Biocoal Briquettes Combusted in a Household Cooking Stove

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Bio-Coal Briquettes Combusted in a Household Cooking Stove: Improved Thermal Efficiencies and Reduced Pollutant Emissions Juan Qi, Qing Li, Jianjun Wu, Jingkun Jiang, Zhenyong Miao, and Duosong Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03411 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on December 31, 2016

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Environmental Science & Technology

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Bio-Coal Briquettes Combusted in a Household Cooking Stove:

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Improved Thermal Efficiencies and Reduced Pollutant

3

Emissions

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Juan Qi1#, Qing Li2#, Jianjun Wu1*, Jingkun Jiang2*, Zhenyong Miao1, Duosong Li3

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National Engineering Research Center of Coal Preparation and Purification, School

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of Chemical Engineering and Technology, China University of Mining and

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Technology, Xuzhou, 221116, China; 2

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State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, 100084, China;

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School of Environment and Surveying and Mapping, China University of Mining and Technology, Xuzhou, 221116, China

12 13 14 15 16 17 18 19 20 21 22 23 24

#

These authors contributed equally to this work. *Corresponding author: (W.J.) Phone: +86-516-83591115; Email: [email protected] (J.J.) Phone: +86-10-62781512; Email: [email protected]

1

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ACS Paragon Plus Environment


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ABSTRACT

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Clean fuels are urgently needed to reduce household cooking emissions. The thermal

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efficiencies (ηth) and pollutant emission factors (EFs) of bio-coal briquettes (made

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from a mixture of biomass and coal powder) burned in a typical cooking stove were

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investigated and compared with those of coal briquettes and biomass briquettes.

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Bio-coal briquette samples were obtained by molding blends of anthracite with 10−30

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wt% crop straw of various types (maize straw, wheat straw or rice straw). The

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optimum proportions for energy savings and PM2.5 EF reduction were found to be

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15−20 wt%. Compared with the ηth of coal briquettes and biomass briquettes, the ηth

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of bio-coal briquettes grew by 81−127% and 88−179%, respectively, with the

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optimum addition ratios of crop straw, while the delivered energy-based PM2.5 EFs of

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the bio-coal briquettes were reduced by 61−67% and 99.0−99.5%, respectively.

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Delivered energy-based EFs of NOx, SO2 and toxic elements (As, Se and Pb) also

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showed a significant reduction. These results indicated that bio-coal briquettes can

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serve as a promising substitute for domestic solid fuel to reduce pollutant emissions

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and save energy.

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2 ACS Paragon Plus Environment


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600

5 Coal 4

400

200

Biomass

Coal 3 Reduction

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Reduction

Fuel consumption (Mt)

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Biomass

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100 Bio-coal

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0

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PM2. 5 emissions (Mt)

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Bio-coal

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INTRODUCTION

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Nearly half of the world’s population uses solid fuels, such as coal, wood, and crop

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straw, for household activities1, especially in developing countries, including China,

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India and Nepal2-4. The direct burning of raw solid fuel in traditional stoves produces

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serious pollutant emission, e.g., PM2.5 (particle matter with an aerodynamic diameter

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less than or equal to 2.5 µm), BC (black carbon), BrC (brown carbon), toxic elements,

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CO, NOX and SO25-10. These emissions give rise to increased health risks11-13 and

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cause tremendous environmental problems14. Household stoves are not likely to be

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replaced in many areas in the near future due to limited economic conditions and

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living habits.

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The combustion properties of solid fuel have been intensively investigated. The

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volatile content of coal is widely identified to be the most important factor influencing

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PM formation8, 15-19. Pollutant emissions increase as the volatile content increases.

