The Effects of Air Flow Rates, Secondary Air Inlet Geometry, Fuel Type

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The effects of air flow rates, secondary air inlet geometry, fuel type, and operating mode on the performance of gasifier cookstoves Jessica Tryner, James Tillotson, Marc E. Baumgardner, Jeffrey Mohr, Morgan DeFoort, and Anthony J Marchese Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00440 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

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Environmental Science & Technology

The effects of air flow rates, secondary air inlet geometry, fuel type, and operating mode on the performance of gasifier cookstoves Jessica Tryner,† James W. Tillotson,† Marc E. Baumgardner,‡ Jeffrey T. Mohr,† Morgan W. DeFoort,¶ and Anthony J. Marchese∗,† 1

†Department of Mechanical Engineering, Colorado State University, 1374 Campus Delivery, Fort Collins, CO 80523-1374 ‡Department of Mechanical Engineering, Gonzaga University, 502 E. Boone Ave., AD Box 26, Spokane, WA 99258 ¶Factor(E) Ventures, 430 N. College Ave., Fort Collins, CO 80524 E-mail: [email protected] Phone: (970) 491-2328. Fax: (970) 491-3827

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Abstract

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Development of biomass cookstoves that reduce emissions of CO and PM2.5 by more

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than 50% and 95%, respectively, compared to a three-stone fire has been promoted as

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part of efforts to reduce exposure to household air pollution (HAP) among people that

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cook with solid fuels. Gasifier cookstoves have attracted interest because some have

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been shown to emit less CO and PM2.5 than other designs. A laboratory test bed and

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new test procedure were used to investigate the influence of air flow rates, stove geom-

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etry, fuel type, and operating mode on gasifier cookstove performance. Power output,

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CO emissions, PM2.5 emissions, fuel consumption rates, producer gas composition,

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and fuel bed temperatures were measured. The test bed emitted < 41 mg·MJ−1 d PM2.5

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and < 8 g·MJ−1 d CO when operating normally with certain prepared fuels, but order

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of magnitude increases in emission factors were observed for other fuels and during

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refueling. Changes in operating mode and fuel type also affected the composition of

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the producer gas entering the secondary combustion zone. Overall, the results suggest

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that the effects of fuel type and operator behavior on emissions need to be considered,

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in addition to cookstove design, as part of efforts to reduce exposure to HAP.

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Introduction

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An estimated 2.8 billion people worldwide rely on solid fuels for cooking 1 and, as a re-

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sult, face a range of health risks associated with household air pollution (HAP). 2–4 Emis-

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sions targets designed to reduce HAP have been established for solid-fuel cookstoves, 5,6

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and questions remain as to whether efforts to reduce HAP should focus on improving ac-

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cess to clean fuels (e.g., LPG and electricity), disseminating improved solid-fuel cookstoves,

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or both. 6–8 Development of “advanced combustion stoves” 8 that result in large reductions

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in carbon monoxide (CO) and particulate matter (PM) emissions compared to a baseline

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three-stone fire has been encouraged, and several discussions 8–10 have hypothesized that top-

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lit up draft (TLUD) gasifier cookstoves have the potential to achieve the low emissions that

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are sought. Cookstoves that utilize the TLUD design, 11,12 in which solid fuel is gasified and

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the resulting producer gas burns in a secondary combustion zone, have attracted interest

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because they often emit less CO and PM than other biomass cookstove designs in both lab-

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oratory 13–15 and field 16–18 settings. TLUDs have been marketed and studied extensively in

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India 10,16–22 and China, 23,24 but there are also reports of TLUDs being marketed elsewhere

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in Asia 12,25–27 as well as in Africa, Central America, and South America. 12,26

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A TLUD is designed to operate as inverted downdraft gasifier. 11 In a stratified downdraft

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gasifier, primary air first passes through unreacted solid biomass before oxidizing the volatiles

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released from the biomass in the flaming pyrolysis zone. The heat released by oxidation 2 ACS Paragon Plus Environment

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reactions drives the pyrolysis and drying of the upstream biomass. The products leaving the

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flaming pyrolysis zone are reduced when they pass through the downstream char layer. 28

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As a result of the presence of oxidizer and high temperatures in the flaming pyrolysis zone,

