Environmental legislation requires that power plants monitor the composition of their ash as well as the impact of the emissions or possible re-usage of the ash on the environment. However, this elemental data has not been commonly used for evaluating workers’ exposure, which was our aim in this study. The results of this study showed equally high elemental median amounts, and estimated exposure between bottom and fly ashes in pellet-, wood- and peat-fired power plants, and clearly higher median amounts, and higher estimated exposure in an SRF power plant. The results of combined element ash data, and measured inhalable dust air concentrations showed good correlations and p-values (p <0.0001) of Mn, Th, Al, and Cd. According to these results, it seems to be important to use environmental elemental ash data in worker exposure assessments in biomass-fired power plants.

• Biomass-fired power plants

• Ash

• Occupational exposure

Jumpponen M, Laitinen J (2016) Usefulness of Biomass-Fired Power Plant Ash in Worker Elemental Exposure Evaluations. Chem Eng Process Tech 2(3): 1031.

In Finland, environmental legislation requires that power plants are aware of the amount and composition of their emissions, and of the impacts of the pollutant sources’ operations on the environment [1]. Therefore, the amount of elements in the fly ash and bottom ashes of biomass-fired power plants are measured regularly [2]. Power plant ash has been reported to contain the major (B, Al, Ca, Fe, K, Mg, Na, P, Si, Ti) and minor (As, Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, TI, V, Zn) elements that are formed from solid biomass, together with Cl and S [3- 5]. The amount of elements in fly ash and bottom ash samples in Finland has been reported in several studies that have used forest residues as fuel [6,7]. In these studies, the highest amounts of elements have been reported in fly ash. However, these studies are focused on explaining the environmental risks of elements or the corrosive environment of boilers [2,7], and therefore ash removal and maintenance workers’ exposure to elements of ash in biomass-fired power plants remains an almost unknown issue.

It is known that workers are exposed to boiler ash when power plant boilers are shut down for ash removal and maintenance operations [8]. Operations start after two days of power plant cool down when ash removal workers go inside the boilers, super heaters, and electrical filters, etc. Inside the boilers, the ash may still be hot, and all surfaces are covered with ash. As a consequence, ash removal workers are exposed to ash and its compounds (i.e. dusts and elements) [3,9,10].

Currently, there are no easy passive methods for assessing workers’ exposure to inhalable dust and its elements. In exposure assessments, air samples are usually taken actively by IOM samplers’ membrane filters from workers’ breathing zones, using sampling pumps. After sampling, the filters are sent on for laboratory analyses, which can take from several days to weeks. Direct reading instruments have been developed to provide immediate concentrations of inhalable dust [11]. However, as there are no low-priced direct reading instruments to provide immediate concentrations of metals, laboratory analyses of metals are needed. Workers’ exposure to elements can be calculated using power plant ash elemental data (which already exist in power plants due to environmental legislation requirements), and the reported average concentrations of   inhalable   dust or concentrations given by a direct reading instrument. This procedure enables the combining of environmental and occupational monitoring benefits, and the occupational exposure assessment of workers to elements can be carried out as easily as through passive methods.

The purpose of this study was to evaluate the usefulness of ashed fuel, and bottom-, and fly ashes, and calculated air concentrations of ash in occupational exposure assessments.

Technical details of biomass-fired power plants

The capacity of biomass-fired power plants ranged from small, 0.3 MW, to large, 110 MW. Pellets (100%) were the main fuel in two of the plants (total thermal input 0.3–0.7 MW), wood (wood chips, saw dust, and bark 95-100% and peat 5%) in three (total thermal input 17–40 MW), and peat (peat 75-100% and wood chips, saw dust, and bark 25%) in two (total thermal input 4.5–110 MW). Solid recovered fuel (SRF), recycled wood (25%) and wood chips (25%) was used in one biomass-fired power plant; peat (50%) being the main fuel.

Material samples and comparison studies of ash

Ash samples (bottom ash, ntot=21; fly ash, ntot=11) were collected from the places in which the workers performed their tasks. Each power plant delivered mixed fuel samples (ntot=9) to our laboratory for fuelashing procedures, and further elemental analysis. Biofuel samples were ashed before element sample analysis at 815ºC. The elements of ash and fuels were analyzed by ICP-OES and ICP-MS, using the ISO 11466 method [12], and a total of 43 different elements were analyzed. Labtium Oy, T025, is an accredited test laboratory (FINAS accreditation services EN ISO/IEC 17025), which specializes in the metal analysis of ash and solid biofuels, and was responsible for the ashing of fuel samples, and the element analysis of ash and fuel samples in this study.

