Btu/ft3. The use of oxygen instead of air produces a medium heating value syngas, 265 to 400 Btu/ft3, suitable for combustion turbine application.
Oxygen production is expensive, and hence indirectly heated gasifiers which utilize twin bed concept similar to fluid catalytic crackers (FCC) that are being used in crude oil refining are being developed for generation of medium calorific syngas using air as fluidizing medium.
The syngas produced from the indirectly heated gasifiers is devoid of the nitrogen contamination, and hence they are also favored for other gas- to liquid-based technologies. The non-requirement of oxygen to generate medium heating value syngas, results in lower capital cost for the gasification plant.
If the gas turbines using fossil fuel (natural gas or diesel) which has heating value of approximately 885 Btu/ft3, needs to be interchangeable with the biomass syngas, higher heating values are preferred. In a natural-gas-fired turbine, the gas is only related to 2% v/v of the flow and the rest is air for dilution and combustion, while in contrast, syngas accounts for 14 to 16% v/v of the total gas.
The IGCC (Integrated Gasification Combined Cycle) uses air as the gasifying agent and requires modified turbine combustors to handle the low heating-value gas (110 to 190 Btu/ft3). The low calorific syngas up to 135 Btu/ft3 is only suitable for firing in the boiler or use in diesel engines.
Indirectly heated gasifiers in general generate syngas with a high percentage of hydrogen, methane, and C2+ compounds which directly contribute for the higher heating value of the syngas.
Similarly for the directly fired gasifiers, the higher range of hydrogen, methane, and C2+ components corresponds to the combustion using oxygen, leading to higher calorific value of the syngas. The absence of nitrogen (inert) in the system largely helps to concentrate the composition of syngas with higher heating value components. Production of tars, chars, and volatile alkalis are problems associated with gasification which requires further cleaning before it is utilized in turbines for power generation.
The product stream at high pressure and temperature needs to be cleaned under hot conditions not lower than 540°C (1005°F) (tar dew point) in order to maximize the energy conversion efficiency. Thus, a hot cleanup system is required, for which either catalytic tar crackers or a thermal tar cracker could be utilized. A catalytic tar reformer will operate at temperatures comparable to gasifier temperature of 825°C (1,515°F), while a thermal cracker will typically operate at 870 to 980°C (1600 to 1,795°F). After the tar reformer/cracker, the product gas will be partially cooled to minimize the amount of alkali vapors, typically to 350 to 650°C. The product will then pass through a filter to remove solids. As gas turbine application limits the alkali to less than 25 ppb, much of alkali as well needs to be removed.
See also: Synthesis Gas Quality.
Biomass – Herbaceous
Herbaceous biomass is biomass from plants that have a nonwoody stem and which die back at the end of the growing season. This biomass includes most agricultural crops and grasses, including bamboo and wheat straw. Also, relatively young and essentially nonwoody parts of trees exhibit similar characteristics. In general, herbaceous biomass will have higher nutrient contents and lower lignin contents than wood. Herbaceous biomass is variable in composition depending on the time of year and on the type of tissue. Composition can also be strongly influenced by the availability of minerals or nutrients in the soil.
Herbaceous biomass is classified into cereal crops, pastures, oilseed crops, tubers and legumes, flowers, herbaceous biomass of gardens, parks, pruning, vineyards, orchards, and mixtures of all these. This biomass can be used raw (direct residues of the field) or processed (from the food industry).
Residues from cereal crops include the use of the following parts of the plant: dried stems, pods, and husks, as well as their mixtures. Of the grasslands are considered stems, shells, and their mixtures. In oil crops, stems, leaves, pods, rinds, and their mixtures are considered. The residues of the tuber crops are integrated by stems, leaves, roots, and mixtures of these. The stems, leaves, pods, and their mixtures can be harvested from legume crops, but it should be noted that the incorporation of legumes in an annual biomass cultivation system could reduce the nitrogen fertilizer requirements, which will reduce the inputs of bioenergy production. However, they also contribute to the anthropogenic emissions of nitrous oxide (NO2) which, in this case, nitrous oxide comes mainly from the decomposition of legume residues.
Finally, from the cultivation of flowers, the waste from the plant (when the quality of these is not adequate), the stems, leaves, as well as their mixture can be used. For its part, processed residues can come from herbaceous residues without chemical treatment, as well as chemically treated herbaceous residues and its mixtures. This type of biomass is used mainly in combustion and co-combustion systems, where the technology is well developed, has high efficiency, and low emissions; nevertheless, herbaceous crops that have a high content of potassium which may be detrimental for biomass combustion equipment. It is also used, albeit to a lesser extent, in gasification systems in mixtures with other sources of biomass.
The most abundant form of herbaceous biomass in the world, particularly in European countries, is straw, which is obtained during the annual production of cereals in a proportion of 0.6 to 0.8 tons of straw per ton of grain in the field. In countries where crop productivity is high (in the order of 8 to 10 tons of grain per hectare), the amount of straw per hectare can be between 6 and 8 tons and much of it has been removed for the next crop cycle.
Herbaceous biomass contains variable amounts of cellulose, hemicellulose, and lignin, and a small amount of other extractive, usually perennial, with looser fibers. The relative proportions of cellulose and lignin is one of the determining factors to identify the suitability of plant species for further processing as energy crops (Table B-21).
Table B-21 Approximate composition of herbaceous biomass gas from different sources.
| Properties | Herbaceous biomass type | |
|---|---|---|
| Wheat straw | Barley straw | |
| Moisture (%, w/w) | 16 | 30 |
| Volatile material (%, w/w) | 59 | 46 |
| Fixed carbon (%, w/w) | 21 | 18 |
| Ash (%, w/w) | 4 | 6 |
| Net calorific value (MJ/kg) | 18.6 | 16.1 |
| C (%, w/w) | 48.5 | 45.7 |
| H (%), w/w | 5.5 | 6.1 |
| O (%, w/w) | 3.9 | 38.3 |
| N (%, w/w) | 0.3 | 0.4 |