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most of the tars released during pyrolysis are consumed and the final tar concentration in

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the producer gas is low. 29 In a conventional updraft gasifier, on the other hand, primary air

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reacts with char in the oxidation zone at the bottom of the fuel bed. Once the oxygen in

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the primary air has been consumed, products of the oxidation reactions are reduced as they

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pass through the remainder of the char layer. Hot gases leaving the char layer then pyrolyze

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and dry the unreacted solid fuel as they flow up through the fuel bed. 28 Because the gases

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and tars released during pyrolysis only pass through low-temperature sections of the fuel

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bed where no oxidizer is present, the final tar concentration in the producer gas is high. 29

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Despite the low emissions frequently observed with the TLUD design, some TLUDs

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have been shown to emit more CO and PM2.5 than a three-stone fire. 15 In a recent study,

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emissions from natural-draft TLUDs were found to be affected by fuel type and, in some

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cases, dominated by large, transient increases in emission rates. 30 Increases in emission rates

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have been observed when stoves transitioned from gasifying volatile biomass to burning the

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char left behind by the gasification process 21,30–34 and when batch-fed stoves were refueled

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by adding volatile fuel on top of a hot char bed. 30,35 Emissions associated with refueling are

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important because real-world users have been found to refuel TLUDs in this manner. 27

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Given the promising yet variable nature of TLUD performance, questions remain as to:

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(1) whether or not TLUDs can meet the Tier 4 performance targets outlined in the ISO

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IWA tiers 5 and (2) what must be done, in terms of cookstove design and dissemination, to

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achieve consistently low emissions. Accordingly, the objectives of the present study were

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to characterize how stove design parameters and fuel properties, as well as refueling and

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transition to char burnout, affect TLUD operation and performance.

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This study expands upon previous work in which the effects of fuel type, 21,30,32 fuel mois-

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ture content, 24,36 fuel particle size, 32,36–38 primary air flow rate, 31,33,34,39 ratio of secondary



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to primary air flow, 21,33,39 secondary air inlet area, 21,40 chimney sections, 38 and operating

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mode 23,32 on gasifier cookstove performance were investigated experimentally. A comprehen-

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sive evaluation of performance, including measurements of CO emissions, PM2.5 emissions,

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useful power output, producer gas composition, and fuel bed temperatures, was completed.

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As noted previously, existing test protocols such as the Water Boiling Test 41 (WBT)

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are not well suited for batch-fed cookstove designs 42 and may fail to capture the range

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of operating modes and emission rates observed in the field. 23,27 Consequently, a new test

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procedure was developed to evaluate the range of performance that can be observed with

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batch-fed TLUDs in various modes of operation including refueling and transition to char

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burnout.

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Experimental

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A description of the test bed used to vary cookstove design parameters is provided below,

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followed by descriptions of the test matrices, the test procedure used to evaluate performance,

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and the equations used to calculate fuel consumption rate, fuel gas production rate, and

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global equivalence ratio in the secondary combustion zone.

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Modular Test Bed

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The modular TLUD test bed, shown in Figure 1, was constructed from stainless steel (SS)

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and is similar to those used in previous studies. 31,32,39 Primary and secondary air flows were

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driven by compressed air, which was regulated to 276 kPa, and flow rates were controlled

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using rotameters. Secondary air was preheated by two electric heaters connected to PID

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controllers. These features allowed the primary air flow rate, secondary air flow rate, and

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secondary air temperatures to be set to known values so that: (1) the influence of these

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parameters on performance could be investigated and (2) the influence of other design pa-

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rameters could be investigated independently of the air flow rates.


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The ring through which secondary air entered the combustion chamber was removable

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so that the inlet geometry could be varied. Secondary air inlets that featured 32 holes

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ranging from 2 to 10 mm in diameter were tested. Inlets that featured 4-mm-diameter holes

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positioned at angles to induce swirl in the horizontal plane or direct the secondary air down

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towards the fuel bed were also tested (see Figure S1, SI Section S1.1). During relevant

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experiments, the arms that supported the pot were adjusted to change the distance between

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the top of the test bed and the pot, constrictions intended to recirculate gases inside the

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stove were inserted before and after the secondary air inlet (see Figure S2, SI Section S1.1),

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and an insulated chimney with a diameter of 108 mm and a height of 100 mm 43 was installed

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downstream of the secondary air inlet. Another assembly allowed 10% of the secondary air

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to be injected earlier in the fuel chamber through six 2-mm-diameter holes spaced equally

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around the circumference (see Figure S3, SI Section S1.1). Figure 1: A photograph and a cross-section of the modular TLUD test bed with key features labeled.

