The Las Pampas biogas CHP plant in Pichidegua, Chile

Credit: MTU Onsite Energy

Using waste materials as fuel for biomass and biogas cogeneration is a win/win proposal – but there are some challenges involved. Tildy Bayar spoke with several companies engaged in waste-to-energy projects to find out what the issues are.

Using waste materials as fuel for biomass and biogas cogeneration is a win/win proposal. Feedstock can often be locally sourced, and problematic waste can be used beneficially rather than disposed of. In addition, projects can benefit from subsidies for renewable power sources.

In Europe, Germany leads in biogas production with nearly 8000 plants, which generated over 26 billion kWh and employed over 41,000 people in 2014.

Other important markets include the UK, Italy, France and the Netherlands. Italy is Europe’s second largest biogas power and heat producer with 1200 plants. Although there are currently only 153 biogas and anaerobic digestion (AD) plants, the nation’s transmission system operator has predicted that biomethane could meet 10% of the nation’s gas demand in future.

The US market is still relatively small compared to the European market, while interest is growing in Latin America although MTU OnSite Energy’s Christian Mueller says there are ‘still only a handful’ of countries, with Chile leading the pack.

Biomass produced from agricultural waste can include straws, leaves, roots, waste wood and animal waste. It can be treated and converted to energy by burning, through fermentation and distillation, or through gasification.

ExxonMobil: managing gas quality

Jarmo Vihersalo, EMEA Industrial Marketing Advisor for Energy at ExxonMobil, says the main challenge for biomass-fired cogeneration plants in the coming years will be managing varying gas qualities if the biomass is gasified, as well as higher specific engine outputs, while still delivering high reliability. Reaching this goal will require attention to a number of factors.

Fuel for biomass-fired plants must be pre-processed, and along with fuel handling (especially for waste-derived fuels) this is one of the biggest financial demands on a plant. Because biomass produced from wood chips contains between 40% and 55% moisture, it must be dried before combustion or gasification to improve the efficiency of the process. However, says Vihersalo, drying may not be economically viable in some cases.

He also notes that the makeup of biomass is another key challenge. Chemical agents are often used to protect crops from diseases, pests and weeds, and the use of fertilizers can mean there is nitrous oxide in the gas.

Gases with high levels of methane can also create difficulties for engine operators. A report from the International Council on Combustion Engines (CIMAC) has indicated that unburned methane in the exhaust gas, referred to as a ‘methane slip’, can lead to higher fuel consumption and increased operational costs, as well as impact on greenhouse gas levels. However, gas engine makers are aware of this and are working on addressing the issue.

Biogases from landfill sites or digester gases from farm waste can also contain high levels of hydrogen sulphide, which can pose major difficulties for engine operators. When biogas is burned in an engine, hydrogen sulphide can condense with water to produce sulphuric acid. If the sulphuric acid accumulates rather than being neutralized, Vihersalo warns, engines are at risk of accelerated corrosion and component failure.

While OEMs build engine components – such as the bearings, cylinder head and crank case – to withstand the corrosion caused by these gases, Vihersalo notes that selecting the optimal lubricant is also key.

‘One factor to consider when selecting a lubricant for an engine running on biogas that contains hydrogen sulphide is the lubricant’s total base number (TBN),’ he says. ‘This identifies its acid neutralizing capacity, and alkaline additives used in higher TBN lubricants can help protect engines by absorbing sulphuric acid and neutralizing its corrosive properties. Higher TBN oils also perform for longer in biogas engines, compared to lower TBN lubricants which degrade more quickly, reducing oil drain intervals.’

Engines running on biogas typically feature shorter oil drain intervals when compared to natural gas-fired engines, Vihersalo says, but choosing the right lubricant can extend these intervals for optimal engine performance.

The Entrade E3 biogas plant Credit: Entrade

MTU Onsite Energy: developing livestock waste projects

MTU Onsite Energy and its North and South American distributor W W Williams have developed multiple biogas CHP projects in collaboration with digester manufacturer DVO. Among their reference projects are a cow manure-fuelled anaerobic digestion (AD) biogas CHP plant at the Statz Brothers dairy farm in the US state of Wisconsin, and a plant developed for South American food conglomerate MaxAgro in Pichidegua, Chile which supplies power to the local community with the added benefit of reducing the odours from a pig-rearing and slaughtering plant. For livestock and dairy farmers, managing the penetrating odour of ammonia is one of the main reasons to consider an anaerobic digester.

