(1) Each gas company must offer by tariff a voluntary renewable natural gas service available to all customers to replace any portion of the natural gas that would otherwise be provided by the gas company. The tariff may provide reasonable limits on participation based on the availability of renewable natural gas and may use environmental attributes of renewable natural gas combined with natural gas. The voluntary renewable natural gas service must include delivery to, or the retirement on behalf of, the customer of all environmental attributes associated with the renewable natural gas.
(2) For the purposes of this section, "renewable natural gas" includes renewable natural gas as defined in RCW
54.04.190. The commission may approve inclusion of other sources of gas if those sources are produced without consumption of fossil fuels.