Particulate emissions: Particles of a solid or liquid suspended in a gas, or the fine particles of carbonaceous soot and other organic molecules discharged into the air during combustion.

Particulate: A small, discrete mass of solid or liquid matter that remains individually dispersed in gas or liquid emission.

Particulates: A fine liquid or solid particle such as dust, smoke, mist, fumes, or smog, found in air or emissions.

Pathogens: is a microorganism—in the widest sense, such as a virus, bacterium, prion, or fungus—that causes disease in its host.

Pay zone thickness: The depth of a tar sand deposit from which bitumen (or a product) can be recovered.

Pelletizing: Pelletizing is the process of compressing or molding a material into the shape of a pellet.

Performance specifications: For mechanical systems, specifications used in design/build and turnkey contracting. Set forth the owner’s minimum requirements for how a system will be configured and function.

Per-unit Sale: A timber sale in which the buyer and seller negotiate a price per unit of harvested wood, and the buyer pays for the timber after it is cut and the volume is determined.

Pesticides: are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.

Petrochemical Feedstock:  Petroleum (hydrocarbon) based substance used as a raw material in an industrial process. Examples of petrochemical feedstocks are ethylene, propylene, butadiene, benzene, toluene, xylene, and naphthalene.

Petroleum: Any petroleum-based substance composed of a complex blend of hydrocarbons derived from crude oil, including motor fuel, jet oil, lubricants, petroleum solvents, and used oil.

Phloem: In plants, the inner bark; the principal tissue in a tree concerned with the transport of sugars and other nutrients from the leaves.

Photoautotroph: An organism, typically a plant, obtaining energy from sunlight as its source of energy to convert inorganic materials into organic materials for use in cellular functions such as biosynthesis and respiration.

Photoconversion: Conversion of light into other forms of energy by chemical, biological, or physical processes.

Photoheterotroph: heterotrophic organisms that use light for energy, but cannot use carbon dioxide as their sole carbon source. Consequently, they use organic compounds from the environment to satisfy their carbon requirements. They use compounds such as carbohydrates, fatty acids and alcohols as their organic "food". Examples are purple non-sulfur bacteria, green non-sulfur bacteria and heliobacteria.

Photosynthesis: A process used by many plants and bacteria to build carbohydrates from carbon dioxide and water, using energy derived from light. Photosynthesis is the key initial step in the growth of biomass and is depicted by the equation:

       CO2 + H2O + light + chlorophyll = (CH2O) + O2.

Pile burner: A type of biomass combustion burner in which a pile of fuel burns on the grates. Primary combustion air comes from above the grates, not below.

Pilot scale: The size of a system between the small laboratory model size (bench scale) and a full-size system.

Plantation: Planted pines or hardwoods, typically in an ordered configuration such as equally spaced rows.

Poletimber trees: Live trees at least 5.0 inches in d.b.h. but smaller than sawtimber trees.

Polymer: A large molecule made by linking smaller molecules (monomers) together.

Polysaccharide: A long-chain carbohydrate containing at least three molecules of simple anhydrous sugars linked together. Examples include cellulose and starch.

Pour point: The lowest temperature at which oil will pour or flow when it is chilled without disturbance under definite condition.

Pre-commercial Thinning: Thinning that occurs when trees are too young, too small, or of species undesirable to be used for traditional timber products.

Prescribed fire: Any fire ignited by management actions to meet specific objectives. Prior to ignition, a written, approved prescribed fire plan must exist, and National Environmental Protection Act requirements must be met.

Present value: The worth of future receipts or costs expressed in current value. To obtain present value, an interest rate is used to discount future receipts or costs.

Primary wood-using mill: A mill that converts round wood products into other wood products; common examples are sawmills that convert saw log into lumber and pulp mills that convert pulpwood round wood into wood pulp.

Process development unit: An experimental facility that establishes proof of concept, preliminary process economics, and engineering feasibility for a pilot or demonstration plant.

Process heat: heat used in an industrial process rather than for space-heating or other housekeeping purpose.

Process steam: Steam used as a high-temperature medium for a variety of industrial purposes.

Producer gas: combustible gas mixture (CO, H2, CH4,CO2,N2) from gasification. see SyngasBio-syngasFuel gas, Natural gasLandfill gas.

Proof: The ethanol content of a liquid at 60 degrees Fahrenheit stated as twice the percent by volume of the ethyl alcohol.

Protein: A protein molecule is a chain of up to several hundred amino acids and is folded into a more or less compact structure. Because about 20 different amino acids are used by living matter in making proteins, the variety of protein types is enormous. In their biologically active states, proteins function as catalysts in metabolism and to some extent as structural elements of cells and tissues. Protein content in biomass (in mass %) can be estimated by multiplying the mass % nitrogen of the sample by 6.25.

Proximate analysis: The determination, by prescribed methods, of moisture, volatile matter, fixed carbon (by difference), and ash. The term proximate analysis does not include determinations of chemical elements or determinations other than those named. The group of analyses is defined in ASTM D 3172.

PTO: Power take-off. Drive shaft from tractor, or other vehicle engine used to drive ancillary machinery. Can be used to power some wood chippers.

Public power: The term used for not-for-profit utilities that are owned and operated by a municipality, state or the federal government.

Public utility commissions: State agencies that regulate investor-owned utilities operating in the state.

Public utility regulatory policies act: (PURPA) A Federal law requiring a utility to buy the power produced by a qualifying facility at a price equal to that which the utility would otherwise pay if it were to build its own power plant or buy power from another source.

Pulp chips: Timber or residues processed into small pieces of wood of more or less uniform dimensions with minimal amounts of bark.

Pulpwood: Round wood, whole-tree chips, or wood residues that are used for the production of wood pulp.

Pyrolysis is the thermal decomposition of biomass occurring in the absence of oxygen. It is the fundamental chemical reaction that is the precursor of both the combustion and gasification processes and occurs naturally in the first two seconds. Many dehydration, cracking, isomerization, dehydrogenation, aromatization, coking, and condensation reactions and rearrangements occur during pyrolysis. The products of biomass pyrolysis include biochar, bio-oil and gases including methane, hydrogen, carbon monoxide, and carbon dioxide.  Depending on the thermal environment and the final temperature, pyrolysis will yield mainly biochar at low temperatures, less than 450 oC, when the heating rate is quite slow, and mainly gases at high temperatures, greater than 800 oC, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is bio-oil. Pyrolysis processes can be categorized as slow pyrolysis or fast pyrolysis. Fast pyrolysis is currently the most widely used pyrolysis system. Slow pyrolysis takes several hours to complete and results in biochar as the main product. On the other hand, fast pyrolysis yields 60% bio-oil and takes seconds for complete pyrolysis. In addition, it gives 20% biochar and 20% syngas. see Conventional pyrolysis.

Pyrolysis oil: Bio-oil produced by fast pyrolysis of biomass. A dark brown, mobile liquid containing much of the energy content of the original biomass, with a heating value about half that of conventional fuel oil. Can be burned directly, either alone or co-fired with other fuels, gasified or otherwise upgraded.

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