Boosting manure based biogas with straw

Agricultural residues in the form of animal manure and straw represent a significant yet under-utilised source of renewable energy and fertiliser. The Baltic Sea Region (BSR) is an area of intensive arable and animal husbandry production, though not without its environmental issues.

Methane and ammonia emissions to the air, groundwater nitrate pollution, run-off leading to freshwater eutrophication and algae blooms in the Baltic Sea, which is one of the world’s largest brackish inland seas and a particularly sensitive marine environment regulated under the 1992 Convention on the Protection of the Marine Environment of the Baltic Sea Area.

According to Baltic Manure, a recently concluded European Union (EU) funded flagship project to address manure management practices around the BSR, some 187 million tonnes of cattle, pig and poultry manure are produced annually in the BSR, excluding Russia, most of which is generated in Poland, Denmark and the northern German states.

EU legislation regulates application rates of animal manure according to its nitrogen content. Especially for pig manure, this can result in over-fertilisation of phosphorous since pig manure is characterised by a high content of phosphorous relative to nitrogen.

Denmark is one of the world’s leading exporters of pork and, according to Statistics Denmark, the stock of pigs in 2013 numbered almost 12 million. The Danish government is keen to both increase biogas production capacity while mitigating the environmental impact of its livestock industry and has set a target of digesting 50 percent of the total annual volume of animal manure by 2020.

Yield hampers economics

Whilst anaerobic digestion (AD) is a suitable technology to treat manure, produce biogas and use the digestate as organic fertiliser, manure-based biogas remains largely untapped compared to its potential.

– For the farmer the economics of manure-based biogas production is often the most serious bottle-neck for increasing capacity, even with generous feed-in tariffs for heat and/or electricity, simply because the biogas yield per tonne of manure feedstock is very low compared to a crop-based feedstock, said Dr Torben A. Bonde, during a visit to Foulum biogas plant in Denmark where the straw pre-treatment technology has been installed and demonstrated.

A specialist in microbial ecology, Dr Torben A. Bonde is a distinguished Danish entrepreneur with a long scientific R&D career. He is a founder and CTO of Biofuels Technology ApS, a company specialised in developing biofuel processes and technologies, as well as a Director and Partner of Kinetic Biofuel A/S, a newly formed company for the patented straw pre-treatment technology.

Straw as substrate

Cattle and pig manure at 8–10 percent dry matter content typically have a methane yield of 15–25 Nm³ per tonne whereas poultry litter does better, at 30 percent dry matter content and 30–100 Nm³ per tonne. Manure pales compared to crop-based feedstock such as maize or grass silage at 33 percent dry matter content and 160–220 Nm³ per tonne not to mention substrates such as fats and crude glycerine which can reach 1000 Nm³ methane per tonne.

– Mixing livestock manure types and energy crops as co-substrate in manure-based biogas plants can improve profitability by increasing the methane yield, but results in additional organic nitrogen applied to the fields and potentially increased leaching. Furthermore, additional land is needed to grow these crops or additional costs are incurred to purchase them and this eats into profitability, said Bonde.

However, profitability can now be considerably improved by using other residues from agricultural production such as cereal straw as co-substrate. And Danish farmers produce a lot of straw. On an annual basis 5.5–6 million tonnes of which roughly one third is used as fuel for heating, one third is used for animal bedding and feeding and the remainder is essentially unused.

– The Danish government’s 50 percent manure-based biogas target can only be achieved by using large amounts of straw to increase methane yield and secure plant economies. All other high yield residues such as from the food processing industry are already taken, said Bonde.

In theory cereal straw has a potential methane yield of 240–320 Nm³ per tonne but up until now has had limited use in biogas plants.

Under the microscope, a strand of cereal straw is a fantastic ligno-cellulosic structure. It is light, strong, hydrophobic and decay resistant with the cellulose and hemi-cellulose locked behind the lignin coated cell walls. The challenge is to make these components readily available for microbial conversion, he said.

Quite a challenge as even shredded or macerated straw that absorbs some degree of water is difficult to mix in with cattle or pig slurry. It is voluminous and will inevitably float causing surface crust or dead-zones in a reactor, and blockages in pumps and pipes. Furthermore, electrical consumption of pumps, stirrers and mixers goes up in addition to the energy needed for shredding or maceration.

Mechanical industrial pre-treatment

Given the above mentioned, the technology solution offered by Kinetic Biofuel comes somewhat as a surprise at first as it too is entirely mechanical, a briquetting press line consisting of a bale conveyor, shredder, stone trap, hammermill, cyclone and filter, feed silo and briquette press. Furthermore, the press itself is a seemingly standard BP 6510 HD 1.5 tonne per hour briquetting press from Danish manufacturers C.F. Nielsen A/S.

As straw briquettes have a much higher bulk density than a big bale, about 550 kg per m³ compared to 150 kg per m³, the immediately obvious benefit is a radical reduction in transport and storage costs.

– This is, of course, an important benefit as it enables cost-effective volume aggregation. Briquetting also preserves straw quality over time with minimum degradation that would otherwise have a negative effect on biogas yield when used, said Torben.

There is though more to it than that. The briquetting process itself also has, as Bonde reveals, a remarkable effect on the straw and its properties.

– In short, it can be described as a mechanically induced steam explosion, said Torben adding that the C.F. Nielsen briquette press has been adapted and modified to optimise this internal process, and patented.

The shredded and milled particles are compressed under high pressure. The kinetic energy on the moment of impact by the piston and sudden pressure drop on its retraction together with the high temperature caused by dissipation of kinetic energy into the straw causes steam explosions rupturing the fibres, cell walls and pores.

– This, as you know, changes everything. The dry straw particles are able to absorb water at high capacity and the cell structures are opened allowing enzymatic and microbial conversion of the contents, said Torben.

The straw briquettes can preferably be fed directly from the briquetting line via a screw conveyor into a biogas reactor or fed into a mixing tank and stirred in liquid manure before pumping the mixture into a reactor.

Full-scale research plant

The entire briquetting line including both reactor feed-in systems has been installed and tested at Aarhus University’s biogas research plant in Foulum. Built in 2007, the Foulum plant is one of the world’s largest of its kind and the University has its own livestock and crop research and production in the surroundings.

Research at the full-scale biogas plant is focused on advanced technologies for improving the biogas production including optimisation of the biogas potential and improvement of methods such as pre-treatment, high technology separation of nutrients, thermal conversion of the residual fibres after digestion, high solid digester technology and upgrading of biogas to vehicle fuel or natural gas quality.

Click here to continue reading the article in Bioenergy International number 4/2015.