Techno-Economic Assessment of Converting Cellulose from sewage into Bio-based Products
Sewage contains about 25-30% of the particulate fraction in the form of cellulose fibers. The source of these fibers is mainly toilet paper due to its increased consumption. Utilizing these valuable materials could reduce the use of natural resources and subsequent carbon dioxide (CO2) emissions and hence, realizing a circular economy. In the WOW project, a techno-economic assessment of the use of these cellulose fibers for bio-based products is performed at a pilot scale.
The cellulose fibers are recovered by using special screens, dewatered, dried, and formed into pellets as shown in Figure. A fast pyrolysis process transforms the pellets into biochar and volatiles that are separated into bio-oil, acetic acid, and pyrolysis gas. The pyrolysis gas is used up internally to provide the heat required for drying the cellulose fibers.
Minimum selling price
The minimum selling price (MSP) estimated in this project for biochar, bio-oil and acetic acid are 38%, 79%, and 41% lower than the market prices, respectively. This is because the feedstock (cellulose) is essentially free and comes at no cost but require further pretreatment. The selling prices and the plant scale are key parameters that have a significant effect on the MSP. At a large scale, which is double that of the pilot-scale, the MSPs of the products decrease significantly. At higher market prices, the selling of biochar and bio-oil will generate higher profits for the plant.
The biochar has limited applications such as combustion for energy, soil enhancer, and water filtration. Therefore, it needs to be activated to increase the surface area and the effectiveness to expand its applications by using either a physical or chemical method. The techno-economic assessment showed that the chemical activation method using a liquid activating agent is more suitable for this type of value chain.
The chemical activation method is done using phosphoric acid in this case. It requires additional equipment and energy compared to physical activation but has a higher activation yield. Besides, the surface area obtained is almost double that of physical activation. Hence, it expands the application possibilities further to air purification, medical uses, gas storage, several chemical, and metallurgical operations. Moreover, the market price of such activated char increases with an increased surface area. Compared to the base case discussed earlier, the capital expenditure and operating expenses increase, the overall yield decreases but the revenue increases significantly.
The performance of this value chain compared to other pyrolysis plants is promising and results in a positive business case under the assumptions made. It would be interesting to investigate further since it has the potential for reducing the MSPs by optimizing plant operation and efficient energy consumption.
Figure A: Cellulose recovery and conversion to bio-based products