Biogas beckons06 April 2010

Biodiesel may have its problems, but what about the various biogases on offer? Brian Tinham talks to engine and technology developers to assess the issues

There's a lot of talk in the industry about biofuels right now – just as there has been, on and off, for years. This subject is, of course, far from new. But how much do we really know about the alternatives, aimed at reducing our dependence on fossil fuels and cutting emissions? And, just as important, do we understand the implications of their sources, energy efficiency, the available infrastructure, costs and their requirements in terms of storage, treatment etc?

What about the engines modifications required, and the ancillary equipment and electronics being developed to run them efficiently? For that matter, what about the operational issues and the implications of different fuel types for maintenance and manufacturers' warranties? Ultimately, which fuels, and combinations of fuels, are likely to work best on what kinds of operations, and why?

Given that Volvo Truck & Bus has now very publicly backed just two alternative biofuels for serious investment – having, just a couple of years ago, boasted of seven vehicles trialling seven fuels – let's start there. Its preferred choices are now methane (natural gas from whatever source) in the dual-fuel arrangement with diesel, and DME (dimethyl ether) gas.

As Volvo director of public affairs Lennart Pilskog puts it: "We think that methane-diesel technology will work well in the European, Asian and American markets, and similarly DME, given the current plans for production in, for example, China, Japan and Iran. If you compare those fuels, methane has the advantage that it exists, although not everywhere, while DME has to be produced and distributed in separate channels. But DME offers as high an efficiency rating as a traditional diesel engine, and at a lower noise level, with massively reduced climate change impact. So scaling up will be faster with methane-diesel than with DME, but DME provides a more optimal fuel for many transport operations."

Dimethyl ether
Sol let's review those options. DME first, and a key point to note is that, while it can be produced from natural gas, it can also be processed from a variety of biomass sources – making it Bio-DME. Most important, the latter can claim a full 95% reduction in overall climate impact, compared with conventional diesel. But don't fall into the trap of assuming that means no tailpipe emissions – the savings there are more like 5%. However, DME also offers a fivefold improvement in transport kilometres per hectare of cultivated land, compared against first generation biodiesels.

From a handling perspective, DME is a gas, but readily transforms into a liquid under low pressure (5bar, as per propane), which means it is not difficult to distribute or store – similar to LPG (liquefied petroleum gas) in fact. That does, however, mean pressurised fuel tanks.

As for the engine, modifications to run DME do seem relatively minor. Back in 2005, Volvo trialled DME on an FM9 rigid two-axle truck, with a 9.4-litre D9 diesel engine, and that needed new electronic unit-injectors (leaving six redundant cams on the camshaft) with the common-rail system. At the time, Transport Engineer speculated that Volvo was profiting from earlier work by Bosch on a common-rail system for a six-litre Cummins ISB engine to run on DME. TE also noted that concerns at the time about fire and explosion risk led to emergency shutdown valves being installed. Needle and nozzle tolerances in common-rail injectors also had to be tightened, with higher grade materials used to minimise wear. Also, non-metallic seals in the fuel system had to be changed, because DME attacks the plastics and rubbers in most diesel engine fuel lines.

Today, Pilskog says simply: "We do change the injectors [which are from Delphi, not Bosch], fuel pump, fuel lines, the electronic management system and, of course, the tanks." He also asserts that the reason for the injector change has noting to do with DME's poor lubricity (which is anyway being handled by additives under development by Total on the way to a world standard for the fuel), but is due to the "much lower pressures" required. "That is also the reason why DME engines are so much quieter," he explains.

As for combustion itself, when vaporised inside the engine, DME has much in common with diesel. It's a compression-ignition fuel, so no spark plugs are needed, and the resulting burn is quiet. Also, the fuel produces no particulates, which not only simplifies exhaust gas after-treatment, but also means that NOX emissions can be cut using high EGR (exhaust gas recirculation) rates, making DME a natural for Euro-6. Interestingly, DME also has a high cetane number (55—60, compared with diesel's 40—55), meaning that it ignites readily as cylinder pressure increases. Incidentally, because particulate emissions are practically zero, the ECU doesn't have to reduce torque at standing start (as it does with diesel), so the engine effectively delivers higher starting torque.

Volvo is currently involved in a significant project with the EU, the Swedish Energy Agency, Danish chemicals specialist Haldor Topsoe, fuel giants Total and Preem and biofuel producer Chemrec, aimed at developing DME from black liquor – a by-product of the forestry industry. Alongside the Chemrec plant in Piteå, northern Sweden, construction has started on a facility for extracting DME from black liquor obtained from the nearby pulp plant. As part of the process energy rebalance, the cooking chemicals produced by gasification of the black liquor will be returned to the pulp plant, creating a closed loop that the team hopes will maximise 'well to wheel' energy-efficiency.

