Thermal treatment plants make possible to exploit the energy content of waste that cannot be recycled or partially recovered: the main challenge posed in their design is therefore to obtain the maximum efficiency of energy recovery, while ensuring full compliance with the regulations on quality of the flue gases released into the atmosphere, for the purposes of environmental protection.
A thermal treatment plant has the task of energetically enhancing what cannot be recycled or partially recovered, thus avoiding sending towards landfill a waste that still has an energy value.
The use of thermal treatment plants, in addition to allowing energy to be recovered from waste, also has the not less important advantage of avoiding the useless occupation of the limited spaces destined for landfills, since the ashes returned by the combustion process represent a significantly lower quantity than the incoming waste.
Furthermore, ashes are a more stable waste, less or not at all dangerous, as they have been deprived of the organic substance which represents the potentially toxic, volatile, odorous and combustible part.
Consequently, the main objective of a thermal enhancement or waste-to-energy plant has always been to extract as much energy from the waste, returning the minimum amount of ash and residual slag, and releasing flue gases into the environment with a level of pollutants which is negligible or, at the very least, acceptable given the most stringent European, state and regional regulations.
In addition to incineration, there are additional thermal processes and dedicated technologies with which to obtain a waste-to-energy process; in this case, the sewage sludge can undergo a process of pyrolysis, gasification or combustion; each of them has a different mechanism of thermal decomposition of the organic matrix and leads to the origin of products of various nature and interest.
From the point of view of performance and therefore of conversion efficiency, however, it must be said that the process that by its nature achieves the best efficiency is that of combustion, which leads to the complete oxidation of organic molecules in a single step, returning as waste only ashes (hence the term â€śincinerationâ€ť), without going through the formation of energy by-products, such as carbonaceous liquids or syngas. The latter in fact would undergo a second thermal process downstream, which completes their oxidation and allows their heat recovery, with further conversion losses.
The overall value of the incineration and waste-to-energy process is therefore assessed on the basis of energy recovery efficiency, the content of unburnt residues present in the ashes, the quantities of flue gases and wastewater released into the environment and the content of pollutants present in them.
Within the PerFORM WATER 2030 project, the waste-to-energy technology selected was that of the incineration or thermal destruction of biological sludge, which appears to be the most promising way for thermal treatment of the matrix consisting of dried sludge, due to the higher degree of oxidation that it is able to achieve, combined with the immediate exploitation of the energy content of sludge, without further energy conversions.
The pilot plant is designed, built and operated by the company VOMM Impianti e Processi S.p.A., which has been present for 50 years in the sludge drying sector, thanks to its patented turbo-technology, and which more recently, starting from 2007, expanded its sphere of interest to the waste-to-energy sector, developing its own flat grate combustion technology.
The flat grid technology represents a novelty in the sludge incineration sector; in fact, the vast majority of waste-to-energy plants use a fluidized bed system when it comes to incinerating sludge, while the grid system is used almost exclusively in the incineration of other types of waste such as MSW, RDF, hospitals, or in the field of biomass, SRF, etc.
The flat grid technology lends itself, much better than a fluidized bed, to working on already dried sludge, as happens in the San Giuliano Milanese Ovest waste water treatment plant, operated by CAP Group, where a VOMM drying system has been present for years, which returns a sludge dried around 85-90%.
The following photo depicts the experimental waste-to-energy plant built and installed by the VOMM company at the San Giuliano Milanese wastewater treatment plant.
The first section of the San Giuliano Ovest plant consists of a big-bag loading system of dried sludge (coming from the nearby drying plant), and its feeding to the pellet machine, which compacts it in the form of small cylinders with a diameter of millimeters. The pellets are then unloaded onto a particular type of vibrating conveyor, which sieves the dusty fraction not compacted by the action of the pellet machine. Subsequently, the pellets are fed through a conveyor belt into the storage hopper, which has the task of ensuring a minimum storage capacity and a quantity of reserve fuel. Further conveyor belts extract the pellets from the hopper and load them into the feeding hopper of the combustion grate. In the following photo you can see the whole described.