Median amounts of elements in ashed fuel samples, bottom ashes, and fly ashes were calculated, and elemental (Mn, Al, Fe, Ba, Cu, Cr, Ni, Pb, Co, As, Se, Th, Cd, and Be) amounts of pellet-, wood-, peat-, and SRF-fired power plants were presented separately after elemental analysis of ashed fuels and ashes. Only data of these elements were presented because these elements (excluding Th) have occupational exposure values (OEL) [23].

Air samples   and   comparison   between   calculated   and measured elemental air concentrations.

For the comparison study, inhalable air dust and its elemental (Al, n=24; As, n=24; Pb, n=24; Cd, n=24; Mn, n=24; and Th, n=22) concentrations were measured in the breathing zones and stationary sites of 35 male workers in eight biomass-?red power plant boilers. These metals were chosen for a workers’ metal exposure assessment based on their carcinogenic and neurological effects. We did not have any possibility to measure all ash elements in the air because of a limited research budget. The inhalable dusts and metals were collected in an IOM sampler ((Millipore-filter, 25 mm AAWP, pore size 0.8 µm), at a calibrated flow rate of 2.0 L/min. This inhalable dusts sampling method fulfils the requirements of European standard EN 481, and collects more than 77 percent of total particles in the air, with an aerodynamic diameter of less than 10µm [13]. After sample collection, we gravimetrically analyzed the filters of all IOM samples. The metal analysis of the samples used the ICP/ MS technique [14]. The Client Services of the Finnish Institute of Occupational Health, T013, is an accredited test laboratory (FINAS accreditation services EN ISO/IEC 17025), and the collection and analysis method of inhalable dust and metals in this study falls under the purview of accreditation.

The concentrations of calculated air elements (CC) were calculated using the measured average concentrations of inhalable dust in the boilers (CI), and the results of the elemental analysis of the material ash samples (CA) (Equation 1).

CC = CI * (CA / 1 000 000)

The following measured inhalable dust air  concentrations (CIs;0.5 mg/m3 ? 290 mg/m3) and amounts of elements [CAs;(Mn (440 mg/kg – 23000 mg/kg), Th (1.1 mg/kg – 6.2 mg/kg), Al (8300 mg/kg – 32000 mg/kg), Cd (0.1 mg/kg – 46 mg/kg), Pb (0.7 mg/kg – 3900 mg/kg), and As (3.2 mg/kg – 410 mg/kg)] were used to determine the concentrations of calculated air elemental (CC). After this, these calculated results were compared to the measured air elemental concentrations (Mn; 0.002-3.1 mg/m3, Th; 0.0001-0.0024 mg/m3, Al; 0.01-5.1 mg/m3, Cd; 0.0001-0.019 mg/m3, Pb; 0.0001-2.5 mg/m3, and As; 0.0001-0.027 mg/m3). The correlation between the measured and calculated elemental concentration data was determined using a linear regression fit. Thep-values, and 95th percentiles ofthe data were also reported.

Comparison of ashed fuel-, bottom- and fly ash amounts

Median amounts of Cr, Ni and Se were higher in fuel pellets than in pellet bottom and fly ashes. Possible sources of fuel Cr and Ni can be metal alloys [15], which are used in many different processes when wood logs are processed to sawdust. A possible source of pellet fuel can be wood pellet additives, i.e. potatoes (saw dust binding agent), which are added to saw dust in the pellet making process [16] (Figure 1).

Figure 1: Median amounts and standard deviation of fuel (pellet, n=2; wood, n=3; peat, n=2; SRF, n=2), bottom ash (pellet, n=6; wood, n=10; peat, n=3; SRF, n=2), and fly ash (pellet, n=2; wood, n=4; peat, n=3; SRF, n=2) elements (mg/kg) in pellet-, wood-, peat-, and SRF-fired power plants

There were no big differences in the levels of elements in bottom and fly ashes when standard deviation of elemental amounts was taken into account. A clear enrichment of elements to bottom or fly ash was not observed. These finding shows that the elemental exposure of workers to bottom ash elements (Mn, Al, Fe, Zn, Ba, Cu, Cr, Ni, Pb, Co, As, Th, Cd, and Be) can be equally high during a work task, i.e. in boiler grate, or during work tasks in the super heater area (Figure 1).

