Making biogas ends meet
Shandong province is major agriculture region in northern China. Traditionally post-harvest straw was burnt in the field and the ash ploughed into the ground as fertiliser. Banned years ago the practice is still widespread causing severe smog. Shandong Baori Bioenergy offers an alternative.
On an annual basis Shandong farmers would typically grow around 1.3 million ha of vegetables along with 4 million ha wheat, 2.67 million ha corn and 0.87 million ha of cotton thus producing significant volumes of straw. Shandong Baori Bioenergy Co., Ltd (SBB) is a newly formed subsidiary of the oil company Chang’an Group, which is based in Dongying City. SBB was founded in June 2011 as a means of exploring and developing bioenergy business opportunities.
Being located in Shandong province, finding a use for abundantly available straw while tackling the smog issue was a clear choice. Today SBB operates three bioenergy plants, two in Dongying and one in Laoling. In Dongying it operates a 3 million Nm3 per annum corn stover biogas plant and a 10 000 tonne per annum sweet sorghum bioethanol plant.
In Laoling the company currently has a 5 million Nm3 per annum corn stover biogas facility, which celebrated its first year of operations in October 2015. The facility is complete with a biomethane upgrading, compression and bottling plant as well as a carbon dioxide (CO2) collection and purification unit. However according to Bin Zuo, Chief Technology Officer for Baori Laoling Biogas Plant, the company has plans to increase the annual biogas capacity at its Laoling plant six-fold, to 30 million Nm3 in another two build phases.
– Corn is a major crop in Laoling County accounting for over 94 percent of the autumn crop harvest. This means that 850 000 Mu (≈57 000 ha) is under corn plantation, explained Bin Zuo.
The second phase is to add another 15 million Nm3 per annum capacity and the final phase will see another 10 million Nm3 per annum. The plant produced 200 000 Nm3 bio-CNG in its first year.
– At the moment the local bio-CNG demand is lower than we had expected. Gas production is controlled by the gas sales demand. To increase biogas production and profitability, we’re including a 600 kW electric output gas engine in the second phase. This is already under construction and is due for commissioning soon, said Zuo.
Phase one of Baori Laoling biogas was completed in October 2014 and the operation processes include feedstock receiving and pre-treatment, three 3 000 m3 anaerobic digesters each with a 1 700 m3 raw biogas tank on top, biogas purification, organic fertiliser production, slurry utilisation, CO2 collection and a bio-CNG refuelling station.
Anaerobic digestion is commonly used for livestock manure and organic municipal waste but straw has low density, it floats and up to 60-80 percent consists of cellulose, hemicellulose and lignin making it difficult for organisms to break down.
– Proper pre-treatment of the stover before anaerobic digestion is essential. The cut stover is loaded into the pre-treatment reactor, mixed with bacteria that produce an enzyme that breaks chemical bonds in straw, which improves its biological degradability. By this ambient temperature, solid chemical pre-treatment, the gas output for corn stover can be increased 50-120 percent after a 3-day pre-treatment, revealed Zuo.
Each digester is equipped with three externally mounted agitators from the German company Suma Rührtechnik GmbH. These form a multi-level triangle position keeping the contents in motion to avoid crust formation and sedimentation while enhancing reactivity within the digester. A gas-fired boiler provides additional heat for the digesters during winter period when ambient temperature is too low.
Novel rental programme
As for most Chinese biomass companies, SBB meets a major challenge when it comes to straw and stover collection and transportation. Farmland in China is fragmented and owned by many small household farmers. It is an onerous and expensive task to negotiate with each individual farmer to ensure consistent supply of stover quality and quantity to the plant.
Last year the company tested a novel new way to improve stover collection – a “free” corn harvester rental programme. SBB purchased 30 harvesters and rent these to local farmers free of charge. In return, the stover harvested by these farmers is delivered to the Baori Laoling plant as payment for the machine rental. Furthermore with the harvester, farmers can provide a harvest service to neighbouring farms earning additional income.
– Many household farmers are happy to pay for this service as it saves time and they can seed winter wheat as soon as the land is cleaned, said Zuo.
The corn stover is shredded into pieces up to 1 cm in length and transported directly to the plant where it can be loaded directly into the pre-treatment reactor or stored in one of the four 5 000 tonne storage clamps.
– It saves money and space if the stover is cut into 1-2cm in the fields. Otherwise, extra handling, labour and operation costs are incurred for resizing at the biogas plant, explained Zuo adding that this is reflected in the different prices paid for corn stover.
For the 2014/2015 season, the company paid CNY 100-120 per tonne for non-shredded stover, CNY 125 per tonne for preliminarily shredded (larger than 2 cm) material and CNY 170 per tonne for fully shredded corn stover. Based on the good experience of the “stover in lieu of cash” harvester rental programme, SBB plan to expand it with additional harvesters this year.
Text & photos: Xinyi Shen