The pellet contained inside the hopper is discharged by gravity into a pair of dosing screw conveyors, which provide the actual fuel supply inside the combustion chamber. In particular, the pellets are deposited on the flat surface of the grid, which is set in slow rotation by a variable speed gearmotor unit. According to the thermal load required, that is to say the thermal power to be used, it is therefore possible to vary the flow rate of the sludge fed, and consequently the volume of flue gases produced by combustion.
The combustion air necessary for the combustion of the sludge is blown under the grate with the help of a centrifugal fan.
The combustion chamber is delimited by a refractory vault and by the walls of the furnace itself; these are also covered with thick layers of refractory brick, suitable to withstand the high temperatures generated by the oxidation process, and capable of thermally insulating the interior of the oven from the external environment. For this reason it is also referred to as a â€śadiabatic ovenâ€ť.
A natural gas burner is installed on one wall of the combustion chamber; it is used to heat the chamber during the start-up phases, in order to reach the temperatures necessary for starting the combustion process of dried sludge.
To reduce the formation of nitrogen oxides (which are often referred to by the collective term NOx), due to the presence of significant quantities of nitrogen within the biological matrix, a flue gas recirculation system is used; it allows control of the peaks of temperature (exceeding 1000°C), to which the production of NOx is mainly linked. The recirculation gases are injected at the beginning of the adiabatic chamber.
The purpose of the adiabatic chamber is to provide a sufficient volume to allow the flue gases generated in the combustion chamber to thermally disintegrate those harmful compounds, such as dioxins and furans, produced by the combustion reactions of the sludge.
Furthermore, VOMM company has decided to apply, in order to further reduce NOx, a SNCR system (Selective Non Catalytic Reduction system) by injection in the adiabatic chamber of a flow of urea which is able to chemically bind to nitrogen oxides and return water vapor and nitrogen. The use of urea is much simpler and safer than the use of ammonia, since the latter is corrosive and toxic
Once in the combustion chamber, the pellet sludge undergoes an initial dehydration phase, which removes the residual moisture content, followed by an intermediate pyrolysis phase with the formation of various compounds, which are then completely oxidized in the final phase of combustion.
The long residence times of the sludge inside the combustion chamber, the high temperatures, the presence of combustion air well distributed along the grate, the conformation of the chamber itself, guarantee a high conversion efficiency and therefore they are factors on which it must be performed a careful designing.
The effluents released by the process are then of two types: the flue gases are evacuated passing through the adiabatic chamber, undergoing a first process of abatement of the pollutants; the bottom ashes, which constitute the solid waste, are extracted at the bottom of the grid, by means of an extraction screw, which unloads them inside a removable box.
The recovery of the energy resulting from combustion and contained in the gaseous flow takes place in a boiler placed at the exit of the dedusting cyclone, which follows the adiabatic chamber seen in the previous section.
The adiabatic cyclone, operating at very high temperatures, is one of the innovative patented technologies introduced by the company VOMM Impianti e Processi. Its function is to immediately remove fine dust particles upon leaving the oven since, although limited thanks to the pelleting technique, they are still present in the flue gases in a small percentage. In this way, not only the work of the more stringent treatment systems present downstream (such as bag filter and scrubber) is lightened, but the tube bundles of the boiler and subsequent heat exchangers are protected from erosion and fouling. This detail is not irrelevant: one of the most frequent causes of plant shutdown is linked to the need of cleaning boilers bundles covered by encrustations.