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The test bed was mounted on a digital balance that recorded the mass of the system at

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a rate of 1 Hz. Thermocouples installed along the height of the fuel chamber tracked the

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progression of the primary combustion zone. Gas entering the secondary combustion zone

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was sampled for compositional analysis via a probe installed between the fuel bed and the

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secondary air inlet. Several additional thermocouples were installed throughout the test bed,

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and temperature data from all thermocouples were logged at 1 Hz. For information on the

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thermocouples used and their locations, see Figure S4 and Table S1, SI Section S1.1.

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Test Matrix

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Experiments were conducted using the two test matrices shown in Table 1. For each test, one

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parameter was varied and all other parameters were held constant at the default values shown

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in bold. Together, the parameters shown in bold represent the “baseline”configuration. For



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more information on the baseline geometry and justifications for the default values see SI

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Section S1.2. Table 1: The two test matrices for which experiments were completed. Baseline configuration data are indicated in bold. Matrix 1 — Secondary air delivery parameters Parameter Values tested Parameter Values tested Secondary to 2 Secondary air 4-mm-dia. holes primary air 3 opening 6-mm-dia. holes flow ratio 4 8-mm-dia. holes (mass basis) 5 10-mm-dia. holes Secondary air 0◦ Secondary air 0◦ swirl angle 15◦ downward 10◦ ◦ 30 angle 20◦ 45◦ 30◦ Pot gap 15 mm Constriction None 30 mm location Before secondary air inlet, 45 mm 2.50:4.25 ratio After secondary air inlet, 2.50:4.25 ratio ◦ Secondary air 100 C temperature 150 ◦ C 200 ◦ C 250 ◦ C 300 ◦ C Matrix 2 Primary air flow rates, fuel properties, and secondary air delivery parameters Parameter Values tested Parameter Values tested Primary air 15 Fuel moisture 0 flow rate 20 content 7 −1 (g·min ) 25 (weight % on 15 30 wet basis) 25 Fuel type Corn cob chips Fuel bulk 174 Eucalyptus chips density (corn 137 Douglas fir chips cobs) (kg·m−3 ) 126 Lodgepole pine pellets Secondary air 4-mm-dia. holes delivery 2-mm-dia. holes parameters Insulated chimney Early secondary air injection

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For the first test matrix, CO emissions, useful power output, fuel consumption rate, and


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mass loss rate were measured while secondary air delivery parameters were varied. Douglas

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fir (Pseudotsuga menziesii ) chips with a moisture content of 7% on a wet basis and a bulk

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density of 156 kg·m−3 (95% CI = 155–157) were used as the fuel and the primary air flow

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rate was set to 25 g·min−1 .

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For the second test matrix, CO and PM2.5 emissions, useful power output, producer gas

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composition, fuel consumption rate, and mass loss rate were measured while primary air

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flow rate, fuel properties, and secondary air delivery parameters were varied. The Douglas

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fir chips used as the default fuel had a bulk density of 160 kg·m−3 (95% CI = 159–161)

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and were more uniform in size and shape than the chips used in the first matrix (see Figure

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S5, SI Section S1.2). The “corn cob chips”and “174 kg·m−3 bulk density”cases shown in

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italics in Table 1 were identical. A complete list of test cases, and test parameter values for

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each case, can be found in Tables S2 and S3, SI Section S1.2. For detailed fuel properties,

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including the results of proximate and ultimate analyses, biochemical compositions, heating

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values, densities, porosities, and moisture contents, see Tables S5 and S6, SI Section S1.2.

