James J. Jetter

This study examines three representative semigasifier cookstove models each burning four types of pelletized-biomass fuel (hardwood, peanut hull, rice husk, and wheat straw) using the International Organization for Standardization (ISO) 19867-1:2018 protocol. ISO tier ratings for fine particulate matter (PM_2.5) and carbon monoxide (CO) emissions ranged 1-4 and 2-5 (where 5 = cleanest), respectively, suggesting that pellet-fueled cookstoves may provide substantial emissions reductions, dependent upon stove/fuel matching and operation, over other biomass-fueled cooking alternatives. PM_2.5 emission factors based on useful energy delivered (EF_d) varied by up to 25-fold, and organic and elemental carbon (OC and EC) EF_d values respectively varied by >200- and ∼100-fold, reflecting complex variability in PM_2.5 composition. These semigasifier cookstoves showed higher ultrafine particle (UFP) emissions but lower bulk PM_2.5 emissions. Operation of pellet-fueled cookstoves at higher firepower resulted in higher PM_2.5 and UFP emission factors and higher EC-to-total-carbon ratios; operation at lower firepower resulted in higher gaseous pollutant emission factors. Results of this work provide technical guidance for stove developers, users, and policy-makers. These ISO-protocol-based emission factors are also pertinent to health and climate modeling efforts.

In 2018, the International Organization for Standardization (ISO) 19867-1 “Harmonized laboratory test protocols” were released for establishing improved quality and comparability for data on cookstove air pollutant emissions, efficiency, safety, and durability. This is the first study that compares emissions [carbon dioxide, carbon monoxide, total hydrocarbons, methane, nitrogen oxides, fine particulate matter (PM_2.5), organic carbon, elemental carbon, and ultrafine particles] and efficiency data between the ISO protocol and the Water Boiling Test (WBT). The study examines six stove/fuel combinations [liquefied petroleum gas (LPG), pellet, wood fan, wood rocket, three stone fire, and charcoal] tested in the same US EPA laboratory. Evaluation of the ISO protocol shows improvements over previous test protocols and that results are relatively consistent with former WBT data in terms of tier ratings for emissions and efficiency, as defined by the ISO 19867-3 “Voluntary Performance Targets.” Most stove types remain similarly ranked using ISO and WBT protocols, except charcoal and LPG are in higher PM_2.5 tiers with the ISO protocol. Additionally, emissions data including polycyclic aromatic hydrocarbons are utilized to compare between the ISO and Firepower Sweep Test (FST) protocols. Compared to the FST, the ISO protocol results in generally higher PM_2.5 tier ratings.

Liquefied petroleum gas (LPG) cookstoves are considered to be an important solution for mitigating household air pollution; however, their performance has rarely been evaluated. To fill the data and knowledge gaps in this important area, 89 laboratory tests were conducted to quantify efficiencies and pollutant emissions from five commercially available household LPG stoves under different burning conditions. The mean thermal efficiency (±standard deviation) for the tested LPG cookstoves was 51 ± 6%, meeting guidelines for the highest tier level (Tier 4) under the International Organization for Standardization, International Workshop Agreement 11. Emission factors of CO₂, CO, THC, CH₄, and NO_x on the basis of useful energy delivered (MJ_d) were 142 ± 17, 0.77 ± 0.55, 130 ± 196, 5.6 ± 8.2, and 46 ± 9 mg/MJ_d, respectively. Approximately 90% of the PM_2.5 data were below the detection limit, corresponding to an emission rate below 0.11 mg/min. For those data above the detection limit, the average emission factor was 2.4 ± 1.6 mg/MJ_d, with a mean emission rate of 0.20 ± 0.16 mg/min. Under the specified gas pressure (2.8 kPa), but with the burner control set to minimum air flow rate, less complete combustion resulted in a visually yellow flame, and CO, PM_2.5, EC, and BC emissions all increased. LPG cookstoves met guidelines for Tier 4 for both CO and PM_2.5 emissions and mostly met the World Health Organization Emission Rate Targets set to protect human health.

Ultrafine particle (UFP) emissions and particle number size distributions (PNSD) are critical in the evaluation of air pollution impacts; however, data on UFP number emissions from cookstoves, which are a major source of many pollutants, are limited. In this study, 11 fuel-stove combinations covering a variety of fuels and different stoves are investigated for UFP emissions and PNSD. The combustion of LPG and alcohol (∼10¹¹ particles per useful energy delivered, particles/MJ_d), and kerosene (∼10¹³ particles/MJ_d), produced emissions that were lower by 2–3 orders of magnitude than solid fuels (10¹⁴–10¹⁵ particles/MJ_d). Three different PNSD types—unimodal distributions with peaks ∼30–40 nm, unimodal distributions with peaks F₃₀) varied among the tested systems, ranging from 13% to 88%. The burning of LPG and alcohol had the lowest PM_2.5 mass emissions, UFP number emissions, and F₃₀ (13–21% for LPG and 35–41% for alcohol). Emissions of PM_2.5 and UFP from kerosene were also low compared with solid fuel burning but had a relatively high F₃₀ value of approximately 73–80%.

This study measures polycyclic aromatic hydrocarbon (PAH) compositions in particulate matter emissions from residential cookstoves. A variety of fuel and cookstove combinations are investigated, including: (i) liquid petroleum gas (LPG), (ii) kerosene in a wick stove, (iii) wood (10 and 30% moisture content on a wet basis) in a forced-draft fan stove, and (iv) wood in a natural-draft rocket cookstove. The wood burning in the natural-draft stove had the highest PAH emissions followed by the wood combustion in the forced-draft stove and kerosene burning. LPG combustion has the highest thermal efficiency (∼57%) and the lowest PAH emissions per unit fuel energy, resulting in the lowest PAH emissions per useful energy delivered (in the unit of megajoule delivered, MJ_d). Compared with the wood combustion emissions, LPG burning also emits a lower fraction of higher molecular weight PAHs. In rural regions where LPG and kerosene are unavailable or unaffordable, the forced-draft fan stove is expected to be an alternative because its benzo[a]pyrene (B[a]P) emission factor (5.17–8.24 μg B[a]P/MJ_d) and emission rate (0.522–0.583 μg B[a]P/min) are similar to those of kerosene burning (5.36 μg B[a]P/MJ_d and 0.452 μg B[a]P/min). Relatively large PAH emission variability for LPG suggests a need for additional future tests to identify the major factors influencing these combustion emissions. These future tests should also account for different LPG fuel formulations and stove burner types.