Combustion and Emissions Properties of RSPB Reed Briquettes Edward Mitchell, Amanda Lea-Langton, Alan Williams, Jenny Jones. Energy Research Institute, University of Leeds, Leeds, UK. LS29JT. 8 th October 2014
1. Executive summary Results are presented from the emissions testing of three fuels in a domestic stove used for space heating. RSPB reed briquettes have been compared with coal and wood logs, which are commonly used commercially available solid fuels. The reed briquettes combustion test showed the highest NO x emissions and the highest particulate emissions. Examination of the ash also revealed evidence of clinker formation, suggesting that reed briquettes had a lower ash melting temperature than the other fuels. This is believed to be a result of a higher concentration of trace elements in the fuel, as previous work has shown via ICP-MS analysis It is suggested that emissions abatement technologies are considered for the briquette fuels in their current formulation. Other option are to investigate pretreatment/ upgrading of the fuels in order to slow down the combustion or decrease the 'flaming combustion' duration and extend the 'char burning' duration. This would result in reduced peak emissions. 2. Introduction Domestic solid fuel burning is typically inefficient and unabated, leading to high emissions of gaseous pollutants such as carbon monoxide (CO), oxides of nitrogen (NOx) and particulate matter (PM) (Williams et al., 2012). Particulate matter emissions legislation typically refers to particles below 10 micrometers (PM 10 ) and 2.5 micrometers (PM 2.5 ). However, many of the particles produced are known to be below 1 µm in diameter which are the most hazardous to health as they can pass deep into the lungs (Bølling et al., 2009). Biomass burning is also associated with high emissions of organics such as polycyclic aromatic hydrocarbons (PAH) which are known to be mutagenic and carcinogenic (Naeher et al., 2007). 3. Materials and Methods 3.1. Sample preparation Biomass fuels were shredded using a Retsch SM100 cutting mill to a size of <1 mm, and were then milled using a SPEX 6770 cryogenic grinder to achieve a very fine particle size require for analysis. Mineral fuels were milled using a Retsch PM100 ball mill. All samples were sieved to ensure a particle size of 90 µm or less. The wood logs were milled and analysed including the bark. 3.2. Fuel characterisation Proximate analysis on mineral fuels was carried out according to BSO ISO 17246. For the biomass fuels, proximate analysis was carried out according to BS EN 14774-3 for moisture, BS EN 15148 for volatile matter and BS EN 14775 for ash. The principle is the same for solid biofuels as mineral fuels, but the moisture is determined in air rather than nitrogen, and the ashing temperature is 550 C rather than 815 C. Ultimate analysis (CHNS) was carried out on a CE Instruments Flash EA1112. Oxygen was calculated by difference in accordance with BS ISO 17247. Calorific value was determined by bomb calorimetry using a Parr 6200 calorimeter.
3.3. Test setup A Waterford Stanley Oisin multifuel stove was used for all fuels. The appliance is rated by the manufacturers as having a maximum non-boiler thermal output of 5.72 kw and an efficiency of 78.8%. The dimensions are 535 x 408 x 415 mm (HxWxD). There is just one (primary) air supply which is manually controlled via a damper. The general arrangement of the test equipment was done in accordance with BS EN 13240, whereby the stove was mounted on a set of scales on a trihedron, as shown in figure 1. Figure 1. Laboratory test rig. Sampling ports were installed in the flue at a height of 1.43 m. The flue is insulated as has an internal diameter of 125 mm. Sampling was done in-stack and there was no dilution of the flue gas. The stove was mounted directly underneath a laboratory extraction system which was used to apply a continuous draught of 12 Pa which is required for the nominal heat output test in BS EN 13240. 3.4. Operating procedure One batch of approximately 3 kg of fuel was tested on each run, with no re-loading. This mass was chosen on the basis of the volume of the combustion chamber. Six standard ZIP kerosene firelighters were used to ignite the fuel and no samples were taken during the first 20 minutes to allow them to burn out completely, and to allow the fire to become established. The damper was fixed at a dilation of 10 mm for all fuels to allow for comparison.