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Test Procedure

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The test procedure that was developed to capture three characteristic operating modes (nor-

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mal, post-refueling, and char burnout) is represented graphically in Figure 2. During the first

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phase (normal operation), the test bed started at room temperature and the fuel chamber

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was filled with 1.5 L of biomass. To ignite the fuel bed, 50 mL of biomass was wetted with 2

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mL of kerosene, placed on top of the fuel bed, and lit with a match. A SS pot containing 2.5

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L of water at 15 ± 2 ◦ C was placed on the pot support, and emissions were measured while

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the water was heated to 90 ◦ C. This volume of water (2.5 L) was selected to ensure that

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a pot of water could be boiled using the lower bulk density fuels. This phase was similar

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to the cold start phase of the WBT 41 and Emissions and Performance Test Protocol. 44,45

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As shown in previous studies 27,30 and the results below, the emissions measured during this

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phase represented the best-case scenario for high-power performance. 7 ACS Paragon Plus Environment


Figure 2: Plot of typical CO emissions (mg·s−1 ) and water temperature (◦ C) as a function of time for the three phase test procedure (normal operation, post-refueling, and char burnout). The CO emission rates shown on the left y-axis are representative of those measured for the baseline test case in Matrix 2.

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When the water temperature reached 90 ◦ C and the first phase ended, the pot of water

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was removed and the test bed was allowed to operate normally until the secondary flame

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extinguished. The latter event signaled that the first batch of fuel had been consumed. For

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the second phase (refueling), 1.0 L of biomass fuel was added to the fuel chamber on top of

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the hot char bed left behind by the first batch of fuel. The test bed was re-lit using 50 mL of

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biomass wetted with 2 mL of kerosene. Emissions and power-output measurements resumed

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when a pot containing 2.5 L of water at 15 ± 2 ◦ C was placed on the pot support and

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continued until the water temperature reached 90 ◦ C or the secondary flame extinguished.

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As shown in previous studies 27,30 and the results below, emissions measured during this

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phase typically represented the worst-case scenario for high-power performance.

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The third phase started when the secondary flame extinguished. During this phase, the

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pot of water was left on the pot support and the char remaining from the two fuel batches

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was allowed to burn while emissions were measured for 20 minutes.

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The producer gas entering the secondary combustion zone was sampled for 30–60 seconds

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starting at ten minutes after the start of Phase 1, five minutes after the start of Phase 2,

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and five minutes after the start of Phase 3. The gas composition was analyzed using a gas

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chromatograph with a thermal conductivity detector. For more information on the fume

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hood in which experiments were conducted, the instrumentation used to measure emissions,

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as well as producer gas sampling and analysis, see SI Section S1.3.

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During tests with Lodgepole pine pellet fuel, two modifications were made to Phases 1

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and 2 of the procedure: (1) 2.5 L of water was boiled twice (i.e., 5 L of water was boiled using

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two identical pots) and (2) the producer gas was sampled 30 minutes after the start of Phase

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1 and 15 minutes after the start of Phase 2. These modifications, and the justifications for


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them, are discussed in SI Section S1.3.

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The number of replicates collected for each data point is listed in Table S4, SI Section

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S1.2. A recent study suggests that 10 or more replicate tests may be necessary for the width

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of the confidence intervals on PM2.5 emissions measurements to asymptote to a constant

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value. 46 In this study, 3-4 replicates were collected for most data points to allow a large

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number of cases to be screened and 90% confidence intervals were calculated for the results.

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Calculations

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The average useful power outputs and dry fuel consumption rates for Phases 1 and 2 were

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calculated using equations similar to those used by Huangfu et al. 24 . Average useful power

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output was calculated using Equation 1: Pusef ul =

mH2O,i cH2O (Tf − Ti ) + hf g,H2O (mH2O,i − mH2O,f ) tf − ti

(1)

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where mH2O,i and mH2O,f were the masses of the water in the pot at the start and end of

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the phase, respectively, in g, cH2O the specific heat of water in J·g−1 ·K−1 , Tf the temperature

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of the water in the pot at the end of the phase in ◦ C, Ti the temperature of the water at the

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start of the phase in ◦ C, hf g,H2O the heat of vaporization of water in J·g−1 , tf the time at

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which the water in the pot reached Tf , in s, and ti the time at which the pot of water was

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placed on the stove in s.

