HYDROTHERMAL TREATMENT AND SILVERADO'S GREEN FUEL PREPARATION

Over the last 40 years researchers around the world have investigated virtually every low-rank coal (LRC), subbituminous, lignitic, and brown coals, drying technology ever proposed. Of all the LRC drying technologies assessed, the most promising is hydrothermal treatment (HT), sometimes called hot water drying. HT is particularly effective for producing Silverado’s concentrated Green Fuel suitable for many liquid fuel applications. Production of Silverado’s Green Fuel with its inherent environmental, transportation and utilization benefits also eliminates the stability problems that have plagued all other LRC drying processes designed to produce a dried LRC that can be safely handled and transported.

HT is an advanced technology, featuring moderate temperature/pressure, non-evaporative drying, which irreversibly removes much of the inherent moisture from LRC. HT upgrading allows fuels derived from LRC to be produced with solids content rivaling those obtained from bituminous coal-water fuels without the uses of costly additives. Silverado’s Green Fuel is a non-hazardous, non-toxic, easily transportable quasi-liquid fuel that avoids all the stability problems of dust generation and spontaneous combustion associated with LRC.

Silverado’s Green Fuel is produced by treating a slurry of pulverized LRC at moderate temperatures, up to 300 degrees C, and the corresponding saturated steam pressure in water, hence the name hydrothermal treatment (HT). HT is similar in many respects to pressure cooking, and the Green Fuel retains all of the desirable combustion characteristics of LRC. Sufficient residence time at reaction conditions is provided in a laminar-flow reactor to ensure that the interior of the largest particle reaches the desired temperature. Upon heating, some water expands out of the coal due to differences in thermal expansion between coal and water. Loosely bound carboxyl groups on LRCs are also released as CO2, expelling additional water from the coal. Some volatile matter, waxy materials in the LRCs, being hydrophobic (water hating), are retained in the micro-pores of the LRC in the pressurized aqueous environment, giving a unique uniform wax distribution, which seals most of the micro-pores and minimizes water re-absorption. By retaining most of the volatile matter, the high reactivity LRC, as well as most of the energy value, is preserved.

Figure 1 is an artist's rendition of what occurs during HT. Following HT, the system pressure is reduced and excess water is removed, leaving Silverado’s Green Fuel. The excess water is reused in the continued production of our Green Fuel.

Left Side - Microscopic View of Raw Low-Rank Coal (LRC) Particle: Water fills macro and micro pores of the raw coal particle. Water is also bound to the coal particle via hydrogen bonding to the oxygen containing sites in the LRC and via electrostatic bonding between oxygen in water and cations (mineral matter) that are bonded to the LRC. This water is called inherent moisture, as opposed to surface moisture and explains why some LRCs containing over 50% moisture appear dry. LRCs include subbituminous, lignitic and brown coals and have inherent or equilibrium moisture values of 20-60%. The high inherent moisture in LRCs increase shipping costs, i.e., a 100-car unit train shipping a LRC with 25% moisture is actually shipping 25 cars of water and only 75 cars of dry coal. This has relegated most LRCs for use as mine-mouth or nearby power plants where the electricity is transported.

Right Side -- Microscopic View of Hydrothermally Treated LRC Particle: Hydrothermal treatment involves heating LRC to coal specific temperatures in an aqueous phase maintained by pressures above the saturated steam pressure somewhat analogous to pressure cooking. Water expands and is expelled from most of the pores when much of the oxygen in LRC is released as CO2 during heating. This eliminates most of the pore bound moisture and that held by the LRC's oxygen functionalities. When CO2 is lost, cations are also released into the water phase eliminating the inherent water associated with LRC cations. However, a key to permanent moisture removal is the evolution of some of the LRC volatile matter as waxy substances upon heating. Waxy material, being hydrophobic is retained on the LRC in the pressurized aqueous environment. Upon cooling it seals most of the micro-pores and limits moisture reabsorption. Following hydrothermal treatment there is a net increase in the energy content of the dry LRC since most of the volatile matter is retained and LRC carbon lost as CO2 has already been oxidized.

