2.5.1 Role of Biomass in the electric power system
In order to avoid dangerous climate change, Europe is striving for zero carbon elec-tricity production by 2050; several are the options explored and biomass could be taken in consideration [11]. Biomass is a renewable energy source used for many centuries and still nowadays is very important in several developing countries. Under UE legislation it is possible to consider that the amount of carbon emitted, through the use of the fuel, is the same of the amount of CO2 absorbed by the plants to produce the same quantity of biomass: this process is called ”carbon cycle”. For this reason biomass source can be classified as renewable source [10].
Nowadays in the market mainly 5 different types of feedstocks used as fuel are present: wood from forestry or wood processing, agricultural crops, food and indus-trial waste, residues from agricultural harvesting and by-products from manufactur-ing processes[11].
It is possible to convert the biomass in order to produce three types of energy vectors:[10]
• Heat: the combustion of solid biomass and biogas generates heat. The systems can be from small to industrial scale;
• Electricity: combined cycle power plants can produce heat and electricity through co-generation;
• Transport fuels: using energy crops is possible to derive biofuels;
The use of biomass has many advantages over conventional energy sources and as well as over other renewable energies; the increasing of the production of biomass energy could have several consequences mainly related to socio-economic and environmental issues[13]. The utilization of the biomass as energy source for domestic heating generates pollution emitting in the air pollutants such as the PM 2.5, PM 10, CO, dioxins and furans.
The combustion of biomass in large-scale power plants is different and, thanks to the new technology, it is possible to capture and remove most of the pollutants. Due to the difficult control of temperature in the combustion chamber and time needed by the fuel to burn completely, the production of the electricity for industrial power plants represents a constant base-load for the electric grid.
2.5.2 Situation worldwide
The world’s population continues to grow and today, in 2019, the population is twice than 1960 and in 2050 is projected to increase further to 9 billion. Because of this trend, the percentage of the global energy used in cities will increase significantly.
Due to the wide availability of by-product of many industrial and agricultural pro-cess, biomass represents a renewable energy source with high growth potential [14].
But in the reality the growth of biomass is very slow; in 2018 the generation grew by only 2% with a similar rate of 2016 and 2017. In the key European countries like Germany and Italy, the growth has slowed and in others like Poland and Finland has even fallen slightly. A few large projects are still existing thanks to the possibility to the reconversion of the old coal power plant [15].
One of the most restriction of biomass is the pollution caused from particulates.
As the following graph shown (FIG 2.6), it is possible to notice that the growth of biomass power plants in Europe projected to 2030 is much lower than the growth of the other types of renewable sources like wind and solar energy.
FIG: 2.6. 2030 projection of renewable electricity share in European Commission’s Long Term Strategy [15]
2.5.3 Technology Overview
In order to study the power generation from biomass source, it is essential to consider the three components that characterize the process:
• Biomass feedstocks: different properties and varieties of the fuels impact the power generation;
• Biomass conversion: in this process the biomass feedstocks are transformed into the energy used then converted in heat and/or electricity;
• Power generation technologies: several are the existing solution in the market;
they depend of the type of the fuel input and the type of the conversion;
The power generation from biomass power plant can be achieved using different type of feedstocks and power generation technologies and the process can be direct (e.g.
gasification) or include intermediate conversion processes. Mainly it is possible to classify the conversion process in two main categories: Thermo-Chemical process and Bio-Chemical process. Moreover the first one can be categorized in other three processes: Combustion, gasification and Pyrolysis.
Direct combustion of biomass is a mature and commercially available technology and can be applied for a wide range of scales (from a few MW to 100 MW). In the world the most important process is the combustion route in which two are the main components: the biomass-fired boiler (stoker and fluidised bed) and the steam turbine. The first is used to produce steam and the second is used to convert the steam to generate electricity.
It is very interesting the use of combined heat and power (CHP) biomass plants for the simultaneous production of electricity and heat from one source of energy.
These systems can achieve high overall efficiency and the provided heat of stream can be utilized for industrial use or for space/water heating in residential buildings (directly or through a district heating network) [16].
The chemical composition of feedstocks is highly dependent on the plant species and for this usually it is very heterogeneous. It is important to take into account the ash content, the density, the particle size and the moisture content because they could create critical problems during the operation of the power plant.[12]
The moisture content of biomass can vary from 10% to 60%; higher is the percent-age and lower is the energy value of the feedstock. Related to this problem is that the transportation costs increases and then in general the total fuel cost on an en-ergy basis. Enen-ergy density is an important parameter and its improving can reduce transportation costs and improve also the combustion efficiency.
Another important consideration is the ash content. It can be formed during the combustion process and then deposit inside of the combustion chamber and gasifier decreasing the performance and increasing the maintenance costs.
The density and the size of the biomass is also important because they have effect on the rate of heating and drying during the process [16].