However, there is nothing to prevent DME from being produced from other sources – albeit not necessarily with the same energy efficiency. "Since DME can be produced from all types of biomass, it may become viable even for countries without any significant forestry industry," explains Per Salomonsson, DME project manager at Volvo Technology. And Lars Mårtensson, environmental affairs director at Volvo Trucks, adds: "From the holistic viewpoint, DME is one of the most promising second-generation biofuels."

Currently, 14 test trucks are being built for use in field tests throughout Sweden, starting this summer. Preem is also building fuel stations so the trucks can be used under normal operating conditions. Says Pilskog: "The field test is on Euro 5 engine trucks, but it's true that Euro 6 is within reach and probably easier with DME than diesel." However, don't expect a lurch to DME that quickly: he also points out that the development krone available for the massive Euro 6 diesel market far exceed those being applied to DME, which is still ahead of the early adopter phase.

So what about methane-diesel? Looking at methane (natural gas), its low carbon content means it also burns very clean – with negligible particulates – and Volvo's Mårtensson emphasises the point that it is by far the most accessible fuel realistically able to knock diesel off its perch. "There are larger reserves of natural gas than oil, but above all, production of climate-neutral biogas is gaining momentum in many countries, which solves the most urgent problem – reducing CO2 emissions," he says.

Volvo's rather grand claim at the close of last year was that it would "be the first manufacturer with an efficient diesel engine fuelled by a mix of methane gas and diesel". Field testing of its methane-diesel trucks would start in Sweden and the UK in 2010, it said – and indeed seven Swedish operators are now running with it commercially, and there are reports of biogas stations appearing in the Nordics.

However, that claim seems a little hasty, given the work already done by others – notably Clean Air Power back in 2006 and subsequently also rival Hardstaff, with its OIGI (oil ignition gas injection) technology (see panel opposite). Between them, they have enabled dual-fuel CNG/LNG (compressed natural gas / liquefied natural gas) and diesel conversions on Mercedes-Benz Actros and Axor tractor units (with Econic coming soon), as well as on DAF CF85 in 4x2 and 6x2 configurations.

But there is a connection: Volvo concedes that, "to optimise and refine the technology, Volvo Trucks is collaborating with technology companies Clean Air Power, Hardstaff Group and Westport". That said, Mårtensson insists: "This technology allows us to combine the advantages of gas with the diesel engine's high efficiency, which is about 30—40% superior to spark ignition gas engines." Except, of course, in multi-drop distribution and urban duties, where dual-fuel engines score less highly, because they need the hot burn of high engine loading to get gas replacing diesel. With that caveat, though, his point is that dual-fuel trucks consume around 25% less energy than gas-only trucks. They also don't suffer their problem of restricted range (typically 150—200km). "Combining methane gas with diesel fuel, and using it in a diesel engine increases range by over 50%, and when liquefied gas is used, that doubles again," says Mårtensson.

As for converting diesel engines for methane-diesel operation, this, too, appears to be no great shakes. Volvo talks of adding special tanks for either LNG/LBG (the B signifying biogas) or CNG/CBG, and adding a separate fuel system with gas injectors for the inlet manifold, where mixing takes place. Then, in operation, a small amount of diesel is injected and ignited by the compression phase, which, in turn, ignites the methane gas/air mixture. Hence no spark plug and hence also the ability to use already efficient diesel engine technology.

"Power and driveability are identical to that of a conventional diesel truck," enthuses Mårtensson. "Processors continuously calculate fuel ratio, according to the current driving pattern. The optimum [highest] proportion of gas is then achieved during smooth, stable driving," he adds. And he reassures operators thinking about the technology that, if the gas runs out, for whatever reason, the truck can continue operating on diesel alone.

How green is dual-fuel? That depends on the truck operation, because the amount of gas injected has to change markedly as the engine moves through the demand cycle. "We expect to be able to run on up to 80% methane gas once the technology has been refined and tested," claims Mats Franzén, who manages engine strategy and planning at Volvo Trucks. "Our field tests will start with a mixture containing up to 70% methane; the remainder will consist of bio-mix diesel [fossil diesel mixed with diesel produced from renewables]," he adds.

Given a fair wind, Franzén reckons that in the long term the technology could cut CO2 emissions by up to 80% from 'well to wheel', compared to diesel – assuming that biogas and 100% second-generation biodiesel are used. He also suggests that low methane prices and strict environmental regulations in many towns and cities will force the pace of demand for what amount to gas-powered trucks on steroids. "Methane gas is currently a relatively cheap fuel in many markets. For example, Volvo Trucks' technology already offers a profitable fuel option for trucks undertaking long daily transport jobs and returning to the same filling station," he says.

