Many thermal conversion processes can be classified as partial oxidation pro­cesses in which the biomass is supplied with less than the stoichiometric amount of oxygen needed for complete combustion. Both air and oxygen have been utilized for such systems. When the oxygen is supplied by air, low — energy gases are formed that contain higher concentrations of hydrogen, carbon monoxide, and carbon dioxide than medium-energy gases. When pure oxygen or oxygen-enriched air is used, gases with higher energy values can be obtained. In some partial oxidation processes, the various chemical reactions may occur simultaneously in the same reactor zone. In others, the reactor may be divided into zones: A combustion zone that supplies the heat to promote pyrolysis in a second zone, and perhaps to a third zone for drying, the overall result of which is partial oxidation.

One system (Fig. 9.6) uses a three-zoned vertical shaft reactor furnace (Fisher, Kasbohm, and Rivero, 1976). In this process, coarsely shredded feed is fed to the top of the furnace. As it descends through the first zone, the charge is dried by the ascending hot gases, which are also partially cleaned by the feed. The gas is reduced in temperature from about 315°C to the range of 40 to 200°C. The dried feed then enters the pyrolysis zone, in which the temperature ranges from 315 to 1000°C. The resulting char and ash then descend to the hearth zone, where the char is partially oxidized with pure oxygen. Slagging temperatures near 1650°C occur in this zone, and the resulting molten slag of metal oxides forms a liquid pool at the bottom of the hearth. Continuous withdrawal of the pool and quenching forms a sterile granular frit. The product gas is processed to remove flyash and liquids, which are recycled to the reactor. A typical gas analysis is 40 mol % carbon monoxide, 23 mol % carbon dioxide, 5 mol % methane, 5 mol % C2’s, and 20 mol % hydrogen. This gas has a higher heating value of about 14.5 MJ/m3 (n).

An example of the gasification of biomass by partial oxidation in which air is supplied without zone separation in the gasifier is the molten salt process (Yosim and Barclay, 1976). In this process, shredded biomass and air are

continuously introduced beneath the surface of a sodium carbonate-containing melt which is maintained at about 1000°C. As the resulting gas passes through the melt, the acid gases are absorbed by the alkaline media and the ash is also retained in the melt. The melt is continuously withdrawn for processing to remove the ash and is then returned to the gasifier. No tars or liquid products are formed in this process. The heating value of the gases produced depends on the amount of air supplied and is essentially independent of the type of feed organics. The greater the deficiency of air needed to achieve complete combustion, the higher the fuel value of the product gas. Thus, with about 20, 50, and 75% of the theoretical air needed for complete oxidation, the respective higher heating values of the gas are about 9.0,4.3, and 2.2 MJ/m3 (n).

Many gasifier designs have been offered for the manufacture of producer gas from virgin and waste biomass, and several types of units are still available for purchase. As mentioned in the introduction to this chapter, thousands of producer gasifiers operating on air and wood were used during World War II, particularly in Sweden, to power automobiles, trucks, and buses. The engines needed only slight modification to operate on low-energy producer gas. Al­though only limited research has been carried out on small-scale producer gasifiers for biomass in recent years, significant design advancements continue
to be made even though the gasifiers have been used for more than 100 years. One of the interesting developments is the open-top, stratified, downdraft gasifier in which the feedstock such as wood chips moves downward from the top as it is gasified and air is simultaneously drawn in from the top through successive reaction strata (LaFontaine, 1988; LaFontaine and Reed, 1991). Low-cost, portable gasifiers can be assembled for captive use from ordinary metal cans, garbage containers, and drums that are manually loaded with fuel from the open top. More sophisticated units can of course be manufactured. The open — top biomass gasifier is simple to operate, is inexpensive, and can be close-coupled to a gas engine-generator set without requiring the use of complex gas-cleaning equipment. The system appears to be quite suitable for small — and moderate-scale engine applications from 5 to 5000 HP and portable electric-power generation systems. The gasifier dimensions are sized to deliver gas to the engine based on its fuel-rate requirements, and minimal controls are needed.