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The average rate of dry solid biomass fuel consumption was calculated using Equation 2: m ˙ dry solid f uel =

msolid f uel (1 − M Cf uel ) te − ti

(2)

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where msolid f uel was the mass of fuel added to the stove at the start of the phase in g,

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M Cf uel the moisture content of the fuel as weight % on a wet basis, ti the time at which

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the stove was lit in s, and te the time at which the secondary flame extinguished at the end

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of the phase in s. The solid fuel consumption rate was calculated as shown in Equation 2, 9 ACS Paragon Plus Environment


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dm dt

, because

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and not simply set equal to the mass loss rate recorded by the digital balance

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some mass of solid char was left behind after the biomass had devolatilized. In this manner,

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the remaining char was treated as a product of the gasification process.

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The mass flow rate of producer gas entering the secondary combustion zone was calculated by applying the conservation of mass to a control volume drawn around the fuel/char bed: dm =m ˙ primary air − m ˙ f uel gas dt

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where

dm dt

(3)

was the rate of change of the mass of the fuel/char bed recorded by the digital

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balance in g·min−1 , m ˙ primary air the primary air flow rate in g·min−1 , and m ˙ f uel gas the mass

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flow rate of gas leaving the fuel bed and entering the secondary combustion zone in g·min−1 .

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The stoichiometric air-to-fuel ratio in the secondary combustion zone was calculated from

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the gas composition and fuel gas flow rate using Equation 4:

(m ˙ air /m ˙ f uel )stoic

Mair = × 4.76 × Mgas

1 1 7 xCO + xH2 + 2xCH4 + xC2 + 5xC3 2 2 2

(4)

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where Mair and Mgas were the molecular weights of air and the fuel gas, respectively,

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in g·mol−1 , and x the mole fraction of each component in the fuel gas. Mole fractions of

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hydrocarbons larger than C3 were negligible. The small concentration of O2 in the gas

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samples was neglected.

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Global equivalence ratios in the fuel bed and secondary combustion zone were calculated using Equation 5: φ=

(m ˙ air /m ˙ f uel )stoic (m ˙ air /m ˙ f uel )

(5)

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where m ˙ air was the mass flow rate of primary or secondary air in g·min−1 and m ˙ f uel the

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fuel consumption rate calculated using Equation 2 or the fuel gas flow rate calculated using

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Equation 3 in g·min−1 .

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The equations used to calculate commonly-reported CO and PM2.5 emissions metrics 10 ACS Paragon Plus Environment

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are provided in SI Section S1.4. Briefly, CO emissions were calculated by integrating the

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concentration of CO measured in the fume hood exhaust at 1 Hz using NDIR spectroscopy.

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Total PM2.5 emissions were calculated, assuming isokinetic sampling, from the mass of PM2.5

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collected on a PTFE filter.

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Results and discussion

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A complete set of experimental results is provided in SI Section S2. The parameters that

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had the greatest influence on performance are discussed below.

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Effects of air flow rates, fuel moisture, and secondary air delivery

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parameters

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Useful power output increased linearly (r2 ≥ 0.99) with primary air flow for the range of flow

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rates tested (see Figure 3). The global equivalence ratio in the fuel bed remained between

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3 and 5 when the primary air flow rate was varied, and the average dry fuel consumption

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rate increased linearly with primary air flow rate (see Figure S21, SI Section S2.2). The

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composition of the producer gas entering the secondary combustion zone during Phase 1 was

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not affected by primary air flow rate (see Figure S27, SI Section S2.3). The producer gas

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consisted primarily of H2 , CO, CH4 , CO2 , and N2 , and the mole fraction of hydrocarbons

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larger than CH4 was less than 2% in all samples collected during the second test matrix. Figure 3: Effects of primary air flow rate and fuel moisture content on average useful power output during normal operation (Phase 1, green triangles) and post-refueling (Phase 2, gold circles). Black symbols refer to the baseline configuration (Douglas fir chip fuel, 7% moisture content, 25 g·min−1 primary air, 75 g·min−1 secondary air, 200 ◦ C secondary air temperature, 4-mm-dia. secondary air holes, no swirl, no downward angle, 15 mm pot gap, no flow constriction, no other features). Error bars represent 90% confidence intervals. Data points have been offset slightly on either side of the values listed in Table 1 to improve the readability of the plot.

