**5.2 Recycling of ash from biomass power plants**

Combustion of biomass to produce energy (heat or electricity) always leaves back ash in various amount and composition depending on the biomass used as raw materials in the furnaces. This ash highly alkaline due to its potassium carbonate content (pH >10). Therefore it cannot be used directly as a fertilizer; only very small amount can be applied even for acidic soils. Since the ash contains all microelements and non-volatile elements in the amount absorbed and used by the plants harvested from the given area, their amount and ratio is optimal for the plant. Its insoluble content (e.g. phosphates) can be converted via digestion to utilizable material. In addition, it can be also supplemented with nitrogen fertilizers or other additives. In this way the processed ash becomes a useful material called

An Integrated Waste-Free Biomass Utilization

other in the structure of this compound.

the soils.

fertilizer can be spread as solid by common facilities.

System for an Increased Productivity of Biofuel and Bioenergy 221

needed (Angyal et al, 2006). Since the formed mortar like mass dries and solidifies easily, after granulating or pelletizing into the usual shape of solid fertilizers, the formed eco-

This method of neutralization has numerous advantages towards the classical neutralization technologies, not only the formation of solid fertilizer instead of dilute liquids, and thus avoiding the high volume expensive reactors during manufacturing, but from chemical viewpoints as well. Normally, the ash formed from straw and energy grass contains a mixed potassium calcium carbonate (Buetschliite), K2Ca(CO3)2 as main components, the second most important phase is the KCl, and K2CO3 and K2SO4 can also be detected by powder Xray diffraction. Similar amount of magnesium hydroxide and sodium carbonate can also be detected. The ratio of potassium chloride and sulfate depends on the soil composition, and the fertilization and type of fertilizer used (KCl or K2SO4) during the production of the wheat of course. Expressing the important metal content in the form oxides are as it follows: ~40 % of K2O, ~10% CaO, 3.5 % MgO and 2.5 % of Na2O. The straw contains a lot of chlorides (~7 %), the other anions as sulfate and carbonate expressed in SO3 and CO2 are ~10 % and ~20 %, respectively. The potassium-calcium carbonate (or potassium and calcium carbonate as well) easily reacts with diluted (~50 %) sulfuric acid, but not only the expected K2SO4 and CaSO4 but their double salts as syngenite (K2Ca(SO4)2.H2O) and polyhalite (K2Ca2Mg(SO4)4.2H2O) are formed as main products. The syngenite is less soluble (but not completely insoluble) in water and has ion-exchange properties toward ammonium ion, because due to their similar sizes of potassium and ammonium ions they can substitute each

 K2Ca(CO3)2 + 2H2SO4 = K2Ca(SO4)2.H2O+ H2O + 2CO2 (8) The excess of sulfuric acid is neutralized to pH=6 with limestone powder and can be used as simple and general potassium and sulfate fertilizer which has opened pore structures which can absorb water and keep it in the pores, this way increasing the water retaining capacity of the soil. This fertilizer contains soluble phosphates and microelements previously digested by the sulfuric acid treatment. Furthermore, via controlling the amount of the calcium carbonate powder in the last step of the manufacturing, acidic (sub-neutralized), neutral or alkaline (over-neutralized) fertilizers can also be produced. No liquid or solid waste form in this technology, all the used components are built into the structure of the product. Since the water absorbing capacity of these materials, due to the porosity controlled by the synthesis conditions is very high (50-120 % of its mass), it is obvious, that not only water but various aqueous solutions can also be absorbed in these pores. This behavior opens new perspectives, namely absorbing different other fertilizers, e.g. nitrogen fertilizers, insecticides, or any other solutions of important compounds which should be injected into