A similar, wood-fueled, downdraft gasifier patterned after Swedish reports from the early years of World War II was initially built in the United States in the late 1970s of mild steel. It was used to power an unmodified 1978 Chevrolet Malibu station wagon equipped with a 3,3-L (200-in.3) V-6 engine for a coast-to-coast trip from Jacksonville, Florida, to Los Angeles, California, a distance of about 4300 km (Russel, 1980). Small pine and hardwood blocks of 15 to 25 wt % moisture content were used as fuel throughout the trip. The gasifier was pulled on a small two-wheel trailer behind the vehicle. The system was subsequently driven a total of 8046 km. Examination of the vehicle and all components showed no significant wear or abnormalities. A typical composi­tion of the low-energy fuel gas was reported to be 18 mol % carbon monoxide, 9 mol % carbon dioxide, 1 mol % methane, 17 mol % hydrogen, 45 mol % nitrogen, and 10 mol % water. On a distance traveled basis, about 3.0 to 3.6 kg of wood fuel was estimated to equate to 1 L of gasoline.

Steam Gasification

Steam is also blended with air in some gasification units to promote the overall process via the endothermic steam-carbon reactions to form hydrogen and carbon monoxide. This was common practice at the turn of the last century, when producer gasifiers were employed to manufacture low-energy gas from virgin and waste biomass. The producer gas from these gasifiers generally had heating values around 5.9 MJ/m3 (n), and the energy yields as gas ranged up to about 70% of the energy contained in the feed.

Study of the steam gasification of biomass in a sequential pyrolysis-steam reforming apparatus has shown that gasification occurs as a two-step process (Antal, 1978). At temperatures in the 300 to 500°C range, volatile compounds are evolved from biomass and some residual char is formed. At about 600°C, the volatile compounds are steam reformed to yield synthesis gases. The con­densable tars, oils, and pitches are reduced by the steam reforming reactions to less than 10 wt % of the original feedstock. Table 9.5 is a summary of the steam gasification of pure cellulose that illustrates the effects of temperature and residence time in the steam reformer on product yields. As temperature and residence time are increased, char and tar yields decrease and gas yields increase as expected. A medium-energy gas was produced in these experiments because of the relatively high concentrations of lower molecular weight hydro­carbons in the product gas.

An obvious improvement in the steam gasification of biomass for synthesis gas production is to operate at higher temperatures and to use catalysts to gasify as much of the char and liquid products as possible. Laboratory-scale experiments have been carried out to examine this possibility (Mitchell et al, 1980). Nickel precipitated on silica alumina (1:1) and a mixture of silica alumina and nickel on alumina were evaluated as catalysts for steam gasification at 750°C and 850°C and atmospheric pressure. The results are summarized in Table 9.6. The function of the silica alumina is to crack the hydrocarbon

intermediates, and the function of the nickel is to promote methane reforming and the hydrogenolysis of higher molecular weight hydrocarbons. It is evident from the data in Table 9.6 that a synthesis gas almost stoichiometric for methanol synthesis can be produced from wood at high yields by catalytic steam gasification in a single-stage reactor at atmospheric pressure. Potential methanol yields over 60 wt % of the wood feedstock were estimated. The advantages of catalytic steam gasification of biomass over steam-oxygen gasifi­cation include elimination of the need for an oxygen plant and shift conversion, higher methanol yields for a stand-alone plant, and less carbon dioxide forma­tion. Using the data from the example in Table 9.6 in which the steam-to — wood weight ratio is 0.71, and assuming wood that contains 20 wt % moisture is fed at 100°C with steam at 850°C, the net reactor heat requirement is estimated to be 2800 kj/kg of dry wood.

The various stoichiometric equations listed in Table 9.1 suggest that synthe­sis gas mixtures from biomass gasification are generally deficient in hydrogen for methanol synthesis; i. e., the molar ratio of H2: CO is less than 2. The use of steam in biomass gasification could conceivably increase hydrogen yields by reaction of residual char, if formed, via the steam-carbon reaction. Steam gasification might also make it possible to use green biomass feedstocks without drying. Under the proper gasification conditions, the use of oxygen or air to meet any heat requirements would be expected to increase the yields of carbon oxides, but an oxygen plant is required in the case of oxygen usage. Gas quality would suffer with air because of nitrogen dilution of the product gases unless air is utilized separately from the gasification process, as already mentioned. However, as just indicated (Mitchell et al, 1980), it has been shown that product gases containing a 2:1 molar ratio of hydrogen to carbon monoxide can be produced without use of a separate water gas shift unit:

C6H10O5 + 3H20 -» 4CO + 2C02 + 8H2.