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The average high-power PM2.5 emissions measured during normal operation remained 11 ACS Paragon Plus Environment


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between 21 mg·MJd −1 and 42 mg·MJd −1 when the primary air flow rate was varied (see

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Figure S18, SI Section S2.2). The secondary flame was difficult to re-light after refueling

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when the primary air flow rate was low and, as a result, the variability in Phase 2 PM2.5

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emissions was high for the test cases with primary air flow rates of 15 g·min−1 and 20 g·min−1 .

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Higher primary air flow rates may have made the test bed easier to re-light by causing fuel

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gas production to resume more quickly. The concentration of CO in the producer gas during

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Phase 2 increased with primary air flow rate (see Figure S27, SI Section S2.3).

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Useful power output and average dry fuel consumption rate decreased linearly with in-

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creasing fuel moisture content (see Figure 3 and Figure S21, SI Section S2.2). Fuel moisture

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content did not affect CO or PM2.5 emissions during normal operation (Phase 1), but Phase

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2 (post-refueling) emissions increased with increasing moisture content (see Figure S18, SI

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Section S2.2). Similar to the test cases with low primary air flow rates, the secondary com-

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bustion zone was difficult to re-light after refueling for higher moisture contents (15% and

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25%), and relighting difficulties likely contributed to the higher Phase 2 emissions that were

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measured for the higher moisture content fuels. Higher moisture contents may have made

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the test bed difficult to re-light by preventing fuel gas production from resuming quickly.

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A minimum in high-power (Phase 1 and 2) CO emissions was observed for secondary to

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primary air flow ratios of 3:1 and 4:1 (see Figure 4). This result is in agreement with previous

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studies 21,39 that recommended a secondary to primary air flow ratio of approximately 3:1

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for high-power operation. Figure 4: Effects of secondary to primary air flow ratio and secondary air opening size (secondary air velocity) on average high-power CO emissions. Average values that were calculated from only two replicates are marked with a ‘+’. Error bars represent 90% confidence intervals for markers with no ‘+’ and the total range of the measurements for markers with a ‘+’. Data points have been offset slightly on either side of the values listed in Table 1 to improve the readability of the plot.

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Higher secondary air velocities resulted in lower CO emissions (see Figure 4). These

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higher velocities most likely led to better mixing between the secondary air and the fuel 12 ACS Paragon Plus Environment

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gas. This result stands in contrast to previous work 21 in which CO emissions decreased

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with increasing secondary air inlet area; however, the total secondary air flow rate was not

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decoupled from the inlet area in that study. 40 The 2-mm-diameter secondary air inlet holes

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tested in the second matrix resulted in average Phase 1 high-power CO and PM2.5 emissions

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that were only 0.9 g·MJd −1 and 8 mg·MJd −1 lower, respectively, than those for the test case

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with 4-mm-diameter holes (see Figure S20, SI Section S2.2). This result may indicate

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diminishing returns associated with further increases in secondary air velocity. Results may

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be different for a natural draft or fan-powered stove. Primary and secondary air flow rates

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were fixed in the modular test bed. In a natural draft or fan-powered TLUD, the secondary

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air inlet area and/or the presence of a chimney section would be expected to affect the

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secondary air flow rate.

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Varying the other secondary air delivery parameters investigated in the first and second

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test matrices did not improve performance compared to the baseline configuration (see SI

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Sections S2.1 and S2.2). For example, the 15◦ swirl angle had a negligible effect on

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emissions, and the 30◦ and 45◦ swirl angles resulted in higher Phase 1 CO emissions relative

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to the baseline case (see Figure S8, SI Section S2.1). Photographs of the secondary

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combustion zone for the 15◦ , 30◦ , and 45◦ swirl test cases are shown in Figure S16, SI

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Section S2.1. In the photograph of the 15◦ case, the secondary flames are attached to

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the air inlets and have a bluish color. In contrast, the flames are not attached, are more

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luminous, and rise further out of the top of the stove in the 30◦ and 45◦ degree cases. The

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higher luminosity suggests that PM2.5 emissions were also higher in these two cases. The

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larger swirl angles appear to have promoted soot formation by increasing the residence time in

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fuel-rich combustion zones rather than enhancing fuel/air mixing. In addition, impingement

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of the flame on the cold cooking surface in the 30◦ and 45◦ swirl cases would be expected to

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quench the reactions that oxidize CO and PM.