3.5. Flue gas sampling procedure Flue gas composition was monitored using a Testo 340 handheld device. The capabilities of this equipment include O 2, CO 2, CO, NO, NO 2 and flue gas temperature. The CO sensor has a limit of 10,000 ppm (1%). Above this value, sampling is automatically halted to prevent damage to the sensor. CO levels in excess of 10,000 ppm were regularly observed in the dying-out phase but an accurate measurement was not possible at the time of study, so a value of > 1% is assigned. Flue gas velocity and flow rate were calculated by measuring the dynamic pressure change in the flue, using a Wöhler DC100 pressure computer and an S-type pitot tube, in accordance with BS EN ISO 16911-1. PM 10 and PM 2.5 was determined following a method based on USEPA Method 201a and BS ISO 25597. Briefly, in the standard methods a probe featuring a set of cyclones, pitot tube and thermocouple is inserted directly into the flue. Flue gas is drawn through a pre-selected nozzle into the cyclone separators and then through a heated probe into a set of impingers, before a dry gas meter. A schematic is shown in figure 2. Figure 2. Schematic of the PM 10 and PM 2.5 sampling train. Source: USEPA Method 201a. However, due to the small flue size of the test rig it was necessary to mount the cyclones externally to the flue. In the standard methods, the cyclones are inserted into the flue for a period of around 30 minutes for temperature equilibration. Due to the cyclones being mounted externally, a heated jacket and PID controller was used in lieu. Isokinetic sampling was not possible due to the extremely low flow rate in the flue (< 1.5 m s -1 ) and a personal communication with the equipment manufacturer (Smurthwaite, 2014) led to the sampling rate being fixed at 10 L min -1. A fixed flow rate is required for Method 201a in order to maintain the cut point of the cyclones. In addition, previous works have shown that isokinetic sampling is not necessary when working with very fine particulate, such as the type originating from stoves (Cottone and Messer, 1987). Sampling was carried out for a period of approximately 20 minutes for each fuel before the filters generally became blocked and it was not possible to maintain the set flow rate. After weighing, the filters were immediately transferred into a fridge for storage before analysis.
Theoretical Ultimate Proximate 4. Results The proximate, ultimate, and higher heating value (HHV) results for each fuel are presented in table 1. Also included in the table are the average burning rates for each fuel, and the theoretical flue gas composition which is calculated from the ultimate analysis results. Polish coal Local cut wood RSPB Reed 1 Moisture (% ar) 7.2 7.8 Volatiles (% db) 39.4 79.3 85.1 Ash (% db) 4.2 0.9 4.6 Fixed carbon (% db) 57.9 20.5 10.0 C (% ar) 74.2 42.1 46.5 H (% ar) 5.1 5.9 6.5 N (% ar) 1.4 0.3 0.4 S (% ar) 0.4 0.0 O (% ar) 9.1 43.6 42.0 HHV (MJ kg -1 ) 33.682 17.325 17.490 Burning rate (kg hour -1 ) 0.79 2.48 1.35 A/F stoic 11.63 7.00 9.36 CO 2 (%wet) 14.5 12.4 10.9 H 2 O (%wet) 11.9 20.8 18.2 SO 2 (ppm) 315 4 N 2 (%wet) 73.6 66.8 70.9 Table 1. Fuel characterisation results for the nine fuels. The fuel characterisation results show a large variation in the properties of the fuels. The reed briquette showed the highest volatile content, which is consistent with the observations of rapid increase in burn rate, as shown in figure 3. 1 Results are estimates for untreated reed by Dan Howard from TGA analysis
Figure 3. Mass loss with time for each fuel. Figure 3 shows a relatively uniform burning rate for wood logs and coal. The burning rate for the wood logs was significantly higher than for coal, due to the higher volatile content, which led to a shorter burn out time for the wood logs. In comparison, the reed briquettes took longer to reach the peak flaming phase than with wood, but when fully lit the fuel was flaming intensely and the burning rate rapidly increased up to 3 kg hour -1. However, following this period of intense flaming the reed briquettes smouldered for much longer than the wood logs. The fuel was still lit and exuding heat after 4.5 hours, which may have extended even further if more than 3 briquettes have been used. The duration of the flaming phase is important to note because the majority of particulate matter emissions are emitted during this time. The PM emissions factor is therefore dependent on the time of sampling. The time of sampling, burning rate and flue gas velocity for coal and reed are shown in figure 4.
Figure 4. Burning rate, velocity profile and sampling period for coal and reed briquettes. As the figure shows, the sampling time for reed briquettes coincided with the peak flue gas velocity and burning rate. PM samples were therefore taken during the most intense period of flaming and PM production, which led to the highest PM emissions factors, as shown in table 2. Emission factor (mg MJ -1 ) Fuel PM t PM 10 PM 2.5 Coal 189 4 185 Wood logs 201 0 201 RSPB reed briquettes 732 8 724 Table 2. PM t, PM 10 and PM 2.5 emissions factors from the cyclone method. The results from the cyclone tests also show that a very small mass of PM greater than 2.5 µm was collected for any of the fuels. This is consistent with observations in the literature which show that the majority of particles are below PM 2.5 and even PM 1 (McDonald et al., 2000). The time at which samples are taken for particulate analysis was found to have a substantial influence on the final figure for the emissions factor for all fuels tested. For example, the higher volatile fuels release a highly carbonaceous dark aerosol during flaming combustion. Sampling during this period leads to a higher emissions factor. Longer sampling times for the higher volatile fuels may extend beyond the flaming phase, when PM production reduces. Therefore, the total PM mass will be divided into a larger sample volume. The peak burning rate was also correlated with peak flue gas temperature and peak NOx emissions. NOx emissions were mostly derived from fuel-nox and so dependent on the N content of the fuel. However, reed briquettes were an exception, as shown in figure 5.