The most common nitrogen fertilizer is the ammonium nitrate, however, the metal-doped NH4NO3 production has serious problems because the ammonium nitrate is prilled from the melt, and the melted ammonium nitrate is easily exploded due to the catalytic effect of metal compounds. Thus, these metal microelements cannot be added to the melted NH4NO3 before prilling. Using ammonium nitrate solutions, the metals compounds can be added in the required amounts without any risk to the porous granulated eco-fertilizer. Although drying after the absorption of the aqueous solutions (e.g NH4NO3 solutions) requires extra energy, but the complete process energetically is still more advantageous, due to the

eco-fertilizer (Angyal et al., 2006). In order to ensure the waste free technological viewpoints, the biomass power plant ash has to transform into fertilizer. It has been known for a long time that the alkaline ash can be neutralized with mineral acids, when potassium containing fertilizers are formed. However, these methods are difficult to perform technically, due to the large volume of gas formed in these reactions. Roughly 150 normal m3 of carbon dioxide evolves in the reaction of 1 ton of ash with intensive foaming. A biomass power plant with electric capacity of 50 MW produces roughly 50-60 tons ash/day which means producing of > 300 normal m3 of CO2 gas/h. Furthermore, a very large volume of dilute fertilizer solution is produced, thus the cost of transportation is very high. The evaporation of the solution is not feasible. In order to solve these problems and ensure recycling the ash components for sustainable biomass production, a new technology has been developed for the neutralization of the biomass power plant ashes. This method ensures complete digestion of potassium and phosphorus content of the ash and decreases the K and P fertilization costs which are essential in the case of plants with fast metabolism such as sugar sorghum or energy grass. The nitrogen supplement is the only one which should be ensured in a usual way with the addition of N fertilizers. In this way the sulfur deficiency of the soils, or extreme sulfur demand as in case of oily plants like rape can also be satisfied (Scherer, 2001).

The new method based on the reaction of sulfuric acid and the biomass ash in a long quasi-closed tube reactor equipped with a screw for moving the reaction mixture. The reaction substantially proceeds during the mixture motion from the one end of the reactor to the other. This technology ensures not only continuous production of neutralized ash, but other important changes also appear in the chemical constitution of the starting ash. Normally, concentrated sulfuric acid does not wet the ash and does not react with it at all, their mixing can be proceed without CO2 evolution. The reaction starts only in the presence of water. The mortar like mass prepared from the ash/limestone mixture and cc. sulfuric acid are mixed with water and reacted in the tube reactor. The reaction starts only after the dilution of the sulfuric acid. The most advantageous concentration is ~50 % (of sulfuric acid) when the carbonates reacts in a self-sustainable way due to producing water in the neutralization reactions. The key element in the reaction is the quasi-closed tube reactor. During moving the mortar like mass toward the opened end of the tube reactor, the neutralization reaction takes place and CO2 gas evolves as usual. However, the reaction mass acts as a plug which ensures that the formed CO2 gas cannot released from the reactor. The in situ evolved CO2 gas makes micro-bubbles in the material, because of the overpressure of the quasi-closed equipment, and the swelled mass fills out the tube reactor. However, the wall of the reactor and the reaction mass as a plug ensures an overpressure within the reactor. Due to the overpressure no fizzing out occurs within the reactor and the micro-bubbles of the carbon dioxide gas are kept in the mortar-like mass. If the amount of the mass is adjusted to be less in its volume than the volume of the reactor, the mass is blown up due to the evolved gas and fills completely the space within the reactor. The carbon dioxide micro-bubbles have a slight overpressure, thus leaving the tube reactor at the opened end, the gas leaves the semi-solidified mass and the places of the micro-bubbles becomes opened pores. Technologically this method ensures processing of the ash with sulfuric acid in a small volume of continuously operated tube reactor having only a volume of ~3 times larger than the volume of the ash that can be processed (~ 60-80 times larger reactors as used in the classical neutralization methods do not

eco-fertilizer (Angyal et al., 2006). In order to ensure the waste free technological viewpoints, the biomass power plant ash has to transform into fertilizer. It has been known for a long time that the alkaline ash can be neutralized with mineral acids, when potassium containing fertilizers are formed. However, these methods are difficult to perform technically, due to the large volume of gas formed in these reactions. Roughly 150 normal m3 of carbon dioxide evolves in the reaction of 1 ton of ash with intensive foaming. A biomass power plant with electric capacity of 50 MW produces roughly 50-60 tons ash/day which means producing of > 300 normal m3 of CO2 gas/h. Furthermore, a very large volume of dilute fertilizer solution is produced, thus the cost of transportation is very high. The evaporation of the solution is not feasible. In order to solve these problems and ensure recycling the ash components for sustainable biomass production, a new technology has been developed for the neutralization of the biomass power plant ashes. This method ensures complete digestion of potassium and phosphorus content of the ash and decreases the K and P fertilization costs which are essential in the case of plants with fast metabolism such as sugar sorghum or energy grass. The nitrogen supplement is the only one which should be ensured in a usual way with the addition of N fertilizers. In this way the sulfur deficiency of the soils, or extreme sulfur demand as in case of oily plants like rape can also