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Effects of fuel type

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Changes in fuel type sometimes resulted in order of magnitude changes in the average high-

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power PM2.5 emissions measured during normal operation (see Figure 5). The test bed

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exhibited the highest emissions when fueled with corn cobs and the lowest emissions when

275

fueled with Lodgepole pine pellets. The producer gas composition during normal operation

276

(Phase 1) was also affected by fuel type (see Figure 6). Differences in producer gas composi-

277

tion were not attributed to differences in elemental composition between the fuels. Ultimate

278

analysis revealed that C:H:N:O ratios for the fuels were almost identical (see Table S5, SI

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Section S1.2). The differences in producer gas composition and emissions may have re-

280

sulted from differences in fuel particle size, density, biochemical composition, and inorganic

281

constituents affecting yields of light gases and tar from the gasification process. 36,47–50 Figure 5: Effects of varying fuel type on high-power CO and PM2.5 emissions during normal operation (Phase 1) and post-refueling (Phase 2). Gray symbols correspond to the test case with the highest bulk density corn cobs (174 kg·m−3 ). Error bars represent 90% confidence intervals. The data points representing the Phase 1 and Phase 2 results have been offset to improve the readability of the plot.

Figure 6: Effects of fuel type on producer gas composition for normal operation (Phase 1), post-refueling (Phase 2), and char burnout (Phase 3). Error bars represent 90% confidence intervals.

282

When the Lodgepole pine pellet fuel was used, average H2 concentrations in the producer

283

gas were 103% and 23% higher during Phase 1 and Phase 2, respectively, compared to the

284

baseline (Douglas fir chips). Varunkumar et al. 32 also reported that pellet fuel resulted in

285

higher H2 concentrations in the producer gas compared to wood chip fuel. The maximum fuel

286

bed temperatures measured during Phase 1 were highest when the Lodgepole pine pellet fuel

287

was used (see Figure S37, SI Section S2.3), and higher temperatures would be expected

288

to result in increased conversion of tars to light gases. 36,51,52 In addition, the Lodgepole

289

pine pellets were 6.4 mm in diameter, whereas pieces of the chipped fuels were ≤ 3-mm14 ACS Paragon Plus Environment

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thick on average. The larger particle thickness may have resulted in a longer residence time

291

for primary pyrolysis products to undergo secondary reactions inside the particle and, as a

292

result, increased conversion of tars to light gases. 47

293

When the chipped corn cob fuel was used, the average H2 and CO2 concentrations were

294

64% lower and 43% higher, respectively, during Phase 1 compared to the baseline. The max-

295

imum fuel bed temperatures measured during Phase 1 were lowest when the corn cob fuel

296

was used (see Figure S37, SI Section S2.3), and lower temperatures would be expected to

297

result in reduced conversion of tars to light gases. 36,51,52 The corn cobs had a lower lignin con-

298

tent and higher hemicellulose content compared to the wood fuels. Hemicellulose pyrolyzes

299

more rapidly and over a narrower temperature range compared to lignin. 48,53 Yang et al. 48

300

observed a higher concentration of H2 in the gaseous products of lignin pyrolysis and a higher

301

concentration of CO2 in the gaseous products of hemicellulose pyrolysis. Worasuwannarak

302

et al. 49 observed higher tar yields from pyrolysis of corn cobs than from pyrolysis of rice

303

husk and rice straw.

304

Higher tar yields could contribute to higher PM2.5 emissions. Primary and secondary

305

tars released during pyrolysis of the fuel can undergo further reaction in high-temperature

306

zones to form PAHs, 54 which are precursors to soot, and condensible organics in the ex-

307

haust can condense onto existing particles or nucleate to form new particles. 55,56 Tar yields

308

were not quantified in these experiments. The manner in which fuel properties affect tar

309

yields, producer gas composition, and overall emissions from the stove are worthy of further

310

investigation.

