The path to zero waste.
Sierra Energy typically designs the preprocessing stage to dry waste down to around 20% moisture by weight. This helps to optimize the efficiency of the gasification stage.
Even though the FastOx gasifier can handle wastes with moisture content up to 50% (by weight on wet basis) without pre-treatment, it makes more thermodynamic sense to pre-dry the wet waste to less than 20%.
Pre-drying can be accomplished using excess heat produced from the gasification process or excess steam from the system. By pre-drying, the FastOx gasifier has less internal endothermic load, allowing for more waste materials to be gasified per unit time, i.e. more conversion of wastes per day for a given gasifier size. In other words, less heat is being used to drive off moisture in the gasifier, which means that heat can be better used to drive the desired reactions in the base of the gasifier.
Additional carbonaceous materials should be co-charged with the waste streams containing low fixed carbon or char.
High moisture content in waste (% by weight) generally lowers the heating value of waste and decreases the resulting amount of energy produced. If waste containing 50% moisture (by weight) is gasified, most of the water is driven off as steam. This high moisture content in the syngas decreases the heating value of wet syngas released from the gasifier. When moisture is condensed out of the syngas during the gas cleaning process, the lower heating value (LHV) of dry syngas increases to 10.2-12.1 MJ per Nm3. Even though the LHV of dry syngas remains unchanged, there is less mass of dry syngas resulting in the lower amount of final energy product (i.e. lower kWhe or gallons of fuel output per metric ton of feedstock processed).
For FastOx gasification, the average amount of fixed carbon and char in waste materials is more important than the moisture content and the LHV. A certain amount of fixed carbon in the waste streams is required to enable oxidation of the char in the tuyere area where the oxygen and steam are co-injected into the FastOx gasifier. The char oxidation reaction provides the energy to self-sustain the endothermic reactions in the other reaction zones (i.e. drying, devolatilization, melting) through the heat exchange with the rising hot syngas from the tuyere level.
Some waste streams may not require additional carbonaceous materials. However, if the desired feedstock has a low fixed carbon content, additional carbonaceous materials such as biomass, tires, or railroad ties should be blended with the wet waste stream to increase the overall fixed carbon. To determine whether the extra carbonaceous material is necessary, Sierra Energy needs to see proximate and ultimate analyses of waste components and the range of compositional fluctuation. Sierra Energy uses the ASTM D-5142 method conducted by a feedstock testing lab to find the fixed carbon value.
Sierra Energy does not typically recommend gasification of liquid waste as it can cause drastic losses in efficiency.
FastOx gasifiers can handle upwards of 50% moisture by weight. However, we typically aim to dry waste down to roughly 20% moisture by weight (using recovered waste heat/hot air from the plant), as it helps optimize the overall efficiency of the gasification process and produces more renewable end products.
It is possible to combine a liquid waste stream with additional waste streams to allow for efficient processing of the waste, however whether or not it is a good idea will have to be determined through a feasibility study.