# Insights into PCDD/Fs and PAHs in Biomass Boilers Envisaging Risks of Ash Use as Fertilizers
## 1 Introduction
For protection of human health, limits of PCDD/Fs in foodstuffs [22] were pushed down to safer values [23], namely for free-range eggs, which are highly sensitive to dioxin effects. [PERSON] et al. [8] evaluated the sources and influence of soils contamination on food safety, for example, 5 ng TEQ/kg of PCDD/Fs + PCBs in soils induces contents above the safe limits in free-range chickens/eggs, exceeding the Tolerable Daily Intake (TDI). Among identified sources, the application of ashes and sludges are referred to as relevant causes of soils contamination. This highlights the urgent need to update soils regulations, as well as regulations and monitoring of products used as fertilizers and pesticides, with protective limits for POPs. Between 2010 and 2017, the efforts to control greenhouse gases (GHG) resulted in an increase of 70% and 37% of biomass used for electricity and heat production, respectively [24]. This may increase pressure on the environment since about 90% consists of forest biomass. Hence, sustainable principles to increase bioenergy must include proper ash management and protection of soils, incorporating the Stockholm POPs Convention orientations. Around 10% of the world primary energy is generated by combustion of biomass, using several combustion technologies, like fluidized bed (FB) systems, grate furnaces (GF) and pulverized combustion (PC) [25]. Combustion processes generate emissions of air pollutants and fine particulate matter (PM), and result in several ash streams like bottom or bed ash, boiler ash, filter ash and electrostatic precipitators (ESP) fly ash. Ash characteristics depend on the biomass used and combustion technology, as well as on air pollution control devices (APCDs) and ash collection strategy. Biomass ashes are essentially composed of inorganic elements contained in the fuels, plus extraneous minerals from adhering soil or sand, minerals used as bed material in the case of FBsystems, and, eventually, additives and sorbents added for pollution control. For clean woody biomass ashes, mostly composed of Si, Ca, Mg, K, P, Na, Al, Fe, Cl and S, only traces of heavy metals are usually present, however, co-combustion with wastes may raise toxic metals to concerning levels [26; 27; 28; 29; 30]. Ash may also contain many Products of Incomplete Combustion (PICs), including POPs, of which the most concerning are PCDD/Fs, PAHs, and PCBs [31; 32; 33; 34; 35]. The reutilization of ashes is highly attractive for a great variety of purposes, driven by management needs and circular economy [36; 37]. Interest of using biomass ashes as soil fertilizers (a traditional practice) has been broadly investigated, mainly concerning physicochemical properties, composition, presence of heavy metals and leachability [38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48]. Some publications address the problem of PAHs [49; 50; 51; 52; 53], but usually PCDD/Fs and other organic pollutants are not mentioned or are considered of minor relevance in biomass ashes to be applied in soils. However, the intensive use of biomass ashes may have significant impacts; for example, the application of industrial wood ashes containing 7.4-36 ng TEQ/kg, at 18-38 ton/acre/year, was estimated to increase PCDD/Fs in soils by 0.15-1.53 ng TEQ/kg a year [54]. [PERSON] et al. [55] also found that wood ash application, containing 16.8 mg/kg of PAHs and 3.4 \(\mu\)g/kg of PCBs, increased by six-fold the concentrations of PAHs in a forest soil. On the other hand, a decrease of PCBs was observed, possibly due to enhanced solubilization, owing to the ash alkalinity. This may indicate that deeper soil levels and water aquifers may become contaminated with PCBs. A previous paper [56] presented the chemical constitution of biomass ashes, revealing low levels of heavy metals and low mineral reachability. Some TEQs and \(\Sigma\)PAHs were also presented to highlight that biomass ash may exhibit POPs contamination, although the EU proposal to regulate the use of ashes as soils fertilizer was not available then.