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More recent research has revealed that pollutant EFs are positively correlated with

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volatile contents ranging from 2.8% to 48.7%20. Biomass produces more serious

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pollutant emissions because of the higher volatile inclusion21. Anthracite has been

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widely acknowledged as a clean fuel with low PM2.5 EFs. In addition to the

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anthracite’s low yield and high price, the low burnout ratio and poor ignition

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performance are significant limitations to its utilization5, 22-25. These poor performance

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factors are related to the density and structure of coal26. Therefore, determining a

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reasonable method to utilize biomass and anthracite is a serious environmental

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problem. Better combustion performance may be achieved if the structure of


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anthracite can be adjusted. Biomass, a renewable energy, is a good candidate to be

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mixed with anthracite for providing sufficient volatile matter and thus improving its

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combustion performance. China is a large agricultural country in which most crop

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straw is treated as waste and usually burned in open fires. Pollutant emissions are

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produced from the direct biomass burning. Haze contamination usually becomes more

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serious during the harvest season. Moreover, powdered coal, whose production

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increases with the development of mining mechanization, poses a direct threat to the

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environment and aggravates energy waste, while anthracite is regarded as a scarce

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resource. If bio-coal briquettes, the mixture of biomass and coal, are proven to be

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feasible for large-scale use in household combustion, it will not only solve the

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problem of direct crop straw burning but also address the issue of powdered anthracite

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utilization and provide a solution that will benefit the environment, energy production

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and the economy. Bio-coal briquettes may be a valuable and feasible way to use the

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mixture of biomass and coal in residential combustion.

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Co-combustion of biomass and coal has been intensively studied in industry27-29.

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The blends provide a sufficiently high burnout ratio29,

30

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production27. The synergistic effect of biomass and coal helps to enhance the

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combustion performance of the mixture, and more volatile matter than expected is

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yielded during the pyrolysis process31, 32. The mixture’s high volatility leads to a

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highly porous char-accelerated combustion27, 33. The degree of uniformity is also

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increased by the addition of cellulose34. These changes are beneficial for heat and

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mass transformation. The flame temperature drops during co-firing, suggesting a

and reduce pollutant



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moderate reduction in thermal NOx formation27, 35. NO formed from volatile N can

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be reduced with the gas-solid heterogeneous reactions of char36. Metallic oxide, which

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has a high melting point, inhibits the volatilization of heavy metals37, 38. Nevertheless,

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the pollutant emission of bio-coal briquettes used in household cooking stoves has not

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been studied.

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Aiming to seek for a substitute solid fuel to overcome the deficiencies of the low

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burnout ratio and poor ignition performance of anthracite, this study investigated the

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emission factors (EFs) and thermal efficiencies (ηth) when burning bio-coal briquettes

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(made from a mixture of biomass and anthracite powder) in a typical household

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cooking stove. Coal briquettes and biomass briquettes were also tested for comparison.

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Environmental implications were also discussed.

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MATERIALS AND METHODS

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Tested samples

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To ensure a good representation of bio-coal briquettes, three types of crop straw,

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which is the by-product of the main crops in China according to China’s rural energy

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Yearbook (2009–2013) and the 2014 China agricultural development report39, 40, were

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used for this study: maize straw, wheat straw and rice straw. They were collected

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from rural Xuzhou in Jiangsu province, East Shanxi anthracite was selected for

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blending. Table S1 presents the characteristics of these raw materials, including

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proximate analysis, elemental analysis and the net calorific value of the received fuel.

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Figure S1 shows a typical thermogravimetric analysis (TGA) of the tested biomass

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and coal samples using a heating rate of 5 K/min under argon and air atmospheres,


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respectively. According to previous work, the ignition point is the temperature at

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which the combustion curve and the pyrolysis curve deviate from each other41, 42. In

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this study, the ignition point was 200−300 °C for biomass and 450−550 °C for

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anthracite. The data sufficiently demonstrate that the ignition point of biomass is

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much lower than that of anthracite.

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To investigate the actual mechanism of combustion of biomass mixed with

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anthracite and to identify the optimum biomass content, bio-coal briquettes for each of

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the above crop straws were prepared with different biomass contents (0 wt%, 10 wt%,

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15 wt%, 20 wt%, 25 wt%, 30 wt% and 100 wt%) in the same process conditions:

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molding pressure of 25 MPa, particle size less than or equal to 1 mm, cylindrical

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shape with 30 mm diameter and 20 mm height, and 10% clay soil added as a binder.