The new method based on the reaction of sulfuric acid and the biomass ash in a long quasi-closed tube reactor equipped with a screw for moving the reaction mixture. The reaction substantially proceeds during the mixture motion from the one end of the reactor to the other. This technology ensures not only continuous production of neutralized ash, but other important changes also appear in the chemical constitution of the starting ash. Normally, concentrated sulfuric acid does not wet the ash and does not react with it at all, their mixing can be proceed without CO2 evolution. The reaction starts only in the presence of water. The mortar like mass prepared from the ash/limestone mixture and cc. sulfuric acid are mixed with water and reacted in the tube reactor. The reaction starts only after the dilution of the sulfuric acid. The most advantageous concentration is ~50 % (of sulfuric acid) when the carbonates reacts in a self-sustainable way due to producing water in the neutralization reactions. The key element in the reaction is the quasi-closed tube reactor. During moving the mortar like mass toward the opened end of the tube reactor, the neutralization reaction takes place and CO2 gas evolves as usual. However, the reaction mass acts as a plug which ensures that the formed CO2 gas cannot released from the reactor. The in situ evolved CO2 gas makes micro-bubbles in the material, because of the overpressure of the quasi-closed equipment, and the swelled mass fills out the tube reactor. However, the wall of the reactor and the reaction mass as a plug ensures an overpressure within the reactor. Due to the overpressure no fizzing out occurs within the reactor and the micro-bubbles of the carbon dioxide gas are kept in the mortar-like mass. If the amount of the mass is adjusted to be less in its volume than the volume of the reactor, the mass is blown up due to the evolved gas and fills completely the space within the reactor. The carbon dioxide micro-bubbles have a slight overpressure, thus leaving the tube reactor at the opened end, the gas leaves the semi-solidified mass and the places of the micro-bubbles becomes opened pores. Technologically this method ensures processing of the ash with sulfuric acid in a small volume of continuously operated tube reactor having only a volume of ~3 times larger than the volume of the ash that can be processed (~ 60-80 times larger reactors as used in the classical neutralization methods do not

be satisfied (Scherer, 2001).

needed (Angyal et al, 2006). Since the formed mortar like mass dries and solidifies easily, after granulating or pelletizing into the usual shape of solid fertilizers, the formed ecofertilizer can be spread as solid by common facilities.

This method of neutralization has numerous advantages towards the classical neutralization technologies, not only the formation of solid fertilizer instead of dilute liquids, and thus avoiding the high volume expensive reactors during manufacturing, but from chemical viewpoints as well. Normally, the ash formed from straw and energy grass contains a mixed potassium calcium carbonate (Buetschliite), K2Ca(CO3)2 as main components, the second most important phase is the KCl, and K2CO3 and K2SO4 can also be detected by powder Xray diffraction. Similar amount of magnesium hydroxide and sodium carbonate can also be detected. The ratio of potassium chloride and sulfate depends on the soil composition, and the fertilization and type of fertilizer used (KCl or K2SO4) during the production of the wheat of course. Expressing the important metal content in the form oxides are as it follows: ~40 % of K2O, ~10% CaO, 3.5 % MgO and 2.5 % of Na2O. The straw contains a lot of chlorides (~7 %), the other anions as sulfate and carbonate expressed in SO3 and CO2 are ~10 % and ~20 %, respectively. The potassium-calcium carbonate (or potassium and calcium carbonate as well) easily reacts with diluted (~50 %) sulfuric acid, but not only the expected K2SO4 and CaSO4 but their double salts as syngenite (K2Ca(SO4)2.H2O) and polyhalite (K2Ca2Mg(SO4)4.2H2O) are formed as main products. The syngenite is less soluble (but not completely insoluble) in water and has ion-exchange properties toward ammonium ion, because due to their similar sizes of potassium and ammonium ions they can substitute each other in the structure of this compound.