311

Effects of operating mode

312

Phase 2 emissions where higher than Phase 1 emissions for all test cases. The concentrations

313

of H2 , CO, and CH4 in the producer gas were highest during Phase 1 (normal operation)

314

(see Figure 6). For the baseline case (Douglas fir chips), the average H2 , CO, and CH4 con-

315

centrations were 61%, 36%, and 23% lower, respectively, during Phase 2 (post-refueling) and 15 ACS Paragon Plus Environment


316

93%, 81%, and 81% lower, respectively, during Phase 3 (char burnout). The concentration

317

of O2 /Ar in the producer gas was low during Phases 1 and 2, but was 3.3× the average of

318

the Phase 1 and Phase 2 values during Phase 3. The concentrations of H2 , CO, and CO2

319

observed during normal operation (Phase 1) and char burnout (Phase 3) were similar to

320

those reported by Varunkumar et al. 32 .

321

The temperatures measured in the fuel chamber revealed that gasification proceeded from

322

the top of the biomass fuel bed to the bottom during Phase 1 and from the bottom to the

323

top during Phase 2 (see Figure 7). During Phase 1, the temperatures in the top of the fuel

324

bed increased first, followed by the temperatures in the middle and bottom. This progression

325

indicates that the stove operated as an inverted downdraft gasifier (i.e., as a TLUD) during

326

Phase 1. After the first batch of fuel had completely gasified, the temperature of the bottom

327

thermocouples remained high. After the second batch of fuel was added on top of the

328

remaining hot char, the temperatures in the middle of the fuel bed increased first, followed

329

by the temperatures in the top and at the gas sampling height. This progression indicates

330

that the stove operated as a conventional updraft gasifier during Phase 2. Conventional

331

updraft gasifier operation would be expected to result in high tar concentrations in the

332

producer gas. 29 Figure 7: Temporal variation in temperature (◦ C) at four different vertical locations in the combustion chamber for the baseline case. See Figure 1 for thermocouple locations. The primary combustion zone progressed from the top to the bottom of the fuel bed during Phase 1, and from the bottom to the top of the fuel bed during Phase 2.

333

Together, the emissions, producer gas composition, and fuel bed temperature measure-

334

ments reveal that TLUDs only really operate as TLUDs, and exhibit the associated low

335

emissions, while the initial batch of biomass fuel is gasifying. The proportion of the mass

336

collected on the Phase 2 filter samples that is attributable to the transient emissions observed

337

immediately after re-lighting vs. steady state post-refueling operation is unknown. The de-

338

gree to which conventional updraft gasification influences steady state PM2.5 emissions, if at


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all, can be investigated by collecting separate filter samples for transient and steady state

340

post-refueling operation in future experiments.

341

Tier 4 high-power emissions were readily achievable when the baseline configuration was

342

fueled with Douglas fir wood chips or Lodgepole pine pellets and operating normally (during

343

Phase 1) (see Figure 5). However, the same stove exhibited Tier 0–3 emissions during normal

344

operation with a low-quality fuel (corn cobs) and post-refueling (during Phase 2). These

345

results suggest that the effects of fuel type and operator behavior on emissions from biomass

346

cookstoves need to be considered, in addition to cookstove design, as part of efforts to reduce

347

exposure to household air pollution resulting from solid fuel combustion. Locally available

348

fuels and user behavior will influence performance in the field, and improving user access to

349

high-quality prepared biomass fuels, such as pellet fuels, may be necessary to achieve Tier 4

350

emissions reliably.

351

Acknowledgement

352

The authors acknowledge support from U.S. Department of Energy award number DE-

353

EE0006086.

354

Supporting Information Available

355

Additional information on the experimental methods and a complete set of results graphs

356

are available in the Supporting Information.

357

358

This material is available free of charge via the Internet at http://pubs.acs.org/.

References

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Graphical TOC Entry

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ACS Paragon Plus Environment

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Page 28 of 34

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Page 30 of 34

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Page 33 of 34

End fuel batch 1

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1000

End fuel batch 2


Page 34 of 34 Gas Top Middle Bottom

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ACS Paragon Plus Environment 0 0

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Figure 7

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Time since start of test (s)

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The Effects of Air Flow Rates, Secondary Air Inlet Geometry, Fuel Type

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