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Coal and biomass were crushed into powder using a crusher. These powders and clay

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were mixed using an electric blender and then formed into briquettes using the

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cold-press molding technique. Figure S2 shows a photograph of the finished

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briquettes, the details of which are shown in Table S2.

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Tested Stove and Operation

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To ensure the repeatability of the sampling process, a stove (see Figure S3) that had

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been used in a typical rural kitchen for 3 years was chosen for the combustion

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experiment. The ηth value of the stove was lower than that of a new stove. A kettle

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was placed on it to test ηth, and the water temperature was recorded with a control

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display unit at the spout. Fuel (bio-coal and coal briquettes fixed at 2.0 kg and

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biomass briquettes fixed at 1.0 kg according to the capacity of the fuel chamber) was



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added from the upper part of the stove. The burn-out (not being put out) method was

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adopted, and ash was cleaned from the bottom. New fuel was not added to the stove

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until the briquettes burned out and the ash was cleaned. For consistency during the

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laboratory experiments, the fuel sample was ignited using propane gas with a fixed

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flow rate of 3 L/min until a stable flame was observed, which took approximately 1−5

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min. The ignition time and extinction time were recorded. A new kettle with 4.0 kg of

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water replaced the previous one as soon as the temperature reached the fixed

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maximum temperature of 90 °C to prevent the water from reaching the boiling

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temperature, at which point more water vapor would have been released into the

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exhaust.

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Sampling System

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This study was conducted on simulated civil combustion test equipment (as shown in

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Figure S4) in a village of Beijing. The sampling method and the testing stove were

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introduced in previous publications23, 43 but are briefly summarized here. A stove and

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a kettle were positioned in a stainless steel box. An air blower built into the side

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pumped filtered ambient air into the box to dilute the smoke, which was then sucked

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into a pipe by a high-power fan fixed at its downstream end. Sampling ports are

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located at the dilution tunnel. Each sample was tested at least three times.

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The flue gas, which was filtered via a quartz filter, flowed continuously into three

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flue gas analyzers (Thermo Scientific™; 48i, 43i and 42i for CO, NOx and SO2

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respectively), CO2 meter (GC-0012; Gas Sensing Solutions Ltd.), and particle

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sampler (URG-2000-30 EH; URG Inc.) over the entire combustion process.

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Simultaneous samples were collected on filters in three branches (PM2.5, PM1.0 and


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TSP), and each branch had two sampling ports for parallel sampling. Particle samples

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were collected on quartz-fiber filters (Pall; 2500QAO-UP; 47 mm diameter) for two

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burning cycles. Teflon membranes were used to collect PM in one burning cycle to

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analyze toxic elements. An X-ray fluorescence (XRF) spectrometer (NAS100; Nayur

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Technology Co., Ltd.) was applied to measure the concentration of toxic elements (Pb,

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As and Se) captured on the Teflon filters containing PM2.5 samples in offline mode.

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An ion chromatograph (Dionex-600, Dionex, Thermo Fisher Scientiﬁc, Inc.) was used

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to measure the concentrations of water-soluble SO42-.

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Analysis method

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The ηth value for fuel was derived from the temperature increase (ΔT (°C)) of the

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water in the kettle. It is expressed as ηth (%) = (Mw × Cw × ∆T) / (Mc × Qc), where Mw

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(kg) represents the water mass in the kettle, Cw (kJ/(k°C)) represents the specific heat

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capacity of water, M c (kg) represents the fuel mass for each test, and Qc (kJ/kg)

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represents the net calorific value of the received fuel.

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The EFs include mass-based (EFm) and delivered energy-based (EFt) factors. They

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can be expressed in terms of pollutant mass per fuel mass and pollutant mass per unit

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useful delivered energy (not per unit of fuel energy). EFt was determined using EFm

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based on the following equation: EFt (mg/kJ) = EFm / (ηth × Qc). EFm can be presented

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as follows: EF (mg/g) = Mi × F / Mc, where Mi (mg) is the collected sample mass, F is

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the ratio of the total diluted flue gas flow rate to the sampling flow rate, and Mc (g) is

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the fuel mass for each test.



