Heating - Biomass system - Industrial / Canada (Ottawa)

Case study assignment

You have been hired by a speciality lumber producer that produces high-quality finish lumber from white pine, basswood, oak and other native tree species. To ensure a high-quality product the producer dries the lumber in kilns, which it keeps at 30 to 50 ºC for long periods of time. The plant currently uses an electrically powered air dehumidification and heating system. They want you to evaluate whether a biomass-fired heating system would be financially viable compared to the existing, fully electric system. They are also considering using a diesel-fired heating system.

Site information

The lumber producer is located in Renfrew, Ontario, Canada; the nearest meteorological station is in Ottawa, 60 km to the east.

The kilns are insulated and use ambient make-up air. Space to locate an energy system is available in the building housing the dry kilns. The estimated winter peak load is 150 kW. The warehouse floor area is 250 m². The producer has examined its energy bills and has concluded that the kilns can be treated as a base load accounting for 60% of the annual energy demand.

The biomass system would be fired with wood waste from the plant. Tests of the wood waste show a calorific value of almost 20,000 MJ/tonne.

Financial information

Typical financial figures for the analysis are provided by the company: inflation of 2.5%, debt ratio of 60%, debt interest rate of 10%, debt term of 10 years, discount rate of 15%, income tax rate of 30%. The biomass energy system capital cost is assumed to be depreciated using a straight-line method over the first 5 years of system use. The system is assumed to last 25 years.

The average price of electricity is $0.07/kWh and diesel oil is delivered for $0.40/L. The fuel cost escalation for the next 10 years is expected to be 2.5%. The plant is presently paying $17/tonne to dispose of the wood waste it produces.

Prepare a RETScreen study, documenting any assumptions that you are required to make, and report on the significant conclusions from this analysis.

Solution

The worked-out solution is the data file selected from within the RETScreen Project Database. The user automatically downloads the Project Database file while downloading the RETScreen software.

Teacher's notes

• By an iterative procedure, 600 W/m² was found to be the heating load for the building cluster that results in a peak load of 150 kW, as specified in the assignment.
• The average annual moisture content is estimated at 30%, as it is likely that the wood waste will include a mixture of dry and wet wood, with a moisture content of 15% and 50%, respectively.
• Building and yard costs of $3,000 have been included, principally to cover the cost of building a wall around the new boiler. Note that a new building and yard is not required.
• The biomass equipment installation cost of $128/kW includes the cost of converting the existing heat distribution system to a hot water system.
• The existing heating system has been kept as a peak load heating system, even though it is not required; it also serves as a back-up system. The cost of the peak load heating system has therefore been set to 0.
• By comparing the net present value of systems with different sizes of boilers, the user can verify that a capacity of 150 kW is near optimal. This reflects the high cost of electricity for the peak load heating system versus the negative cost of fuel for the biomass system.
• RETScreen can be used to compare the biomass system to a diesel-fired system. Once again, the biomass system achieves very high returns.
• The base load of 60% implies that the energy requirement of the kilns is largely weather independent. This is not entirely true: the sensible heat losses from the kiln will be affected by the outside temperature while the heat required to evaporate the water in the wood will be temperature independent. To be more precise, the degree-days below the kiln temperature, rather than the degree-days below 18 ºC, should be used, and the base load lowered so that it would account only for the heat required to evaporate the water from the wood. Since these figures for degree-days are not readily available, this approximate approach is sufficient. With the returns on the biomass system being so high, however, this is really not a concern.
• Since the main problem for Opeongo Forest Products is what to do with waste wood, the RETScreen model could also be used for quickly determining how much wood could be used by adding additional heat loads and biomass boilers.

Real project

Results

Opeongo Forestry Service, a speciality lumber producer in Renfrew, Ontario, Canada, installed a biomass boiler in late 1997 for the purpose of providing heat to its lumber drying kilns. The biomass boiler heats a mill shed and two dry kilns located inside a lumber storage building. Approximately 50,000 m³ (2 million board feet) of lumber are dried annually in the two 1,000 m³ (40,000 board foot) kilns, resulting in each kiln being loaded 25 times per year. The kilns utilise a combination of dehumidification by heat pumps and air heating by biomass. The biomass system is fired by waste wood products from the plant. The use of biomass for kiln heating is very cost effective. This is due primarily to the availability of wood waste that would otherwise have to be disposed of at a cost of $17/tonne.

System description

The plant installed a 146 kW Bioblast biomass burner system designed and built by Grove Wood Heat, located in Prince Edward Island, Canada. Wood chips are stored in a bin and fed to the combustion chamber with a screw auger. The biomass combustion system consists of two chambers. The primary chamber uses a fixed grate with under-fire air. It has a water chamber to preheat the water. The burning gases enter the secondary combustion chamber. It is equipped with a thermostatically-controlled induced draft system that regulates the combustion temperature. Slab wood can also be fed into the secondary chamber. The boiler is located above the secondary chamber. The system efficiency is estimated to be 65% when using fuel with 20 to 30% moisture content on a wet basis. The fuel moisture content is usually slightly higher during the winter months, reducing the system output.

During the summer months the plant is operated at full capacity during the initial stage of the heating cycle and then is set back to a pilot state. In the winter the biomass system operates continuously at 100% capacity.

Lessons learned

• Kiln space heat and mill floor heat can be provided by cheap onsite wood biomass with minimal daily labour requirements. This includes preheating (thawing or warming lumber) and base load heating.
• Reduced loads on the heat pump dehumidifiers can be achieved by adding dry heat to the kiln and allowing the rejection to outside of moist air instead of recirculating all air through the dehumidification cycle.
• Wood waste can be diverted from a landfill site and turned into a profitable resource with the installation of a biomass energy system.

The big picture

Drying of lumber in many forest product companies uses heat pumps for air dehumidification. The kiln is preheated to 32 °C (90 °F) with electric heaters and heat pumps being used to dehumidify the kiln (thereby drying lumber) by extracting moisture using the latent heat of condensation. Dry heated air is re-injected to the kiln. This method is used for speciality lumber because it is a slower less heat intensive method - about 10% of Canada's lumber is dried this way. Without lower cost supplemental heating annual electricity cost for kilns of the size used by Opeongo Forest Products would be about $50,000 at $0.08 / kWh.