The highest median amounts in pellet bottom and fly ashes were Mn, Al, and Fe, the amount of which varied between 11800- 24000 mg/kg. The median amounts of other elements in bottom and fly ashes decreased in the following order Ba>Zn>Cu>Cr>N i>As>Co>Pb>Cd>Th>Se>Be, and Zn>Ba>Cu>Cr>Ni>Pb>As>Co>C d>Th>Se>Be, respectively (Figure 1).

The median amounts of elements in wood fuels were at the same level as amounts in wood bottom and fly ashes. The median amounts of Al, Pb, Co, and Se in wood fly ashes were higher than in wood bottom ashes, which can predict higher exposure levels of these elements in work tasks when workers are exposed to wood fly ash when compared with possible bottom ash element exposure to these elements, because of enrichment of these elements to wood fly ash (Figure 1).

The highest median amounts in wood bottom and fly ashes were Al, Fe, and Mn (2400-19000 mg/kg); however, Mn amounts were smaller in wood bottom- and fly ashes than in pellet bottom- and fly ashes. The median amounts of other elements in bottom and fly ashes decreased in the following order Zn>Ba>C u>Cr>Ni>As>Pb>Co>Th>Be>Se>Cd, and Zn>Ba>Pb>Cu>Cr>Ni> As>Co>Cd>Th>Se>Be, respectively (Figure 1).

In peat fuels the median amounts of Al, Fe, Th, and Be were higher than the amounts of these in peat bottom and fly ashes. High peat Al and Fe amounts can be due to Al and Fe additives which are added to peat soils to bind fertilizer phosphorous to minimize leaching [17]. Peat itself can contain Th and Be, and soil in the peat layers can also contain Th, which is the possible source of peat fuel Th and Be [18,19]. Peat fuel Mn, Zn, Ba, Cu, Cr, Ni, Pb, Co, As, Se, and Cd amounts were at the same level as the amounts of these in peat bottom and fly ashes (Figure 1).

The median amounts of As and Cd in peat fly ashes were higher than in peat bottom ashes, which can expose workers to As and Cd more when workers are working with peat fly ash. The median amounts of As in peat fly ash were clearly higher than in pellet and wood fly ashes. The amounts of Mn, Al, Fe, Zn, Ba, Cu, Cr, Ni, Pb, Co, Se, Th, and Be were in the same range between bottom and fly ash, which could predict equally high exposures of these in bottom and fly ash exposures (Figure 1).

The highest median amounts in peat bottom and fly ashes were Fe, Al, and Mn (440-66000 mg/kg); however, the Mn amounts were clearly smaller in peat bottom and fly ashes than in wood or pellet bottom- and fly ashes. The median amounts of other elements in bottom and fly ashes decreased in the following order Ba>Zn>Cr>Cu>Ni>Pb>As>Co>Th>Se>Be>Cd, and Ba>Cr>Z n>Cu>As>Ni>Pb>Co>Th>Se>Cd>Be, respectively (Figure 1).

In SRF fuels the median amounts of Ba and Ni were higher than the amounts of these in SRF bottom and fly ashes. The high amount of Ba in SRF fuels may be due to recycled building materials used in SRF fuels [20] or concrete impurities in wood fuels [21]. High Ni amounts in SRF-fuel can be due to the fuel itself [21,22] or impurities like metal alloys and steel dust [20], which get into fuel during its processing (Figure 1).

SRF fly ashes contained bottom ash at higher median amounts of Mn, Al, Zn, Cu, Cr, Pb, Co, As, Se, Cd, and Be, and showed that fly ash may expose workers to much more than bottom ash during work tasks with fly ash. Especially with Cu, Pb, and Cd the median amounts were several times higher in SRF fly ashes than in bottom ashes, and were enriched clearly to fly ashes. Only the Ba, Ni, and Th median amounts in SRF bottom and fly ashes were at the same level.

