Keith Morris, DynaMotive Technologies Corporation, Vancouver, Canada
Raj Thamburaj, Orenda Aerospace, Mississauga, ON, Canada
BioOil a greenhouse gas ‘neutral’ fuel is fast becoming an alternative to fossil fuels. DynaMotive Technologies is testing BioOil in small gas turbine units, with the aim of reaching performance and capital cost parameters comparable with current gas turbine power packages.
The scientific consensus on global warming is clear. If atmospheric concentrations of greenhouse gases continue to rise in this century the way they did during the twentieth century, global ecosystems will be disrupted in ways that will alter the environmental conditions in which human civilizations have developed and thrived.
Figure 1. Application of pyrolysis oil to gas turbine operation. OGT2500 – a 2.5 MW industrial gas turbine packaged by Orenda Aerospace Corporation, a Magellan Aerospace company
Combustion of fossil fuels for power generation is a significant contributor to global warming. Biomass has long been identified as a sustainable source of renewable energy. However, power generation using a solid fuel has had significant limitations with respect to materials handling requirements and efficient energy conversion. Converting biomass fuel into a liquid addresses these issues and makes possible the use of higher efficiency combined cycle systems for power generation.
Liquid BioOil fuel can be stored, pumped and transported similar to petroleum based products and can be used as an alternative to fossil fuels in gas turbines, diesel engines and boilers. However, because it is produced from biomass wastes, it is greenhouse gas neutral and produces no SOx (sulphur dioxide) emissions and significantly less NOx (nitrogen oxide) emissions than diesel fuel during combustion.
The nearest term commercial application for BioOil is as clean fuel for generating power and heat from gas turbines, small stationary diesel engines and boilers. DynaMotive is a technology development company that has already commercialized two technologies and is now in the process of commercializing its BioOil production technology. It is a world leader in the development of fast pyrolysis technology for the production of BioOil fuels, and is working with Orenda to test and develop BioOil fuels for power generation using their OGT 2500 gas turbine package, with an ISO rating of 2.85 MWe.
DynaMotive produces BioOil using a patented ‘fast pyrolysis’ process that has been developed and successfully demonstrated, on a continuous basis, in a pilot plant rated at a capacity of 2 tonnes/day (t/day). The company is currently building a larger 10 t/day pilot plant to be commissioned later this year, and is designing a 25 t/day commercial demonstration plant to be built in 2001. Full scale, 100-200 t/day commercial plants will follow.
DynaMotive’s BioOil production process utilizes a deep bubbling fluidized bed technology combined with a novel BioOil recovery system that was originally patented by Resource Transforms International Ltd. The overall simplicity of DynaMotive’s BioOil process provides major competitive advantages over earlier pyrolysis technologies including low capital and operating costs, higher product yield, a significantly higher quality BioOil and the flexibility to process a wide variety of feedstocks.
Feedstock for the fast pyrolysis process can be any biomass waste material including wood byproducts and agricultural wastes. Preparation includes drying the feedstock and then grinding the feed to small particles (see Box).
A fuel for gas turbines
As a clean fuel, BioOil has a number of environmental advantages over conventional fossil fuels:
- CO2/greenhouse gas neutral: because BioOil is derived from biomass (organic waste), it is considered to be greenhouse gas neutral and can generate carbon dioxide credits.
- No SOx emissions: as biomass does not contain sulphur, BioOil produces virtually no SOx emissions and, therefore, would not be subject to SOx taxes.
- Low NOx: BioOil fuels generate more than 50 per cent lower NOx emissions than diesel oil in gas turbines.
- Renewable and locally produced: BioOil can be produced in countries where there are large volumes of organic waste. As BioOil has unique properties as a fuel, it requires special consideration and design modifications. Some of these properties are presented in Table 1 and are compared to those of diesel fuel.