The boileris of flue gas tubes type: a cylindrical casing contains the heat transfer fluid, in this case water, which surrounds a bundle of tubes crossed by the flue gases. The result is the limited overall dimensions and the construction simplicity
The water cools down the flue gases and, in the meantime, heats up, pushed to circulate in a closed circuit by a special circulation pump; the stored heat can be usefully used in several ways. Instead of water, obviously, any other heat transfer fluid can be used, depending on the needs. This is the case of diathermic oil, which can reach high temperatures, capable of providing valuable heat, to be used for important thermal processes, such as that of drying the sewage sludge itself. This is the case where the combination of the two technologies finds its natural synergy, capable of optimizing the entire recovery process.
Once cooled down inside the boiler, the flue gases still have a certain energy content, which is used to heat the combustion air that flows under the incinerator grate. In this way the energy recovery efficiency increases and the combustion process is facilitated.
This second phase of energy recovery takes place inside an air/flue gases exchanger, placed at the outlet of the boiler and before the bag filter, visible in the following photo, which depicts the heat recovery unit and the boiler before their installation in San Giuliano Milanese.
Once the heat recovery process is completed, the flue gases generated by the furnace combustion process must be treated to reduce the polluting components as much as possible. Among these, the fine particles emitted by the solid inert part of the fuel, carried by the gaseous stream through the previous sections of the plant and, due to their fine particle size, surviving the sequestration action of the adiabatic cyclone, are captured by a bag filter placed immediately downstream of the combustion air preheater.
The bag filter is so called because it contains numerous â€śbagsâ€ť arranged on metal cages, through which the gases flow to be dedusted is passed at low speed. The sleeves are made of very fine-woven polymer fabric, which retains the fine dust of the fumes that pass through it, causing it to deposit on the external surface. A jet of compressed air periodically shakes the bags, causing the dust accumulated to precipitate downwards, where the latter are deposited inside a collection box attached to the filter.
The joint action of the cyclone deduster and the bag filter would be sufficient to return fumes with a negligible content of dust, and full compliance with the regulations for industrial-sized incinerators.
However, VOMM has chosen to install and test within the experimental project an additional device for the sequestration and capture of the finest powders: it is a patented process that makes use of a scrubber-type washing tower, where an aqueous solution specifically designed to capture dust is sprayed into the gaseous stream to remove the last particles present.
In fact, in view of new and more stringent emission limits foreseen for the next decades, the goal is to be able to have and immediately apply a technology capable of returning flue gases virtually free of fine particles, with zero impact on the increase of particulate matter in the environment, an increasingly topical problem.
Once the dust has been captured, the gaseous pollutants must be removed before returning the flue gases to the environment. The treatment of gaseous emissions from combustion of sludge is a more complex operation than what happens for the combustion of biomass or other types of waste, because the sludge, depending on its origin, can contain appreciable quantities of nitrogen, sulfur, chlorine and fluorine, as well as mercury and other heavy metals.
In fact, already in the combustion phase, abatement techniques have been adopted, in particular within the adiabatic chamber, with regard to dioxins, furans and nitrogen oxides; in the relevant paragraph, the SNCR system for removing NOx by urea was described, and it was shown how the high temperature residence volume offered by the chamber allows thethermal abatement of chlorinated compounds.
The acid components present in the flue gases at the outlet of the bag filter are therefore represented by sulfur oxides and hydrochloric acid, which must be eliminated through a treatment system that uses a chemical reagent capable of converting the polluting molecules into inert products.
The treatment can take place both dry and wet; in the case of the San Giuliano Ovest plant, it was decided to use a wet scrubber-type treatment system, located downstream the first powder washing tower. A basic solution is continuously dosed in the Scrubber to treat the acids present in the gases, from which a small liquid effluent derives, which is sent to the waste water treatment plant.
After the first phase of basic treatment to remove the acid components, a final treatment is carried out on the gases, which are sucked in by the main fan, and pushed into a fixed bed system of activated carbon; the latter are used in the form of small diameter cylindrical pellets.
Due to its porous structure, activated carbon has a very high contact surface per unit of volume, and therefore has the ability to adsorb the last polluting particles escaped from the previous capture and removal systems.
