Novel biogas plant feasts on culinary cultivar
While Belgian endives may have been discovered by accident, the biogas plant at endive grower Joluwa N.V. is purpose built. Commissioned in September 2010 the fully automated plant uses spent chicory roots as feedstock. It is the first commercial installation of a novel high yield digester technology developed by another Belgian firm GreenWatt SA.
In the culinary world the term “Endive” denotes a lettuce like leaf vegetable, Cichorium endivia, from the chicory genus. Often used in salads they are especially popular in France, Belgium and the Netherlands. Somewhat confusing Belgian endives or Witloof is a cultivated variety of the common chicory, Cichorium intybus, whereby the blanched, tight heads known as Chicons are produced by forcing mature chicory roots to grow in darkness. Once the chicon is harvested the root is discarded.
Located in countryside just outside the town of Nivelles is the Witloof grower and producer Joluwa N.V. The company currently produces around 4 000 tonnes of forced chicory roots per year at its growing, harvesting and packing facility. According to Joost Depaepe from Joluwa the company needed an environmentally suitable, cost effective and long-term solution for the growing volume of residue from spent chicory roots. The expanding business was having issues with odour and leaching from storage piles of rotting roots. Disposal costs at external processing facilities were increasing and the company had heat and power needs but had no particular desire to run a biogas plant.
– It is important for us to be independent and cost efficient in our waste and energy management but biogas in itself is not our business or interest. We wanted to address these issues with a turnkey solution that was easy to manage and not time consuming, said Joost Depaepe from Joluwa.
A project to utilise the energy potential of the chicory roots was set-up in 2009 with GreenWatt SA, a Belgian company specialised in the design and installation of on-site biogas plants for farms and agribusinesses. A research spin-off from the Catholic University of Louvain, GreenWatt developed a novel high yield flushing anaerobic digestion (HYFAD) reactor and the Joluwa project is the first commercial installation of this patented technology.
– We specialise in designing biogas plants based on fruit and vegetable waste as substrate. Each project is tailored to fit the residue generated, said Baptiste Genin, R&D engineer at GreenWatt. The different stages of the anaerobic digestion process have conflicting operating parameters which need strict regulation management. A change in the composition or volume of the feedstock input in a conventional biogas plant risks destabilising the process through acidification damaging the methane producing bacteria.
– We have removed this risk entirely. The methane production is stable and consistent regardless of feedstock quantity variations. Our multi-stage technology process is based on the separation and specialisation of these phases into two or three stages to control the operating parameters of these different reactions independently of one another, Genin said.
Two-stage AD plant
The Joluwa facility is a two-stage anaerobic digestion (AD) biogas plant with three main process steps: a hydrolysis tank, the HYFAD reactor and a post-digester. On average the plant produces 850 m3 biogas a day 75 percent of which is produced in the reactor and the rest in the post-digester. The solid digestate is used as fertiliser and the fluid is discharged to the public wastewater system after decantation. The biogas has an average methane content of 54 percent and is used to fuel an on-site a combined heat and power genset with 104 kW power rating. All the electricity and around 65 percent of the heat is used by Joluwa for the chicory processing plant and self-consumption of the biogas plant. An additional 30 percent of the heat is supplied via a small network to a printing company located about 1 km from the biogas plant.
– The overall heat loss is only 5 percent which is very low for a biogas plant, Genin pointed out.
In the first step, hydrolysis and acidogenesis, the chicory roots along with other vegetable matter are loaded into the receiving hopper using a front loader. An X-ripper grinder pump is used to get the material into the hydrolysis tank where it is converted into a liquid loaded with volatile fatty acids. In the second step, methanogenesis, this acid liquor is fed into the HYFAD fixed bed reactor to produce biogas. The HYFAD reactor consists of two 25 m3 vessels that are side by side and connected to each other. The vessels are packed with vertical honeycomb columns which function as the fixed bed media. The honeycomb structure provides a very large surface area in relation to the reactor volume. This leads to a high concentration of bacteria attached to the bed as biofilm enabling a high methane yield. Organic loading rates (chemical oxygen demand) of up to 35 kg COD/m3 per day have been achieved with this reactor. The reactor has an anti-clogging system that cleans the columns and recovers the biofilm.
The temperature in the HYFAD phase is kept at 38oC and the pH is always kept neutral and remains stable even if the liquefier is overfed. The post-digester in the final step allows the digestion of organic material that had not been fully processed in the hydrolysis tank. The global hydraulic retention time (HRT) for the plant is 45 days.
The plant uses on average 11 tonnes of chicory roots per day but can handle up to a 30 tonnes per day peak. According to Genin, chicory roots yield about 38 m3 methane per tonne.
– This is quite low compared to other feedstocks and is due to the high water content, around 85 percent, said Baptiste Genin.
Start-up: September 2010
Technical: Two-stage anaerobic
digestion plant: hydrolysis tank, High Yield Flushing Digester (HYFAD) reactor
& postdigester. CHP with 104 kWe rating
Feedstock: 4 000 tonnes/year of forced
chicory roots and vegetable waste
CH4 potential: 38 m3 per tonne (chicory roots)
Hydrolysis tank: 30 tonnes/day,
165 tonnes/week, 500 tonnes/month
HYFAB: Up to 35 kg COD/m3 per day
Biogas: 280 000 m3 at 54% methane content
Electricity: 500 MWh
Heat: 760 MWh