Acidification of slurry and biogas can go hand in hand
Henrik B. Moller & Veronica Moset
Department of Engineering
Aarhus University, Denmark
Production of biogas and acidification of slurry are two very promising environmental technologies, which each serve their own purpose. But so far very little has been done to combine the two technologies, which is considered a precondition, when it comes to reaching the Danish targets of degassing 50% of all manure in biogas plants by 2020 and at the same time reducing ammonia evaporation from animal production.
This forms the background for the cooperation between Aarhus University, Department of Engineering and Infarm A/S, which is a company that develops acidification technologies. The cooperation should utilize the maximal synergies from the two technologies.
Generally large amounts of sulphur will reduce the biogas yield, but because the acidification ensures reduced methane loss in the stable this could increase the biogas yield and the total CO2 gain. Furthermore it appears that additional small amounts of sulphur in the reactor tank have had a positive influence on the biogas yield. However, it is important to know the critical amounts, since there is a fine balance between the positive gain and a reduced biogas yield. To avoid the negative effects it is therefore recommended not to add more than approximately 10% of acidified slurry to a biogas plant. When using the separated fiber fraction from acidified manure the percentage can be increased considerably and when replacing 30% of the biomass with acidified fiber there was no negative effect and the gas production was increased by 50% compared to normal slurry.
Picture: Separation of cattle manure with screw press (left) and dismantled screw press (right). (Photo: Infarm A/S).
Utilization of acidified slurry
A series of tests have been carried out to clarify the effects of sulphur on the biogas process and to determine the optimal mixture proportion of acidified and non – acidified slurry. The tests were carried out in four different scales: 1 liter, 120 liters, 10 m3 and 30 m3 reactors, where the tests in 30 m3 are to be considered as commercial scale.
By comparing the different tests the main conclusions are that a smaller amount of sulphur apparently can result in an increased production and with up to 10% substitution of non – acidified slurry with acidified slurry there are positive effects and only a minimal increase in the sulphur concentration in the biogas. At higher levels the biogas production is inhibited and when using more than 20% the sulphur content in the gas increases to 13,000 ppm.
Figure 1: Methane yield at 100 days batch digestion dependent on proportion of acidified slurry with direct addition of sulphur to the slurry and the average sulphur concentration in the gas.
Use of 30% acidified manure combined with non – acidified manure was tested for more than 100 days in a 30 m3 reactor, which resulted in approximately 20% less gas production and a sulphur content in the gas of 18,000 ppm. The optimal solution will therefore be to use 10% acidified manure, since this appears to increase gas production with a minimal increase in sulphur content in the gas. In plants which use other sulphur containing biomasses, this will most likely not be the case. In Figure 1 results are shown from addition of different amounts of sulphuric acid to manure ranging from 0 – 100% acidified manure. When the amount exceeds 20% the methane yield starts to decrease and at amounts larger than 40% the process will essentially stop. With a share of up to 20% acidified manure, it appears that the sulphur content in the gas will not increase.
Separation of biogas and acidified slurry
Integration of slurry separation and biogas production in new ways could increase the gas yield from biogas plants. However there are a number of conditions, which have to be considered in relation to the process, and also it is essential to know the effect of which type of separation method is used. The separation method should be chosen according to fulfillment of the purpose at the lowest possible cost. The purpose of the separation in relation to biogas production on farm level is to transfer as much dry matter as possible to the fiber fraction, so that as much as possible of the biogas potential is maintained.
Different methods for separating acidified and non – acidified slurry have been tested and a selection of the produced fractions have subsequently been tested in Work Package 3. The focus was on three types of separation: Screw press (Fibermaster, figure 2), drum sieve (Reko) and belt sieve (UTS). The separation equipment was tested at three pig production farms and one dairy cattle farm. On each of the farms a number of settings were tested (Table 1) and all fractions were analyzed in relation to nutrient content and biogas potentials.
Table 1: Types of separation and settings
Figure 2 shows the separation efficiency of organic matter in pig and cattle manure at different separation methods. Cattle manure has a higher separation efficiency than pig manure. In the tests the separation efficiency was higher in acidified compared to non – acidified manure. The screw press had a lower separation efficiency than the other methods in cattle manure, but in return the dry matter content was higher in the fiber fraction compared to the other methods.
Figure 2: Separation efficiency of organic matter (VS) dependent of manure type and separation method
In figure 3 the methane yield in the solid fractions after separation are illustrated. It is shown that the biogas potential per ton material is significantly higher when using the screw press compared to the other methods. The solid fractions produces 3 – 8 times more gas than normal manure from cattle and pigs. The drum sieve gives the lowest yield, but in return the fraction from the drum sieve can be pumped and in pig manure a larger share of the original organic matter will be utilized for biogas production. With shorter transport distances it will be sensible to use the drum sieve since the fraction can be picked up with the vacuum tanker, while it is an advantage at longer distances to have as high a gas potential per ton material as possible.
The biogas yield from the solid fraction of acidified cattle manure, when processed by the screw press, has been tested in continuous tests in different mixtures with non – acidified manure. The tests were carried out in a thermofilic reactor with an average retention time of 14 days. The biogas yield and process conditions were evaluated during two months, where cattle manure was replaced with 10%, 20% and 30%, respectively, from the solid fraction of acidified cattle manure.
Figure 3: Methane potential in the solid fraction after separation. The red line indicates the normal gas potential in pig and cattle manure at 5 and 8% dry matter, respectively.
Figure 4 shows the average methane production at different proportions of acidified fibers. The methane yield increases with increasing amount of solid fraction from acidified cattle manure. The methane yield in the reactor with 30% acidified fibers was therefore 50% higher than the reference based on only cattle manure, both in relation to daily reactor yield and yield per ton of biomass. In the reactor with 30% acidified fibers the content of hydrogen sulphide increased from 400 ppm to 4000 ppm in the test period and therefore there is a heightened need to clean the gas, however the level is so relatively low that normal biological cleaning will be sufficient.
Figure 4: Average methane yield at different percentages of acidified cattle manure fibers.
The influence of different sulfate concentrations on biogas production has been tested in cattle and pig manure and it appears that inflow of 10 – 20% acidified slurry can stimulate the methane yield with almost 20%, while larger amounts can cause serious reduction of the methane yield and increasing hydrogen suplhide content. However, when using slurry separation there are very promising results with addition of 30% fiber fraction, where the gas yield increases with approximately 50% compared to a biogas plants utilizing only slurry.
Three different separation methods were tested: screw press, drum sieve and belt sieve. Overall the separation efficiency was highest in cattle manure.
The screw press produced a solid fraction with higher gas yield than the other fractions, but in return the amount of organic matter which was transferred from a given amount of manure was lower from a screw press than when using the other methods.
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