Phosphorus (P) rich ash from biomass incineration is a potential promising alternative for non-renewable phosphate rock. This study considered the P recovery potential of poultry manure ash, sewage sludge ash and meat and bone meal ash through wet chemical extraction. X-ray diffraction analysis showed that these three ash types had a distinct P mineralogy. If inorganic acids were used for the extraction, the P extraction efficiency was not or only slightly affected by the P mineralogy. Contrarily, for the organic acids, alkaline extraction liquid and chelating agents considered, the P extraction efficiency was highly affected by the P mineralogy, and was also affected by the elemental composition of the ash and/or the chemical characteristics of the extraction liquids. Alkaline extraction liquids showed in general low heavy metal co-extraction, in contrast to the inorganic acids. From an economic point of view, of all extraction liquids considered, sulfuric acid was the most interesting to extract P from all three ash types. Oxalic acid could be a more sustainable option for P extraction from sewage sludge ash. In addition, extraction of poultry manure ash with ethylenediaminetetraacetic acid showed a relatively high P extraction efficiency combined with relatively low heavy metal co-extraction.Graphic Abstract

Unfortunately, Table 1 was published incorrectly in the original publication of the article. The correct version of Table 1 is provided here

The overall aim of this research was to evaluate the valorization potential of the poultry litter ash produced in the biomass power plant of BMC Moerdijk (the Netherlands), since the ash contains phosphorus (P) and potassium (K), which are both essential nutrients. As a first step, the ash was characterized by means of chemical analysis. Ash collected in the superheater section of the boiler had the highest P concentration (10.6%). Furthermore, the P concentration in the ash decreased as it was collected further downstream in the boiler and flue gas cleaning part of the installation. K showed an opposite concentration trend, that is, its concentration was the lowest in the superheater ash (9.4%) and increased to 15.5% in the electrostatic precipitator ash. Based on the results of the chemical analysis, different valorization options could be considered. Although poultry litter ash has the same heavy metal/P ratio as poultry litter and is free of pathogens and toxic organic substances, its recycling as a P/K fertilizer is hindered by legal constraints. Furthermore, the use of the ash in/as animal feed is not straightforward because of its origin (animal feces) and waste status. Besides P and K, other ash elements such as calcium, silicon, magnesium, iron and aluminum can also be valorized, for example by using the ash as building material or in cement production. However, in these applications the high P and K concentration of the ash can be a technical obstacle rather than a benefit. In this regard, it can be interesting to separate the fertilizer elements, that is, P and K, from the rest of the ash by means of for example a wet chemical extraction after which the remaining solid residue better meets the composition requirements for building material or raw material for cement production.

When manure is introduced in a hot fluidized bed for gasification or combustion, its inorganic compounds will undergo chemical transformations upon heating. The phosphorus containing salts, which are mostly hydrogen and dihydrogen phosphates or alkali and alkaline earth metals, melt at low temperatures (200–300 °C), before they decompose. In their liquid state, these compounds may drive the formation of ash coatings on bed particles of silica sand, and irreversibly agglomerate multiple particles if they are locally present in high amounts. The recent first time observation of the latter phenomenon led to a new interpretation of an earlier concept, melt induced agglomeration. Decomposition of the aforementioned (di)hydrogen phosphates and reaction of the fresh coatings with silica bed material drives the fluidized bed towards chemical equilibrium upon the ash’s increasing temperature, in which K2Si4O9 is formed aside from Ca3(PO4)2 and CaSiO3. In this situation of (near) chemical equilibrium, a melt is formed if the amount of K2Si4O9 is high and the amount of CaSiO3 is low. Low CaSiO3 may result from a low calcium concentration in the fuel, or by a high phosphorus concentration, since Ca3(PO4)2 is the more stable form. The particle’s silicate melt is concentrated at the interior of the coating, due to the abundance of silica at this location, and any Ca3(PO4)2 makes up the exterior of the coating. If enough silicate melt can find a way through the solid exterior of the coating, entrained particles may deposit onto an inclined refractory wall above the fluidized bed. We support particle deposition as the initiating deposition step, as opposed to gaseous condensation, primarily because of the aluminum and potassium silicate chemistry involved. After deposition, SEM-EDX analysis of a deposit’s cross section revealed a chemical anchoring by chemical reaction between K2Si4O9 and the alumina rich refractory material, creating a strong solid bond of for instance potassium feldspar, which has a high melting point. Deposits can therefore grow larger before they break loose and cause bed disturbances, thus damaging the refractory wall. The comprehensive theory on transient thermal ash transformations, presented in this paper, will allow to adept the design of future thermal energy applications for manure by selecting appropriate additives and refractory bed and/or wall materials.