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This study reported the ηth and pollutant emission factors reduced by modifying

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biomass content mixture with anthracite. Multi-objective optimization method was

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employed to evaluate a balance point between the two parameters. The corresponding

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mathematical model with objective function is described as: f1 = max (ηth) & f2 = min

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(EFt). For convenience, this function was converted to the general form of

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multi-objective optimization as: f1 = max(ηth) & f2 = max(1 / EFt ). The overall

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optimization objective function was defined as follows: Max f = a × η+ b × (1−c),

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where f is the objective function, η is normalized ηth, c is normalized PM2.5 EFt, and a

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and b, the ratio of two criteria. The weights of thermal efficiency and PM2.5 emission

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factor were considered to be equally important in this analysis, namely a = 0.5 and b =

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0.5.

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The mass-weighted average (EFb-c), which was interpolated between the values for

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100% biomass and 100% coal according to mass inclusion of biomass and

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coal, was calculated as follows: EFb-c = EFb × b% + EFc × c%, where EFb and EFc

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represent EFs (mass-based or delivered energy-based) of biomass briquettes and coal

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briquettes, respectively; b% and c% are the mixing proportions of biomass and coal,

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respectively, in each bio-coal briquette.

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The modified combustion efficiency (MCE) was determined using the following

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formula43,

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fire-integrated excess molar mixing ratios of CO2 and CO, respectively, and refer to

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the EFs of the overall combustion process of CO2 and CO.

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: MCE = ∆CO2 / (∆CO2 + ∆CO), where ∆CO2 and ∆CO are the

The burnout ratio can represent fuel’s combustion completeness 23, 45, which can be


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calculated as follows: ηbr = (1 - Abot) / (1 - Ad) × 100%, where Abot is the ratio of

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bottom ash mass to the fuel mass in a combustion cycle, and Ad represents ash on a

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dry basis obtained by proximate analysis.

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RESULTS AND DISCUSSION

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Improved thermal efficiency

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Figure 1 shows that the addition of biomass increased ηth and reduced the PM2.5 EFs.

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However, the proportions of biomass for the peak ηth values were not completely in

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agreement with those for the lowest PM2.5 EF values. The former occurred when the

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biomass content in the bio-coal briquettes was 15 wt% (maize straw), 15 wt% (wheat

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straw) and 20 wt% (rice straw). The biomass ingredient composition values were 15

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wt%, 20 wt% and 20 wt% when the delivered energy-based EFs of PM2.5 declined to a

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minimum, as illustrated in Table S3 (the mass-based EFs of PM2.5 are presented in

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Figure S5). The parameter f was introduced to simplify the analysis and certify the

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optimum biomass content in bio-coal briquettes for achieving the best energy savings

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and PM2.5 EF reduction (see Table S4). Accordingly, 15 wt% (maize straw), 20 wt%

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(wheat straw) and 20 wt% (rice straw) were determined to be the optimum

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compositions for bio-coal briquettes in the tested stove.

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One-way ANOVA analysis indicated that ηth of bio-coal briquettes was

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significantly higher (p = 0.008) than that of coal briquettes and biomass briquettes,

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while the delivered energy-based PM2.5 EFs (p = 0.008) were significantly lower than

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that of biomass briquettes, as shown in Figure 1. This result implies that biomass

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plays a positive role in promoting the combustion properties of anthracite. In addition



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to the ηth increase, a further reduction in the PM EFs was also achieved, which is

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important for potential industrial production, proving the tested process to be feasible

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for addressing powdered anthracite and crop straw. The convincing experimental

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results are presented and analyzed below featuring ηth and PM2.5 EFs.

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Figure 1 shows that the average ηth of the three bio-coal briquettes ranged from

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6.33±0.26% to 8.53±0.3 7% (maize straw), 8.29±0.35% to 8.86±0.37% (wheat straw)

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and 6.06±0.24% to 10.02±0.63% (rice straw), demonstrating a common trend of first

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increasing and then declining for increased biomass. In contrast, biomass-only and

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coal-only briquettes exhibited lower ηth, ranging from 3.83±0.35% to 4.70±0.28%.

