Phosphorus recovery from the biomass ash: A review
Introduction
During the last years, an increasing interest for sustainable energy production has been seen globally. This development is mainly caused by the international concern regarding CO2 emissions. Biomass fuels are CO2-neutral and their use for energy production could substantially decrease the greenhouse effect at a global level [1], [2]. Thermo-chemical technologies, especially in the form of combustion are thought to be promising solutions for energy production from biomass. Its most advanced form is considered to be fluidized bed combustion [3], [4], [5]. Biomass fuels usually have a high content of alkali metals, which, together with other mineral components of the ash, give rise to severe ash deposition. Ash deposition can reduce the heat transfer and increase the boiler tube corrosion [6], [7], [8]. Although biomass ash can produce some disadvantages, due to some biomass ash such as olive ash, sludge ash, meat and bone meal (MBM) ash, etc. containing a large of valuable nutrient element such as phosphorus and potassium, people can reuse biomass ash for recovery nutrients. Especial for P, the merging of food and fuel economies has increased the demand of mineral P fertilizer, and its price increased over 200% in 2007 [9], [10]. So about P recovery from biomass ash is also very important for biomass energy utilization.
Apatite minerals are the main raw materials used in the manufacturing of fertilizers. However, the resources of apatite in the world are estimated to last only for about 200 years. Therefore, sustainable methods to recycle the phosphorus used in the society are needed [128]. Phosphorus is a limited non-renewable resource, which is indispensable as an essential nutrient for the growth of organisms in most ecosystems, and cannot be replaced by other elements. It is a very important element for many industries as well. The demand for phosphorus fertilizer alone increased with the increase in the world's population from 9 × 106 to 40 × 106 metric tons between 1960 and 2000 and was expected to increase further to 20 × 106 metric tons by 2030 [123]. Phosphorus, being a limited natural resource, is used in the industry in many different applications. As primary phosphorus compounds in sewage sludge constituents of organic origin were identified. The wastewater de-phosphorization by the biological cleaning stages enriches the phosphorus concentration in the organic residues, mostly as adenosintriphosphate (ATP). A share of approximately 10% is bound in stable iron or alumina phosphates during the chemical precipitation step [130]. Generally, the phosphorus is enriched in the fly ash together with the trace elements. An important issue in the development of sludge ash utilization strategies is whether the concentration of trace elements is low enough to allow the use of sludge ashes or products thereof as fertilizers [126]. The ash has a relatively high P content (approximately 5.4% by weight) and could therefore, with advantage be recycled as a P fertilizer on agricultural land. Recycling of P is especially important seen in the light of the shrinking global phosphate rock reserves and increases in demand for P fertilizer in agriculture [124], [125]. In MBM, a significant share of the phosphates is hydroxyapatite, the major constituent of bones. A percentage of approximately 10% is also organically bound phosphates similar to sewage sludge. Due to the low boiling points of organic phosphorus compounds, it is assumed that these are released as gaseous phosphorus oxides in the boiler. The phosphorus oxides condense in the temperature range of 400–600 °C, forming primarily phosphorus-oxide, P4O10, and, in the likely presence of water, orthophosphoric acid, H3PO4 [131]. As an alternative to phosphorus rock, phosphorus can be recycled from phosphorus-rich residues, such as meat and bone meal (MBM), municipal sewage sludge, phosphorus-rich ashes, agricultural residues, etc.[128]. The plant nutrients such as phosphorus, potassium, magnesium, and sodium, are nearly completely recovered in the agriculturally usable ash-mixture of bottom ash and cyclone fly ash. Thus, returning the ash to the grassland could close the cycles of these nutrients [127].
The objectives of this paper were to: (1) research a variety of bio-fuels and its ash composition characteristics, so that we have a general understanding of the composition of bio-fuel and its ash as the base of ash research; (2) systematically summarize and analysis phosphorus characteristics in bio-fuel ash from the biomass thermal conversion technology; (3) examine these potential technologies for P recovery from the biomass ash.
Section snippets
Bio-fuel composition analysis
Several typical bio-fuels are investigated about their composition. The ultimate analysis of different fuels is shown in Table 1. From Table 1, we can know some characteristics about the bio-fuels composition. The first, comparing to the coal, carbon content of biomass fuels is less. The highest carbon content of biomass fuels is about 53%, equivalent to the carbon content of lignite which was generated from the less age. The fixed carbon content of biomass fuels is also significantly less than
Ash composition properties from bio-fuel combustion
The biomass fuel ash composition influences boiler ash deposit formation and agglomeration, boiler tube corrosion, and ash use. Table 3 shows the chemical composition of some typical biomass ashes which data are from the literature. From Table 3, we can know that the ash mainly include K, Na, Ca, Mg, Si, P, Al, Fe, S and so on elements, especially for alkali metal and alkali earth metal content prominent. To make a better comparison of the different fuel ashes possible, we have used a
Bio-fuel agglomeration temperature
Seen from Fig. 5 and Table 6, the initial agglomeration temperatures for most of bio-fuels were from 900 °C to 1000 °C. Among these bio-fuels, the highest initial agglomeration temperature is 1020 °C for baggasse sugar cane and rapeseed meal, and the lower initial agglomeration temperatures are respectively alfalfa, wheat straw and Lucerne. So seen from the whole initial agglomeration temperature situation, bio-fuels are easy to agglomerate because the normal operational temperatures in the
Biomass thermal conversion technology for phosphorus
Phosphorus is a limited non-renewable resource, which is indispensable as an essential nutrient for the growth of organisms in most ecosystems, and cannot be replaced by other elements. Apatite minerals are the main raw materials used in the manufacturing of phosphorus fertilizers. However, the resources of apatite are estimated to last only for about 200 years [72]. So the methods of recovery phosphorus used in the world are needed. One way of recovery phosphorus is from biomass ash because
Phosphorus recovery by the bioleaching process
Loss of nitrogen and phosphorus has been reported while carrying out bioleaching of heavy metals from sewage sludge and soil–sludge mixtures [113], [114]. Shanableh et al. reported the loss of 76% phosphorus and 38% nitrogen during sludge bioleaching [115]. With increase in solid content the sludge bioleaching was enhanced, which was represented by the acceleration of sludge acidification, oxidizing environment formation, and substrate (sulfur) utilization. Higher solid content was more
Summary
To meet the growing energy demand and reduce the CO2 emissions, the dependency in biomass for power generation and disposal of biomass ash will continue to increase. So how to utilize the biomass ash as a resource is more and more important. In view of the above discussion about phosphorus from biomass ash, the salient points from this extensive review could be summarized in the following sections:
- (1)
Biomass ash composition characteristics
- (i)
Biomass ash mainly includes K, Na, Ca, Mg, Si, P, Al, Fe, S
- (i)
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