$$\text{K}\_2\text{Ca(CO}\_3\text{)}\_2 + 2\text{H}\_2\text{SO}\_4 = \text{K}\_2\text{Ca(SO}\_4\text{)}\_2\text{H}\_2\text{O} + \text{H}\_2\text{O} + 2\text{CO}\_2\tag{8}$$

The excess of sulfuric acid is neutralized to pH=6 with limestone powder and can be used as simple and general potassium and sulfate fertilizer which has opened pore structures which can absorb water and keep it in the pores, this way increasing the water retaining capacity of the soil. This fertilizer contains soluble phosphates and microelements previously digested by the sulfuric acid treatment. Furthermore, via controlling the amount of the calcium carbonate powder in the last step of the manufacturing, acidic (sub-neutralized), neutral or alkaline (over-neutralized) fertilizers can also be produced. No liquid or solid waste form in this technology, all the used components are built into the structure of the product. Since the water absorbing capacity of these materials, due to the porosity controlled by the synthesis conditions is very high (50-120 % of its mass), it is obvious, that not only water but various aqueous solutions can also be absorbed in these pores. This behavior opens new perspectives, namely absorbing different other fertilizers, e.g. nitrogen fertilizers, insecticides, or any other solutions of important compounds which should be injected into the soils.

The most common nitrogen fertilizer is the ammonium nitrate, however, the metal-doped NH4NO3 production has serious problems because the ammonium nitrate is prilled from the melt, and the melted ammonium nitrate is easily exploded due to the catalytic effect of metal compounds. Thus, these metal microelements cannot be added to the melted NH4NO3 before prilling. Using ammonium nitrate solutions, the metals compounds can be added in the required amounts without any risk to the porous granulated eco-fertilizer. Although drying after the absorption of the aqueous solutions (e.g NH4NO3 solutions) requires extra energy, but the complete process energetically is still more advantageous, due to the

An Integrated Waste-Free Biomass Utilization

respectively.

decreased.

**7. References** 

2006.

0020-2274

0887-0624

**6. Conclusion** 

System for an Increased Productivity of Biofuel and Bioenergy 223

with absorbing of aqueous liquids ensures the recycling of the by-product of the biomass power plant providing energy and electricity for the biofuel (biodiesel and biobutanol and supplemented) plants. This way we can sustain the production of the renewable energy plants, e.g. sugars sorghum, while the soil quality is maintained and improved,

By proper selection of biomass available from a given area, the sugar and energy sources, and the relative amount of the vegetable oil produced can be adjusted. In order to decrease the processing cost of raw materials into sugar containing mash for fermentation plants, the classical sugar sources as corn can be replaced with sugar sorghum, which can be processed similar to sugarcane. Combustion of the residual biomass in power plants or their digestion into biogas depend on the water and protein content of the residue and the heat or electricity demand of fuel-producing (biodiesel, biobutanol, acetals, etc.) or waste processing (fertilizer production) plants. Generally, it is more advantageous to use biomasses of high protein and water content in biogas plants. Burning the biogas or by using it as fuel in gas-engines the amount of heat and electricity can be controlled. Wastes of high cellulose content can be advantageously burned in power plants, sometimes after drying with the low heat value warm water streams of energy production. Wastes of fuel production can be utilized by combination these two methods of energy production. The ash and the solid residues from biomass power plants can be utilized as fertilizers by mixing them with potassium sulfate or calcium sulfate formed during recovery of the catalyst (KHSO4 or H2SO4) in biodiesel or acetal plants. Finally, there are two other wastes. The first is K2SO4 from the biodiesel technology, and the other is the ash from the combustion. Beyond the integration of energy producing and consuming plants and controlling the ratio of the raw materials and the type of the energy (heat or electricity), the production technology is also to be changed mainly in biodiesel, biobutanol and fertilizer plants. In this way the energy consumption of each technological step can be

Andersen, V. F., Anderson, J. E., Wallington, T. J., Mueller, S. A. & Nielsen, O. J., *Energy &* 