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The maximum ηth of bio-coal briquettes with maize straw, wheat straw and rice straw

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increased by 81.4±10.5%, 88.4±10.7% and 127.3±12.62%, respectively, compared

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with coal briquettes and by 87.5±14.1%, 98.7±12.8% and 178.9±18.96%, respectively,

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compared with biomass briquettes. Measured ηth for these stove/fuel combinations are

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lower than those of other stove/fuel combinations reported in the literature

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addition to the aging of the used stove tested here, the heating effect of the kettle was

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also not included.

46-48

. In

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Figure 2 shows that the briquette structures exhibited significant differences, which

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possibly resulted in the different ηth values among these briquettes. Sufficient oxygen

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can enter bio-coal briquettes via pore penetration, leading to complete combustion of

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the anthracite component and releasing more energy. To support this assertion, we

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calculated the burnout ratio from the bottom ash mass and the MCE from the

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measured EFs of CO2 and CO. All three bio-coal briquette types exhibited a similar


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trend. Taking maize straw-coal briquettes as an example, as shown in Figure 3 (details

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are in Table S5), both the burnout ratio and the MCE significantly increased when

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maize straw was added, indicating that biomass can have a major effect on improving

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the completeness of anthracite combustion. An upward trend was found in both the

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burnout ratio and the MCE (not obvious) until 20 wt%; a similar trend was also

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identified for ηth with a turning point at 15 wt%, indicating that increased maize straw

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was beneficial for enhancing ηth over a specific range.

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The ηth declined when the biomass exceeded 15 wt%, 15 wt% and 20 wt% for

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maize straw, wheat straw and rice straw inclusion, respectively. The subsequent

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reduction trend was largely attributable to the unburnt volatile matter that was

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released (due to higher biomass content) during the fuel ignition stage and a low

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burnout ratio15,

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combustion was fostered by increased biomass content, and more energy was lost; this

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conclusion is supported by the declining trend in the MCE after 20 wt% (see Figure 3,

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details are in Table S5).

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Reduced PM2.5 EFs

23

. Due to the short reaction time of volatile matter, incomplete

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Figure 1 shows that the delivered energy-based PM2.5 EFs from bio-coal briquettes

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were much lower than the calculated mass-weighted averages (delivered energy-based

260

averages), which were interpolated between the measured values for 100% biomass

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and 100% coal. The former covered comparatively wide ranges (0.34 mg/kJ to 5.33

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mg/kJ for maize straw, 0.36 mg/kJ to 3.55 mg/kJ for wheat straw and 0.30 mg/kJ to

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5.19 mg/kJ for rice straw in the bio-coal briquettes; see details in Table S3). The



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corresponding maximum reductions peaked at 93±2%, 96±2% and 98±1%; these

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values were obtained at contents of 15 wt%, 20 wt% and 20 wt% and were compared

266

with the mass-weighted averages (delivered energy-based averages) with an optimum

267

addition ratio. The values declined by 63±9%, 61±18% and 67±5%, respectively,

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compared with those of coal briquettes, and decreased by up to 98.8±0.3%, 99.0±0.4%

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and 99.5±0.1%, respectively, compared with those of biomass briquettes.

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The PM2.5 EFs were also closely correlated with the volatile matter content.

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Unburnt volatile matter acting as a PM precursor can positively contribute to the

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formation of particles20. In accordance with the above analysis, combustion

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completeness increased with the addition of biomass, decreasing the amount of

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unburnt volatile matter. Therefore, the PM2.5 EF curves decreased over a specific

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range (see Figure 1; details in Table S3). When the biomass content exceeded a

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certain point (15 wt% for maize straw, 20 wt% for wheat straw and 20 wt% for rice

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straw), the PM2.5 EFs exhibited an upward trend due to the increase in unburnt

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volatiles that occurred as biomass increased (see Figure 1; details in Table S3). This

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finding also illustrates that almost all of the various PM2.5 EFs from the biomass

280

content exhibited an opposite trend to ηth (see Figure 1; details in Table S3).

