Energy demand is increasing, such as waste production: could we solve both problems with one unique solution?
As the world population grows and living standards continue to rise, the consumption of goods and energy is increasing too, which has significant environmental consequences. The correlation between income and energy consumption remains very strong; additionally, higher consumption per capita has led to an increase in waste generated mainly by households and other sources, such as shops, offices, and public administrations.
Figure 1 Municipal waste generated, 2005 and 2019.
If the waste production increases and the energy demand increases, it could be possible to kill two birds with one stone: we could reduce on the one hand the amount of waste sent to landfilling and, on the other, the amount of CO2 otherwise produced by conventional power plants. Indeed, the diversion of waste from landfills prevents the production of methane emissions, which has up to 86 times a stronger impact on global warming than CO2, considering a 20-year period. The Intergovernmental Panel on Climate Change (IPCC) says that “Compared to landfilling, waste incineration and other thermal processes avoid most GHG generation, resulting only in minor emissions of CO2 from fossil C sources.”
Regarding the generation of energy using waste, given that the average heating value of municipal solid waste (how much heat could be produced by a unit mass of waste) is approximately 10 MJ/kg, it could be stated that using waste to produce energy could contribute to solving the dilemmas of waste management and energy demand, and it could ease the transition to a more sustainable model of production and consumption. Moreover, with around 39 TWh of electricity and 90 TWh of heat produced in Europe annually, Waste-to-Energy (WtE) could prevent the production of up to 50 million tons of CO2 emissions that would otherwise be generated by fossil fuels.
Let’s look up some data.
In 2018, 247 million tons of MSW were treated in the EU. Of this, 30% was sent to recovery operations for recycling, 17% to composting operations and 23% was disposed of through landfill dumping. A further 47% of the waste was disposed of through incineration, either simple incineration or incineration with energy recovery. The quantity of MSW incinerated in the EU has risen from 32 million tons (67 kg per capita) in 1995 to 70 million tons (136 kg per capita) in 2018, an increase of 117%. These changes and new European legislation on renewable energy and waste have created a significant growth in energy generation from MSW.
Figure 2 MSW, 1995-2018
According to Eurostat, in 2018, overall energy production from waste amounted to 40.4 MTOE (million tons of oil equivalent), about 2.4% of the total energy supply in the EU.
Nowadays, there are around 2,500 Waste to Energy plants (incineration with energy recovery) active worldwide, but only some countries, such as Germany (7.1 MWh in 2018) and Netherlands (2.2 MWh), are already using this resource at maximum extent, with little room for further expansion. The results of WtE suitability analysis for the other European countries show that the MSW resources are underexploited for energy production. There is a potential to implement around 248 new WtE plants in the EU, with a total capacity of 37 million tonnes.
Why did we think about using waste to produce electricity?
Around the end of the 18th century, the first incinerator was built due to the rising issues linked to the disposal in the agricultural fields and to the big productions in the bigger cities. This solution was able to prevent the spreading of new pathogens due to poor hygiene and to reduce the volume of waste. For the first MSW incinerators, energy recovery was not a goal! On the contrary, modern waste-fuelled power plants extended their application to the production of electricity and/or heat. This led to the development of new environmental threats linked to air pollution. We had to wait until the end of the 60s to see the first plants equipped with an air control system: new regulations to control furans, dioxins and heavy metals (especially mercury) started to be released.
Even if the production of electricity and heat has become the focus, it must be kept in mind that the output of the metabolism of the anthroposphere contains large amounts of hazardous organic materials that cannot be recycled. For this reason, the importance of using WtE power plants to destroy organic compounds has been kept as a priority. Well designed and operated incinerators are excellent ‘‘final sinks’’ for most hazardous organic substances. To summarize today’s goals of waste management we could list hygienisation, volume reduction, environmental protection, immobilisation of hazardous compounds, resource conservation and affordable costs.
How does a Waste-to-Energy Power Plant work?
