Anaerobic digestion of crop and waste biomass: Impact of feedstock characteristics on process performance
11
Citation
46
Reference
20
Related Paper
Citation Trend
Abstract:
Anaerobic digestion provides an array of positive environmental benefits such as reducing greenhouse gas emissions, replacing mineral fertilizers, producing renewable energy and treating waste. However, pitfalls in anaerobic digestion such as poor methane yields, process instability, process failure and regional shortages of feedstock have limited the full exploitation of the anaerobic digestion process.
The research presented in this thesis deals with the assessment of the possible negative or positive impacts of feedstock characteristics on the efficiency of anaerobic digestion. In addition, it investigates ways of enhancing the methane yield of the feedstock by improving the feedstock characteristics. The feedstocks investigated were various energy crops, food industrial waste and sewage sludge. The improvement methods investigated were ensiling, nutrient supplementation, co-digestion and anaerobic pretreatment.
It was found that ensiling crops results in insignificant losses in energy, total solid and wet weight. In addition, no significant difference was found in methane yields between the ensiled and fresh crop samples. The importance of correcting for losses of volatiles in total solids determination was pointed out and it was shown that failing to do so could be the main reason why many previous publications report increased total solid based methane yields after ensiling. Increased methane yield in silages may therefore be an effect of an analytical error rather than an effect of using ensiling as a pretreatment prior to anaerobic digestion.
Anaerobic digestion of crop biomass is known to be particularly limited by nutrient availability. Direct nutrient supplementation in crop mono-digestion in this research demonstrated an efficient biogas process at the shorter hydraulic retention times commonly applied in co-digestion of crop biomass and manure. The high degradation efficiency was evidenced by high methane yields, comparable to maximum expected yields generated under controlled conditions, and low volatile fatty acids accumulation. As a result of nutrient addition, the digestate could comply with certification standards for bio-fertilizer. Also, viscosity problems commonly reported for crop mono-digestion were not observed in this study, which could be another effect of nutrient addition.
Co-digesting of waste biomass and crop biomass led to significant improvement in methane yield per ton of feedstock and carbon to nitrogen ratio as compared to digestion of only the waste biomass. Biogas production from crops in combination with waste biomass also eliminated the need for addition of micronutrients normally required in crop mono-digestion. Co-digestion was also presented as a means of feedstock supplementation to curb feedstock shortages in waste-based anaerobic digestion processes. In addition, inhibitors in anaerobic digestion such as free ammonia and light metal ions were diluted, a condition which can lead to an overall viable biogas process
Anaerobic pre-treatment led to the solubilisation of particulate organic matter in sewage sludge. This solubilisation could have led to the improved methane yield, methane production rate and reduction in volatile solids.
Applying different feedstock improvement solutions to the various feedstocks investigated, i.e. nutrient addition, co-digestion and pretreatment, were demonstrated as effective means of enhancing the methane yield of the feedstock thereby improving the overall anaerobic digestion process.Keywords:
Biogas
Digestate
Energy crop
Food Waste
Digestion
Biodegradable waste
Cite
Biogas
Bioconversion
Energy crop
Anaerobic respiration
Digestion
Cite
Citations (3)
Biogas
Digestion
Digestate
Cite
Citations (20)
Biogas
Sewage sludge
Food Waste
Cite
Citations (60)
Biogas
Hydraulic retention time
Lignocellulosic Biomass
Cite
Citations (455)
The increasing demand of energy supply requires the development of systems of energy production based on the exploitation of renewable energy sources as an alternative to fossil fuels in common use. Through the process of anaerobic digestion it is possible to convert into biogas agricultural biomass, zootechnical waste, sewage sludge and organic fraction of municipal solid waste. After that it is possible to generate energy from biogas through the process of cogeneration. More recent concerns about global warming have stimulated further anaerobic digestion application and the improvement the processes in order to maximize biogas production, which is a renewable and versatile energy source that can be used for heat and electricity production, and as transportation fuel. It is in the interest of operators of anaerobic digestion plants to maximize methane production whilst concomitantly reducing the chemical oxygen demand of the digested material. The pre-treatment of solid waste is regarded as a prerequisite of the anaerobic digestion process to reduce volume and increase methane yield. The aim of the mechanical treatment is the reduction of the size of the biomass and its degree of crystallization, in order to increase the surface area available to enzymatic hydrolysis. This generates an increase on biogas production and a decrease in the time required for the digestion. In this work the link between mechanical pretreatment and the increase of methane yield of some samples of a dedicated crop (triticale) was discussed.