Agglomeration of ash in fluidized bed combustors may result in defluidization and subsequent downtime of the installation. Previous research has shown that Ca-based additives can prevent agglomeration, but the added amount was determined arbitrarily and testing occurred only on lab scale or pilot scale. This paper presents a statistical approach, based on a newly developed agglomeration index, to calculate the amount of CaO that should be added (in the form of a Ca-based mineral, e.g., CaCO3) to the fluidized bed in order to prevent agglomeration. The agglomeration index is based on an understanding of the reactions occurring in the ash, for instance, the formation of potassium silicates with low melting points, and the formation of calcium phosphates and calcium silicates with high melting points. Full-scale testing of partial replacement of silica sand by calcite (CaCO3) as fresh bed material showed that the increased CaO concentration in the ash, with respect to normal operation, appears to reduce both wall and in-bed agglomeration problems. As a measure for agglomeration risk, differential bed pressure variations were statistically analyzed. In the test periods during which CaCO3 was added, the bed pressure variations were smaller and less frequent, and the severity of agglomeration was thus reduced. The proposed strategy can be applied for fuels that are commonly perceived as difficult or unsuited for fluidized bed combustion, and also for other additives than CaCO3, e.g., Al-based minerals.

The combustion of poultry litter, which is rich in phosphorus, in a fluidized bed combustor (FBC) is associated with agglomeration problems, which can lead to bed defluidization and consequent shutdown of the installation. Whereas earlier research indicated coating induced agglomeration as the dominant mechanism for bed material agglomeration, it is shown experimentally in this paper that both coating and melt induced agglomeration occur. Coating induced agglomeration mainly takes place at the walls of the FBC, in the freeboard above the fluidized bed, where at the prevailing temperature the bed particles are partially molten and hence agglomerate. In the ash, P2O5 forms together with CaO thermodynamically stable Ca3(PO4)2, thus reducing the amount of calcium silicates in the ash. This results in K/Ca silicate mixtures with lower melting points. On the other hand, in-bed agglomeration is caused by thermodynamically unstable, low melting HPO42− and H2PO4− salts present in the fuel. In the hot FBC these salts may melt, may cause bed particles to stick together and may subsequently react with Ca salts from the bed ash, forming a solid bridge of the stable Ca3(PO4)2 between multiple particles.

The oversupply of organic fertilizers causes an urgent need for alternative treatments of manure. CO2 neutral electricity is produced from poultry manure, a renewable fuel which is relatively dry and has a heating value of 6–8 MJ/kg. The electricity production from manure saves emissions from fossil fuel combustion, resulting in a reduced environmental impact in the impact category climate change. Moreover, as manure contains a large amount of ammoniacal N, and due to nitrification and denitrification processes, land spreading of poultry manure causes larger emissions of NH3, N2O and NOx than combustion. Electricity production from manure therefore outperforms land spreading in the impact categories terrestrial acidification, particulate matter formation, marine eutrophication and photochemical oxidant formation. The fluidized bed combustor of BMC in the Netherlands generates zero waste, as the ash is recovered as a PK fertilizer, which is odorless, dry, sterile and has a lower mass and volume than the manure, making it more suitable for export to regions with a high P demand. The ash does however cause technological problems, such as agglomeration and deposition.