The highest median amounts in SRF bottom ashes were Fe, Al, and Mn (640-10500 mg/kg), and in fly ashes Al, Fe, Zn, and Pb (3900-21200 mg/kg). The median amounts of other elements in bottom and fly ashes decreased in the following order Zn>Cu>Ba>Cr>Pb>As>Ni>Th>Co>Se>Be>Cd, and Cu>Mn>As>Cr>Ba>Cd>N i>Co>Se>Th>Be, respectively (Figure 1).

Comparison between calculated and measured elemental air concentrations

The comparison of calculated and   measured   elemental air concentrations of Mn, Th, Al, Cd, Pb, and As is presented in Figure (2).

Figure 2: Comparison of measured and calculated elemental (Mn, n=24; Th, n=22; Al, n=24; Cd, n=24; Pb, n=24; As, n=24) concentrations (mg/m3) in air, 95th percentiles, correlations, and p-values.

The results concerningMn (r=0.9031), Th (r=0.8829), Al (0.8228), and Cd (0.6128) showed good correlations (r) with the calculated and measured data, and the p-values (p<0.0001) of these elements were also statistically significant. The measured air concentration of Mn, Al, and Cd were 1.0-1550%, 0.5-255%, and 0.5-95% of their Finnish occupational exposure limits (OELs, Mn=0.2 mg/m3, Al=2.0 mg/m3, and Cd=0.02 mg/ m3), respectively. Pb and Asresult showed weaker correlations, at0.6026 and 0.5400, respectively, and their p-values also had the same trend (Pb; p=0.0018 and As; p=0.0056). The measured air concentration of Pb and As were 0.1-2500% and 1.0-270% of their Finnish occupational exposure limits (OELs, Pb=0.1 mg/m3 and Cd=0.02 mg/m3), respectively.

This finding proves that material sample element data regarding Mn, Th, Al, and Cd can provide valuable information on workers’ exposure assessments when inhalable dust concentrations are known and elemental ash data are present. The results showed   that   the   measured   air   concentrations of Mn, Al, Pb and As exceeded their OELs by several times, and also how important workers’ exposure assessments are. However, elemental data on power plant ashes is still rarely used in workers’ exposure assessments, and routine available information regarding the elemental composition of ash should be used more often in occupational risk evaluations at biomass fired power plants.

The amounts of the elements in the bottom and fly ashes of power plants have been reported widely in the literature [2,6,7], but these results have not been routinely used in workers’ occupational exposure assessments. The results of this study showed that the ashed fuels and bottom and fly ashes contained fifteen different elements that are potentially harmful to workers, and that the air concentrations of Mn, Al, Pb and As exceeded their OELs by several times, indicating potentially high exposures for workers.The results of ashed fuels showed that fuel processing, fuel additives, and soil additives may have effects on fuel elemental quality. Fuel quality is linked to the amounts of elements in thebottom and fly ashes of power plants, as well as to worker elemental exposure in tasks in which workers are exposed to power plant ash and its elements. Therefore, using high-quality fuel in power plants could be the first step towards protecting workers against ash elements.

The results of bottom and fly ashes showed that there were no clear differences between the median levels of Mn, Fe, Zn, Ba, Cu, Cr, Ni, Th, and Be between bottom and fly ashes of pellet-, wood- and peat-fired power plants (when standard deviation of results were taken into account), which tells us that the bottom ash and fly ash of these can be equally harmful to workers. However, the median amounts of Pb, Co, As, Se, and Cd were higher in fly ashes in wood- and peat-fired power plants, which can result in higher exposures of these when workers are exposed to the fly ashes of these. The results of SRF fly ashes showed that they contained higher median amounts of Mn, Al, Zn, Cu, Cr, Pb, Co, As, Se, Cd, and Be, and may expose workers to much more than bottom ash, and only the median amount of Ba, Ni, and Th in SRF bottom and fly ashes were at the same level.

The comparison of calculated and measured elemental air concentrations of Mn, Th, Al, and Cd showed good correlations and p-values (p <0.0001) between the calculated and measured data, and showed that the elemental data of ash of these can be used in the risk assessment of workers to power plant elements.

