Receiving, treating and transporting fuel
When extracting energy from waste, the economics of the process are greatly affected by the composition of the waste. A high content of inert substances in the waste reduces its calorific value. A large amount of energy in the plasmatron is then consumed converting the inorganic portion of waste into liquid slag.
For this reason, when using waste for energy it is desirable to eliminate as much waste as possible that does not contain any energy (ash, rubble, metals, glass...) by using sorting lines.
Before the actual sorting, all feedstock received is crushed and homogenized in a slow speed crusher. This is followed by a metal separator and drum sieve, which removes the majority of fine rubble and stones. A ballistic separator removes any residual inorganic components. The last machine on the sorting line is another crusher, this one designed to make sure the feedstock is the optimal size (max. dimension of particle size) before it enters the drying line. The material that has been sorted out is collected in open bays. The metal is then taken to be recycled and the rest goes to a landfill.
Plasma reactor with plasmatrons
The plasma reactor is a vertical, metal vessel lined on the inside with material resistant to high temperatures. The thickness and chemical composition of the refractory materials used is different in individual parts of the reactor. The upper part of the reactor has enough space to hold the synthesis gas that is created long enough to allow for maximum disintegration of complex organic molecules and reduce the speed of the gas.
The plasmatrons are located in the central part of the reactor. The number of torches, their capacity and exact location is determined by the chemical composition of the feedstock. The bottom part of the reactor is designed to collect melted slag. The melted slag flows from the bottom of the reactor through a drain onto a conveyor belt with a water bath. There it cools into glassy, vitrified slag.
The reactor is typically equipped with three plasmatrons placed along the perimeter. The plasmatrons introduce the necessary energy into the system via ionized superheated gas. This enables the operator to control the gasification process in the reactor independently of the kinetics of the ongoing reactions. Each plasmatron creates low-temperature thermal plasma (approx. temperature 3-5000 °C). The plasma produced by the plasmatrons can increase the specific energy of the working gas from two to ten times more than in conventional combustion systems (high energy density >100 MW/m³).
Cooling and purifying synthesis gas
Before it can be purified, the temperature of synthesis gas must be decreased from 1,250 °C to approximately 200 °C. The heat energy obtained from the synthesis gas is used to create high-pressure steam in a Heat Recovery Steam Generator (HRSGI). The parameters of the purification technology depend on the end use of the synthesis gas (cogeneration unit, gas boiler, combustion turbine, Fischer-Tropsch synthesis). Typical processing of synthesis gas involves the removal of acidic gases (HCI, H2S), solid pollutants and excess moisture. Specific requirements for purification may include, for example, the reduction of sulphur content to ppm for use in an engine-generator or adjustment of the ratio of the basic components of synthesis gas (CO/ H2) to produce synthesis fuels through Fischer-Tropsch synthesis.
Possible uses of synthesis gas
Single-stage production of heat and electricity
Synthesis gas produced in the reactor is sent to a combustion chamber where it is burned. The thermal energy of hot flue gases is used in a heat recovery steam generator (HRSG II) to produce high-pressure steam. Along with the steam produced by cooling the synthesis gas in the HRSG, the high-pressure steam is then used to generate electricity in a steam turbine. After cooling and scrubbing, the flue gas is released into the atmosphere.
Combined production of heat and electricity
Depending on the amount of synthesis gas produced, it is possible to use the chemical energy of synthesis gas to produce heat and electricity in a cogeneration unit. This combined production of electricity may include a steam turbine. The steam for its operation is created in two systems, in HRSG I (operations boiler) by cooling the synthesis gas coming from the reactor, and in HRSG II (combustion boiler) by transferring the heat energy of flue gas from the engines. Units with a higher hourly waste processing capacity use a combination of combustion and steam turbines, preceded by purification and compression of the synthesis gas.
The ratio of the basic elements of synthesis gas (H2:CO) depends on the gasification technology used and the character of the medium used for partial oxidation of the synthesis gas created. This ratio must always be optimal for the technologies of the end user of the synthesis gas that may require, for example, a reduction in hydrogen content (BTL, gas turbine, methanol production) or the primary aim may be to only produce hydrogen. To produce hydrogen it is possible to use the membrane process of separating hydrogen, which is efficient with respect to the low consumption of energy and other media (steam, water, chemicals). This technology is delivered as a packaged unit with simple installation and operation control. After hydrogen separation, the synthesis gas can further be used for combustion in a boiler, turbine or engine of a cogeneration unit.
Production of synthetic motor fuels
After adjusting the CO:H2 ratio, synthesis gas can be used as a raw material to produce synthetic motor fuels through Fischer- Tropsch synthesis. The FT reactor uses a specific temperature, pressure and catalyst to convert the synthesis gas into a mixture of hydrocarbons (paraffins, olefins) that is then processed and refined by standard processes. The type of reactor, select reaction conditions and catalyst determine the characteristics of the raw products and the individual final products after processing (e.g. naphtha, petrol, diesel, kerosene, waxes). In addition to these products, a considerable amount of heat is available from the cooling of the reactor (strong exothermic reaction) and incondensable gas products. Both of these sources of energy can be used to generate electricity, which not only covers internal demand, but for the most part is sold back into the grid.