was not feasible to clean up all of the groundwater contamination,” designating a “technical impracticability zone” where the groundwater “is expected to remain contaminated for the foreseeable future.”6 |
| Mason City, Iowa — coal gasification plant | 1900–1951 | PAHs, BTEX and coal-tar DNAPL in groundwater. The April 2023 EPA five-year review found that in the deeper (intermediate) aquifer, “levels of benzene and benzo(a)anthracene were increasing at some locations” — roughly seventy years after the plant closed.7 |
The pattern is not confined to old gas plants. An analysis of the federal monitoring data required under the 2015 coal-ash rule found that groundwater near 242 of 265 U.S. coal plants with data — about 91 percent — contained unsafe levels of at least one ash-related pollutant, with arsenic exceeding safe levels at 52 percent of sites and lithium at 60 percent.20 The heavy metals driving that pattern — arsenic, cadmium, chromium, lead — are the same metals measured as leachable in municipal-waste incineration and gasification ash.15
Commercial history
The Commercial Track Record
Beyond emissions, MSW gasification has a documented history of technical and financial failure at commercial scale. The Global Alliance for Incinerator Alternatives review concludes that “there are numerous examples of plants that have been forced to shut down due to technical failures and financial failures,” and titles the technology “high risk, low yield.”14
The failures are named, dated and costly. A formal Audit Scotland review of the Caithness Heat and Power project concluded it “failed because the company procured ‘experimental’ and high risk gasification technology which could not be commissioned successfully,” at a cost to the council of about £13.8 million.13 The largest documented loss came when Air Products, a Fortune 500 company, abandoned two plasma-gasification plants at Tees Valley in England in 2016 after a pre-tax charge of roughly $900 million to $1 billion.25 In the United Kingdom, a briefing by the UK Without Incineration Network catalogued more than a dozen failed gasification projects, and linked the developer New Earth Solutions alone to six abandoned gasification schemes.21 One of them, a 60,000-tonne-a-year plant proposed at Easter Langlee near Galashiels, Scotland, was scrapped in 2015 after the council terminated its contract, forcing a write-off of about £2.4 million spent on the procurement.24 These and other named, dated commercial and financial failures are collected on the Defeated Projects page. Repeated failures like these are why a demonstrated commercial and technical track record, not just a permit application, is a fair thing to ask of any new proposal.
Questions and answers
Common Questions
Is gasification the same as incineration?
Not in the first step: gasification heats waste with limited oxygen to make a synthetic gas rather than burning it outright.2 But in a waste-to-energy plant the syngas is then burned to generate electricity, so European Union law defines such plants as waste incineration and applies the same emission limits, including limit values for dioxins and furans.1
Why do dioxin emission limits matter?
Because dioxin is one of the most potent pollutants regulators track. IARC classifies 2,3,7,8-TCDD as Group 1, carcinogenic to humans, and the U.S. EPA sets one of the lowest reference doses in its entire database for it (7 × 10−10 mg/kg/day).811 That same class of dioxin is already documented in municipal-waste ash. The full hazard classifications and the PM2.5 health evidence are on the Health page.
What are the three stages of the proposed Plainfield process?
Sorting the incoming trash into refuse-derived fuel and separated organics; gasifying the refuse-derived fuel into a syngas that is burned for electricity; and anaerobic digestion of the separated organics to capture biogas.223
What byproducts does the process leave behind?
Fly ash, bottom ash and slag, flue gas and process wastewater. Peer-reviewed testing of municipal-waste ash documents leachable heavy metals such as cadmium, chromium and lead, and finds fly ash is often classified as hazardous waste.15
Does gasification destroy PFAS “forever chemicals”?
Not fully. A 2021 peer-reviewed study of waste-incineration plants measured residual PFAS in fly ash (mean 16.4 ng/g), bottom ash (mean 14.6 ng/g) and much higher levels in leachate (mean about 215 ng/mL); a 2023 study made the first confirmed measurement of PFAS in a waste-to-energy plant’s flue gas.1617 A 2022 review found full mineralization needs at least about 1,000 °C, a threshold typical municipal-waste furnaces run below, so the chemicals redistribute into ash, stack gas and water rather than being eliminated.18
Do MSW gasification plants work reliably at commercial scale?
The record is poor. A government audit found one project’s gasification technology “could not be commissioned successfully,” and independent reviews and expert briefings document numerous MSW gasification and pyrolysis projects worldwide that have shut down or been abandoned for technical and financial reasons.131421 See the Defeated Projects page for the named cases.