1. Directive 2008/1/EC of the European Parliament and of the Council of 15 January 2008 concerning integrated pollution prevention and control (Codified version) (Text with EEA relevance). 2008. Official Journal of the European Union.
2. Rönkkömäki H, Pöykiö R, Nurmesniemi H, Popov K, Merisalu E, Tuomi T, et al. Particle size distribution and dissolution properties of metals in cyclone fly ash. Int. 2008. J. Environ. Sci. Tech. 2008; 5: 485-494.
3. Khan AA, de Jong W, Jansens PJ, Spliethoff H. Biomass combustion in fluidized bed boilers: Potential problems and remedies. 2009. Fuel processing technology. 2009; 90: 21-50.
4. Obernberger I, Brunner T, Bärnthaler G. Chemical properties of solid biofuels-significance and impact. Biomass and Bioenergy. 2006; 30: 973-982.
5. Emilsson S. International Handbook. From extraction of forest fuels to ash recycling. Swedish Rorest Agency. 2006.
6. Korpijärvi K, Ryymin R, Saarno T, Reinikainen M, Räisänen M. Utilisation of ashes from co-combustion of peat and wood – Case study of a modern CFB-boiler in Finland. ASH. 2012: 25-27.
7. Pöykiö R, Rönkkömäki H, Nurmesniemi H, Perämäki P, Popov K,Välimäki I, Tuomi T. Chemical and physical properties of cyclone fly ash from the grate-fired boiler incinerating forest residues at a small municipal district heating plant (6MW). 2009. J Hazard Mater. 2009; 162: 1059-1064.
8. Yager JW, Hicks JB, Fabianova E. Airborne Arsenic and Urinary Excretion of Arsenic Metabolites during Boiler Cleaning Operations in a Slovak Coal-fired Power Plant. Environ Health Perspect. 1997; 105: 836-842.
9. Jumpponen M, Rönkkömäki H, Pasanen P, Laitinen J. Occupational exposure to solid chemical agents in biomass-fired power plants and associated health effects. Chemosphere. 2014; 104: 25-31.
10. Hicks J. and Yager J. Airborne crystalline silica concentrations at coal- fired power plants associated with coal fly ash. J Occup Environ Hyg. 2006; 3: 448-455.
11. Freestone JL, Pahler LF, Thiese MS, Larson RR. A comparison of real- time monitoring of select metal concentrations in a copper smelter workplace compared to standard pump air sampling monitoring methods. Journal of Chemical Health and Safety. 2011; 18: 13-20.
12. ISO 11466, Soil quality- Extraction of trace elements soluble in aqua regia, 1995.
13. EN 481. Workplace atmospheres – Size fraction definitions for measurements of airborne particles. European Committee for Standardization, Rue de Stassart. 1993; 36: 1050.
14. Niosh 7303 elements by icp (hot block/hcl/hno3 ashing): method 7303. 2003. Niosh manual of analytical methods (nmam), fourth edition, 2003.
15. Lyyränen J, Ohlström M, Moilanen A, Jokiniemi J. Selvitys Raskasmetallipäästöistä                Suomen      Energiantuotannossa Tutkimusselostus. 2004.
16. Urbanowski E. Strategic analysis of a pellet fuel opportunity in northwest British Columbia. 2005. Simon Fraser University.
17. Metla Project 3264. Maintenance of sustainable nutrients status in peatlands stands. 1999-2003. The Finnish Forest Research Institute, Oulu Unit, PL 413, FI-90014 Oulunyliopisto, FINLAND.
18. Hansson SV, Kaste JM, Chen K, Bindler, R. Beryllium-7 as natural racer for short-term downwash in peat. 2014. Biogeochemistry. 2014; 119: 329-339.
19. Rowe PJ, Richards DA, Atkinson TC, Bottrell SH, Cliff RA. Geochemistry and radiometric dating of a Middle Pleistocene peat. GeochimicaetCosmochimicaActa. 1997; 61: 4201-4211.
20. Korri J. Selvitys kierrätyspolttoainejakeiden ominaisuuksista ja soveltuvuudesta kaasutus-CHP-laitoksen polttoaineeksi. 2011.Pro- gradu – tutkielma. Jyväskylän yliopisto. In Finnish.
21. Swart DW. The Utilization of Alternative Fuels in The Production of Portland Cement. 2005. B.S., Auburn University.
22. Wilen C, Moilanen A, Hokkinen J, Jokiniemi J. Fine Particle Emissions of Waste Incineration. VTT Project Report. 2007.
23. Ministry of Social Affairs and Health. 2014. HTP VALUES 2014. Concentrations known to be Harmful. Publications of the Ministry of Social Affairs and Health 2014: 2. In Finnish.