Biomass, Combustion, Gasification, Pyrolysis
Next: Anaerobic Digestation and Esterification
Let’s look a little at biomass. Its the most obvious source or local energy.
Biomass
Biomass is in principle CO2 neutral. When trees are cut down, new ones grow to replace them. If wood is used to make furniture, houses etc. the use of forests is carbon negative as the carbon gets locked in for decades into structure. But when used for energy, it is neutral. Of course it is possible to capture the CO2 when biomass is burned and either store it underground or use as carbon source for the Fischer-Tropsch process to make all kinds of chemicals. This is much easier than capturing carbon from the air as the concentration of CO2 in flue gases. Typically the concentration is 12-15%, but it depends a lot of efficiency of burning, type of wood etc.
The most obvious use is to use solid biomass like wood, peat etc. Solid biomass can be used in three different ways: combustion, gasification and pyrolysis
Combustion requires enough oxygen to happen - at least stoichiometric amount. Stoichiometry is way of calculation used to balance chemical equations, here it means that there is enough oxygen for all flammable material to react. The process is exothermic meaning that more heat is generated than consumed.
Combustion is the overall most common method to use biomass and most common use is for heating. For example, in Finland with 5,5 inhabitants there are 2 million wood burning fireplaces and 1,5 million stoves in saunas. Many of these in free time huts used on holidays or as backup or additional heat sources.
Gasification turns solid or liquid fuels into combustible mixture of gases. This happens where there 50-70% stoichiometric amount of oxygen. The result is a flammable non-condensable mixture of gases containing carbon-monoxide (CO), methane (CH4), hydrogen (H2) and water (H2O). This is an endothermic reaction, meaning that it requires energy to happen.
Pyrolysis happens when there is no oxygen and biomass is heated to a high temperature. Biomass pyrolysis is usually conducted at or above 500 °C, providing enough heat to deconstruct the strong bio-polymers. Biomass is converted into liquid, gas and solids. Pyrolysis is also used to produce charcoal. It offers higher efficiencies compered to combustion. Anything carbonaceous can be used for gasification: wood, coal, energy crops, plastic wastes.
Combustion
Combined heat and power plants (CPH) provide base load to network while renewables like wind and solar are intermittent. They can use biobased fuels (albeit today more often fossil fuels like coal, natural gas, oil etc.). They all are based on conventional steam power operations
Steam power consists of a boiler where burning happens, water is pumped through the boiler and it heats up. Steam is lead through turbine that generates electricity. In condenser steam is again turned into liquid and pumped again through the boiler. Cooling water can come from the sea or inland cooling towers are used. Some constructions can have two phases of turbines
Flue gases need to be purified to prevent acid rain and sometimes even carbon dioxide could be taken out (at least in future) to be used as a feedstock in various processes for example to make hydrocarbons.
There is a wide variety of boilers.
A fire-tube boiler is a type of boiler in which hot gases pass from a fire through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.
Their advantage over flued boilers with a single large flue is that the many small tubes offer far greater heating surface area for the same overall boiler volume. The general construction is as a tank of water penetrated by tubes that carry the hot flue gases from the fire. The tank is usually cylindrical for the most part and this cylindrical tank may be either horizontal or vertical.
Not suitable for power plant boiler, water type boiler is more economical. Generally heated with coal or oil but also biomass or methane. This is the old “locomotive” style steam generation
Old locomotive fire tube boiler sectioned. Rabensteiner - Own work CC BY 3.0
A high-pressure water-tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which boils water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam. There are three main variants (natural, forced, straight through).
Natural circulation: vaporized water flows in pipes, water first heated near saturation point in pre-heater, portion of water vaporizes in vaporizer pipes and steam directed to superheater before turbine. This is relatively simple, cheap and reliable construction with low pumping costs.
Forced circulation: water is led to vaporizer with forced circulation pumps. Power required normally 0.5% of generated electricity. Vaporized pipes can be smaller and in any position. Typical in industrial and power plant boilers.
Straight through boilers. (Benzon and Sulzer: by steam circuit and evaporation point)
Sulzer: water separating flask before superheater. Constant evaporation point (temp and pressure where evaporation happens?)
Benson: Evaporation point varies based on load. Common in large condensing power plants
Both are suitable for large power plants needing high efficiency
Both can be described as long pipe with outside heating where water goes in on one end and steam comes out the other. No cylinder, nor internal circulation.
Simple and reliable.
The burning process depends on the fuel type. Solid fuels are burned on grate for wood, wood chips and solid waste. Bubbling fluidized beds are used for wood, peat and granulated biomass and circulating fluidized beds also for wood, peat and granulated biomass.
Liquids, Gas and pulverized fuels can be burned in a burners as a flame (for biogas and methane) or with burners in a boiler where fuel is fed together with air to the combustion chamber.