For dairy farm projects such as the Statz Brothers installation, says Patrick Ahern, manager of the Gas Power Systems group at W W Williams, the water must be cleaned before use to remove phosphorus runoff, constituting an additional step. ‘The other part that’s important and that takes a lot of work,’ he adds, ‘is how we size the hot water reclamation system off of the engine, and how much of the hot water is used by both the dairy farm and the digester to keep different parts of the dairy warm and to keep the digester at a temperature to optimize the amount of gas that it can make. We have to make sure we get the right heat balance, and also make sure we properly cool that hot water when it’s not being used to keep the engine happy. If the water comes back too warm to the engine, a radiator is used to cool away the heat. The hot water system has to fit seamlessly into the customer’s digester system.

Biogas can create difficulties for gas engines

Credit: ExxonMobil

‘We have a mechanical engineer on staff who sizes the pumps and heat exchangers to make sure we have the proper equipment to interface not only with the MTU equipment but with the customer’s equipment.’

Ahern notes that his firm’s first few AD projects were challenging ‘because they were our first ones, so there was a learning curve – and there are so many moving parts on any digester project. Luckily it was the same digester, from DVO, for both projects,’ he adds, ‘so we learned a lot in a short period of time.’

At the Statz Brothers farm, the power offtake proved to be an issue. ‘We designed the system to self-generate all of the electricity for the dairy farm,’ Ahern says. ‘The utility would not purchase any more power, and the line where we wanted to tie in to the utility was at its maximum amount so costly modifications would have been necessary for the customer to sell.’ Self-consumption reduced the overall capital cost of the project, as well as allowing the unit to operate independently from the grid.

‘Although the diary farm is still purchasing power from the local utility, our system limits the amount of power the customer purchases as much as possible,’ Ahern says. ‘When our engine is running at optimal load, the farm is only buying about 60 kW from the utility.’

In terms of technical challenges, he notes that there were ‘not as many as you would think’ on either project. The gas supply, for example, is taken care of: ‘The manufacturer of the digester system guarantees a set amount of gas based on the manure put into the digester. That part is really easy. DVO designs and builds the digester system and supplies us with gas.’

‘The biggest challenge with any waste-to-energy project is cleaning and conditioning the gas to make sure it’s within specifications to burn in the engine,’ he explains. ‘We install a dehydration system and a compression system to take the moisture out of the gas; we compress and dry it for combustion in the engine. Our team, as the distributor, designs and procures that equipment and makes sure it works properly. It’s pretty much bespoke – the only changes in the system design are the amount of gas. There will still be a chiller, there will still be heat exchangers, pumps and valves – that equipment gets bigger or smaller based on the flow rates.’

Christian Mueller, Sales Engineering Manager at MTU Onsite Energy, adds that the amount of sulphur in the gas is variable. ‘The design and cost of the gas treatment system to remove the sulphur varies from one site to another,’ he says. ‘The nice thing about our Statz partner DVO is that their digester technology already reduces the amount of sulphur in the raw gas, and in addition they use a phosphorus recovery system.’ The phosphorus then constituted an additional revenue source for the farmer, as it can be used as a fertilizer.

For the Pichidegua project in Chile, Mueller says the customer ‘looked for a partner to supply a complete solution. We supplied the complete containerized unit, engineered in-house and easy to install for the customer at site. The system was made according to the specifications of the site – the amount of gas available, the quality of the gas, there definitely is gas treatment as well… The contaminants in the gas are worse than from cow manure and there was more extensive treatment needed at that site. We really used our experience from biogas systems we’ve done by the hundreds in Europe and then transferred that knowledge.’