A first generation fuel system and combustion system has been designed and tested, demonstrating the capability to operate a 2.5 MW industrial gas turbine on BioOil. These tests not only revealed the feasibility of operation but also demonstrated that similar performance could be achieved for BioOil and diesel. Although CO and particulate emissions were higher than diesel, testing revealed that NOx emissions were about half that of diesel fuel and the SO2 emissions levels were so low as to be undetectable by the instrumentation.
The engine being utilized for this programme is the 2.5 MW OGT2500 industrial gas turbine engine. The OGT2500 offers distinct technical advantages over other engines. Unlike aero-derivative engines, it has been designed as an industrial engine with durability being one of the main design criteria and not weight. In addition to the ruggedness, the distinct ‘silo’ type combustion system allows for easy access and modifications to the entire combustion system, which is one of the critical systems for the adaptation of the engine to BioOil.
In addition to the engine design, important design modifications are necessary to compensate for the unique properties of BioOil.
BioOil has an energy density approximately half that of diesel fuel. Therefore, to meet the same energy input requirement, the flow rate must be double. This requires design changes to the fuel system to be able to control higher flow rates and also modify the fuel nozzle to accommodate this larger flow. This lower energy density also can affect combustion since physically there must be twice as much fuel in the combustion chamber as with diesel. This, however, is another advantage of using an industrial engine in the fact that the combustion chambers are designed with a significantly longer residence time (and therefore a larger volume) for a given power output.
The high viscosity of the fuel reduces the efficiency of atomization, which is critical to complete combustion. Proper atomization is addressed in three ways.
Firstly, the fuel system is designed to deliver a high pressure flow since atomization is improved with larger pressure drops across the fuel nozzle. Secondly, the fuel is pre-heated to lower the viscosity to acceptable levels. Thirdly and most importantly, the fuel nozzle has been redesigned to improve spray characteristics. These improvements are important for complete combustion and effectively reducing CO emissions.
Due to its relatively low pH, material selection is also critical for all components wetted by BioOil. Typically, 300 series stainless steels are acceptable metallic materials and high-density polyethylene (HDPE) or fluorinated HDPE for polymers.
The water content in the BioOil fuel has some advantages. It is helpful in reducing the viscosity, since it is a relatively low viscosity fluid. In addition, it is a factor in lowering thermal NOx emissions.
The solids content is a combination of ash and char fines which have carried-over to the liquid part of the BioOil. The effect of these solids is to cause sticking of close tolerance surfaces and in addition, they can result in particulate emissions because of the long residence time required to fully combust. It is important that the solids level in the BioOil is controlled to be less than 0.1 wt%.
The ash content in the fuel represents the material that cannot be combusted. Depending on the elements in the ash, it can result as a deposit on the hot gas path components that will reduce the turbine efficiency. The solution is a turbine wash system. This typically consists of two separate systems in which an abrasive medium is injected during operation to physically ‘scrub’ off the deposits. This allows turbine cleaning without any downtime. The second system is an offline process which injects a cleaning fluid and allows a soak period to loosen the deposits which are then removed when the engine is started.
Figure 2. The 2 t/day BioTherm pilot plant in Vancouver, Canada. DynaMotive is building a 10 t/day pilot plant to be commissioned later in 2000, and is also designing larger, commercial plants
Within the ash are alkali elements, which can result in hot corrosion of the hot gas path components with sodium and potassium being the most critical elements found in BioOil. These elements form low melting temperature compounds, which, as a liquid, will stick to the hot gas path components and then react and corrode the component. This effect can be mitigated through the use of fuel additives. As with the turbine wash systems, this technology was developed for the use of heavy fuel oils in gas turbines and has been in use for decades.
Due to the poor ignition characteristics of BioOil, one other key design requirement is a BioOil specific ignition system or process. To overcome this, the OGT2500 system starts on diesel fuel flowing through the primary channel in the fuel nozzle. Following a warm-up period, BioOil is fed into the secondary channel at an increasing rate while the diesel fuel flow is reduced until 100 per cent BioOil flow is achieved.