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Lower EFs for NO2, SO2 and toxic elements

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Lower EFs for NO2 and SO2 were discovered in all three types of bio-coal briquettes,

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each displaying a similar tendency. For example, in the bio-coal briquettes with the

284

most obvious trend, wheat straw (see Figure 4; details in Table S6), measured

285

delivered energy-based EFs for NO2 and SO2 were significantly lower in comparison


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to the calculated mass-weighted averages. The values of NO2 fluctuated between

287

1.12±0.02 mg/kJ and 1.62±0.21 mg/kJ. A downward trend was found for the SO2 EFs

288

for increased wheat straw contents. At the minimum NO2 (1.12±0.02 mg/kJ) and SO2

289

(0.24±0.10 mg/kJ; see details in Table S6), the maximum margin between the

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measured values and the mass-weighted averages (delivered energy-based averages)

291

of NO2 and SO2 reached 62±2% and 91±4%, respectively. Bio-coal briquettes resulted

292

in 68±2% and 93±3% lower NO2 and SO2 EFs, respectively, than coal briquettes. The

293

results can be explained by the porous structure of bio-coal briquettes (see Figure 2),

294

which induces the intermediate product of NO to form N2 instead of NO2 via the

295

disoxidation of C36, 49. Meanwhile, the richness of alkaline-earth metals in biomass

296

contributes to the formation of alkaline earth metal sulfation, leading to more S in the

297

ash and reducing the SO2 EFs. To test this hypothesis, we measured the water-solution

298

ionic SO42- in the ash converted per unit of fuel. Compared with the mass-weighted

299

averages (received mass-based averages), the measured values increased, indicating

300

that more water-soluble sulfate was formed (see Figure S6; details in Table S7).

301

Wheat straw additions to coal of up to 30 wt% were not sufficient to reduce NO2 and

302

SO2 emission levels compared to those of pure wheat straw.

303

EFs of toxic element (As, Se and Pb) from the three types of bio-coal briquettes

304

were also significantly reduced for increased biomass contents according to the

305

experimental results. The trend was also illustrated in an example of wheat straw–coal

306

briquettes (see Figure 5; details in Table S8). A downward trend was found for

307

increased wt% of wheat straw, and the minimum values appeared at 30 wt% biomass



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308

input. The maximum decreases were 91±2%, 94±3% and 96±1% based on the

309

mass-weighted values, respectively. The phenomena are closely linked to the enlarged

310

oxygen inlet through the porous structure (see Figure 2) in the wheat straw–coal

311

briquettes, as analyzed above, possibly leaving more toxic elements in the form of

312

oxides in the ash instead of releasing them into the air via flue smoke.

313

ENVIRONMENTAL IMPLICATIONS

314

To address the low burnout ratio of anthracite in household stoves, we investigated

315

the influence of adding biomass to anthracite. Compared to coal briquettes and

316

biomass briquettes, bio-coal briquettes showed a positive effect of dramatic ηth

317

improvement and pollutant EFs reduction. The biomass content exerted a significant

318

influence on the two important parameters. When the biomass composition remained

319

at 15 wt% (maize straw), 20 wt% (wheat straw) and 20 wt% (rice straw), the

320

briquettes displayed the most desirable performance in terms of both ηth and the PM2.5

321

EFs. The maximum ηth increase in bio-coal briquettes was 81−127% compared with

322

that of coal briquettes and 88%−179% compared with that of biomass briquettes. The

323

delivered energy-based PM2.5 EFs decreased sharply to as low as 0.30−0.36 mg/kJ for

324

bio-coal briquettes with the optimized ingredient composition. In addition, the PM2.5

325

EFs in bio-coal briquettes decreased by 61−67% compared with those in coal

326

briquettes and by approximately 99% compared with those in biomass briquettes. In

327

addition, the delivered energy-based EFs for NO2, SO2 and toxic elements (As, Se and

328

Pb) were also considerably reduced.