Mueller says that, in addition to being of lower quality, the composition of the gas made from pig waste varied due to the addition of energy crops, added to supplement and increase biogas production. ‘It takes some time for the bacteria to digest something – maybe the next day you add less of it. It took some time for the operator to gain experience on how much they have to add to the digester at any given time to get a very stable gas flow,’ he explains. ‘Our biogas engines are designed to cope with varying gas quality, but there are some implications on how good the engine runs and the amount of power it puts out – the amount of electricity going out varies if the amount of gas going in varies. The operator had to fine-tune their way of operating the digester. That’s not out of the ordinary – it is very normal: another learning curve.’

There are multiple waste streams available for biogas CHP projects as well as livestock and agricultural waste and energy crops. ‘On a waste-to-energy project, waste can be anything,’ says Ahern. The company is currently working on an AD project using food waste from restaurants, grocery stores, food manufacturing facilities and even a nursing home, which would otherwise be landfilled. The waste includes ‘spoiled food and scraps’ according to Ahern.

Much of the challenge of such a project, he says, involves ‘permitting the site to operate in the way we want it to, to accept the waste. We know how much gas we’re going to make, and how to design the system – those challenges are pretty much ironed out. It’s the local and government hurdles that we have to jump over now.’

Entrade: customizing feedstock

Julien Uhlig is CEO of Entrade, which manufactures what it says is ‘the world’s smallest biomass plant’, the 150 kW E3. The containerized, pellet-burning plant can fit comfortably on a pickup truck, and the company regularly does promotional tours where it drives a plant somewhere, sets it up in 15 minutes and ‘can provide power within 1.5 hours’, Uhlig says. ‘We have four of our machines in one container, which comes with a pelletizer that can support 10 machines. One machine can power the pelletizer.’

The German firm currently has E3 plants in operation in countries across Europe as well as in the US, and it aims to become a leading provider of decentralized energy solutions in sub-Saharan Africa.

To do this, Entrade will need to utilize local waste feedstocks. In Nigeria, Uhlig explains, the company is working with waste products such as nut shells as well as planting fast-growing trees to ensure a sustainable feedstock supply. ‘Roughly 50% to 60% of our feedstock is pelletized wood – a constant factor which keeps the process stable – and the rest is unpelletized nut shells,’ he explains. ‘We want to maximize the percentage of organic waste that we can use.

‘We can also put together customized fuel or feedstock that’s regionally available. Countries or companies send us their feedstock, for example sunflower oil production waste products, or sawdust. We put it in the lab and see if we can turn it into a feedstock. As long as it’s organic or carbonaceous, we haven’t found anything that doesn’t work at all, except metals or glass.’

Some fuels are more practical than others, he notes. ‘Sewage sludge, with over 40% ash content, is not a lot of fun. It produces huge amounts of ash – there’s as much coming out as going in. It can be worth it if you do it right next to a water treatment facility, but I wouldn’t recommend it right now.’

‘We have a complete testing lab at the university in Graz [Austria], where we do feedstock testing 24/7. This week we’re working with hay. For our UK plants we’re looking into hay and straw as well as the types of wood pellets available in the UK market; we’re also looking at the use of plastic bags and bottles and for smaller cities to partner with us on this. From a permitting point of view [plastics are] a separate question, but [as a fuel] we’re quite sure that it works.’

Uhlig notes that the most critical aspect of biomas gasification – or, as Entrade calls it, a high-temperature reactor or conversion – is the tar content of the fuel. ‘Tar is what’s left if you don’t fully convert carbons into syngas that you can use in the engine,’ he explains.

‘Every manufacturer should put out the data on how much tar they have. Ours is 0.1 g/m3. You can build a beautiful reactor but if your tar levels are too high, your engine will die. It’s very rare to see a reactor with under 1 g of tar. Ours has been validated by the Fraunhofer Institute in long-term testing. With the gas quality we have, even for operation of over 20,000-30,000 hours, the engine should last because the gas is clean enough not to do any damage. Our first client unit in the UK had 92% uptime in its first month, with 1200 operating hours.’

Uhlig says the company is currently developing a larger unit, but at the moment is focusing on scaling up the current one. One project in Poland features 10 12-metre containers, and he says several similar projects are underway in Nigeria.