Maize silage has long been a mainstay of the oper ation of AD plants and varieties with a higher biogas production potential will have an big efficiency impact.
Someone who understands the importance of variety selection is Mark Voss, operations director with Ixora Energy. He is responsible for a total of eight AD facilities, including three gas-to-grid units, each with a 2MW equivalent output.
The largest units use 14,000-16,000 tonnes of maize silage per year.
He says: “The business model for AD production is actually very simple. With prices fixed, we have to maximise gas output from feedstocks while reducing feedstock costs.
“With feedstock accounting for 30-50% of costs, it is our number one priority, and we have to pay close attention to quality of inputs.
“Maize is a really important input, as not only does it yield about 350cu. metres of methane per tonne of dry matter, but it allows us to increase the proportion of poultry manure, slurry and farmyard manure [FYM] we can use, as it balances nitrogen in waste.”
Tim Richmond, maize product manager for Limagrain UK, says originally maize for the AD market was dominated by very late maturing, high yielding, but generally lower quality, varieties. However, growers are becoming more interested in the attributes of varieties grown, such as dry matter and energy content, and their ability to produce more methane.
Mr Richmond says: “Many growers have focused solely on crop yield, without paying sufficient attention to quality attributes. For optimum efficiency, you do not want a large quantity of low dry matter, low energy feed, yet this is exactly what growers get when they focus on varieties selected solely on yield.
“It is also important to select early maturing varieties, as they have significant environmental benefits, helping meet the terms of the biogas.org.uk Voluntary Code of Practice.
“In particular, by being harvested earlier and in better conditions, there is less chance of damaging soil structure and a greater opportunity to establish successor crops.”
Mr Voss says in the Ixora Energy business, variety choice is largely left to contracted growers. He says it is important the variety suits the farm and fits the rotation. Early varieties are preferred as they allow successor crops to be established, primarily wheat for wholecrop which is used in the digester.
“As we pay on a tonne of dry matter basis with a target of 31%, our growers have been focusing on varieties with high dry matter yields, but increasingly, we are looking closely at quality attributes, principally ME content and cell wall digestibility.
“A key driver of efficiency is the rate of fermentation. The faster and more completely feedstock is fermented, the faster gas is generated and the greater the total output.
“At one of our 2MW gas-to-grid units, we will be adding 75t of feedstock per day into the 10,000cu.m tank, including about 44t of maize plus wholecrop, grass silage, pig slurry and FYM. “On average, this will stay in the plant for 130 days. If we could reduce this to 120 days, we could ferment an additional 5t of maize feedstock daily, which would generate 1,000cu.m of biogas, which would be worth about £440.
“For an electricity plant, we have a set volume of gas we can use, so a higher quality feedstock would mean we can hit the target from reduced total feedstocks.
“To help increase fermentation rate, we chop maize much shorter than a typical forager to increase the surface area, which means it will be more accessible to microbes. We target 95% at less than 18mm.
“The other thing we need to drive up is ME content and digestibility. With 50% of the total energy being in the vegetative part of the plant, increasing cell wall digestibility is becoming increasingly important.
“A low ME, less digestible plant will take longer to ferment. We may get the same gas yield, but it will take more time to get it.
“Higher cell wall digestibility helps us increase energy output per day spent in the digester.
“Ideally, we would like to be growing early maturing varieties with high dry matter yield, combined with excellent ME content and superior cell wall digestibility. This will allow us to drive output and control costs, which is essential with a fixed price.”
TIM Richmond says the BSPB/NIAB Maize Descriptive Lists contain data which assesses the quality characteristics of varieties to influence their suitability for AD. He says data allows operators to make more balanced judgements regarding potential varieties.
“The system tests the very latest varieties in UK conditions against other well-known forage maize varieties. Results include all the key criteria affecting quality, including dry matter content, dry matter yield, ME content and ME yield.
“Growers can assess varieties based on characteristics which will influence their potential to produce methane. While well-proven varieties, such as Atrium, Asgaard and Fieldstar, continue to rank highly, the 2018 list contains two new varieties showing exceptional potential, delivering exactly the attributes required to increase methane yield and AD plant efficiency.
“Gatsby is an impressive variety producing high yields of energy rich silage, scoring highly on ME yield and relative ME yield, thanks to a combination of high dry matter yields and excellent ME content, a result of high starch content and excellent cell wall digestibility.
“Being earlier maturing, it can fit seamlessly into arable rotations, enabling the establishment of successor crops.
“LG31.211 is highly digestible with massive yields combined with excellent quality, meaning it will support excellent gas yields.
“On favourable sites, these varieties produce over 1t dry matter/hectare more than the average of evaluated varieties, and over 2.5t DM/ha more than the lower performing varieties. This equates to 350cu.m more methane/ ha than the average, worth £260/ha for no increase in growing costs.
“At the same time, improved digestibility will increase the rate of digestion, increasing overall plant throughput. For a 500kW digester, requiring about 220ha of maize, this could add up to over 77,000cu.m more methane than if an average performing variety is grown, worth £59,000.
Therefore it has a huge impact on performance and financial returns.”