Waste-to-Energy plants make the conversion of a wide variety of waste into electricity and heat through combustion. A WtE power plant represents a highly case-specific system that is tailored to fit in the prevailing boundary conditions, such as local stack emissions limits, fuel composition, and the desired output product such as power and/or heat (typically supplied as low-pressure steam). The stable operation of the plant is prioritised with respect to reaching the optimal efficiency, even if a lower efficiency leads to major consequences in terms of the environmental impact of the power plant and its economics.
The operating phases of the incineration plant are essentially five: the waste receipt and storage, the combustion, the steam generation, the flue gas purification and, eventually, the cogeneration of electricity and heat.
First of all, the waste enters the plant and, after being weighted, it is unloaded into the bunker. Thanks to a feeding system, the waste is sent to the combustion chamber and the oxidation process takes place: heat is released, and, in the boiler, it is exchanged counter currently with the fluid evolving in the boiler’s tube banks, which can be either diathermic oil or, more often, steam. In WtE plants, the boiler is designed in a less sophisticated way than in coal-fired power plants due to the much smaller sizes and corrosion problems. The latter is an important issue that constrains most of the optimal operating conditions, such as the maximum pressure and maximum temperature of the cycle. Thus, the efficiency can be increased up to a maximum value set by technical constraints. Compared to coal, waste is characterised by the presence of chlorine, which forms, during the combustion process, gaseous or liquid chloride-containing compounds that are well known to accelerate the corrosion rates. There is also a high presence of molten salts containing chlorine which have been considered a major cause of rapid corrosion attacks. The maximum pressure of the system is 65-70 bar while the maximum superheating temperature is around 420-450 °C. Usually, no re-heating is taken into consideration in order to reduce the tubes and material exposed to the corrosion phenomenon.
The last stage of the power plant is the fume purification stream, whose asset can vary based on a techno-economical trade-off.
How can I solve the problem of CO2 emissions?
There are major advantages in promoting CCUS-equipped WtE power plants (CCUS – Carbon Capture Utilization and Storage).
It has been proven that to have an avoidance of CO2 emitted greater than the power plant’s emissions, an efficiency of more than 25% is required: the net electrical efficiency of a typical WtE is in the range of 10 – 30%, because of the low energy content of the raw MSW, its fluctuation in composition and size, as well as the various corrosive species contained in the MSW reducing the effectiveness of heat recovery. WtE plants represent a stationary CO2 emitter that is reasonably large for the implementation of efficient CCS processes. Waste-to-Energy power plants equipped with Carbon Capture and Storage (CCS) are seen as a critical pathway to limit global warming to 1.5◦C above pre-industrial levels (IPCC, 2018), as specified in the Paris Agreement. Concerning the life cycle effects of a CCS process chain integrated into conventional power plants, the global warming potential decreases by up to 75%. On the contrary, several other environmental impact categories increase because of efficiency penalty, additional infrastructures and waste generation related to the capture process.
Municipal solid waste (MSW) represents a suitable fuel source for a CCS system considering its large organic waste fractions (about 10-20% is organic carbon) compared to coal and fossil fuels that, in general, don’t have any organic carbon. Thereby, the benefits of an efficient MSW treatment and the achievement of negative CO2 emissions are combined. Additionally, major disadvantages of CCS applications based on cultivated energy crops, such as land system change, biosphere integrity or freshwater use are avoided, once the already available MSW is used as the biogenic carbon source.
The main advantages can be summarised as follows: first, the plant produces steam that could be used for CO2 regeneration when using solvent-based CO2 capture. Furthermore, CCS from such a plant could be used to unlock negative emissions as nearly two-thirds of the plant emissions are biogenic in nature. Finally, the problems related to the implementation of a large share of renewable energy sources would be partially solved by providing dispatchable, globally distributed low-carbon electricity, and with a lower space encumbrance and costs than other solutions. Taking these aspects under consideration, the future power generation assets will be valued not only for energy, but also for a diverse set of services like grid services, inertia, and frequency provision, or turn down capabilities and dispatchability. All these services are essential for a reliable and affordable power supply. This is a key reason why traditional cost metrics like the levelized cost of electricity (LCOE) are often inadequate when evaluating the economics, as they do not reflect the total cost and value of generation assets to the operation of the power system.