Examples

INDIRECT FIRED GAS TURBINE         EXTERNALLY CO-FIRING EXISTING BOILERS

An example of such a system as a “pure” power plant is given in the paper presented by Lefcort to the 4th Biomass Conference of the Americas in Oakland, CA in 1999. There, two 70 Million Btu/h EnvirOcyclers, each disposing of RDF (Refuse Derived Fuel) from an MRF (Material Recycling Facility) on a California landfill, each fire 50,000 lb/h, 600 psia/680°F WHB's.  The 100,000 lb/h of steam is routed to a  condensing steam turbine which drives a 10 MW_e generator. Because of electricity deregulation in California the power generated on this landfill is sold to customers in the local community.

In this example two-thirds (typically) of the sensible heat in the steam is lost to the condenser cooling medium - either water or air. This is an unavoidable consequence of having no process - such as a number of lumber dry kilns - in which to condense the exhaust steam prior to it being routed back through the boiler.

However, if a large amount of process heat is required then a back pressure turbine is used to generate power. All the low pressure steam leaving this turbine is used for process heat.  The system then becomes a “true” co-generation system, i.e., one which generates both power and process heat. Of course, there is a consequence: the amount of power generated is typically reduced by a factor of 2.

Compared to the common WHB/condensing steam turbine approach, combined cycles reduce the amount of sensible heat dumped to the condenser.  This is because 75% to 80% of the power in a combined cycle is generated by the gas turbine; the condensing steam turbine generates only between 25% and 20% of the total power. If follows then that less power generated by steam means less heat dumped to the condenser.

In the present case an 825°F (440°C) waste heat boiler converts the sensible heat in the exhausts from both the turbine and the recuperator to about 40,000 lb/h of modest pressure steam for process heat.  3.5 MW_e of power and 40,000 lb/h of process steam is an excellent fit for sawmills which, in addition to requiring power, also require significant heat to dry their lumber prior to planing. This approach can be converted into a "poor man's" combined cycle by using a higher pressure WHB and making power in a small condensing steam turbine generator set.

FIGURES 11, 12 and 13 show how EnvirOcycler products of combustion are supplied to the top of the recuperator and drawn down through the recuperator by an induced draft fan located at the bottom of the heat exchanger.  The recuperator is located close to the gas turbine to minimize the length of high pressure, high temperature piping between the recuperator and the turbine inlet.

Compressor discharge air from the gas turbine is piped over to the inlet header at the bottom of recuperator.  The header connects a series of interconnected horizontal rows of tubes that route compressor air up through the recuperator to the discharge header at the top of the recuperator.   The temperature ratings of the tube rows increase from the bottom of the recuperator (carbon steel) to the top of the recuperator (special alloy steel).

While it is possible to indirectly fire any gas turbine by designing a special set of  outs  and ins  - ducts or castings which, in the first case, route compressor air from the compressor discharge annulus "out" to an external flange and which, in the second case, route hot air from the recuperator to an external flange "in" to the turbine inlet annulus - doing so is rather expensive. 

Accordingly, at the present time HEI restricts gas turbine generator sets to those that are provided with  external  combustion chambers - combustion chambers whose axes are at right angles to the axis of the gas turbine. The GE/Nuovo Pignone 5 MW_e PGT-5 or the 10 MW_e PGT-10, the GSS Borsig 5 MW_e THM-1203 or the 10 MW_e THM-1304, the 4 MW_e Allison 501 KM and the 1.2 MWe Dresser-Rand (Kongsberg) KG2-3C are suitable candidates.

It should be noted that the higher the turbine inlet temperature the more expensive the recuperator. HEI and Broach have determined that, in order to guarantee a recuperator life of 100,000 hours, the optimum EnvirOcycler discharge temperature is 1,850°F (1010°C) and the optimum turbine inlet temperature is 1,550°F (843°C). 

Operating a given turbine at 1,550°F, that was designed to operate at turbine inlet temperatures of 1,700°F (927°C) to 1,900°F (1038°C), does derate turbine output.   The following table shows the derating of a typical gas turbine: the GE/Nuovo Pignone PGT-5

  

Turbine Inlet Temperature Power Output MWe 1,500°F (816°C) 3.130 1,550°F (843°C 3.423 1,600°F (871°C) 3.717 1,700°F (927°C) 4.301 1,796°F (980°C) 4.849

Power Generation by Externally Co-firing an Existing Fossil-fuel-fired Boiler

Existing boilers, fired by natural gas, oil or coal, can be "externally co-fired" with the 2,000°F ± 200°F products of combustion of various solid wastes disposed in an EnvirOcycler. These hot  "flue gases" are typically ducted into the boiler furnace section through openings in the furnace water wall. The waterwall tubes in the path of the openings are bent around the openings.

One reason for externally co-firing with the products of combustion of various solid wastes is to displace some of the existing fossil fuel with a zero cost fuel, or with one for which a tipping fee may actually be charged.  Another reason, particularly for coal-fired boilers, is to take advantage of the CO₂ offsets available when burning "renewable" fuels in this post-Kyoto era. CO₂ from the types of fuel considered herein are considered to be of renewable fuel origin and so qualify for CO₂ offsets.

FIGURE 9 & FIGURE 10 show a system in which three 90 Million Btu/h EnvirOcyclers externally co-fire a pulp mill power boiler with the products of combustion of wood waste too wet, and too dirty, to be burned on the grates of this boiler.  As shown, the three EnvirOcyclers burn 65% moisture content wood waste, primarily bark, and dump their 1,800°F products of combustion into a common stack.   A breeching from the stack conveys the products of combustion over to the power boiler.

The boiler ID fan draws a controlled amount of flue gas into the boiler. The system shown disposes of wood waste - from a very large, 30 year old, wood waste pile in BC from which leachate runs off into nearby pristine salmon bearing streams - and displaces some of the natural gas burned in the boiler.

As pointed out above, US utilities are being urged to co-fire their coal burning boilers with wood residue to reduce their greenhouse gas emissions.  To that end HEI has proposed that three 75 Million Btu/h EnvirOcyclers be used to externally co-fire a 270 MW_e coal burning boiler at the TVA's Allen Plant in Memphis, TN.  Their 2,200°F products of combustion would displace 10% of the Btu's supplied by the coal and generate 27MW_e at a cost of about US $8 Million installed.

Finally, disposal of poultry litter is a major problem in the Delmarva (Delaware, Maryland and Virginia) peninsula of the US.  Phosphorus and nitrate runoff contaminate local rivers leading to outbreaks of toxic pfiesteria piscicida. The most recent outbreak was in the summer of 1997. The runoff is from the fields of chicken farmers to which poultry litter is liberally applied.  Restrictions on poultry litter as a fertilizer are being imposed.

One solution is to externally co-fire existing oil-fired boilers in poultry processor plants with the products of combustion of poultry litter disposed in an EnvirOcycler.  The oil displaced by the poultry litter can be used to pay the chicken farmers a nominal $5/ton for poultry litter delivered to the plant.  This solution is discussed at length in the paper Martin and Lefcort presented to the 17th Annual Pittsburgh International Coal Conference, September 2000.    

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