Biogas
Energy crop
Cite
Citations (11)
Digestate
Food Waste
Biogas
Green waste
Digestion
Cite
Citations (14)
Biogas
Renewable resource
Arable land
Energy crop
Lignocellulosic Biomass
Cite
Citations (59)
The role of anaerobic digestion in the present bioeconomy concept exceeds the boundaries of on-site electricity and heat production, as it can serve as a process for renewable energy recovery and the production of bio-based products. To apply this concept, three agricultural feedstocks were evaluated for potential biogas production: cocoa waste, pumpkin and animal manure.
First, the optimization of cocoa waste anaerobic digestion was evaluated employing batch and fed-batch reactors in high (dry AD) and low (wet AD) total solids content of the feedstock. Dry AD performed significantly better than wet AD. A case study included a theoretical energy potential calculation for a full-scale plant with data obtained experimentally. AD from cocoa waste could supply up to 82% of the electricity demand of a rural region of Ecuador where cocoa is grown (20,000 inhabitants).
It was confirmed that the use of synthetic nutrients and cow manure improved biogas and methane yields from cocoa waste, significantly. Interestingly, the treatment of synthetic nutrient addition and co-digestion with sterile cow manure had no significant differences. However, the best treatment was co-digestion with raw cow manure in terms of stability and methane yields in the long-term operation. A possible explanation behind the long-term stabilization of co-digestion with raw cow manure might be the high presence of Metanosaetaceae in the cow manure feedstock, which remained stable throughout the experiment.
After AD optimization, a cyclic valorization of agricultural residues with different processes was performed. AD was integrated with slow pyrolysis to evaluate the energy and mass balances towards new products. The integrated process of co-digestion (cocoa waste + cow manure), followed by the slow pyrolysis of the digestate at 500 °C recovers most of the intrinsic energy of the waste into the gas phase in the form of biogas and non-condensable gases (up to 48%). In synthesis, these integrated processes recover more energy than each of processes separated, being equivalent to 61% mass conversion from the feedstock to fuel (liquid and gas).
Finally, we demonstrated that electrochemical biogas upgrading is a successful method to separate CO2 from biogas. The process achieved almost perfect CO2 removal efficiency. Electrochemical biogas separation produces ‘customized’ gas mixtures. The final cathode and anode off-gas blend varied with the current applied, indicating that it is possible to have specific gas blends according to our necessities by changing operational parameters of the electrochemical upgrading unit. Animal feed, in the form of microbial protein, were generated from real stream biogas, cathode and anode off-gases. Cathode off-gas achieved the highest biomass concentration and the highest protein content in the microbial biomass.
In conclusion, co-digestion proved to be a powerful technology in dealing with agricultural residues and brings stability and buffer capacity in the overall anaerobic digestion process in the long term. Slow pyrolysis has a strong potential for dealing with agricultural waste. Digestate, biogas, biochar, bio-oil and even syngas can supply agricultural areas that have a high demand for fuel and soil amendments. The best-case scenario is the integration of co-digestion and slow pyrolysis. This alternative has the highest energy efficiency in terms of gross energy recovery and in terms of net energy return. Finally, new products can be obtained from agricultural waste, such as microbial protein using anaerobic digestion as central technology.
Biogas
Cow dung
Renewable resource
Cite
Citations (0)
The principles and overview of research, development and implementation of anaerobic digestion for hog wastes are discussed. Based on economic evaluations, an anaerobic technology is cost-effective, especially for a larger herd and becomes more competitive with aerobic treatment. Nevertheless, the rate of treatment is more sensitive and dependent on the particular fraction of manure being processed. Considering the different factors affecting anaerobic digestion, a complete mixed reactor with solids recycle (having high solids retention time and low hydraulic retention time) was found to be the more reliable system with regards to methane generation and manure stabilization. By solids recycle one can obtain significant saving in the reactor volume required, while still achieving the expected degree of treatment. It was also found that even though treatment using advanced anaerobic systems when compared with simple anaerobic systems is more expensive, the rate of return on investment and efficiency of the process are higher.
Biogas
Anaerobic respiration
Hydraulic retention time
Cite
Citations (0)
Different mixture ratio of household organic waste (HOW) and cow manure (CM) were investigated for biogas production. The objective was to explore possible significant synergistic effect obtained from the combination of these different substrates in Reactor 1 – 4 (R1 – 4) batch experiment. The highest methane yield, 247 mL/g VS was obtained from R3 and 243 mL/g VS in R4. Co-digestion in R4 increased to 9% for CM and 78% for HOW in methane production. The results clearly demonstrate synergistic effect from nutrient balanced that improves the stability of anaerobic process.
Cow dung
Biogas
Biodegradable waste
Digestion
Biogas Production
Chicken manure
Cite
Citations (16)