With this last treatment, VOMM wanted to ensure the highest quality in the removal of pollutants, adopting every strategy for their sequestration, in a deliberately redundant way, using multiple systems in series. In fact, with a view to experimentation, the planned waste-to-energy plant also allows to test the punctual effectiveness of the individual abatement systems. For this purpose, a multiple bypass system has been designed and built that allows to isolate a single treatment system, from time to time, to assess its impact on stack emissions.
The described emissions treatment system, photographed during the installation phase, is visible in the following image.
Once it has crossed the bed of activated carbon, the gas stream has been deprived of all potential pollutants and is released into the atmosphere through the chimney which is the last component of the smoke line.
Thanks to the multiple sequestration actions, the released gases reach abatement levels that make them less polluting than the gaseous emissions released by pellet stoves and home fireplaces, rather than from the exhaust of most diesel vehicles.
Historically, in the sludge waste-to-energy sector, the technology used has always been that of fluidized bed furnaces, due to the consistency of the sludge produced by the wastewater treatment plant, which can be fed, after a dehydration process, directly into the fluidized bed reactor.
So far, the experimental incineration and waste-to-energy plant, installed in San Giuliano Milanese thanks to the PerFORM WATER 2030 project, therefore represents the first plant in Italy operating in conditions of sludge mono-incineration with mobile flat grate technology; this constitutes the first important innovation made by VOMM within the project.
Experimentation on these innovative sludge thermal valorization technologies are strictly connected to the theoretical modelling of the grate-combustion phenomenon (for more details see Measurements and modelling for sludge thermal valorization). The pilot plant installed is in fact adequately instrumented to continuously detect the main parameters of interest: particularly significant in this regard is the multiple bypass system, already mentioned in the previous paragraph, which allows the individual abatement systems to be tested (even separately).
VOMM has always chosen to use a flat grid in combination with the pelleting treatment, which reduces the dried sludge into pellets, which are easier to transport, store, and feed into the combustion chamber. The reasons for this technological choice derive mainly from being able to use dried mud and being able to compact the dusty part. In this way, in fact, the ashes returned by the combustion process are mostly heavy and not fly ashes, unlike what happens in fluidized bed systems.
The presence of a limited amount of ash in the flue gases, together with the use of the grate incinerator, results in a number of noteworthy advantages:
Simplification / reduction of fly ash capture systems (dedusting cyclones, bag filters, ESP, etc.)
Protection of the boiler tube bundles and heat recovery units from erosion, which results in a longer life for these components
Accumulation of 90% of the ashes in a single point, at the end of the grid, which facilitates their evacuation
Phosphorus extraction and material recovery facilitated, because the ashes are concentrated and free of other elements (such as sand added in the fluidized bed, or the reagents added in dry treatment systems), as well as having a residual structure
Furthermore, being able to operate with the sludge in which most of the moisture has already been removed, following a drying process, gives further advantages:
allows greater thermal recovery (and, for this reason, it appears as a strategy to improve the combustion process also in the recent document on the best available techniques, the BAT 2019 published by the European Commission;
allows the use of smaller sections of the flue gas line and treatment systems (saving space and costs),
allows a reduction in the consumption of reagents;
returns a lower flow of flue gases to the chimney, consequently reducing the impact (in this regard, note the extremely small dimensions of the pilot plant chimney shown in the photo).
In conclusion, the waste-to-energy process of sludge that does not have the characteristics to be reused, is a virtuous process that is good for the environment because:
eliminates a source of danger and pollution for soils or waters
safeguards environmental spaces because it avoids being disposed of in landfills
allows an important energy recovery
avoids the use of fossil fuels
returns purified flue gases in compliance with the most stringent regulations in terms of environmental protection
The use of flat grid technology for the mono-incineration of sludge is configured as a promising choice due to the advantages it can achieve compared to more traditional technologies, such as the fluidized bed.