Integrated AD-Gasification waste-to-energy system
1. Introduction
Lignocellulosic biomass waste, a heterogeneous complex of biodegradables and non-biodegradables, accounts for large proportion of municipal solid waste. Due to limitation of single-stage treatment, a two-stage hybrid AD-gasification system was proposed in this work, in which AD (Anaerobic Digestion) acted as pre-treatment to convert biodegradables into biogas followed by gasification converting solid residue into syngas. The proposed waste treatment process which combines AD and gasification are shown in Figure 1 (without heat recovery) and Figure 2 (with heat recovery). Energy performance of single and two-stage systems treating some typical lignocellulosic wastes was studied using both experimental and numerical methods.
Figure 1. Process flowsheet of two-stage waste disposal process.
Figure 2. Energy flow of single-stage and two-stage hybrid systems for lignocellulosic biomass waste treatment: (a) single-stage, (b) two-stage.
2. Equipment and materials
For the test bedding of AD process, two types of AD reactors are used. One is the 30L HSAD (High-Solids Anaerobic Digester) and the other is a 1000L semi-continuous AD reactor. Feedstock was mixed externally before being fed into the reactors. In both reactors, temperature and mixing rate can be controlled through the control panel. For test bedding of gasification, three types of gasifiers were used, two down-draft gasifiers (10kW and 20kW) manufactured by All Power Labs and one 1MW gasifier developed by our industry collaborator, Leong Siew Weng Engineering Pte Ltd. Some of the feedstocks we have tested are shown in Figure 3.
Figure 3. Characterization of the tested feedstocks
3. Results
Feasibility of a two-stage hybrid WTE system was justified using both experimental and numerical methods in this work. Main findings are shown in Figure 4-5. In detail, at the first stage, higher I/S (inoculum/substrate) ratio could help improve AD performance while higher organic loading rate may inhibit AD process. At the second stage, gasification was added as post-treatment for AD residue. Experimental results showed higher mass ratio of AD residue may lower down the quality of syngas produced from co-gasification of AD residue and woodchips. Maximum mass ratio of AD residue for gasification was 20%. Furthermore, for lignocellulosic biomass wastes with high BMP, pH would be the main inhibition factor when increasing OLRs. Thus, wet AD is recommended; for lignocellulosic biomass wastes with low BMP, increasing of OLR could considerably help decrease the energy consumption for drying and enhance overall efficiency, thus HS-AD was recommended. Last but not least, the hybrid system could effectively help improve quality of produced gas compared with single-stage gasification system and shows potential in enhancing total gas energy production for some typical biomass wastes.
Figure 4. Evaluation of overall energy performance in the two-stage system
Figure 5.Comparison between the integrated system and one-stage system
4. Publications
- Yao, W. Li, X. Kan, Y. Dai, Y. W.Tong, C.H. Wang, “Anaerobic Digestion and Gasification Hybrid System for Potential Energy Recovery from Yard Waste and Woody Biomass”, Energy, 124, 133-145 (2017).
- X. Kan, Z. Yao, J. Zhang, Y.W. Tong, W. Yang, Y. Dai, C.H. Wang “Energy performance of an integrated bio-and-thermal hybrid system for lignocellulosic biomass waste treatment”, Bioresource Technology, 228, 77-88 (2017).