STAT 498

Background material

The Port Townsend Paper Corporation is a pulp and paper mill located in Port Townsend, Washington; which, as part of its facilities, operates a waste heat boiler. The paper making process is a complex process which encompasses many stages. However, the waste heat boiler is only involved in the recovery of chemicals used in the pulping and washing stages of paper production. It is used to rejuvenate the chemicals expelled from the chemical pulping and washing processes.

The function of the waste heat boiler is to assist in producing energy through the burning of waste chemicals from the pulping process and to facilitate the recovery of the inorganic chemicals which are to be reused in the pulp making process. As with all pulp process operations in Washington State, the operation of waste heat boilers is strictly regulated in order to control the level of pollution being put into the atmosphere.

The Pulping Process

The pulp making process, by itself, is a relatively simple process. The wood (or other fibrous material) enters the system and is mixed with a chemical solution (white liquor) in the "digester." This mixture is then cooked with steam at a pressure above 150 psi for a period of 2 to 6 hours, depending on the grade of pulp desired. Once the cooking is complete, the pulp is removed from the system and placed in the various stages to produce paper or other wood products.

The cooking of the wood/chemical solution requires large quantities of steam and chemicals. Within the digester, sodium hydroxide and sodium sulfide dissolve the fibers' ligneous substances that serve to hold the cellulose fibers together. Throughout this process, approximately 1/2 of the wood substance is dissolved in the chemical mixture and forms what is called black liquor. After the cooking, the non-dissolved cellulose fibers are washed to remove the spent cooking chemicals and any residue which may be present.

Next, the cellulose fibers which were dissolved in the black liquor are separated from the chemicals. This i accomplished through a countercurrent rotary vacuum in which the black liquor is repeatedly rinsed to form progressively weaker black liquor solutions which contain large amounts of dissolved carbonaceous matter. Once all the cellulose has been removed, it is further refined and subsequently converted to paper. The weak black liquor, on the other hand, is recycled in order to recover the washing chemicals and obtain the carbonaceous matter to be used in the generation of steam and energy.

Before the black liquor can be used in the recovery boiler, the excess water must be removed. The weak black liquor which comes directly from the pulping process contains only about 16 to 18 percent dissolved carbon solids. This ratio must be increased to approximately 62 to 65 percent (minimum) in order to use the solids as fuel in the steam generation process (the Port Townsend plant concentrates around 72% indirect solids). In order to achieve this solid solution, the water is removed through evaporation techniques, leaving a solution known as strong black liquor which is high in dissolved carbon solids.

The waste heat boiler is a combustion chamber which, using the strong black liquor as input, burns the carbonaceous matter, converts sulfur compounds into NaS, smelts the inorganic salts for removal, and generates heat from which steam is produced. Once the carbon matter has been burned and the heat generated, the remaining smelt flows into a dissolving tank where it is washed to form green liquor (a solution of NaCO and NaS). The green liquor is pumped into a caustizing system where Ca(OH) is present and reacts with the NaCO to produce a solution of sodium hydroxide. The calcium carbonate is further reduced to calcium oxide and hydrolyzed to calcium hydroxide for reuse in the caustizing process. The liquor which exits the caustizing process is now clean and free of byproducts. As such, it is recycled and ready for reuse in the pulping process.

Environmental Concerns

Waste heat boilers, as with any chemical recovery process, are potentially damaging to the environment, and therefore are strictly regulated. In the case of Port Townsend Paper Company, this regulation comes in the form of controlling the opacity of the exhaust plume emitted by the boiler. In Washington State, the opacity of the exhaust plume cannot exceed 35% for more than 6 minutes in any given hour. Penalties for standards violations are set by the Legislature. The Washington Department of Ecology issues a regulatory order for the use of the waste heat boiler. Penalties for violating these standards are stiff. For each violation (not more than one can be issued in each 60 minute period) the fine cannot exceed $10,000, possibly with imprisonment of up to 1 year. Both the company and the individual responsible for the violation can be liable for the penalties. This places a burden both on the Port Townsend Paper Corporation and on the operators, supervisors, and managers of the plant.

Since this is a waste heat boiler (or Kraft recovery furnace), the primary contributor to the opacity of the exhaust plume is the particulate content of the gases as they rise through the exhaust stack. The waste heat boiler is normally operated at a black liquor firing rate which matches current levels of production. The opacity of the exhaust gases emitted by the waste heat boiler are continually monitored and tend to place an upper limit on the firing rate as conditioned by the regulatory limits on opacity. The Port Townsend Paper Corporation's waste heat boiler is equipped with an electrostatic precipitator (ESP) to collect the dust particles; however, while the precipitators are very efficient, there are various parameters inherent to the operation of the waste heat boiler which can dramatically effect their performance and, as a result, the opacity of the exhaust plume.

Factors Influencing Opacity

The variables identified as contributors to the exhaust plume capacity are as follows:

Black liquor firing rate

The fuel burned in the waste heat boiler is concentrated waste black liquor--a byproduct of the pulping process. If this combustion is not complete, there will be particulate matter leaving the combustion chamber which will contribute to opacity. Higher firing rates tend to lead to higher opacity die to the amount of matter being burned, and the incompleteness of the combustion process. For the Port Townsend plant, the firing rate normally is between 70 and 120 klbs/hr.

Fuel firing pressure

The fuel is sprayed into the combustion chamber via nozzles. The nozzle pressure (fuel firing pressure) influences the size of the liquor droplets--the higher the pressure the smaller the droplets and vice versa. Small droplets have a greater tendency to be carried upward and into the flue gases prior to combustion. In the flue, droplets increase the load on the precipitator and contribute to opacity. However, large droplets are not as easily burned. The normal fuel firing pressure is usually between 13 and 20 psig.

Excess oxygen in the flue gases

The higher the level of excess oxygen (above that required for combustion) in the flue gases, at a given firing rate, the greater the volume that must be treated by the precipitator. Higher volumes of gas tend to increase the velocity at which the flue gases are passing through the precipitator, and therefore lead to lower collection efficiency and higher opacity. Normally, the excess oxygen as a percent of total gas volume is maintained between 3 and 7%.

Exit temperature of the flue gases

The temperature of the flue gases after combustion has taken place cannot be directly controlled, but is important since increased temperature change the resistivity of the particulates and reduce the efficiency of the precipitator, leading to higher opacity. The exit temperature typically varies between 500 and 625 degrees Fahrenheit.

Black liquor solids content of the liquor

Higher proportion of black liquor solids require less fuel for combustion to take place. Also, lower proportions require more water to be evaporated prior to the combustion. Lower solids content, therefore, increases the volume of the flue gases and lead to a decrease in the precipitator efficiency and higher opacity. Port Townsend typically operates between 67 and 74% solids.

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