The last boiler type is waste heat boiler. In it heat is recovered from flue gases.
Grates
Grates also come in a set of varieties. Stationary grates are used in low power boilers. Mechanized grates are used in larger boilers, where motion or vibration conveys the fuel. They can have oblique, chain or roll gates for movement.
Fluidized bed combustion systems have a bed of sand or other granular material. This is fluidized by air (air flow lifts the sand particles). The idea is that the granular material intensifies the combustion (more hot surface to start the burning). Fuel is fed on top of inside the bed. These combustion systems operate at 750-900 Celsius temperature.
In Bubbling Fluidized Bed (BFB), the material is just bubbling and the bubbling is controlled by air flow. Circulating Fluidized Bed (CFB) systems have a higher air flow and in them the hot flue gases flow into cyclone where bed material is separated from flue gases and directed back to the boiler
Fluidized bed combustion boilers can cope with wide variety of fuel quality. Almost all solids are suitable. Liquid and gaseous fuels as additional fuel can also be used. They also have low CO, HC and NOx emissions.
Gasification
Gasification turns solids or liquids into a combustible mixture of gases in an environment where there is not enough oxygen that combustion would occur. Normally this is 50-70% of required amount for combustion. The main components in the gas are carbon monoxide and hydrogen (basically syngas) but smaller amounts of other components like carbon dioxide, methane and steam (in steam gasification particularly). Gasification is endothermic – i.e. it requires energy, but this energy can be generated in the process as some combustion. Such processes are called autothermal.
Anything that contains carbon and can be burned, can in principle be gasified. This includes wood, agricultural wasters, liquid and solid hydrocarbons, plastics, even coal etc. When gasifying waste, there normally needs to be an external energy source (like another material stream)
Gasification offers higher efficiencies than combustion making it quite attractive.
The syngas generated can be used in Fischer-Tropsch process creating synthetic gasoline, diesel et al. that in turn can be used to power a wide range of chemical products as discussed earlier.
The gases can be directly burned in combined heat and electricity production or stored as a fuel for all kinds of engines or turned into other liquid fuels like DME, methanol. The hydrogen produced can be separated and used either for electricity or as transport fuel as well.
In other words, it can be used to produce all kinds of fuels.
There are different methods of gasification. Air gasification is particularly suitable for small setup as it does not require extra oxygen and is relatively simple. The product gas has low heating value as it contains up to 60% nitrogen. Fixed bed gasifiers are old and simple construction used in small scale air gasification.
Oxygen gasification has higher product gas quality but it requires extra oxygen increasing costs and has safety issues. It is normally used in big setups.
Third option is steam gasification. It is traditionally used in gasification of coal and produces product gas with medium heating value. There is no combustion happening in this process. Steam is first heated and then gasification done with it.
Gasification happens as a series of reactions. In air or oxygen gasification first, the material dries up and starts releasing gases and contained water vapor.
The gases start breaking into hydrocarbons in pyrolysis reaction where also tar and charcoal are formed. Partial combustion starts where part of the gases burn releasing heat.
Final step is reduction where in Bouduard reaction solid carbon reacts with carbon dioxide (CO2) to produce carbon monoxide (CO) (Co2+C ⇌ 2CO). And in water gas reaction carbon monoxide and water vapor form carbon dioxide and hydrogen (CO + H2O ⇌ CO2 + H.)
The gasifiers themselves come with different formats: fixed bed (either downdraft or updraft), fluidized bed (bubbling and circulating) and entrained flow. As mentioned, fixed bed are used in small units. Biggest difference between downdraft fixed bed and updraft is that downdraft units produce low amounts of tar. In fluidized bed gasifiers the particles (usually sand) are kept in constant motion by a flow of air, oxygen or steam. They are used in mid to big units. Entrained flow gasifiers are used in big units, steam drives gasification and usually use coal.
Main gasifier types
By Kopiersperre - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=37292688
Gasification product gases contain impurities such as chlorine as in HCL and KCl. Both of these are highly corrosive especially. in high temperatures. No material can withstand them. It is essential to purify them. Also various sulphur compounds, for example sulphur oxides. These cause bad odour and are also catalyst toxics. And for example nitrogen oxides. Both sulphur dioxide and nitrogen oxides (NOx) form acids when released into the atmosphere and cause acid rain causing environmental damage like death of trees and are essential to purify. Finally, some alkali metals, tars and fly ash. Alkali metals are corrosive and can cause clogging
Gases needs to be purified. This can happen for example in cyclone, granular bed filter, electrostatic precipitator, water scrubbing, granular bed filters etc.
Almost always there is some type of cyclone where separation happens via vortex. Heavy particles move to the walls, strike it and fall to the bottom of the cyclone. Electrostatic precipitator is used a lot in large scale industry. In it an electrostatic charge is induced to particles that are then collected.
An example of open source engineering developing gasification designs is GEK project.