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The bio-coal briquettes demonstrated the desirable properties of high ηth and low

330

pollutant EFs based on the experimental results, rendering them a promising substitute

331

for conventional fuels. The amounts of PM2.5 from residential biomass (430 Mt) and

332

coal (90 Mt) combustion were 3.16 Mt and 0.78 Mt, respectively50. For a simplified

333

estimation, the total amount (i.e., 3.94 Mt) can be reduced to approximately 0.07 Mt if

334

the currently consumed biomass and coal are replaced by bio-coal briquettes of

335

approximately 97 Mt while providing the same amount of energy. If bio-coal

336

briquettes are comprehensively adopted in household activities throughout China, not

337

only will the problem of crop straw and anthracite coal powder waste be solved but a

338

striking PM2.5 reduction will also occur. The reduction could be as high as 98% based

339

on simplified estimates according to previously reported inventory results from

340

China’s anthropogenic emission sources23, 51 , and approximately 410 Mt biomass and

341

20 Mt coal would be saved. Bio-coal is expected to be highly marketable as a

342

substitute for conventional fuels to curtail air pollution from residential sources and

343

promote life quality and health in China.

344

ACKNOWLEDGEMENTS

345

This work was funded by the National Key Basic Research Program of China (No.

346

2012CB214900 & 2013CB228505) and the National Natural Science Foundation of

347

China (51574239, 41227805, 21422703, and 21521064).



Page 18 of 26

348

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Measured EF 1.0

(a) Maize straw

30

f

0.8 0.6 0.4 0 .2

9

0.0 10% 15% 20% 25% 30%

25

8

20

7

15

6

10

5

5

4

50 0

12 3 f

PM2. 5 EF (mg/kJ)

40

1.0 0 .8 0 .6 0 .4 0 .2 0.0 10% 15% 20% 25% 30%

10

30

8

20

6

10

4

800

12 2

PM2. 5 EF (mg/kJ)

(c) Rice straw

f

60

1.0 0.8 0.6 0.4 0.2 0.0 10% 15% 20% 25% 30%

10 8

40 6 20

4

0

2 0%

503 504 505 506 507 508 509

ηth (%)

(b) Wheat straw

502

10

ηth (%)

PM2. 5 EF (mg/kJ)

ηth

η th (%)

Calculated EF 35

Page 22 of 26

10%

15%

20%

25%

30%

100%

Content of biomass

Figure 1. Delivered energy-based PM2.5 EFs, mass-weighted averages (interpolated between the values for 100% biomass and 100% coal), and ηth for the bio-coal briquette samples mixed with different contents of biomass: (a) maize straw, (b) wheat straw and (c) rice straw.


Page 23 of 26


510 511 512 513

Figure 2. Cross-section SEM images of typical samples: (a) coal briquette, (b) bio-coal briquette and (c) biomass briquette.


Burnout ratio (%)

Burnout ratio

Page 24 of 26

MCE

100

99

95

96

90 93 85 90

80 75

87

70

84 0

514 515 516 517

MCE (%)


5

10

15

20

25

30

100

Content of maize straw (%)

Figure 3. MCE values and the burnout ratios as a function of various maize straw contents in the bio-coal briquettes.


Page 25 of 26


NO2 (mg/kJ)

4

Calulated flue smoke EF

Measured flue smoke EF

3 2 1 40 0%

10%

15%

0%

10%

20%

25%

30%

100%

30%

100%

SO2 (mg/kJ)

3 2 1 0 518 519 520 521

15% 20% 25% Content of wheat straw

Figure 4. Delivered energy-based EFs for NO2 and SO2 compared with mass-weighted averages (interpolated between the values for 100% biomass and 100% coal) for the bio-coal briquette samples mixed with different contents of wheat straw.



Caculated metal EF

01.2

Page 26 of 26

Measured metal EF

01.0 As (ug/kJ)

0.8 0.6 0.4 0.2 0.5 00.0 0%

10%

15%

20%

25%

30%

100%

0%

10%

15%

20%

25%

30%

100%

0%

10%

15%

20%

25%

30%

100%

Se (ug/kJ)

0.4 0.3 0.2 0.1 00.07

Pb (ug/kJ)

6 5 4 3 2 1 0

522 523 524 525 526 527

Content of wheat straw

Figure 5. Delivered energy-based EFs for As, Se and Pb compared with calculated values (interpolated between the values for 100% biomass and 100% coal) for the coal briquette samples mixed with different contents of wheat straw.


Biocoal Briquettes Combusted in a Household Cooking Stove

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