‘First generation’ systems and design modifications have been developed and tested under this programme. This has demonstrated both the feasibility and significant benefits in utilizing BioOil for the operation of a gas turbine. Efforts are now being placed on the development of second generation designs to achieve performance and durability levels required for commercial operation. This means providing high efficiencies, maintaining high availability, typical time between overhauls and capital cost comparable with current gas turbine power generating packages.
Key to this work is the use of a variety of BioOils to ensure designs accommodate as wide a range of fuel characteristics as possible. This will maximize the applicability of the BioOil gas turbine system to a variety of bioenergy applications. Technically, this work is proceeding down two main avenues:
- The optimization of the combustion system and the determination of the improved engine operating and emission characteristics.
- To develop and test a turbine wash system based on current systems being utilized on the line of other Mashproekt engines.
- To design and test fuel system equipment and components for long term operation.
- Develop hot section coatings.
- Develop a fuel treatment system to upgrade the fuel quality through filtering, additives injection and alkali removal.
Fuelling the future – BioOil production technology
Figure 3. The fast pyrolysis process: feedstock is introduced into the reactor and the thermolysis reaction takes place at temperatures of between 450°C and 500°C
Fast pyrolysis – more accurately defined as “thermolysis” – is a process in which a material, such as biomass, is rapidly heated to high temperatures in the absence of oxygen. The biomass decomposes into a combination of solid char, gas, vapours and aerosols. When cooled, most of the volatiles condense to a liquid referred to as ‘BioOil’. The remaining gases comprise a medium calorific value non-condensable gas.
In this particular fast pyrolysis process, biomass feedstock is introduced into a thermolysis reactor having a bed of inert material, such as sand, with a height to width ratio greater than one. The biomass is shredded to sufficiently small dimensions so that its size does not limit significantly the production of the liquid product fraction. Simultaneous introduction of pre-heated, non-oxidizing gas at sufficient linear velocity performs two principal functions: firstly, as a medium for fluidizing the hot sand bed and secondly, to cause automatic elutriation of the product char from the fluidized bed reactor. The process includes removing the elutriated char particles from the effluent reactor stream and rapidly quenching the gas, aerosols and vapors to produce a high conversion yield of liquid BioOil. For maximum yield of liquid, the thermolysis reaction must take place within a period of a few seconds at temperatures ranging from 450°C to 500°C. The products must then be quenched as soon as possible to prevent cracking of the newly produced BioOil.
Feedstock for the fast pyrolysis process can be any biomass waste material including wood byproducts and agricultural wastes.
Preparation includes drying the feedstock to less than ten per cent moisture content to minimize the water in the BioOil and then grinding the feed to small particles to ensure rapid heat transfer rates in the reactor.
Feedstock conversion yields are in the range of 60 to 70 per cent BioOil, 15 to 20 per cent char and 15 to 20 per cent non-condensable gas. The heat required for thermolysis is the total heat that must be delivered to the reactor to provide all the sensible, radiation and reaction heat for the process to proceed to completion. The heat of reaction for the fast pyrolysis process is marginally endothermic. The total heat requirement to produce BioOil is approximately 2.5 MJ/kg of BioOil produced. The net heat required from an external fuel source, such as natural gas, is only 1.0 MJ/kg of BioOil when the non-condensable gas produced in the process is directly injected into the reactor burner. This represents approximately five per cent of the total calorific value of the BioOil being produced.
BioOil is a dark brown liquid that is free flowing. It has a pungent smoky odour. BioOil contains several hundred different chemicals with a wide-ranging molecular weight distribution and is a mixture of oxygenated compounds containing various chemical functional groups, such as carbonyl, carboxyl and phenolic. BioOil is made up of the following constituents: 20-25 per cent water, 25-30 per cent water insoluble pyrolytic lignin, 5-12 per cent organic acids, 5-10 per cent non-polar hydrocarbons, 5-10 per cent anhydrosugars and 10-25 per cent other oxygenated compounds.