Angyal, A., Hujber, O., Kótai, L., Legeza, L. & Sajó, I. E., *HU Patent Appl. 0600390*,

Arlie, J.-P., *Revue de l' Institute Francais du Petrole*, Vol. 38, No.2. (1983), pp. 251-257. ISSN

Barrault, J., Pouilloux, Y., Vanhove, C., Cottin, K., Abro, S. & Clacens, J. M., *Chemistry and* 

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following considerations. Ammonium nitrate solution is prepared in an exothermic reaction of a ~65 % of nitric acid with ammonia gas, when an aqueous solution of NH4NO3 solution (roughly 80% ) is formed. Evaporation of this solution at high temperature leads to the melt of ammonium nitrate which is prilled in the next step of manufacturing. Since we use the NH4NO3 solution, which is contacted with the granules, the water removal, without melting of the NH4NO3, requires less energy than the final step of the solid NH4NO3 manufacturing. This concentrated NH4NO3 solution has acidic character, and easily react with the syngenite and other components of the eco-fertilizer. The concentrated NH4NO3 solutions are not only physically absorbed and imbibed in the pores of the eco-fertilizer, but chemically reacts with its components, as well.

#### K2Ca(SO4)2.H2O (syngenite) (NH4)2Ca(SO4)2.H2O(koktait)

The potassium ions can be substituted with ammonium ion with the formation of partially or completely ion-exchanged syngenite-like isomorphous compounds. The completely substituted product is called to be koktaite, (NH4)2Ca(SO4)2.H2O, which is less soluble in water, thus releases nitrogen slowly into the soil (Angyal et al., 2006; Coates and Woodward, 1988; Von Maessenhausen et al., 1988). The formed potassium nitrate transformed into a solid solution with the excess of the ammonium nitrate, the typical composition of this product was the K0.27(NH4)0.73NO3. The koktaite and NH4-syngenites are sparingly soluble in water, thus the ammonium ion concentration liberated in the presence of water is constant at a given temperature and ionic strength of sulfate ion. Since not the full amount of the ammonium ion is liberated, no damages to the plant and losses by washing away, respectively occur even if using in high doses. When the plant absorbs the ammonium-ion from the soil, due to the equilibrium conditions, a part of the solid will dissolve and supply the water with a new amount of ammonium ion. Since the equilibrium concentration is closely constant, the amount of water will control the amount of the released ammonium ion, namely, the release of the ammonium ion from this koktaite type compounds is controlled by raining or irrigating. In drought situation, when there is no absorption of ammonium ion from the soil by the plant, there is no dissolution of ammonium syngenites and releasing ammonium ion which would be decomposed by the soil bacteria as it happens in case of water soluble ammonium ion containing fertilizers. Besides ammonium nitrate, other fertilizer components can also be used to adjust the main element concentrations, such as K, P or N, and to change the available form of these elements in various chemical compounds. The K2HPO4 does not react at all with other components of the ash. It is interesting, that ammonium salts as NH4Cl and (NH4)2SO4 cannot transform the syngenite completely into (NH4)2SO4, even if the ammonium sulfate is in excess, but in the presence of urea, the transformation is complete. Both KCl and K2SO4 decompose the ammonium syngenite, but the mixture of the K2SO4 and the NH4Cl produces (NH4)2Ca(SO4)2.H2O. Thus, the main factor is probably the ammonium to potassium ion ratio. There is an important difference between the behavior of the potassium sulfate and potassium chloride. The latter compound is more reactive, and KCl, KNO3 and NH4MgCl3 are also formed in its presence. Using various additives not only the ratio of the agriculturally important elements (K, P, N, S) are controlled but their chemical forms can also be altered. Using various kind of soil bacteria and supplementary materials to ensure theirs intensive growing is another possibility for nitrogen-fixation in the treated area. By using the eco-fertilizer technology supplemented with absorbing of aqueous liquids ensures the recycling of the by-product of the biomass power plant providing energy and electricity for the biofuel (biodiesel and biobutanol and supplemented) plants. This way we can sustain the production of the renewable energy plants, e.g. sugars sorghum, while the soil quality is maintained and improved, respectively.
