Biomass in Microturbines

However, the unique characteristics of biomass provide limited opportunities for co-firing and centralized biomass power production. Unlike coal, which has high energy density and can be mined en mass from a single location, biomass is generally highly distributed and has low energy density. This limits the potential of co-firing and centralized biomass power production to a small fraction of available biomass around the world. Transporting biomass for co-firing becomes economically prohibitive beyond a radius of around 50 miles from the power plant. For centralized biomass power production, the plant must be co-located near a large source of readily available biomass, such as a paper mill or large sawmill.

This leaves the majority of potential biomass resources non-economical for power conversion. In the U.S., up to 6,000 GWh of potential biomass power annually is disposed of. Developing technology that can utilize this resource provides dual economic and environmental benefits. Disposal costs are reduced and the sale of electricity provides a secondary income source. From an environmental perspective, not only is landfill space reduced, but emissions from transporting the biomass to the landfill are eliminated and power is produced with zero net greenhouse gas emissions.

However, conventional boiler systems are non-economical in sizes below approximately 10 MW. Development of a distributed power system, optimized for power production at 1 MW to 5 MW, would provide the enabling technology to utilize these biomass resources near the point of resource production.

Biomass Market Barriers

The key to developing a commercially viable distributed biomass power system lies in understanding the market characteristics, designing a power system that overcomes many market barriers and maximizes the benefits. The key barriers of the biomass market are:

• The biomass resource is typically highly distributed and has low energy density. This makes transportation costs uneconomical beyond approximately 50 miles.
• The physical and chemical characteristics of biomass vary significantly from source to source. No turnkey system currently exists that can utilize a wide variety of biomass types.
• Biomass producers are not energy companies and do not want to become one. In addition, the ratio of operating and maintenance costs (O&M) to power production is high relative to much larger centralized power systems. Power systems require specialized training. The prospect of hiring additional engineers and technicians specifically for that system can eliminate any return on investment for the producers.
• Current gasification or combustion systems may produce other types of emissions and wastes, such as SO2 and tars, which require additional costs to mitigate or dispose of.

These barriers are daunting and have stifled utilization of these biomass resources. However, there are unique characteristics of this market that can–given the right technology–make it profitable. These include:

• The market for renewable energy is growing at an average of 50 percent a year, with no sign of leveling off in the near future. Many states in the U.S. have set goals for replacing up to 40 percent of their energy usage by the year 2025 with renewable energy.
• The economics of renewable energy are becoming increasingly more favorable with emerging markets in renewable energy credits, carbon offsets, production tax credits and methane avoidance credits.
• Power systems co-located near a source of biomass residue production may obtain the fuel at minimal cost and, in some cases, for a slight profit if the biomass producer is currently paying to dispose of the fuel.
• There are no competing technologies in this market space.

Designing Biomass Power Systems

To take advantage of these unique characteristics the right design philosophy is needed. And that is the emphasis of the current biomass power system being developed at the University of North Dakota Energy & Environmental Research Center, with project funding provided by customers of Xcel Energy through a grant from the Renewable Development Fund. The system requirements, in order of importance, are reliability and safety, low operation and maintenance cost, fuel flexibility and capital costs.

Notice that one thing missing from this list is efficiency. Economic studies have shown that efficiency is a secondary consideration to the other requirements. For residue biomass, where the cost of the biomass is low or even free, efficiency affects system economics only if it substantially increases the system’s O&M and capital cost.

Understanding the relative importance of these characteristics helps frame the constraints around the type of system being developed. The requirement for reliability and safety largely rules out high-pressure gasifiers and boilers. It also means the system should be operated far enough from population centers and facilities so any unanticipated catastrophic failure results in minimal damage outside the system.

The requirement for low O&M disfavors the use of internal combustion gensets. These require frequent maintenance when operated continuously and must be co-fired with a slipstream of natural gas or diesel for reliable operation on syngas. The requirement for fuel flexibility means the feed system must be given the same priority as the rest of the system. This is often a secondary consideration. However, during operation around two-thirds of problems requiring system shutdown are due to material flow through the feed system and gasifier.

Microturbine and Biomass Gasifier

With these constraints in mind, Xcel Energy and the EERC are developing a 30 kW pilot-scale biomass power system that combines a microturbine and biomass gasifier. Work started with a complete redesign of the feed system. Issues encountered with the feed system often include bridging (when the biomass is left to freefall in the hopper or any vertical pipes) and plugging (when the biomass is pushed through horizontal piping). Feed system problems were eliminated by ensuring that all vertical drop was minimized and, when unavoidable, that any freefall piping or hoppers used a diverged cone from top to bottom. Pinch points were designed to be as close to the bottom augers as possible to allow the auger to interrupt bridging before it could begin. Feeding through the augers and into the gasifier was performed with oversized augers geared down to low rpm for maximum torque. Auger feeding into the gasifier was maintained until just short of the combustion zone in the gasifier using high temperature stainless steels.

