Feasibility analysis and evaluation of making biochar from surplus sludge
Under the background of carbon neutrality, surplus sludge resources and energy utilization have become the direction of future research and application. It seems that making biochar from excess sludge produced after sewage treatment has attracted people's attention and research for a time. Biochar refers to the pyrolytic solid components remaining after the heat-dried biomass is heated to 300~950℃ under the condition of inert gas (mainly N2, CO2, etc.).
There are two common methods to prepare biochar: slow pyrolysis and hydrothermal carbonization. Slow pyrolysis, as the most commonly used pyrolysis method, has the advantages of easy control of preparation conditions, high yield of biochar and rich types of surface functional groups. However, the properties of biochar prepared by this method are greatly influenced by the physical and chemical properties of raw sludge and are difficult to predict. Different from traditional pyrolysis methods, hydrothermal carbonization does not require high water content of raw sludge, so it can avoid consuming a lot of energy in sludge drying process. In addition, hydrothermal method can effectively retain elements such as C and O in raw materials, so the content of oxygen-containing functional groups in hydrothermal carbon is higher than that in traditional pyrolysis method. In addition, there are high-pressure or vacuum pyrolysis, microwave low-temperature pyrolysis and other treatment methods.
Sludge biochar has many advantages, such as abundant elements such as P and K, rich kinds of oxygen-containing functional groups on the surface, high porosity and large specific surface area. Therefore, it has been widely studied in the fields of agriculture and environment, and is considered as a cheap and easily available soil improver and adsorbent material. In addition, the application of biochar in the fields of energy and materials has also been developed in recent years. Biochar can not only be used as fuel for direct incineration, but also replace granular activated carbon and graphite particles as anode materials of fuel cells, which can improve performance and reduce energy supply cost and carbon footprint.
However, is sludge biochar worth studying and applying? On the basis of briefly describing the general preparation principle and process of biochar, the energy consumption, cost and performance of sludge biochar manufacturing process were quantitatively evaluated and analyzed through detailed calculation, and compared with straw biochar horizontally.
The results show that:
1. The preparation of sludge biochar requires a lot of energy (-695 kW×h/t dry sludge), which is difficult to offset its production capacity, and the production cost is as high as 930 yuan /t biochar. In contrast, the production of straw biochar is a net energy output process, and there is still 1400 kW×h/t straw surplus energy after balancing the input energy;
2. The preparation cost of sludge biochar is much higher than that of straw biochar, which is 930 yuan /t biochar and 575 yuan /t biochar respectively;
3. In terms of performance, the "carbon fixation" content of sludge biochar is only 20% of that of straw biochar, but the ash content is twice as high as that of straw biochar, and the heavy metal content is much higher than that of straw biochar.
Compared with straw biochar, sludge biochar is not dominant in the preparation process or the application of end products, especially the high energy consumption, high cost and heavy metal content in the preparation process limit its practical application, that is, the ideal is full, but the reality is very skinny. Moreover, the dominant straw biochar has not yet achieved large-scale production and application when the technology is quite mature. Therefore, the study of sludge biochar will go to a dead end.
Source：https://mp.weixin.qq.com/s/ZXK2yiB95aGScTw1lWiuFw