Technique title: High steam parameters (emerging techniques) Description Numerous techniques are also emerging to help boost the energy
efficiency of conventional incineration to above 30%. Compared to fossil-fuel-fired LCP boilers, waste-fuelled boilers have low electrical generation efficiency. This is primarily because of the severe corrosive environment created by waste incineration which limits steam
temperatures and pressures to around 425°C and 50 bar.
In incineration of MSW, the major parts of the corrosive species are released in the first part of the combustion grate and thereby in the front of the furnace. The rear parts of the grate are characterised by a burnout of a relatively clean char, thereby releasing relatively clean combustion products which are much less corrosive. This phenomenon can be exploited to split the flue-gases from the grate into two or more fractions, one of which exhibits high heat flux and a low chlorine
concentration. That fraction could then be used in a high-temperature superheater to increase the steam high-temperature and thereby the electrical efficiency of waste-fired power plants. In order to ensure the separation of the two flue-gas fractions in the furnace, a water-cooled membrane wall is installed above the middle of the combustion grate. When the two streams of flue-gases enter the post-combustion chamber, they are then mixed by the secondary air system for final burnout.
The basic idea of the concept is to use all the advantages of a modern waste-fired power plant combined with an integrated final superheater. The final superheating increases the steam to, for example, 500°C and 80 bar and results in an increase in electrical efficiency of 3 percentage points over the baseline steam conditions of 400°C and 45 bar. The overall objective is to achieve a net electrical efficiency of between 27% and 33%, depending on the design of the cooling system for the
condenser.
The concept has been trialled in a modified operational waste plant in Denmark and the results have shown that the concept is feasible87.
Sulphur recirculation is an emerging technology that is able to reduce high-temperature corrosion in superheaters.
Alternatively, it can increase electricity generation at waste incineration installations, if the superheater steam pressure and temperature are raised.
In the process, sulphur from a wet flue-gas cleaning system is returned to the furnace. The recirculated sulphur raises the SO2 concentration in the furnace and reduces the chlorine to
sulphur ratio in deposits and ashes, and the environment becomes less corrosive. Furthermore, the formation of dioxins
Technique title: High steam parameters (emerging techniques) is reduced, and the proportion of sulphates in the effluent water discharged from the wet flue-gas cleaning is reduced.
The process works in two stages. First, sulphur dioxide is removed from the flue-gases in the wet flue-gas cleaning stage. The removed sulphur compounds are then sprayed into the boiler through nozzles with a surrounding carrier gas. In this way the level of sulphur in the water is raised. Thus each sulphur atom passes through the furnace several times.
The process has been demonstrated in Gothenburg, Sweden.
Dioxin samples, impactor measurements, deposit probe measurements, ash samples and 1,000-hour corrosion
measurements were taken in full-scale trials with and without sulphur recirculation. With sulphur recirculation, corrosion rates in the superheaters for all materials evaluated (16Mo3, Sanicro 28 and Inconel 625) were reduced by more than 50%
compared to the reference case88.
Criteria Rating Notes
Net annual average
energy efficiency
High steam parameters offer year-round net electrical efficiencies of up to 33%.
Applicability
Traditionally, high steam parameters have been restricted to the largest plants due to the high costs of corrosion. It is too early to determine whether these techniques will lower the costs of operating WI plants at higher temperatures and pressures.
Exclusion
criteria No None noted.
TRL 7 Small-scale tests in commercial WI facilities have been conducted with encouraging results.
88 Sulphur Recirculation for Low-corrosion waste-to-energy, available at:
www.iswa.org/uploads/tx_iswaknowledgebase/Andersson.pdf
Technique title: Use of the mass and energy balance method to measure waste biogenic content
Description Municipal solid waste is an extremely heterogeneous feedstock and, unless properly managed and mixed before firing, can cause
significant variation in combustion control and pollution abatement.
Use of the mass and energy balance method to measure waste biogenic content is a measurement technique developed by the Technical University of Vienna89.
It was originally designed to provide a method to determine the biogenic content in order to facilitate carbon accounting and access to renewable benefit schemes. It is an approved method for reporting and obtaining applicable renewable energy support credits. The balance method is based on the mathematical solution of theoretical balance equations for materials, substances and energy together with plant data such as flue-gas volume, steam production and bottom ash mass. It utilises operational plant data and can provide a continuous output of results.
The method determines biogenic content (ratio of green energy), fossil CO2 emissions and calorific value. These results, properly analysed and interpreted, can assist operators with improving both the reception and mixing of waste prior to firing and the operation of combustion and pollution control systems, effectively providing an improved conversion efficiency and reduced operational costs. For example, reducing the variations in fuel quality leads to improved efficiency of combustion and therefore greater energy recovery per tonne of waste.
Criteria Rating Notes
Net annual average
energy efficiency
Some improvement in energy efficiency will be obtained through more stable process conditions.
Applicability The technique can be applied to most waste incineration plants relatively easily.
Exclusion
criteria No None noted.
TRL 8 A number of trials have been conducted in operational WI plants across the EU-28.
Technique title: Heat and power decoupling through heat pumps Description Heat pumps can be used to decouple heat and power recovery in a
waste-fired plant district heating application90.