In an ideal world the gasifier would take in biomass and convert it to a pure syngas for conversion downstream to electricity. In the real world, however, the conversion process is incomplete. Along with the syngas, the gasifier will output hydrogen sulfide, ammonia, hydrogen chloride, particulates and heavy oil-like tars. Of these contaminants the particulates and tars are the most problematic. Particulates require a series of filters to remove them from the gas stream. These filters must be replaced on a regular basis. The amount of particulates tolerable in the syngas depends on the components downstream of the gasifier. For an internal combustion genset, particulates can plug the input valves and significantly decrease maintenance intervals for the engine. For a gas turbine, particulates are even more critical, as they affect the high-speed turbine blades which operate as high as 60,000 rpm. This can corrode the blades in short order, requiring turbine replacement.

Tars are an even larger problem than particulates. At the output of the gasifier, tars in the syngas are hot enough to be in the gas phase. However, most downstream applications require the syngas to be cooled. Once cooled, the tars start to condense within the pipes and equipment downstream of the gasifier and plug the system. This requires all of the pipes and equipment downstream of the gasifier to be cleaned out on a regular basis. Even more problematic, biomass tars are known to harden if not cleaned out soon enough. Once hardened, the only option often is to replace the piping altogether.

Tars are the largest barrier to commercializing conventional gas turbine-based biomass power systems. Figure 1 (above, left) shows a simplified system design for a gas turbine based power system. Compressed air is preheated by a recuperator and then injected into a combustor. The hot, pressurized gas exiting the combustor turns the expansion turbine which, in turn, operates the compressor and electric generator. To utilize syngas from a gasifier, it must be cleaned of particulates and acid gases and compressed to high pressure to inject into the combustor. The compressor cannot handle hot input gases, instead requiring cooling of the syngas before compression. This, in turn, requires extensive syngas scrubbing systems between the gasifier and compressor. The capital and operating costs of the syngas scrubbing system in this type of design may exceed those of the gasifier and gas turbine, making this system uneconomical for distributed power production.

Another option is an indirectly heated gas turbine, as presented in Figure 2 (below). In this system, hot syngas is fed to an atmospheric combustor which then heats high-pressure air through a high-temperature heat exchanger. Since the syngas never contacts the high-speed turbine, particulate cleanup requirements are greatly reduced. The compressor is eliminated as is the need to cool the syngas below the condensation temperature of tars. This eliminates tar fouling in the pipes and reduces the particulate cleanup requirements.

The use of a high-temperature heat exchanger to heat the air instead of a combustor presents several challenges and benefits to the system. The heat exchanger must be capable of transferring heat at temperatures greater than 1,500 F to high-pressure air at high flow rates. The primary benefit is that the scrubbing requirements of the syngas are greatly reduced. Particulates never impact the high-speed turbine blades. The temperature of the syngas can be kept high to prevent condensation of tars in the pipes. The tars and other contaminants are then combusted and exit the system as flue gas.

There currently are few if any manufacturers that produce heat exchangers made for these temperatures and pressure differentials. In addition, gas turbine manufacturers currently produce gas turbines with the combustor in the high-pressure zone, typically for use with clean natural gas or diesel. The EERC is currently modifying an off-the-shelf 30 kW microturbine to move the combustor out of the high-pressure zone and into the low-pressure zone. In addition, a high-temperature heat exchanger is being incorporated into the modified microturbine to work with the stock recuperator.

The EERC is also systematically designing and testing each component, from feed system, to gasifier, to microturbine in an effort to demonstrate the feasibility of this system for distributed biomass power system. To date, the feed system has been proven to work as designed. A new gasifier, designed to extract clinkers before they coalesce into larger chunks, has been constructed and connected to the feed system. Cold flow testing of a variety of biomass fuels through the feed system and gasifier has been successful.

The microturbine modifications with integrated high-temperature heat exchanger have been finalized and work is progressing on schedule. Hot shakedown of the gasifier and construction of the modified microturbine is scheduled for completion by the end of the 3rd quarter of 2010. By the end of 2010, construction and operation of the full system should be completed.

Successful demonstration of this system would provide the enabling system level technology to profit from the large amount of biomass residues currently being disposed of.