Even after HT, LRCs retain some of their oxygen functionality and are still somewhat hydrophilic, or water loving. Therefore, they have a much lower tendency than their bituminous coal-water fuel counterparts to settle rapidly. Some LRC produced fuels have shown almost no settling tendencies even when stored for months. For LRC produced fuels that do settle over time, the tendency is to produce a soft pack suspension that can be re-suspended by stirring. As opposed to some of their bituminous counterparts, no LRC produced fuels created to date, regardless of particle size distribution, have shown any tendency towards dilatent (tending to solidify), shear thickening, behavior. Consequently, the higher the shearing force applied, the lower the viscosity, which accounts for their ease of atomization. Therefore, generally no costly stability enhancing or viscosity reducing additives are used with LRC produced fuel. Instead, stirring and/or agitation are used to maintain a constant feed in storage and feed tanks. Typically the only additive recommended for Silverado’s Green Fuel is a biocide to prevent biological growth in fuel that is to be stored for some period of time before use.

The technical feasibility of hydrothermal treatment has been demonstrated in pilot plants in Australia, Japan, and the US. These pilot plants have operated for thousands of hours with LRCs from around the world. As a general rule, the increases in energy content or density for LRC produced fuels versus coal water slurries prepared from untreated coals are around 30% for subbituminous coals, about 50% for lignites, and well over 100% for brown coals, peat, and biomass. What remains to be done for commercialization is production and end-use testing in a demonstration-scale facility to establish commercial production costs, establish derating values in an oil-designed boiler, and to produce thousand tonne quantities of Silverado’s Green Fuel for potential end-users to test in their own facilities or independent laboratories.

SILVERADO'S GREEN FUEL COMBUSTION

Since most of the LRCs volatile matter is retained in the coal particle in the form of waxy material, the high reactivity and rapid carbon burnout of LRCs, in comparison to bituminous coals, is maintained. In addition, LRCs, even after HT remain non-agglomerating, Figure 2. In all reported combustion tests fuels produced from LRC burned significantly better than bituminous coal-water fuels (CWFs). Fuels produced from LRC ignited more rapidly and gave higher carbon burn out, typically over 99%, when compared with commercial bituminous CWFs. Tests in an Allison coal-fired turbine, using a fuel produced from subbituminous coal from the Powder River Basin in Wyoming, demonstrated the superiority of fuels produced from LRCs over a commercial bituminous CWF. The best carbon burnout obtained with the micronized bituminous coal-water fuel under optimum operating conditions was slightly over 97%, whereas the fuel produced from LRC, which had an average particle diameter three times larger, gave carbon burnout of over 99%, under all operating conditions.

1) Photo of LRCWF combustion looking across the throat of a vertical injector in a coal fired boiler. Notice the intense, bright flame and almost complete lack of “sparklers” that are indicative of agglomeration and incomplete combustion. Contrary to bituminous coal, LRC doesn’t agglomerate, but explodes upon heating for rapid ignition and complete carbon burnout.     2) Same shot with a commercial bituminous CWF. Note the “sparklers,” which are agglomerates many times larger than the feed coal and the poor flame quality. Some gglomerates are ash covered spheroids, containing unburned carbon, that are so large they exceed the entrainment velocity and fall to the boiler floor. This leads to poor carbon burnout and loss of efficiency.

During HT, alkali cations (positively charged ions) associated with the coal particles are released and dissolved in the water. Alkalis, particularly sodium, are the principal cause of severe boiler tube fouling. With their removal as part of the HT process, fuels produced from LRCs are inherently less fouling than their parent raw coal, and in some cases reduce the fouling to levels considered allowable in industry for high sodium coals that have little market value without hydrothermal treatment.

Test quantities of LRCWF were produced from Alaska's ultra-low sulfur subbituminous coal from the Beluga Coal Field and tested in a combustion test facility. Beluga LRCWF proved to be an excellent fuel with less than 4% ash and only 0.07% sulfur. It had a low fuel ratio (ratio of fixed carbon to volatile matter), with volatile matter content close to the level of the raw Beluga coal. Carbon burnout was excellent at over 99.8%. Gaseous emissions were monitored during the test and indicated a very low level of sulfur dioxide (SO2) in the flue gas. Ash deposition was minimal, consisting of a fine white powder easily removed by normal soot blowing operations.

Advanced applications in boilers designed for oil, combustion turbines, and/or diesel engines have placed new demands on the fuel's combustion characteristics due to greatly shortened residence times allowable for combustion. In these cases, the inherent higher reactivity of LRCWF versus bituminous CWF, makes them a superior fuel.