An innovative design has been proposed whereby, to maximise turbine power generation efficiency, no steam bleeds are provided to tap off steam for district heating energy and a condensing turbine set-up for maximum power recovery is specified. The resulting turbine condensate is relatively cool so an array of heat pumps are used to increase the temperature of the turbine condensate from (approx.
40°C) to a temperature more suitable for district heating purposes (70°C). To enable this, electrical energy can be drawn from the grid when there is an excess of electrical energy available (e.g. peaks from wind power and otherwise the grid is not accepting power) to be transformed into heat energy within the district heating system. When there is no demand for heat, the heat pumps would not operate and only power export from the plant would occur.
In this way heat and power can be produced independently according to demand and thus providing a way of storing excess grid power generation capacity. Although the system is highly flexible, it is
anticipated that the overall energy efficiency will be low compared to a state-of-the-art heat-enabled waste incineration plant with a
condensing turbine.
A small-scale operational example of a similar proposal is located in Drammen, Norway. The heat pump energy source is deep water (rather than WI plant turbine condensate), but, in a similar way, the scheme employs heat pumps to extract energy from a
low-temperature source to produce district heating water at a suitable temperature.
The main benefit of this technique is flexibility, not energy efficiency. Although an overall analysis has not been performed, it is thought unlikely that drawing excess grid power to operate heat pumps is more energy-efficient than using surplus heat from a turbine bleed point.
Applicability
The applicability is restricted to a small number of district heating schemes.
Exclusion
criteria No None noted.
TRL 9 There are one or two small-scale examples in Norway and Russia.
90 Ricardo Energy & Environment.
Technique title: Use of ilmenite as a bed material in a circulating fluidised bed (CFB) reactor
Description A new combustion concept has been developed by Chalmers University of Technology in Gothenburg, Sweden, developed from steel industry applications91.
The principal of the new concept is to replace the inert silica sand bed material conventionally used in a CFB reactor with a metal oxide, ilmenite. Ilmenite is the titanium-iron oxide mineral with the formula FeTiO3.
Silica sand has one main purpose in a CFB reactor and that is to act as a heat carrier. Where metal oxide is used as a bed material, as well as carrying heat, the metal oxide carries oxygen for the combustion reaction and absorbs fly ash.
The benefits of this concept are that it enables the input of up to 4%
more heat energy to the boiler and, with better oxygen distribution, there is considerably less CO in the stack emissions.
The concept has gone from lab-scale in 2013 to a commercial-scale demonstrator at the Handeloverket waste treatment plant in Sweden.
This plant has a thermal input of 75MW.
The cost of ilmenite will be higher than silica sand. No data has been provided on the operational costs to replenish the ilmenite bed material.
Criteria Rating Notes
Net annual average
energy efficiency
With the input of up to 4% more heat energy to the boiler, a small gain in plant energy output may be realised.
Applicability
CFB technology is not widely applied to the larger waste streams such as household waste but may treat prepared waste-derived fuels such as SRF and is well suited to waste wood. Therefore, the applicability is somewhat limited. Where pretreatment of mixed waste is required to produce SRF, this will require an additional energy input of approximately 1.7% of the waste input energy.
Exclusion
criteria No None noted.
TRL 9 It has been demonstrated on a commercial scale in Sweden.
Technique title: Organic Rankine Cycle (ORC) turbine for low-grade heat utilisation
Description Waste heat is often of a low temperature and it can be difficult to efficiently utilise the heat contained. In these cases, the ORC turbine can bring some additional benefit to raise the overall plant efficiency.
The ORC turbine utilises this otherwise wasted energy and converts it into power.
The Organic Rankine Cycle is named for its use of a working fluid with a boiling point occurring at a lower temperature than water/steam which is used in conventional Rankine Cycle turbine applications. The fluid allows the Rankine Cycle to recover energy as heat from lower temperature sources such as incinerator waste heat. The working fluid used is normally a refrigerant fluid which must conform to the
requirements of the Montreal Protocol (non-ozone-depleting).
The working principle of the ORC turbine is the same as that of a conventional turbine; the working fluid is evaporated using (low-grade) heat from the incineration process and passes through the turbine at pressure to produce mechanical energy. The fluid exits the turbine to a condenser heat exchanger where it is finally recondensed.
Because of the low working temperatures of the ORC, heat transfer inefficiencies are highly prejudicial and result in a low overall energy efficiency. Suitable equipment is required to prevent any fires related to the working fluid.
The net average annual electrical efficiency of the ORC turbine is estimated to be around 19%92. This is mitigated by the fact that the ORC turbine can utilise low-grade heat which would otherwise be emitted to atmosphere. A conventional turbine can still recover high-grade heat at a higher efficiency in tandem with an ORC turbine.
Applicability
Most WtE plants recover waste low-grade heat which could be used to provide an energy source for an Organic Rankine Cycle turbine.
Exclusion
criteria No None noted.
TRL 9 There are many commercial examples in operation but these are uncommon in WtE plants.
92 http://www.energy.siemens.com/mx/pool/hq/power-generation/steam-turbines/downloads/brochure-orc-organic-rankine-cycle-technology_EN.pdf.