Gasification of biomass for methanol synthesis under these conditions would offer several advantages if such processes can be scaled to commercial size.

Commercial methanol synthesis is performed mainly with natural gas feed­stocks via synthesis gas. Synthesis gas from biomass gasification could conceiv­ably be used as a cofeedstock in an existing natural gas-to-methanol plant to utilize the excess hydrogen produced on steam reforming natural gas. Examina­tion of a hypothetical hybrid plant has been shown to have significant benefits (Rock, 1982). Typical synthesis gas mixtures from the steam-oxygen gasifica­tion of wood and the steam reforming of natural gas are as follows:

From wood: 2CO + C02 + 1.8H2

From natural gas: 5.2/3(2CO + C02 + 10H2)

Combined: 5.5CO + 2.7C02 + 19.1H2.

This combined synthesis gas mixture is stoichiometric for methanol synthesis:

5.5CO + 2.7C02 + 19.1H2 -* 8.2CH3OH + 2.7H20.

The stoichiometry for methanol from the unmixed gases is

2CO + C02 + 1.8H2 + 0.73H2O 1.27CH3OH + 1.73C02

5.2/3(2CO + C02 + 10H2) 5.2CH3 OH + 5.2H2 + 1.73H20.

The unmixed synthesis gases produce 6.47 mol of methanol, of which 1.27 mol comes from wood, and the mixed synthesis gases yield 8.20 mol of methanol. In theory, the use of the combined synthesis gases provides 24% more synthesis gas, but methanol production is increased by 58% over that from natural gas alone. Since hydrogen in the purge gas in the reformed natural gas case has been largely consumed in the hybrid case, the total purge gas stream is greatly reduced. This purge gas is normally used as fuel in the reforming furnace and its reduction must be balanced by firing additional natural gas or other fuel for reforming. The use of natural gas and fuel is about 25% lower for the hybrid design than when using natural gas only for the production of the same amount of methanol. In addition, the hybrid version has eliminated the water gas shift and acid gas removal equipment from the wood gasification process alone. This serves to reduce both capital and operat­ing costs associated with wood-derived synthesis gas.

The stoichiometry of this particular hybrid process is approximately as follows:

Wood: 0.5C6H10O5 + 1.102 — 2CO + C02 + 1.8H2 + 0.7H2O

Natural gas: 5.2CH4 + 6.93H20 — 3.47CO + 1.73C02 + 17.33H2

Methanol synthesis: 5.47CO + 2.73C02 + 19.13H2 — 8.2CH3OH + 2.73H20 Net: 0.5C6H10O5 + 5.2CH4 4- 3.5H20 + 1.Ю2 -► 8.2CH3OH.

By use of the enthalpy of formation for dry poplar wood of 840.1 kj/g-mol (361,440 Btu/lb-mol) of cellulosic monomeric unit at 300 K, which is calculated from its measured heat of combustion and the standard enthalpies of formation for the other components, the enthalpy changes for wood gasification (with oxygen) to synthesis gas, the steam reforming of natural gas, and methanol synthesis, are calculated to be —363.8, 1001, and —631.5 k], respectively. In theory, the overall enthalpy change is almost zero, 5.7 kj. Biomass gasification can of course be carried out in several ways, and the gas compositions used for this analysis are idealized. But this type of analysis makes it possible to calculate several parameters of interest. For example, assuming 100% selectivi- ties for intermediates and products, or that no by-products are formed, and that poplar wood and natural gas are accurately represented by (C6H10O5) and CH4, the feedstock rates for a 907-t/day (1000-ton/d) methanol plant are estimated to be 0.4 million m3 (n)/day (288 t/day) of natural gas and 280 t/ day of dry wood.