Gas and diesel on steroids
Mercedes-Benz experience of methane-diesel operations and engine conversions sheds useful light on the dual-fuel story. The company's interest goes back a few years to the time when US-based Clean Air Power – then one of the leaders in CNG/LNG and diesel technology for diesel engines – was working with Kingston-on-Soar based Hardstaff Haulage, which had itself been doing pioneering work with natural gas-powered heavy transport.

Clean Air Power and Hardstaff went their own ways – the former with its Genesis system and Hardstaff on its OIGI (oil ignition gas injection) alternative, developed with help from Loughborough University, which reputedly harnessed aerospace technology for the job (Transport Engineer, April 2009, page 18). Although both use diesel compression to propagate ignition of the gas and both use a box of electronics to enable variable gas intake to match engine demand, that's where the similarity ends.
Nick Blake, sales engineering manger for Mercedes-Benz vans and trucks, explains that his organisation went with Hardstaff, partly because of Hardstaff Haulage's clear commitment to gas-powered heavy-duty operations in its own truck fleet (that has since yielded several Axor tractive unit sales to Hardstaff – although Sainsbury's gas truck operations, also on Axors, are using Clean Air Power retrofits) and partly because Mercedes felt the OIGI approach had an edge.

What edge? "There were several factors," says Blake. "Their system for treating methane slip [across the inlet and outlet ports] seemed better and they introduced a high-temperature catalyst methane trap in the exhaust." That matters because methane gas emissions are orders of magnitude worse than CO2 in terms of climate change potential. "But their equipment also delivers slightly better methane substitution and it works completely independently of our engine management system, so we were able to approve it for retrofit under our engine warranty," he adds.

Mercedes-Benz has now been working in earnest with Hardstaff for around two years, and also has several converted Actros and Axor tractor units in operation with other customers. Blake cites Howard Tenens and Newark Haulage on heavyweight operations, and Plaxton on Midibuses in Lincolnshire (on the Mercedes 904 engine, as seen at last year's Millbrook show). Late last year, Optare announced a separate deal with Hardstaff, also using Mercedes engines, and Blake mentions "orders for over 60 vehicles to be converted". So this has become very familiar territory, and he says intervention at the engine level is minimal.

"Broadly, Hardstaff inserts a sandwich plate between the cylinder head and inlet manifold and manages sequential injections of the gas into the manifold, not the cylinder itself, to get a good gas-air mixture before it hits the combustion chamber," says Blake. "Because we maintain a flow of diesel through the injectors, lubricity is not a problem and there aren't the issues with lubricant dilution in the bottom half of the engine, as with biodiesel. Also, the system reverts to diesel if the gas supply runs out or the ESP [electronic stability programme] kicks in. But for as long as it's running on gas, there's a significant reduction in CO2, NOX and particulates, approaching Euro 6 standards," he adds.

Optimising the injector design and the electronics for the gas management system was the project tackled by Loughborough University, and the result is a system that elegantly substitutes gas for diesel, while also managing injection timing, without interfering with the Mercedes ECU or the CANbus. Few details are available, but what is known is that Hardstaff's OIGI intercepts the signal from the ECU to the diesel injectors and uses that to control gas injection and reduce diesel volume. It also does so without causing problems for the vehicle's other functions, such as the automatic transmission, which needs to believe that fuel is flowing in line with the demand signal. And it uses a fast-acting lambda oxygen sensor in the exhaust to gauge combustion efficiency and adjust the diesel/gas ratio accordingly.

As for vehicle conversion costs, Hardstaff quotes from £13,000 for a single-cylinder CNG on a Euro 3 vehicle, to £25,000 for a three-cylinder CNG system on a Euro 5 truck. Clean Air Power says simply that £22,000 is an average cost. Either way, Blake concedes that payback is only going to be within two years if your operation involves high horsepower double- or triple-shifting, with some serious mileage and returning to base for regular refuelling. So it's not ideal for low mileage trunking but can work for municipal vehicle operations where duty cycles equate to high fuel consumption.

That said, for Mercedes, the issues of gas/diesel dual fuel are no longer technical; it's all about the CNG/CBG supply infrastructure. Sainsbury's experience, with its dual-fuel Axor tractors on Clean Air Power's Genesis, makes the point eloquently. In its case, LBG comes from Warwick-based Gasrec's anaerobic digester at a landfill site in Albury, Surrey – so it's carbon neutral. But it's then carried by tanker to a Chive Fuels LNG station at the Severn View service area, close to Sainsbury's distribution depot in Bristol – so it's no longer carbon neutral.

The day purified biogas is connected to the grid and integrated into filling stations is the day this approach can really score – from every perspective. And with fossil-based diesel depleting, and natural gas with it (albeit with a projected 60 years to go, as opposed to oil's mere 40), ramping up biogas production and making it available efficiently would appear to be a sensible move.

Brian Tinham

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