The advantages relative to the use of briquetted straw as raw material for biogas are the following:
1 – Direct feeding into biogas reactor
A briquetting line consists of a straw-line including conveyor band for big bales, shredder, stone and sand trap, a hammer mill before the actual briquetting and direct feeding into the biogas reactor. The briquetting offers a systems solution for actual managing and feeding big bales of cereal straw into the bioreactors.
This is possible due to the altered properties of the straw.
It has been demonstrated for more than 3 years, at the biogas plant at Aarhus University full scale experimental facility at Foulum, that substantial quantities of straw briquettes can be directly injected into the reactors.
2 – Water absorption – the sponge effect
Straw is rendered highly water absorbent due to the briquetting treatment, and is therefore immediately dispersed and suspended after addition to the bioreactor. Because water penetrates into the straw matrix, the soaked straw, taking up 10-12 times its own weight, achieves the same density as water, and it will be evenly distributed in the reactor volume. No floating covers will develop, no sediments, dead volumes or clogging will appear.
Addition of straw briquettes does not require additional stirring effect.
3 – The straw pores are exploded
Because all pores of the straw are exploded and water can penetrate into the straw, it also renders access for those microorganisms and enzymes that are digesting the straw into biogas.
This ensures a relatively quick and effective digestion in the biogas plant.
4 – Straw briquettes do not impact the hydraulic capacity of any biogas plant.
The direct feeding of dry briquettes into bioreactors does not impact the hydraulic capacity of the biogas plant. It is unnecessary to suspend the briquettes in raw manure or digested manure under severe stirring or such before adding to reactors. No extra tanks fitted with stirrers are needed.
5 – Straw briquettes add heat and circumvent heat exchangers.
In the case of a briquetting plant being installed at the biogas plant and briquettes being added directly when produced, process heat is also added to the reactors, i.e., the kinetic energy dissipated into the straw is recycled into reactors. The electric power used is recycled as heat into the biogas process.
Also, the straw briquettes do not pass heat exchangers and possible clogging here is avoided.
6 – Straw briquettes are evacuated of air
The compressed straw is emptied for atmospheric air, notably free nitrogen and oxygen, and air is therefore not added to the biogas, when using straw briquettes. This is highly advantageous, when the biogas is upgraded to natural gas quality.
7 – The straw plant may be situated locally at the straw-resource or at the biogas plant.
Local circumstances and conditions will determine if it is advantageous to develop local satellite stations for local collection and processing of straw into briquettes before storing and transportation to one or more biogas plants.
8 – An energy optimal process.
Briquetting uses very little energy and no water for the pre-treatment. The electrical power use of app. 75 kWh per tons straw is mainly reused as heat in the biogas reactor if the processing plant is placed at the biogas plant.
Conventional pre-treatment of straw before a biogas or bioethanol process such as steam explosion at 180-200°C and 30-40 % dry matter is a highly water and energy demanding process, which also impacts the hydraulic capacity of the biogas plant negatively.
9 – A mechanical and robust and stable technology.
Straw inevitably contains gravel, soil or sand, or other impurities. A straw line removes all impurities before the briquetting. The mechanical press is not exposed for this wear and tear and the impurities are not imported into the biogas facility.
10 – High biogas yields.
Briquetted straw gives rise to high and stable biogas yields.
Straw generally contains 18-19 MJ/kg dry matter primary energy corresponding to app. 500 Nm3 methane per tons straw dry matter.
The straw composition is 60-70 % cellulose and hemicellulose, 15-20 % lignin, other organics (5-10 %) and minerals (app. 5 %). Normally, it is the bulk of cellulose and hemicellulose, which, after briquetting, is digested into biogas, while lignin is largely un-digested.
With briquetting, biogas yields of app. 250 Nm3 methane per tons straw can be achieved increasing to app. 300 Nm3 methane per tons under optimal conditions.
A hydraulic retention time of 30-40 days at thermophilic temperatures are needed to achieve these yields. The highest yields can be achieved with addition of potassium lye during briquetting.
The quantity of straw, which can be added to a manure based biogas plants – the organic loading rate – is equally important. It is clearly demonstrated that 10 % straw briquettes relatively to the liquid manure can be added without any difficulties. See a comparison below:
Biogas yields using dry straw; increased production
Pre-treatment Methane yield
(m3 methane/tons straw) *Estimated max quantity to be added manure plants
(in % of slurry) Increased yield per tons slurry (in Nm3 methane)
Shredded straw 200 1-2 % 2-4 10-20
Other mechanical processing of straw 225 2-6 % 5,5-13,5 27,5-67,5
Briquetted straw 250 8-12 % 20-30 100-150
Briquetted straw with catalyzer 300 10-12 % 30-36 150-180
11 – Straw briquettes contribute to a stabile biogas process.
Several manure based biogas plants, if not most plants, are at the brink of ammonia inhibition. In any case it is virtually impossible to add significant quantities of nitrogen containing wastes such as poultry manure.
Straw briquettes help alleviating ammonia inhibition and stabilize the digestion process. An increased yield of slurry is achieved when co-digested with straw briquettes.
12 – A win win
Mechanical briquetting of straw for biogas purposes is the most tested, documented, and safest technology of using straw for biogas purposes.
We are proud to offer this cost effective solution for industrial utilization of straw for biogas.
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