Figure 5. Comparison of NOx emissions for each fuel. The undergrate (usually fine ash) and overgrate (usually unburnt fuel) losses are shown in figure 6. Figure 6. Undergrate (dark) and overgrate (light) losses for the three fuels. As the figure shows, the reed briquettes had a low percentage of total ash or residual fuel remaning compared to the coal (5.7%). However with most fuels there is a clear difference between the overgrate and undergrate losses because the overgrate losses are generally large pieces of residual fuel. In the case of reed briquettes, there was very little residual fuel but the ash remained densely packed into the original shape of the briquette, as shown in figure 7.
Figure 7. Reed briquettes before and after the stove run. It is also very important to note the presence of clinker in the ash residue. This is ash that has melted and fused into glass-like pieces. The total mass of clinker recovered from the ash was 7.19 g, and the largest size was approximately 40 mm, as shown in figure 8. Figure 8. Clinker pieces recovered from the stove ash after the reed briquette testing. Ash melting has not been observed in any similar tests using commercially available solid fuels. The presence of clinker in the reed ash is consistent with previous results of ICP-MS analysis which revealed a high trace element content in the reeds, particularly chlorine.
Figure 9. Fused ash piece recovered from the reed ash. 5. Conclusions and Recommendations Proximate and ultimate analyses of the nine fuels has revealed substantial differences in fuel properties such as moisture and volatile content, which are correlated with the PM emissions. The highest PM emissions factors, in excess of 700 mg MJ -1, were associated with the reed briquettes. However, repeat testing is recommended to confirm this, as flue gas sampling was done during the period of peak flaming and peak particulate emission. It was found that the majority of particles are below PM 2.5. The reed briquettes also had the highest NOx emissions (< 240 ppm at 11% O 2 ). Work is ongoing to reduce uncertainty by repeating tests under different combustion conditions. It is recommended that the tests be repeated to verify results with particular focus on the particulate emissions tests, which were very high compared to the other fuels. However, as discussed, this may be a result of the delay in the briquettes catching, which led to the sampling time covering only the smokiest phase of combustion. Due to the discovery of clinker in the ash after the stove run, it is also recommended that ash fusion tests be carried out on the reed briquettes to establish the characteristic ash melting temperatures, in accordance with is DD CEN/TS 15370-1:2006. Further work should also examine the characteristics of the soot collected during the tests (extractions, SEM and TGA). The impinger water should also be analysed for chlorine content and compared with the other fuels. The evidence of clinker formation, combined with previous work (ICP-MS) suggests the reed briquettes have a high trace element content. Of particular interest is the chlorine content, as the burning of high chlorine fuels may lead to the emission of dioxins, which are highly carcinogenic organic compounds. It is suggested that emissions abatement technologies are considered for the briquette fuels in their current formulation. Other option are to investigate pre-treatment/ upgrading of the fuels in order to
slow down the combustion or decrease the 'flaming combustion' duration and extend the 'char burning' duration. This would result in reduced peak emissions. 6. References BØLLING, A. K., PAGELS, J., YTTRI, K. E., BARREGARD, L., SALLSTEN, G., SCHWARZE, P. E. & BOMAN, C. 2009. Health effects of residential wood smoke particles: the importance of combustion conditions and physicochemical particle properties. Particle and Fibre Toxicology, 6, 20. COTTONE, L. E. & MESSER, E. 1987. Test Method Evaluations and Emissions Testing for Rating Wood Stoves, Research Triangle Park, NC 27709, U.S.EPA. MCDONALD, J. D., ZIELINSKA, B., FUJITA, E. M., SAGEBIEL, J. C., CHOW, J. C. & WATSON, J. G. 2000. Fine Particle and Gaseous Emission Rates from Residential Wood Combustion. Environmental Science & Technology, 34, 2080-2091. NAEHER, L. P., BRAUER, M., LIPSETT, M., ZELIKOFF, J. T., SIMPSON, C. D., KOENIG, J. Q. & SMITH, K. R. 2007. Woodsmoke Health Effects: A Review. Inhalation Toxicology, 19, 67-106. SMURTHWAITE, M. 2014. RE: Personal communication. Type to MITCHELL. WILLIAMS, A., JONES, J. M., MA, L. & POURKASHANIAN, M. 2012. Pollutants from the combustion of solid biomass fuels. Progress in Energy and Combustion Science, 38, 113-137.